Citation
Factors affecting sicklepod (Cassia obtusifolia L.)) competition in Florida-grown soybeans

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Title:
Factors affecting sicklepod (Cassia obtusifolia L.)) competition in Florida-grown soybeans
Alternate title:
Cassia obtusifolia
Creator:
Perry, Kevin McDonald, 1959-
Publication Date:
Language:
English
Physical Description:
x, 95 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Agronomy thesis Ph. D
Dissertations, Academic -- Agronomy -- UF
Senna obtusifolia ( lcsh )
Soybean -- Diseases and pests ( lcsh )
Soybean -- Weed control ( lcsh )
City of Gainesville ( local )
Soybeans ( jstor )
Row spacing ( jstor )
Tillage ( jstor )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Includes bibliographical references (leaves 88-94).
Additional Physical Form:
Also available online.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Kevin McDonald Perry.

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Full Text

















FACTORS AFFECTING SICKLEPOD (Cassia obtusifolia L.)
COMPETITION IN FLORIDA-GROWN SOYBEANS









By

KEVIN McDONALD PERRY


















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA 1987














ACKNOWLEDGEMENTS



I wish to express my sincere appreciation to Dr. Wayne Currey, Dr. Barry Brecke, Dr. David Hall, Dr. Cliff Hiebsch and Dr. Jerry Sartain for their support in preparing this manuscript. I especially acknowledge Dr. Wayne Currey, chairman of my committee, for allowing me to continue my education at the University of Florida and for providing of his time and experience.

I would especially like to thank Mr. David W.

Studstill, whose technical assistance in conducting this research was an essential ingredient.

Thanks are extended to my fellow graduate students and co-workers for sharing with me their ideas, and fellowship and most importantly their friendships.

I thank my parents, Mr. and Mrs. Ben C. Perry for giving of their time, love and understanding and for an upbringing which instilled the necessary character to reach this goal in my life.

Finally I thank my loving, wife JoAnn, for her

understanding and patience throughout my education. It is her love and kindness that has enabled me to pursue this degree and makes all these sacrifices worthwhile.


ii









TABLE OF CONTENTS


PAGE

ACKNOWLEDGEMENTS ..................................... ii

LIST OF TABLES.......................... ************** V

LIST OF FIGURES........... ....................***** *** viii

ABSTRACT ........ ................................. ..... ix

CHAPTERS

I INTRODUCTION.................................... 1

II SICKLEPOD (Cassia obtusifolia L.) SPHERE OF
INFLUENCE IN SOYBEANS........................... 4

Introduction.................................... 4
Materials and Methods .......................... 6
Field and Planting Conditions .............. 6
Soil Moisture Measurements...................... 7
Experimental Design .......................... 8
Data Collection Procedures.................. ....... 9
Results and Discussion.......................... 11
Weed-Crop Biomass Accumulation.................. 11
Sicklepod Interference Effects on Vegetative
Growth ..................................... 19
Sicklepod Interference Effects on Soybean Seed
Yield....................... ................. 27
Sicklepod Interference Effects on Soil Water.. 33

III EFFECT OF ROW SPACING, TILLAGE SYSTEM AND PLANTING DATE ON SICKLEPOD COMPETITIION IN
SOYBEANS...................... 37

Introduction ....................... .. 37
Materials and Methods....................... 40
Soybean-Sicklepod Competition Studies.......... 40
Competition of sicklepod with soybeans-1985..................................... 41
Competition of sicklepod with soybeans-1986.......... ..... ............. .......... 42
Sicklepod Competition--Planting Date Study.... 43
Results and Discussion......................... 45
Soybean-Sicklepod Competition Studies......... 45
Row spacing-tillage system 1985............ 45
Row spacing-tillage system 1986............. 53



iii









Sicklepod biomass accumulation and soybean
yield--1985.............................. 59
Sicklepod biomass accumulation and soybean
yield--1986 .............................. 61
Sicklepod Competition-Planting Date Study.... 63
Planting date--1984 ......... ............... 63
Planting date--1985 .............................. 66
Planting date--1986........................ 72

IV SUMMARY AND CONCLUSIONS........................ 81

APPENDICES

A GAINESVILLE, FLORIDA MAY TO NOVEMBER 1984
PRECIPITATION ................................ 85

B GAINESVILLE, FLORIDA MAY TO NOVEMBER 1985
PRECIPITATION...... .......................... 86

C GAINESVILLE, FLORIDA MAY TO NOVEMBER 1986
PRECIPITATION ..... ................... 87

LITERATURE CITED ............. ............... .... 88

BIOGRAPHICAL SKETCH................................... 95




























iv








LIST OF TABLES



TABLE PAGE

2.1 Growth parameters for soybeans grown without weed interference 1984, 1985 and 1986....... 13

2.2 Growth parameters for sicklepod grown with soybeans 1984, 1985 and 1986... ............ 14

2.3 Relationship of soybean stem dry weight reduction to distance from sicklepod ......... 20

2.4 Relationship of soybean leaf dry weight reduction to distance from sicklepod......... 21

2.5 Relationship of soybean leaf area reduction to distance from sicklepod ................... 22

2.6 Relationship of soybean pod dry weight reduction to distance from sicklepod......... 28

2.7 Relationship of soybean seed weight reduction to distance from sicklepod................... 29

2.8 Relationship between sicklepod canopy width and sicklepod leaf area and its effect on
soybean leaf area reduction, 1986............ 31

2.9 Relationship of soil moisture reduction to distance from sicklepod ..................... 34

3.1 Effect of soybean row spacing and tillage system on soil moisture (millibars) at 15 cm
depth, 1985 ........................ ....... 46

3.2 Effect of soybean row spacing and tillage system on percent canopy closure............. 47

3.3 Effect of soybean row spacing and tillage system on soil moisture (millibars) at 30 cm
depth, 1985................. ............... 51

3.4 Effect of soybean row spacing and tillage system on soil moisture (millibars) at 15 cm
depth, 1986 ................................ 54

3.5 Effect of soybean row spacing and tillage system on soil moisture (millabars) at 15 cm
depth. Sampling time 6, 1986................ 55

v








3.6 Effect of soybean row spacing and tillage
system on soil moisture (millibars) at 30 cm
depth. Sampling time 6, 1986................ 55

3.7 Effect of soybean row spacing and tillage
system on soil moisture (millibars) at 30 cm
depth, 1986.................................. 58

3.8 Effect of soybean row spacing and tillage
system on sicklepod biomass accumulation and
soybean grain yield, 1985..................... 60

3.9 Effect of soybean row spacing and tillage
system on sicklepod biomass accumalation and
soybean grain yield, 1986.................... 62

3.10 Effect of row spacing, tillage system and
planting date on soybean grain yield, 1984... 65

3.11 Effect of row spacing, tillage system and
planting date on soybean grain yield, 1985... 67

3.12 Effect of row spacing and tillage system on
sicklepod biomass accumulation and soybean
grain yield, 1985. Planting date A.......... 68

3.13 Effect of row spacing and tillage system on
sicklepod biomass accumulation and soybean
grain yield, 1985. Planting date B.......... 69

3.14 Effect of row spacing and tillage system on
sicklepod biomass accumulation and soybean
grain yield, 1985. Planting date C.......... 71

3.15 Effect of row spacing, tillage system and
planting date on soybean grain yield, 1986... 73

3.16 Effect of row spacing and tillage system on
sicklepod biomass accumulation and soybean
grain yield, 1986. Planting date A........... 74

3.17 Effect of row spacing and tillage system on
sicklepod biomass accumulation and soybean
grain yield, 1986. Planting date B.......... 75

3.18 Effect of row spacing and tillage system on
sicklepod biomass accumalation and soybean
grain yield, 1986. Planting date C.......... 76

3.19 Effect of soybean row spacing and tillage
system on percent canopy closure. Planting
date A (May 15). Average of 1984 and 1985... 78

vi








3.20 Effect of soybean row spacing and tillage
system on percent canopy closure. Planting
date B (June 1). Average of 1984 and 1985... 79

3.21 Effect of soybean row spacing and tillage
system on percent canopy closure. Planting
date C (June 15). Average of 1984 and 1985.. 80












































vii








LIST OF FIGURES



FIGURE PAGE

2.1 A digramatic representation of the experimental technique used for the
sicklepod/soybean sphere of influence
studies ...................................... 10

2.1 Total dry weight and leaf area accumulation of soybean grown with sicklepod, 1986....... 14

2.2 Total dry weight and leaf area accumulation of sicklepod grown with soybeans, 1986....... 16

2.3 Total leaf accumulation of soybean grown at varying distances ftom sicklepod, 1986....... 23

2.4 Comparison of heights of competing sicklepod and soybean plants (average of 3 years),
Gainesville.......... .............. .......... 25






























viii















Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy


FACTORS AFFECTING SICKLEPOD (Cassia obtusifolia L.)
COMPETITION IN FLORIDA-GROWN SOYBEANS By

KEVIN McDONALD PERRY

May, 1987


Chairman: Dr. W. L. Currey Major Department: Agronomy


Field studies were conducted to monitor the

interference effects of one sicklepod (Cassia obtusifolia L.) plant on soybean [Glycine max (L.) Merr. 'Braxton'] development and seed yield from 1984-1986 to quantify the length and area of soybean row influenced by a single sicklepod plant. The sicklepod's first measurable effect on soybean development occurred eight weeks after planting (WAP) in 1984 and 1985, and occurred at 10 WAP in 1986. Weed interference effects were greatest on soybean plants closest to the weed and decreased linearly with increasing distance from the weed. Sicklepod reduced soybean seed weight by 26, 12 and 19% when calculated for 2 m of row in 1984, 1985 and 1986, respectively, although the yield




ix








suppression was determined by regression analysis to be for a total length of 1.6, 1.26, and 1.3 m of row for the same periods. Sicklepod influenced available soil water at a distance Of 0.32 to 0.56 m of row from July 15 to October 2, 1986. It was determined that soybean leaf area was the most sensitive developmental parameter monitored, ie. reduced by an average of 21%. Sicklepod competition was not found to significantly reduce soybean height. Area of influence methodology can accurately monitor the effects of weed-soybean interference over the extent of a growing season as well as sicklepod effect on seed yield.

Additional field studies were conducted to evaluate the effects of soybean row spacing, tillage system and planting date on sicklepod competition in soybeans. In all three years, soybeans grown in 25 cm rows yielded more highly than those grown in 50 or 75 cm rows, under similiar sicklepod densities. Soybeans grown in 25 and 75 cm rows had a significantly higher rate of soil water depletion in both years. Mean soil water use rate did not differ between tillage systems, although at individual sampling times during stress conditions water use was higher under no-tillage conditions. Narrow-row soybeans had higher yields when planted on two of the three planting dates tested, with 25 cm soybeans having a yield advantage when planted on the two early season planting dates. Narrow-row soybeans did not have a yield advantage when planted on June 15.

X














CHAPTER 1
INTRODUCTION


Sicklepod (Cassia obtusifolia L.) is considered to be the most troublesome weed in soybean [Glycine max (L.) Merr.] production in several southeastern states and is a problem in many other important agronomic crops. In the absence of a reliable herbicide program that provides season-long control without crop injury many growers have relied on cultural management techniques such as cultivation, row spacing and planting dates to assist in managing heavy sicklepod infestations. In the coarse textured sandy soils of Florida, multiple tillage operations often lead to loss of soil structure resulting in water and wind erosion and reduced aeration and water infiltration. Selective herbicides have made possible the development of no-tillage cropping systems which considerably reduce soil erosion and increase soil water retention due to the organic mulch on the soil surface.

In recent years the push to produce a higher profit

margin per unit area has caused growers to initiate changes in production practices used by past generations. The advantages of reducing row spacings to better utilize essential growth factors such as water and light has been


1








readily received by farmers in the northern region of soybean producing areas. The inability to use mechanical cultivation in narrow-row soybean production would lend itself readily to no-till production practices.

Competition for water appears to be an important factor involved in the mechanism of interference between sicklepod and soybeans. Several researchers have investigated the effects of weeds on soil water depletion (20,49,58,65,66, 75). However, few have investigated the effects of tillage practices and row spacings on'weed interference and soil water use in soybeans. The objectives of this research were to determine the effects of tillage and row spacing on the interference effects of a uniform sicklepod density, and to determine whether the added mulch and shading effect of reduced row spacing influenced soil water depletion.

Determination of damage thresholds for sicklepod and other weeds is essential for the effective development of economical and reliable weed management systems. Several weed scientists have indicated that to increase accuracy in predicting weed-crop interactions, weed interference effects should be integrated into crop simulation growth models (39, 56). Density oriented competition studies have often been used to provide the necessary support data to validate existing prediction models. These types of competition studies, although very reliable, are also very laborious and difficult to maintain. A new research methodology (29) referred to as 'area of influence' in which the competitive








effects of a single weed plant on a linear row of crop was evaluated as an alternative to the conventional method. A cooperative research project was initiated in hopes of providing a more feasible research technique and also to provide the necessary information to validate a weed-crop phenology model which is presently being implemented. Research objectives in which the application of grower production practices and the initiation of a unique research technique both serve to provide a better source of information to aid the farmer.














CHAPTER 2
SICKLEPOD (Cassia obtusifolia L.) SPHERE OF INFLUENCE IN SOYBEANS.


Introduction

Weed competition is one of the primary limiting factors in soybean production throughout the world. McCormick (37,44,68) lists sicklepod as one of the most common and troublesome weeds in soybeans in the southeastern United States. Teem et al. (70) reported that sicklepod at 3, 5 and 7 weeds/m of row reduced soybean yields 19, 25 and 38%, respectively.

Many of the studies on weed competition such as work in soybeans by Coble and Ritter (10) and many others (5,6,14,18,28,32,46,60,71,73) have emphasized weed densities and duration of competition. Competition studies aid growers in determining the feasibility of controlling particular weeds at various densities. Determination of economic thresholds for sicklepod and other specific weeds and weed complexes is essential for the effective development of practical and reliable weed management systems (36).

The development of models is one feasible approach to the study of weed biology and its application to crop


4








production (56). Growth simulation models for weeds will be utilized to their fullest extent as we determine how the growth and development of weeds not only respond to their environment, but also how weeds change the environment of an associated crop (or weed).

In a study conducted by Johnson and Coble in which the competition of five different broadleaf weeds in soybeans was evaluated using a replacement planting technique, sicklepod was found to be the least competitive species studied (30). Since there are numerous literature citations (7,14,18,32,70,71,73) detailing that sicklepod can be a very damaging weed in soybeans throughout the southeast, these results need further explanation. It has been shown by Patterson et al. (47) that the replacement technique using microplots in which only short term vegetative studies are done does provide a good indication of ultimate yield and/or crop yield reduction. Therefore a new method of determining crop-weed interactions as an alternative to conventional methods of weed competition research was evaluated.

Experiments were designed to measure the effects of sicklepod on the vegetative and reproductive growth characteristics of soybeans at various intervals during the growing season. These data, combined with information obtained from concurrent studies of weed competition in soybeans, were used to develop a growth simulation model to predict crop response to varying levels of broadleaf weed








competition. The data required for calibration of the model and validation of results include dry weight of individual soybean plants, leaf area, plant height, pod weight and seed weight. Objectives were to determine the effects of a single sicklepod plant on the growth and development of a soybean variety common to soybean production areas in the southeastern coastal plains of the United States.



Materials and Methods



Field and Planting Conditions

Field experiments were conducted in 1984, 1985 and 1986 at the University of Florida Agronomy Farm in Gainesville, Florida. The soil at this site was a Bonneau fine sand (Loamy, Siliceous, Thermic, Arenic Paleudults) with a pH of 6.2. 'Braxton' soybeans were seeded in 75-cm wide rows which were planted in a north-south orientation, to a depth of 3 cm with a double-row planter on July 6, 1984, June 19, 1985, and June 5, 1986. On the same dates, local biotypes of sicklepod seed were hand planted 10 cm from every other soybean row to a depth of 3 cm and were spaced 2.5 m apart in 10 m long soybean rows to prevent interference effects between weeds. Sicklepod seed were randomly planted on either the east or west side of the soybean row. Prior to planting, sicklepod seed were scarified using a mechanical sandpaper scarifying device. To promote uniform soybean and sicklepod emergence, the area was irrigated with 2 to 3 cm






7

of water after planting. Soybeans and sicklepod emerged 4 to 5 days after planting. Approximately 10 days after planting, soybeans were thinned to 25 plants/m. In all three years of this study, test plots were treated preemergence with oryzalin (3,5-dinitro-N4, N4-dipropylsulfanilamide) at a rate of 0.84 kg/ha to control annual grass species and small seeded broadleaves; other unwanted weed species were removed by hand weeding. Soil Moisture Measurements

Relative soil moisture was recorded using single unit mercury manometers in 1985 and using tensiometer tubes in conjunction with a soil moisture TENSIMETER1 in 1986. No soil moisture measurements were taken in 1984. Tensiometer tubes were placed at a depth of 25 cm in five replications at 5 different sicklepod plants in which only nondestructive data were obtained, on June 30 and June 20 for 1985 and 1986, respectively.

1985. The tensiometers consisted of a 50 cm tube with a 10 cm porous ceramic cup. The tube was placed to the 25 cm depth in the soil and connected to the above-ground manometer scale by a single, transparent, plastic tube that served as the manometer measuring tube as well as the connecting link between the manometer assembly and the tensiometric tube. The plastic tube was inserted into a reservoir containing 30 grams of mercury, with the opposite end in the water filled cup tube. The manometer scale was


1TENSIMETER, Soil Measurement Systems, Las Cruces, NM.






8

graduated in millibars of soil tension, a standard unit of measurement for soil moisture. Tensiometers were placed 5 cm laterally from one of the soybean rows and adjacent to a sicklepod plant. Tensiometers were placed at a distance of 0, 15 and 30 cm from each of five different sicklepod plants which were growing within 10 cm of the soybean row. Tensiometric readings were recorded ten times from July 19 to October 20, 1985.

1986. A different technique was used for soil moisture measurements in 1986. Soil moisture tensiometer tubes were placed at the same depth and in an identical arrangement as in 1985. In 1986, a soil moisture TENSIMETER replaced the above ground manometer unit. The Soil Measurement Systems TENSIMETER is a hand-held meter that gives digital read-out for tensiometers. It includes a high quality pressure transducer with attached enclosed syringe needle, and a digital read-out. To operate the TENSIMETER the needle is inserted through the septum stopper of the tensiometer and the tension inside the tensiometer is read directly in millibars. Tensiometric readings were recorded nine times from July 3 to October 9, 1986. Experimental Design.

Each experiment consisted of two weed-soybean treatment combinations: sicklepod growing adjacent to soybeans and soybeans growing without weed interference. Weed interference effects on soybeans were tested in a completely randomized design. Treatment effects were analyzed using






9

analysis of variance and regression procedures. Completely randomized design was employed using the assumption that the weed-free and sicklepod infested areas were uniform in response to growth characteristices of the soybeans. Eight subsamples were taken at each harvest interval and were used to generate the error term, no block effects were employed. Data Collection Procedures.

Destructive vegetative harvests of weeds and soybeans were conducted at 2-week intervals beginning 4 weeks after planting (WAP) and ending 10 WAP in 1984 and 1985 and 14 WAP in 1986. Soybeans were harvested in all three years at distances of 0-25, 25-50 and 50-100 cm from either side of the sicklepod plant (Figure 2.1). Soybeans grown without weed interference were sampled at the same distances from a central point.

Plant height (ground to the most distal node), growth stage, leaf and stem dry weight and total leaf area (1986 only) were recorded for each soybean and sicklepod harvested. Weed canopy width in 1986 was determined by measuring across the widest point of the apex of the sicklepod plant. Plants were harvested at soil level and taken back to a laboratory work area where leaves were removed and stored in refrigeration until leaf area could be measured. Leaf area was measured with an automatic leaf
2 o
area meter2. Leaves and stems were dried at 60-65 C for 48 hours before dry weights were obtained. Soybean yield data


2Li-COR 3100, LI-COR, Inc., Lincoln, NE.





10








































S SOYBEAN S= SICKLEPOD


Figure,'2.1. A diagramatic representation of the experimental technique used for the sicklepod/soybean sphere of influence studies.






11

were collected at seed maturity (11-30-84, 11-27-85 and 11-13-86). The total pod and seed weights were recorded for each soybean plant harvested at the various specified distances from the weed. Rainfall data were collected at the experimental site for all three years.

Data for soybeans sampled at identical distances from a weed combined to give an average value for each distance. Data were analyzed separately for each year. All soybean parameters were analyzed at each harvest date using analysis of variance and linear regresssion procedures. Each soybean parameter was regressed over distance from the weed, and a test of significance was performed on the slope of each regression line. Parameters found to be significant by the regression procedure were compared to the same parameters for soybeans grown without interference to determine the range of influence of an individual weed on a row of soybeans.



Results and Discussion



Weed-Crop Biomass Accumulation

Soybeans growing in competition with sicklepod did not show significant reductions in vegetative growth until eight WAP in 1984 and 1985, and ten WAP in 1986. These findings coincide with those of Gunsolus and Coble (22,23) and others (70,71,73).





12

The ratio of dry matter partitioning between stems and leaves for soybean and sicklepod did not change significantly over the three years (Table 2.1 and 2.2). However, the overall dry matter production for these same parameters did differ between years and therefore the data for each year were analyzed separately. Differences in vegetative development over the three years appear to be a function of rainfall (Appendices A, B, C), but could also be due to differences in planting date as well as soil fertility. Soil water was vety limmiting in the month of August in 1984, during the onset of the pod fill growth stage. Overall low dry matter production in 1986 may be due to limiting soil nutrients (Table 2.1). Leaching rains throughout the summer may have reduced the availability of potassium during critical periods of high requirement.

For up to eight WAP sicklepod has a stem-to-leaf dry

weight ratio of less than one. After eight weeks sicklepod directs a larger proportion of energy towards stem growth. It is during this period that sicklepod becomes a strong competitor.

Soybeans grown with sicklepod had a stem-to-leaf dry weight ratio of 0.7, while sicklepod grown in competition with soybean had a stem-to-leaf dry weight ratio of 2 at 12 WAP (Figures 2.2 and 2.3). Other workers have observed that normally soybean has a stem-to-leaf ratio much closer to two (22,76). These apparent discrepancies may be explained by the fact that in our study the soybean leaf petiole was









Table 2.1. Growth parameters for soybeans grown without weed interference-Gainesville 1984, 1985 and 1986.



1984 1985 1986
Harvest Leaf Stem Leaf Stem Leaf Leaf Stem date weighta weight weight weight area weight weight


(weeks) ---(g/plant)--- ---(g/plant)--- (dm2/plant) ---(g/plant)--4 0.75 0.32 0.95 0.45 0.72 0.41 0.17 6 3.75 1.17 4.03 1.27 2.45 1.22 0.62 8 8.90 3.80 8.40 3.41 4.95 2.56 1.40 10 10.31 5.06 12.68 6.85 12.87 7.20 3.40 12 b 15.20 8.51 5.54 14 b 17.91 9.23 6.58


aAll weights are recorded as dry weights.

bNo harvest at this week.





-JA











Table 2.2. Growth parameters for sicklepod grown with soybeans--Gainesville
1984, 1985 and 1986.


1984 1985 1986
Harvest Leaf Stem Leaf Stem Leaf Leaf Stem date weighta weight weight weight area weight weight (weeks) ---(g/plant)--- ---(g/plant)--- (dm2/plant) ---(g/plant)--4 0.23 0.10 1.09 0.20 0.93 0.63 0.44 6 1.43 0.38 3.08 1.24 4.41 2.69 1.24 8 6.13 4.38 12.63 9.00 43.58 17.00 17.38 10 8.89 8.75 26.13 42.38 57.26 29.88 '42.38 12 _b 69.66 28.63 54.5014 _b 112.45 38.88 90.38 18 _b 44.88 30.63 172.75 Harvest 39.88 188.60 192.80

aAll weights are recorded as dry weights.

bNo harvests taken that week.










,16
in-,-,m LEAF AREA y=(-0.52) + 0.03x + 0.001x2

,u...... LEAF DRY WT. y=(-0.35) + 0.02x + 0.0006x2
6- STEM DRY WT. y=(-0.08) + (-0.008)x + 0.0006x2 12

x = number of days / N


4.
8




t4





0 2 4 6 8 10 12 14 WEEKS AFTER PLANTING
Figure 2.2. Total stem and leaf dry weight and leaf area accumulation of soybean grown with sicklepod--Gainesville, 1986.
& ~0,











100 100

.i.m LEAF AREA y=(0.79) + (-0.21)x + 0.01x2

,,,,,nesI STEM DRY WT. y=0.07 + (-0.46)x + 0.014x2
75. .75
2 // I .m LEAF DRY WT. y=(-0.79) + 0.06x + 0.004x

x = number of days
50 50


F-J
C. 20
25 25






0 2 4 6 8 10 12 14 WEEKS AFTER PLANTING
Figure 2.3. Total leaf and stem dry weight and leaf area accumulation of sicklepod grown with soybeans--Gainesville, 1986.






17

included as a morphological and anatomical component of the leaf, while other researchers often include the petiole in the stem dry weight (24,25,76). The added weight of the petiole would have therefore adjusted the ratio in favor of the leaves to a value of less than one. Another explanation is that sicklepod competition alone altered the partitioning between leaf and stem growth of soybeans. Under limiting light conditions due to shading from sicklepod, the soybean stem may have become somewhat elongated and spindly in an attempt to garner more light, thus altering the stem-to-leaf ratio. However the stem to leaf ratio between soybeans grown alone and soybeans grown with sicklepod did not change significantly (Table 2.1 and Figure 2.2), thus indicating that the data collection technique may be a more determining factor in the stem-to-leaf ratio changes than was sicklepod interference. The high stem-to-leaf ratio of sicklepod is a primary component of the mechanism of competition.

Soybean leaf area is positively correlated with leaf dry weight (Figure 2.2). Soybeans had an increase in leaf area and leaf dry weight of 64 and 63% respectively, from 8 to 14 WAP. In contrast, sicklepod leaf area was more positively correlated to stem dry weight (Figure 2.3). Sicklepod had an increase in leaf area and leaf dry weight of 77 and 64% respectively, from 8 to 14 WAP, while sicklepod stem dry weight had an 81% increase over the same period. Such a large increase in sicklepod leaf area without a corresponding increase in leaf weight may be due






18

to increasing leaf size without a substantial increase in weight. As the sicklepod plant gets larger, the canopy at the top expands in diameter allowing more light penetration down to the main stem exposing previously shaded leaves to more intense light, the result being an expansion in area of the existing shaded leaves without a corresponding increase in weight gain.

The recommended planting dates for 'Braxton' soybeans in Florida range from May 15 to June 15. In 1984, the initial year of this study, soybeans were planted on July 6, or approximately three weeks past the recommended planting date for this variety. The differences in sicklepod dry matter production between years is due to planting date (Table 2.2). Sicklepod biomass accumulation for stems and leaves is appoximately 68 and 79% less, respectively for sicklepod which emerges 3 to 4 weeks after June 1. Delayed emergence due to the later planting date in 1984 altered the stem to leaf ratio of sicklepod. In 1985 and 1986 the ratio was approximately two; however, in 1984 where sicklepod had less overall dry matter production the ratio was reduced to one (Table 2.2). Sicklepod is usually classified as a determinate, photoperiod sensitive plant (53) in which the onset of short days stimulates flower initiation. The onset of reproductive growth for both full season soybeans and sicklepod occurs about the first of August in Florida. It appears from data presented in (Table 2.2) that when sicklepod germinates at June 1 or earlier it has the






19

characteristics of an indeterminate plant. Biomass accumulation for leaves and stems continues to increase even after the onset of reproductive growth which was approximately 10 WAP in 1985 and 1986. However, in 1984, when sicklepod germination and emergence did not occur until July 6, sicklepod was more of a determinate plant. Any cultural practice that reduces the vegetative growth phase of sicklepod before flowering appears to greatly reduce sicklepod competition. Therefore, the manipulation of the soybean planting date in order to reduce vegetative growth of sicklepod but still remain within the recommended crop planting period could reduce sicklepod competitiveness and therefore is a parameter which must be considered in determining the area of influence of sicklepod in Floridagrown soybeans. Oliver (42) reported that velvetleaf (Abutilon theophrasti Medik.) competition was significantly reduced by delaying soybean planting date. Sicklepod Interference Effects on Vegetative Growth

The effects of sicklepod interference on soybean leaf and stem dry weight and leaf area are presented in Tables

2.3 to 2.5. Weed interference effects were greatest on soybean plants closest to the weed and decreased linearly with increasing distance from the weed. These results coincide with work done by James et al. (29) on cocklebur and by Gunsolus and Coble (22) who evaluated both cocklebur and sicklepod interference on soybeans. The linear regression equation presented in Tables 2.3 to 2.5










Table 2.3. Relationship of soybean stem dry weight reduction to distance from sicklepod--Gainesville.



Stem dry wt. Length of Stem wt. Harvest Regressi n 2 weed-free soybean row reduced/2m date equation R soybeans affected of row (weeks) (g/plant) (m) (%)
------------------------------Gainesville 1984-----------------------------8 y = 1.51 + 0.03xb 0.51 3.80 1.52 23 10 y = 2.52 + 0.03x 0.58 5.06 1.70 21

------------------------------Gainesville 1985-----------------------------8 y = 1.40 + 0.03x 0.77 3.41 1.34 20 10 y = 3.35 + 0.05x 0.56 6.85 1.40 18

------------------------------Gainesville 1986-----------------------------12 y = 3.15 + 0.04x 0.57 5.54 1.20 13 14 y = 3.30 + 0.05x 0.62 6.58 1.32 15

aSlopes were significant at the p = 0.01 level and are valid until y equals the appropriate weed-free value.
x = distance of soybean from weed (cm); y = soybean stem dry wt.
0










Table 2.4. Relationship of soybean leaf dry weight reduction to distance from sicklepod--Gainesville.



Leaf dry wt. Length of Leaf wt. Harvest Regressign 2 weed-free soybean row reduced/2m date equation R soybeans affected of row (weeks) (g/plant) (m) (%)
-----------------------------Gainesville 1984-------------------------------8 y = 3.73 + 0.07xb 0.60 8.90 1.48 21 10 y = 4.94 + 0.07x 0.56 10.31 1.52 20

-----------------------------Gainesville 1985-------------------------------8 y = 4.50 + 0.06x 0.67 8.40 1.30 15 10 y = 6.30 + 0.09x 0.59 12.68 1.42 18

-----------------------------Gainesville 1986-------------------------------12 y = 4.75 + 0.05x 0.65 8.51 1.50 17 14 y = 4.67 + 0.06x 0.61 9.23 1.52 19

aSlopes were significant at the p = 0.01 level and are valid until y equals the appropriate weed-free soybean value.
b= distance of soybean from weed (cm); y = soybean leaf dry wt.
X = distance of soybean from weed (cm); y = soybean leaf dry wt.











Table 2.5. Relationship of soybean leaf area reduction to distance from sicklepod--Gainesville,1986.



Leaf Area Length of Leaf Area Harvest Regressign 2 weed-free soybean row reduced/2m date equation R soybeans affected of row


(weeks) (dm2/plant) (m) (%)

-------------------------------Gainesville 1986-----------------------------10 y = 5.33 + 0.ll1xb 0.79 12.87 1.38 20 12 y = 6.10 + 0.13x 0.65 15.20 1.40 21 14 y = 6.60 + 0.15x 0.65 17.91 1.46 23


aSlopes were significant at the p = 0.01 level and are valid until y equals the appropriate weed-free soybean value.
x = distance of soybean from weed (cm); y = soybean leaf area (dm /plant).






23

summarizes the responses of the various soybean vegetative growth parameters on each side of a sicklepod plant. These regression equations explain the area of influence up to a distance at which the regression equation is equal to an appropriate weed-free soybean parameter; beyond this distance the values for the vegetative parameters for soybeans grown with weeds were not significantly different from those for soybeans grown without sicklepod interference. The total length of soybean row affected by sicklepod was considered to be two times the distance explained by the regression equation; this accounts for the interference on opposite sides of the weed. The degree of influence of a weed on a row of soybeans was determined by multiplying half the total length of soybean row affected by the difference between the weed-free parameter and the y-intercept of the regression equation. The degree of influence was expressed in Tables 2.3 to 2.5 as a percent reduction per two meters of row as compared to the same 2 m of row without weed interference.

The effects of sicklepod on soybean leaf and stem dry weight and leaf area differed over years and over harvest dates within a particular year. The three vegetative parameters did not show the same percent reduction as compared to 2 m of weed-free soybean row, indicating that different soybean plant parts were not equally affected by weed interference. Of the soybean measurements taken, sicklepod competition had the greatest effect on leaf area.





24

Final harvest of soybean vegetation indicated a leaf area reduction of 42% in the 25 cm row segment closest to the sicklepod (Figure 2.4). Soybean leaf area was reduced proportionally to a measurable distance of 73 cm from the sicklepod. Final harvest of soybean vegetation indicated a leaf dry weight reduction of 33% and a stem dry weight reduction of 31% in the 25 cm of soybean row next to the sicklepod. Soybean leaf and stem weight was reduced proportionally to a measurable distance of 76 and 66 cm, respectively, from the sicklepod plant.

When averaged over the three years of the study there was no height differential between sicklepod and soybeans for the first six weeks (Figure 2.5). After six weeks the sicklepod grew above the soybeans and was therefore competitive for light. This height differential continued to increase and reached 43 cm 10 WAP. In 1986 sicklepod height differential was 82 cm at 14 WAP (Figure 2.5). In all three years, sicklepod height surpassed the soybean height at or near the onset of reproductive growth. The time sicklepod first interfered with soybeans also coincided with the beginning of soybean reproductive development 8 or 10 WAP, for 1984 and 1986, respectively. Therefore sicklepod was able to compete for light during the reproductive phase of soybean development. Barrentine (2) and Oliver et al. (43) indicate that soybeans may be more susceptible to weed interference during the reproductive development stage than during their vegetative growth stage.










16 16

..-.. 0 25 cm y=(-0.31) + 0.018x + 0.0011x2 s"en""'"* 25 50 cm y=(-0.77) + 0.071x + 0.0007x2

12 """" 50 100 cm y=(-0.49) + 0.03x + 0.0015x2 12 x = number of days E


S 8 8











WEEKS AFTER PLANTING
Figure 2.4. Total leaf area accumulation of soybean grown at varying distances from sicklepod-Gainesville, 1986.
IUI
0 2 4 6 8 10 12 14 WEEKS AFTER PLANTING
Figure 2.4. Total leaf area accumulation of soybean grown at varying distances from sicklepod-Gainesville, 1986.
L,











100

*' SICKLEPOD

SOYBEAN /
75.




50.





25






0 2 4 6 8 10 WEEKS AFTER PLANTING Figure 2.5. Comparison of heights of competing sicklepod and soybean plants (average of 3 years), Gainesville.





27

Sicklepod competition did not have a significant impact on soybean height.

Sicklepod Interference Effects on Soybean Seed Yield

Differences in degree of influence on seed yield

between years is a function of differences in available soil water and probably limiting soil nutrients. The effects of sicklepod on soybean yield components are presented in Tables 2.6 and 2.7. Weed interference effects reduced soybean pod weight (Table 2.6) per plant as well as seed weight (Table 2.7) per plant. Researchers (22,32) have reported that the number of pods per plant was the primary yield component most influenced by weed interference. The present study only dealt with pod and seed weight per plant, but it is fair to assume that pod number may have also been the yield component most affected.

The linear regression equations presented in Tables 2.6 to 2.7 summarize the responses of the various soybean yield components on each side of a sicklepod plant. Total length of soybean row affected and the percent reduction in yield parameters as compared to the value for 2 m of weed-free soybean row was calculated as described for the vegetative parameters.

In 1984, sicklepod reduced seed weight by 26% per 2 m of row (Table 2.7). Yield reductions measured using this techique correspond positively to density work done by Jordan (32) who reported yield reductions of 25% with one sicklepod plant per 2 m of row. In 1985 and 1986, the










Table 2.6. Relationship of soybean pod dry weight reduction to distance from sicklepod--Gainesville.



Pod dry wt. Length of Pod wt. Harvest Regressi n 2 weed-free soybean row reduced/2m date equation R soybeans affected of row (weeks) (g/plant) (m) (%)
-------------------------------Gainesville 1984----------------------------10 y = 1.06 + 0.02xb 0.48 2.45 1.38 20

-------------------------------Gainesville 1985----------------------------10 y = 0.80 + 0.03x 0.67 2.39 1.06 18

-------------------------------Gainesville 1986----------------------------12 NSc 0.34 14 y = 1.13 + 0.02x 0.79 2.71 1.58 23


aSlopes were significant at the p = 0.01 level and are valid until y equals the appropriate weed-free soybean value.
x = distance of soybean from weed (cm); y = soybean pod dry weight.

CSlope was not significant at the p = 0.05 level.
CO










Table 2.7. Relationship of soybean seed weight reduction to distance from sicklepod--Gainesville 1984 and 1985.



Seed weight Length of Seed wt Harvest Regression 2 weed-free soybean row reduced/2m date equation R soybeans affected of row


(g/plant) (m) (%)

-------------------------------Gainesville 1984----------------------------11/30/84 y = 3.54 + 0.07xb 0.75 9.15 1.60 26

-------------------------------Gainesville 1985----------------------------11/27/85 y = 4.90 + 0.05x 0.55 8.03 1.26 12

-------------------------------Gainesville 1986----------------------------11/13/86 y = 3.26 + 0.07x 0.80 7.80 1.30 19


aslopes were significant at the p = 0.01 level and are valid until y equals the appropriate weed-free value.
x = distance of soybean from weed (cm); y = soybean seed weight (g/plant).





30

reduction in soybean seed yield was 12 and 19%. Estimating from the report by Thurlow and Buchanan (71), one sicklepod plant per 2 m of row reduced soybean seed yield by 4%. In North Carolina (22) the seed yield of soybean was reduced by 3 and 9% for 2 different years. The higher percent yield reduction observed in this study occurred primarily because of differences in water-holding capacity between the soils at Gainesville and the other experimental sites.

Seed yield reductions (Table 2.7) in any given year are very similar to percent reductions in vegetative dry matter production (Tables 2.3 to 2.5) indicating that a reduction in vegetative dry matter is a good indicater of sicklepod interference effects on soybean yield. Late season reductions in leaf area and dry weight have been correlated to seed yield reductions by other researchers (20,23,24,43).

Weed canopy width measurements were taken throughout

the growing season in 1986 to estimate the length of soybean row shaded by a sicklepod plant (Table 2.8). Sicklepod canopy width as measured was positively correlated to sicklepod leaf area. Using canopy width to predict weed leaf area, cannot be clearly defined by one years work and is not a dependable prediction technique at this time. However, weed canopy width used as a parameter to predict the area of influence relative to partitioning the mechanism of competition is a very valid parameter. In 1986, mean canopy width increased from 52 to 63 cm from 10 to 14 WAP










Table 2.8. Relationship between sicklepod canopy width and sicklepod leaf area and its effect on soybean leaf area reduction--Gainesville, 1986.



Sicklepod Sicklepod Length of Harvesta Average Average 2 soybean row date Canopy Width Leaf Area R affected (weeks) (cm) (dm2/plant) (m)
-------------------------------Gainesville 1986---------------------------4 7 0.93 6 16 4.41

8 28 43.58 10 52 64.00 0.67b 1.38 12 55 69.66 NSc 1.40 14 63 112.44 0.58 1.46



aTere is a quadratic relationship between sicklepod canopy width and time (R = 0.91).
bA linear relationship exists between sicklepod canopy width and sicklepod leaf area.
CSlope was not significant at the p = 0.05 level.





32

(Table 2.8). During this time the distance of soybean row affected as related to soybean leaf area reduction (Table

2.5) was 1.38 and 1.46 m of row. Therefore, under these conditions sicklepod is competitive to a distance of 2.4 times its canopy width. Sicklepod as shown by McWhorter and Patterson (38) does not exhibit any allelopathic effects. Therefore the observed growth effects at a distance of 2.4 times its canopy width must be the result of competition for soil moisture, nutrients and/or light. It may be important to note that the soybean rows were planted in a north-south direction, indicating that during the early morning and late afternoon the sun's angle is such that considerable shading beyond the canopy diameter is possible.

Researchers in North Carolina (22,23) have determined

that sicklepod at 16 WAP was competitive to a distance equal to or below its canopy diameter. The large differences between research in North Carolina and the present results are primarily a result of a much more aggressive sicklepod biotype in Florida and a larger accumulation of vegetative dry matter before the start of reproductive growth. Research showing the differences between northern and southern biotypes revealed that the vegetative growth phase was 30 days shorter for the selections from Tennessee when compared to Florida biotypes (53). In the North Carolina study, sicklepod grown in competition with soybeans had a total leaf area of approximately 50 dm2/plant at 14 WAP, while for the same period sicklepod in Florida obtained a






33

total leaf area of approximately 100 dm2/plant when grown with soybeans.

Sicklepod Interference Effects on Soil Water

Soil moisture reduction as a function of distance away from a sicklepod plant is represented in (Table 2.9). Relative differences in available soil moisture were not detected until 7 and 12 WAP in 1985 and 1986, respectively. Determination of differences in available soil water using this technique can be variable relative to time, due to sporadic rainfall. Under high soil moisture conditions, water is not a limiting factor and therefore competition for water does not take place. Therefore, depending on reading date and its proximity to the last rainfall, the competition for soil water as it relates to increasing root growth with time may go undetected. This may explain the late date at which sicklepod was found to be competitive for soil water in 1986.

Across both years sicklepod was found to be a moderate competitor for soil water. Shurtleff and Coble (61) in a greenhouse pot study reported that soybeans and sicklepod had a root:shoot ratio of 1.39 and 0.71, respectively at 11 WAP. They concluded that soybean was a better soil water competitor than sicklepod due to a more extensive root system. Banks et al. (1) reported that sicklepod density had minimal effect on early season water usage by soybeans. They reported that during soybean pod filling stage of growth, soybean water uptake from the soil profile decreased as sicklepod density increased.









Table 2.9. Relationship of soil moisture reduction to distance from sicklepod--Gainesville.


Soil Moisture Length of
Reading Regressign weed-free soybean row date equation R soybeans affected (millibars) (m)
-------------------------------Gainesville 1985----------------------------July 15 y = 150.4 1.42xb 0.67 128 0.32 Aug. 5 y = 138.5 1.01x 0.50 116 0.44 Sept. 16 y = 135.9 0.97x 0.53 110 0.54 Oct. 2 y = 181.0 2.03x 0.87 125 0.56

-------------------------------Gainesville 1986----------------------------Aug. 11 NSc 129 Aug. 28 y = 145.4 0.90x 0.42 133 0.28 Sept. 18 y = 188.5 2.53x 0.78 120 0.54 Oct. 2 y = 280.6 2.66x 0.68 210 0.52


aSlopes were significant at the p = 0.01 level and are valid until y equals the appropriate weed-free soybean value.
x = distance of soybean from weed (cm); y = soil moisture (millibars).

CSlope was not significant at the p = 0.05 level.






35

Sicklepod influenced available soil water from 0.32 to 0.56 m of soybean row from July 15 to October 2, 1985 (Table 2.9). Similiar areas of influence, disregarding time, were detected in 1986. In 1986 sicklepod's ability to effectively compete for available soil moisture coincides with a exponential increase in total leaf area. Sicklepod was competitive for soil water to a distance almost equal to its canopy diameter at 14 WAP. Therefore, sicklepod competition was quantitatively proven to be additive to a distance equal to its canopy width and competitive for only light beyond this distance up to a factor of 2.4 times its canopy width.

Results indicate that the area of influence methodology can accurately monitor the effects of weed-soybean interaction over a growing season as well as effects of sicklepod on soybean seed yield. This technique is a valid alternative to conventional additive competition studies and has the potential of developing new criteria for competitive indices for different weed species. Criteria for comparing the competitiveness between weed species could be established by determining the distance at which a 25% growth inhibition (1-25) occurs for a given weed and via comparison of 1-25 values a competitive index could be established.

Area of influence methodology measures weed and soybean interference effects over time and distance. This technique will quantify the dynamic relationship of weed-soybean





36

interference. In conjunction with environmental variables, it will allow the integration of weed interference effects into existing soybean growth simulation models.














CHAPTER 3
EFFECT OF ROW SPACING, TILLAGE SYSTEM AND PLANTING DATE ON SICKLEPOD COMPETITION IN SOYBEANS.


Introduction

Weed competition is one of the primary limiting factors in soybean [Glycine max (L.) Merr.] production throughout the world (52). In the southeastern United States, sicklepod (Cassia obtusifolia L.) is a major competitor in soybeans, cotton (Gossypium hirsutum L.) and peanuts (Arachis hypogaea L.) (5,6,17,18,21,28,60). Sicklepod is considered to be one of the ten most common and troublesome weeds in the major agronomic crops in the southeast (37, 44,68). Sicklepod is a summer annual broadleaf weed which reaches a maximum height of 2 to 2.5 m in the high rainfall conditions of the south. Its persistence as a weed can be attributed to its tolerance of a wide range of temperature, fertility and pH (13,19). Its prominance as a damaging weed is also characterized by abundant seed production (32) and the long persistance of these seeds in the soil profile (4, 13,19).

Soybeans have traditionally been planted in 75 to 100 cm row width. The widespread use of 100 cm rows for soybeans developed because equipment for corn culture has


37






38

been adapted for use on soybeans, and because of the need for cultivation for weed control (9). However, with the introduction of selective postemergence herbicides (11,14, 17,32,54,59,78) narrow-row soybeans are a feasible alternative (48). Many researchers both in the southern (9, 32) and northern United States (26,34,51) have reported increased yields for narrow-row soybeans. Research (12,41, 64) indicates that if weeds are controlled during early season in narrow-row soybeans, then shading from the crop will effectively reduce weed competition for the latter part of the season. If postemergence herbicides are needed in this system, lower use rates could be used. Increased crop yield observed with narrow-rows is partly due to better distribution of soybean plants over the soil surface and more efficient use of available water, light, and nutrients (9,69). Smith (63) and Hartwig (27) indicated that late planted soybeans are more likely to show a greater response to narrow rows than those planted in the spring, due to rapid canopy closure and better light utilization.

Mannering and Johnson (35) reported that narrow-row soybeans provide advantages other than increased weed control and yields. Narrow-row soybeans provided significantly greater ground cover than wide row soybeans during the early growing season resulting in 24% greater water infiltration and 35% less soil loss to erosion. Jordan (32) reported that sicklepod dry weight, seed production and light interception were all decreased by






39

soybeans in narrow-row spacings. He reported that mean water use was greater in the 25 and 50 cm rows than in the 75 cm rows. However, under water stress conditions, there was no difference in water use between row spacings when soil moisture was measured at a depth of 25 cm. Peters (49) reported that decreased row widths increased the overall water loss via transpiration.

In the absence of weeds Banks et al. (1) reported no yield advantage for soybeans grown in non-tilled mulched conditions over that of conventionally tilled plots with no mulch. When sicklepod was present, however, higher soybean yields were obtained with the mulched plots than with the tilled plots at similiar sicklepod densities. He reported that soil water usage was greater in the no-till treatments during the soybean pod-filling growth stage. Blevins et al.

(3) determined that the component primarily responsible for the increased yields under no-tillage conditions was the more efficient use of soil water. The decrease in evaporation and the greater ability to store moisture under no-tillage produces a greater water reserve in moderately well drained silt loam soils. This more efficient use of soil moisture under no-tillage conditions is reflected in increased yields.

Reduced soil erosion (33), increased water intake (31) and reduced water loss through decreased evaporation (72) have all been suggested as bonus advantages of narrow-row spacings and reduced tillage systems. The following






40

research was designed to monitor the effects of reduced row spacing, tillage systems and planting date on the competitiveness of sicklepod for light and soil moisture. Much research has been done detailing the added benefits of herbicidal weed control in reduced row spacings (9,11,32,40, 51) and tillage systems (9,11,54,78,79). These experiments are designed to determine whether changing the cultural parameters alone, without the added use of herbicides, would alter sicklepod competition in soybeans.



Materials and Methods



Soybean-Sicklepod Competition Studies

Studies to evaluate the competition of sicklepod with

soybeans planted in various row spacings and tillage systems were conducted during 1985 at the University of Florida Agronomy Farm (Green Acres) in Gainesville, Florida. Competition studies were continued at this location with modification during 1986. The soil type was a Bonneau fine sand (Loamy, Siliceous, Thermic, Arenic Palendult) with a pH of 6.2 and an organic matter content of less than one.

A split-plot design with tillage systems as main plots and row spacings as subplots was utilized. Subplots were

3.1 m by 9.2 m and replicated four times. Oryzalin (3,5-dinitro-N4, N4-dipropylsulfanilamide) was applied at

0.85 kg/ha over the entire experimental area for control of annual grasses and small seeded broadleaf weeds. Soybean






41

row spacings were 25, 50, and 75 cm. Tillage systems used were conventional and no-tillage. Conventional tillage systems consisted of one mold board plowing followed by discing for seedbed preparation. No-Till soybeans were planted into a wheat straw stubble, with 2000 kg/ha of straw returned to the soil surface. No-Till plots were not subsoiled. Sicklepod density was 5 plants/m2. Analysis of variance, and Duncan's new multiple range test were utilized to analyze data.

Competition of sicklepod with soybeans--1985

'Braxton' soybeans, maturity group VII, was planted on June 1, 1985. Soybean seeding rates were 67, 84, and 100 kg/ha of soybean seed for the 75, 50, and 25 cm row spacings, respectively. Sicklepod populations were established after weed emergence and maintained by hand hoeing for the remainder of the growing season.

Relative soil moisture content was monitored by placing mercury manometric tensiometers at a depth of 15 and 30 cm in each plot of all four replications on June 20, 1985. These depths were chosen so that season long competition for soil water could be monitored. Early season competition between soybeans and sicklepod was monitored at the 15 cm depth and late season competition for soil water monitored at the 30 cm depth. Soybeans possess a large fibrous root system which can penetrate to depths of 30 cm (57,67) and field observations reveal that sicklepod has a large tap root system with fibrous roots primarily concentrated near






42

the soil surface. The tensiometers consist of a 50 cm plastic tube with a 10 cm porous ceramic cup on the distal end. The tube is placed to the desired depth in the soil and connected to the above ground manometer scale by a single, transparent, plastic tube that serves as the manometer measuring tube as well as the connecting link between the manometer assembly and the tensiometer tube. The plastic tube was inserted into a vial containing 30 grams of mercury with the opposite end in the water-filled tensiometer tube. The manometer scale was graduated in millibars of soil water tension, a standard unit of measurement for soil moisture. Tensiometer tubes were placed 15 cm from one of the middle soybean rows and within close proximity to a sicklepod plant. Tensiometer readings were recorded ten times at ten day intervals from June 30 to October 10, 1985. Soybean yields were hand harvested from a 9.0 m2 area on November 10, 1985 and yields recorded.

Competition of sicklepod with soybeans--1986

'Braxton' soybeans were planted on June 4, 1986.

Soybean seeding rates were 67, 84, and 100 kg/ha of soybean seed for the 90, 60 and 30 cm row spacings, respectively. Sicklepod density was established after weed emergence and maintained by hand hoeing for the remainder of the growing season.

A different technique was used for soil moisture

measurements in 1986. Soil moisture tensiometer tubes were






43

placed in an identical arrangement and at the same depths. The above ground manometer unit was not used in 1986, and in its place was used a soil moisture TENSIMETER The TENSIMETER consists of a hand held meter that gives a digital read out for tensiometers. It includes a high quality pressure transducer with attached enclosed syringe needle, and a digital read out. To operate the TENSIMETER, the needle is inserted through the septum stopper of the tensiometer and the tension inside the tensiometer tube is read directly in millibars. Tensiometric readings were recorded nine times on ten day intervals from July 3 to October 9, 1986. Soybean yields were hand harvested from a

9.0 m2 area on November 20, 1986, and yields recorded. Sicklepod Competition--Planting Date Study

Studies to evaluate the effects of planting date,

tillage system and row spacing on sicklepod competition in soybeans were conducted in Gainesville, Florida from 1984 to 1986. In all three years the planting dates used were May 15, June 1, and June 15, all of which are within recommended planting periods for 'Braxton' soybeans in Florida. Hereafter, these three planting dates will be referred to as A, B, and C. The row spacings and tillage systems were the same as those used in the sicklepod competition studies.

A split-plot design with tillage systems as main plots and row spacings as subplots was utilized. In all three years of the experiment, each planting date was considered a


1TENSIMETER, Soil Measurement Systems, Las Cruces, NM.






44

separate test; therefore under these conditions data from different planting dates as they relate to row spacing and tillage system cannot be statistically compared.

Soybean canopy closure was evaluated using an

integrating radiometer/photometer and line quantum sensor that measured photosynthetically active radiation (PAR) in
-1 -2
microeinsteins sec m The line quantum sensor, has a sensing area of 1 m by 12.7 mm, and the light intercepted is integrated across the area of the sensor, eliminating the need for multiple measurements with small sensors. Canopy closure was determined progressively throughout the season for the various row spacings, tillage systems and planting dates, in 1984 and 1985. Determinations were made by placing the sensor above the soybean canopy to measure total incoming radiation at times ranging from one hour either side of solar noon. The sensor was then placed on the ground perpendicular to the soybean row and the output again recorded. Canopy closure was expressed as the percent of the difference between total incoming radiation and radiation at the ground divided by total incoming radiation. Sicklepod morphological data were obtained by randomly selecting plants from each plot and recording plant height and plant dry weights. Sicklepod biomass harvests were made using a harvest area of one square meter per plot.






45

Results and Discussion



Soybean-Sicklepod Competition Studies Row spacing-tillage system 1985

Water use did not differ between soybean row spacings

or between tillage systems at sampling times 2, 4, 5, 7, and

8 (Table 3.1). Analysis of variance did not indicate a significant interaction between row spacing and tillage system at any time. Interactions were only analyzed when soil water tension across row spacing was greater than 100 millibars, thereby omitting sampling time 4, 5, and 7. This procedure was followed because at water reserves of less than 100 millibars there was no immediate competition occurring. For 70% of the sampling times there was no significant difference in soil moisture use among row spacings or tillage systems at the 15 cm depth (Table 3.1) At sampling time one, 25 cm soybean rows had a signicantly higher water use than soybeans grown in 75 cm rows. At sampling time three, soybeans grown in 25 cm row had soil tension levels of 32 and 26% greater than the 50 and 75 cm rows, respectively. Sampling time three corresponds to approximately six weeks after planting, when soybeans are still in the vegetative stage. At this growth stage soybean canopy closure was 86, 70, and 59% for 25, 50 and 75 cm row spacings, respectively (Table 3.2). The more rapid canopy closure by the narrow-row soybeans reduces direct light penetration to the soil surface (55) and therefore should










Table 3.1. Effect of soybean row spacing and tillage systems on soil moisture (millibars)
at 15 cm depth, 1985.


Sampling Times+ 1 2 3 4 5 6 7 8 9 10 X Row Spacing (cm)

25 109a 153a 151a 63a 78a 556a 64a 89a 125a 120a 151a 50 105ab 178a 103b 64a 87a 230c 68a 101a 108a 115a 116a 75 93b 150a 112b 70a 78a 399b 64a 92a 105a 98a 126a Tillage System

Conventional 99a 143a 79b 65a 76a 400a 61a 93a 97b 98b 121a No-Till 106a 177a 166a 66a 86a 386a 69a 95a 128a 126a 141a


+Means within a column within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.









Table 3.2. Effect of soybean row spacing and tillage system on percent
canopy closure, 1985.


Row Spacing Weeks after planting
(cm) 4 6 8 10
-------------percent-------------25 61a 86a 95a 98a 50 44b 70b 91a 96a 75 33c 59c 78b 90a Tillage System

Conventional 48a 68a 89a 96a No-Tillage 44a 72a 84a 94a


+Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.






48

measurably reduce soil water loss due to evaporation at soil depths of 15 cm when compared with wide-row soybeans. Therefore under these conditions, the appreciablely higher soil water deficits which are measured for 25 cm soybeans must be due to a higher soil water usage (Table 3.1). Increased water use may be due in part to a better distribution of soybean plants over the soil surface which results in a more rapid although more efficient water use rate. Tillage system had a highly significant impact on soil moisture at sampling time 3 (Table 3.1). Soybeans grown under no-till mulch conditions had 52% higher soil tension levels than those grown under conventional seedbed conditions. Results indicate that whenever tillage system was found to be significant, the no-till condition always had a higher soil moisture deficit than the conventional tillage method at the 15 cm depth. The addition of 2000 kg/ha of wheat straw back onto the soil surface in the no-till plots may have enhanced water infiltration into the soil profile as has been reported in the literature (31). Tne higher soil tension levels under no-till mulched conditions could therefore be attributed to higher soil moisture use by the soybeans or sicklepod.

At sampling times 9 and 10, soil tension levels were approximately 23% higher under no-till conditions than for conventional tillage systems (Table 3.1). Under these conditions, soil moisture usage may indeed be higher for soybeans grown under mulched conditions. Banks et al. (1)






49

reported that soil water depletion was greater in the no-till treatments during the pod filling stage when soybeans were grown in competition with sicklepod. The mean soil water use (Table 3.1) of all row spacings shows that the 25 cm rows did not significantly differ from the mean water use of the 50 and 75 cm spacings. Between tillage systems, mean water use did not differ significantly for the 15 cm depth. However, after examining each sampling date where stress occurred (average above 200 millibars for at least one row spacing), there was a significant difference between row spacings. Under stress conditions, the water use levels were highest for the 25 cm rows and was followed by 75 and 50 cm rows in decreasing order. This indicates that during conditions where moisture is limiting (sampling time 6) or not limiting (sampling time 3), narrow-row soybeans are using more water thus potentially producing more photosynthate. Although this may or may not increase yields, it should increase early leaf area and therefore provide quicker canopy closure. In addition, when soil moisture is most critical, the period during flowering through pod fill (8,15,62,67) (sampling times 7-10), there were no significant differences between row spacings. Under stress conditions, the no-till mulched plots were not significantly different from the conventionally prepared plots. At the shallow depths (15 cm), soil moisture depletion was higher during the pod fill stage under the no-till conditions. The 15 cm depth in a coarse sandy soil






50

is very much influenced by rainfall and evaporative water loss and therefore may not be a true indicator of soybean water usage. During the early vegetative growth stage of soybeans when both the crop and weed root systems are concentrated in this depth, a competitive advantage between row spacing or tillage system could be adequately monitored.

Soybean row spacing and tillage system did not

significantly effect soil moisture at the 30 cm depth at sampling times 4, 5, 7, 8, 9, or 10 (Table 3.3). During the early growing season (sampling times 1, 2 and 3), narrow-row soybeans consistently gave a lower soil moisture reading. Differences in soil moisture at these early sampling dates are similiar to differences measured at the 15 cm depth (Table 3.1). Since soybean or sicklepod roots are not present in large numbers at this depth this early in the growing season (57,77), differences in soil moisture may be in part due to reduced water infiltration or higher moisture usage. The higher soil water deficits measured at the 15 cm depth have a direct effect on the water content of the soil below.

The mean water use at a depth of 30 cm (Table 3.3) of all row spacings shows that 25 cm rows used significantly more water than the 50 cm rows but was not different from the 75 cm rows. There were no measurable differences between row spacings during the reproductive growth stage of soybeans. During moisture stress conditions, soil moisture use was similiar to the mean water use, where 25 cm row spacings had a higher water demand than did the 50 cm rows.









Table 3.3. Effect of soybean row spacing and tillage systems on soil moisture (millibars)
at 30 cm depth, 1985.


Sampling Times+ 1 2 3 4 5 6 7 8 9 10 X Row Spacing (cm)

25 119a 145a 307a 166a 85a 284a 201a 123a 100a 102a 163a 50 100b 107b 175b 115a 93a 162b 206a 110a 110a 123a 130b

75 113ab 98b 192b 160a 89a 243ab 226a 139a 84a 104a 145ab Tillage System

Conventional 113a 108a 189b 171a 85a 187b 192a 127a 97a 108a 138a No-Till 108a 125a 260a 122a 93a 273a 234a 121a 104a llla 155a Means within a column within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probabilty, as determined by Duncan's new multiple range test.






U,






52

Higher soil water depletion by the 25 cm rows at both depths is a function of the increased plant population and also a more uniform arrangement of plants across the soil surface (74). Therefore the proportion of soil water used by the crop may be larger than that made available for use by sicklepod. These conclusions agree with Jordans (32), who concluded that soybeans in 25 cm rows effectively utilized the additional water while in 75 cm rows, sicklepod was able to increase its biomass dramatically at the expense of the crop. The wider row soybeans have a high moisture use rate because of increased water loss to evaporation, that results from slower canopy closure (Table 3.2) and may in fact be due to sicklepod being the more aggressive species and obtaining a larger share of the available soil water.

Mean water use did not differ between tillage systems at the 30 cm depth (Table 3.3). However at sampling dates of approximately 6 and 9 weeks after planting soil water depletion was greatest under the no-till mulched conditions. During stress conditions at the 30 cm depth, soil water use was greater under no-till conditions at both sampling times 3 and 6. The reduction in soybean root growth expansion as reported by Geddes et al. (20) during the onset of the reproductive growth stage could be a possible explanation for no differences between row spacings during this period.






53

Row spacing-tillage system 1986

Differences in soil moisture readings between years

were primarily due to amounts and dates of summer rainfall. The soil moisture measuring technique used in 1986 reduced the variability between replications and was also a reason for analyzing the data for the two years separately.

Water use did not differ at the 15 cm depth between soybean row spacing or tillage system for 50% of the sampling times (Table 3.4). Analysis of variance did indicate a significant interaction between row spacing and tillage system at sample time six. At sampling times 4, 7, and 8, the soil water depletion was greatest for soybeans grown in 25 cm row spacings. At these sampling times, the 25 cm rows were significantly higher in soil water usage than either the 50 or 75 cm rows. Under these conditions soil moisture usage was identical for the soybeans grown in 50 and 75 cm rows.

A significant interaction between the row spacings and tillage systems exist at sampling time six (Table 3.5). Sample time six corresponds to approximately eight weeks after planting and is nearing the onset of the reproductive growth stage. Under conventional tillage, the soybeans grown in 25 cm rows have the highest soil water reserves while the 50 cm rows have the highest level of soil water depletion at the 15 cm depth (Table 3.5). The trend is reversed for no-tillage where the 25 cm rows have the highest water usage and the 50 and 75 cm rows are










Table 3.4. Effect of soybean row spacing and tillage system on soil moisture (millibars)
at 15 cm depth, 1986.

+ *
Sampling Times+ 1 2 3 4 5 6 7 8 X Row Spacing (cm)

25 186a 257a 176a 241a 66a 215 229a 247a 202a 50 155a 198a 127a 170b 67a 211 168b 182b 160b 75 186a 198a 172a 175b 61a 188 162b 175b 165b Tillage System

Conventional 157a 235a 174a 211a 70a 157 190a 183a 172a No-Till 194a 200a 143a 180a 61a 252 183a 270b 185a


Means within a column within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.

Interaction of soybean row spacing and tillage system exists.






55



Table 3.5. Effect of soybean row spacing and tillage
system on soil moisture (millibars) at 15 cm
depth. Sampling time 6, 1986.


Row Spacing Tillage System
(cm) Conventional No-Tillage

25 bl30b a299a 50 a189a a233b 75 a153ab a223b


+Means within a row preceded by the same letter, or means within a column followed by the same letter, are not significantly different at the 5% level of probability, as determined by Duncans's new multiple range test.







Table 3.6. Effect of soybean row spacing and tillage
system on soil moisture (millibars) at 30 cm
depth. Sampling time 6, 1986.


Row Spacing Tillage System+
(cm) Conventional No-Tillage

25 b86b a 180a 50 al30a a142b

75 allla a152ab


Means within a row preceded by the same letter, or means within a column followed by the same letter, are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.





56


significantly lower. Narrow-row soybeans at eight weeks after planting had almost complete canopy closure under both no-tillage and conventional tillage systems. The 50 and 75 cm rows under conventional tillage are susceptible to moisture loss from solar evaporation as well as competitive moisture loss from sicklepod at the 15 cm depth. The higher soil water depletion in 25 cm rows versus 50 or 75 cm rows under no-tillage conditions could be due to higher soil moisture use rate.

Soil water usage by soybeans in 25 cm rows is

significantly different between tillage systems (Table 3.5). Soil water tension measurements are 57% higher under no-tillage conditions than conventional seedbed preparation. There are no differences in soil moisture between tillage systems at the 50 and 75 cm row spacings. It could be proposed that the increased row spacings tend to negate the advantages obtained by adding mulch back to the soil surface to reduce solar evaporative water losses. The mean water use (Table 3.4) between row spacings indicates that the 25 cm row was significantly higher than the 50 or 75 cm rows. These results coincide with Jordans (32) which reported that at the 15 cm depth narrow rows used more water in all conditions. He reported however that under stress conditions narrow-row offered no advantages over the wider rows in regards to soil moisture usage. During the reproductive growth stage the 25 cm rows had significantly higher soil water use rates than the 50 or 75 cm rows at the






57

15 cm depth (Table 3.4). Mean water use did not differ between tillage systems at the 15 cm depth (Table 3.4).

Water use at the 30 cm depths did not differ between

soybean row spacings or between tillage systems at sampling times 1, 2, 3, 5 and 7 (Table 3.7). Soil moisture usage for narrow-row soybeans was significantly greater than soybeans grown with 50 or 75 cm spacings at sampling times 4 and 8. The mean soil water use for all row spacings indicates that soybeans grown in 25 cm rows have the capacity to use more soil water via transpiration than the wide rows. No significant difference exists at the 30 cm depth between tillage systems in relation to soil moisture. Therefore in 1986, the addition of 2000 kg/ha of wheat straw back onto the soil surface did not significantly effect the water holding capacity or the soil water use rate in this coarse sandy soil.

In sampling time six, a significant interaction exists at the 30 cm depth between row spacings and tillage systems (Table 3.6). The response of these parameters are very similiar to those reported for 15 cm depths. Significant differences between tillage systems exists only for the 25 cm row spacings. This condition may be explained by the presence of a compaction layer or 'tractor pan' at or above this depth under no-tillage conditions. Conventional plots were plowed to a depth of 20-30 cm prior to planting, whereas no soil preparation was done to the no-till plots. Under these conditions, the soybean roots in the narrow-row











Table 3.7. Effect of soybean row spacing and tillage system on soil moisture (millibars)
at 30 cm depth, 1986.


Sampling Time+ 1 2 3 4 5 6 7 8 X Row Spacing (cm)

25 133a 136a 109a 161a 67a 133 122a 176a 130a

50 llla 138a 114a 135ab 64a 137 120a 124ab 118ab

75 114a 134a llla 118b 68a 132 101a 108b 110b Tillage System

Conventional 113a 141a 117a 142a 67a 109 115a 122a 116a No-Till 125a 131a 105a 135a 66a 158 113a 192b 128a


Means within a column within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.

Interaction of soybean row spacing and tillage system exists.




CO






59

soybeans would be more uniformly distributed within and limited to the upper soil profile and would have the capacity to deplete the water resevoirs from this area rapidly. Field observations indicated that no-till plots showed visual drought stress symptoms when the soybeans grown in conventional plots would not. However due to the abundance of rainfall (Appendix C) in August, these periodic drought stresses were not sustained long enough to decrease crop yields.

Sicklepod biomass accumulation and soybean yield--1985

The effect of soybean row spacing and tillage system on sicklepod biomass accumulation and soybean yield for 1985 is presented in (Table 3.8). Soybeans grown in 25 cm row spacings had significantly higher seed yields than the wider row soybeans. However there was no significant difference in sicklepod biomass accumulation between row spacings. Narrow-row soybeans must have used their essential growth factors such as light and water more efficiently to obtain increased yields under similiar weed pressures. Narrow-row soybeans had a higher soil water use rate early in the growing season which hastened development of the soybean canopy to better utilize the available light and possibly shade the competing sicklepod plants. Soybeans grown in 25 cm rows also responded with a higher water use rate under stress conditions which may have resulted in increased yields.









Table 3.8. Effect of soybean row spacing and tillage system on sicklepod
biomass accumulation and soybean grain yield, 1985.


Soybean Soybean Sicklepod Sicklepod Row Spacing Grain Yield Height Dry Weight Height
(cm) (kg/ha) (cm) (kg/ha) (cm) 25 850a 68a 11780a 84a 50 590b 64a 13810a 90a 75 615b 60a 15630a 90a Tillage System

Conventional 672a 64a 14567a 92a No-Till 698a 60a 13853a 88a Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.






61

There were no significant differences between row

spacings for soybean or sicklepod plant heights, although there appears to be a trend for increased height at closer rows and higher populations. These results contradict those of Jordan (32), who reported a height increase for narrow-row soybeans as well as a height reduction for sicklepod grown in competition with narrow-row soybeans. Under present conditions in which no herbicides are used to slow early season competitiveness of sicklepod, the narrow-row soybeans alone do not have the ability to suppress weed growth as effectively. Neither soybean yield or sicklepod dry matter production was significantly different between tillage systems. Sicklepod biomass accumulation and soybean yield--1986

The effect of soybean row spacing and tillage system on sicklepod biomass accumulation and soybean yield for 1986 is presented (in Table 3.9). Soybeans grown in 25 cm rows had a 23 and 21% higher yield than soybeans grown in 50 or 75 cm row, respectively. Sicklepod dry matter production did not differ among row spacings. As reported in Tables 3.4 and 3.7, the mean water use for all row spacings was greatest for the 25 cm rows in 1986 at both the 15 and 30 cm depth. In this case, the higher soil moisture use rate is positively reflected in a substantial yield increase. Even under similiar weed pressures the soybeans grown in 25 cm rows were able to more effectively utilize the limiting growth factors to obtain higher yields. There were no









Table 3.9. Effect of row spacing and tillage system on sicklepod biomass
accumulation and soybean grain yield, 1986.


Soybean Soybean Sicklepod Sicklepod Row Spacing Grain Yield+ Height Dry Weight Height
(cm) (kg/ha) (cm) (kg/ha) (cm) 25 734a 67a 11805a 79a 50 562b 64a 11730a 76a 75 580b 65a 11905a 76a Tillage System

Conventional 619a 65a 12440a 83a No-Tillage 655a 66a 11187a 71a


Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncans's new multiple range test.








I\)






63

significant differences across row spacing or tillage systems in plant heights. It can therefore be concluded that by varying the soybean row spacing from 75 to 25 cm with a uniform sicklepod population and without the use of herbicides, an approximate 30% yield increase can be obtained.

Sicklepod Competition--Planting Date Study

The effects of row spacing and tillage systems on

sicklepod competition in soybeans was evaluated in three planting dates starting at May 15 and occurring in two week intervals up until June 15, from 1984 through 1986. Data were analyzed separately by year due to differences in overall weed pressure and variable amounts and dates of rainfall.

Planting date--1984

Overall yields were higher in 1984 due to a reduced and subsequently late competing sicklepod population. The area in which this experiment was established and maintained for the following three years had been in bahaigrass (Paspalum notatum L.) pasture for the last three years and was essentially free of agronomic weed pressures. During the winter of 1984 prior to planting the wheat (Triticum compactum Host) cover crop, sicklepod seed was spread evenly at the rate of 22 kg/ha over the experimental area. However even at this high seeding rate the sicklepod population was not sufficient, due to poor emergence, partial control by discing of conventional tilled plots and






64

use of the burn down herbicide treatments in the no-till plots. After soybean emergence sicklepod was transplanted into this area, but was not as competitive as full season sicklepod would have been. This was especially evident in the later planting dates in which sicklepod competition was even more limited by the shorter growing season.

Even though the weed pressure was low (less than 5 plants/m2) sicklepod still caused significant yield differences between row spacings in the intitial year of this experiment (Table 3.10). There are reports in the literature (21) that narrow-row soybeans are most beneficial in late planted soybeans. However these citations are indicative of soybean production to the north, in which the growing season is considerably shorter and maximizing early season canopy closure is more critical. Under Florida conditions narrow-row soybeans were significantly more productive at the earliest planting date, in which full season sicklepod competition was most severe. At the earliest planting date soybeans grown in 25 cm rows had seed yields approximately 30% higher than soybeans grown in either 50 or 75 cm rows. There were no significant differences in soybean yields between row spacings at planting dates B or C. Tillage system did not have a significant effect on soybean yield at any planting date in 1984.






65




Table 3.10. Effect of row spacing, tillage system and
planting date on soybean grain yield, 1984. Planting Date May 15 June 1 June 15


------------Yield (kg/ha) -----------Row Spacing
(cm)

25 1797a 1594a 1511b 50 1318b 1575a 1824a

75 1385b 1762a 1675ab Tillage System

Conventional 1564a 1658a 1716a No-Tillage 1436a 1630a 1631a +Means in a column within a row spacing or tillage system followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's multiple range test.






66

Planting date--1985

In 1985, when the sicklepod density and growth was

adequate for full season competition there were significant differences in both row spacings and tillage systems (Table

3.11). At planting date A, 25 cm rows had significantly higher seed yields than the 75 cm rows, but had yields similiar to the soybeans grown in 50 cm rows. The narrow-row soybeans at planting date B had significantly higher yields than than either the 50 or 75 cm rows. Thus indicating that under conditions of full season sicklepod competition the reduction in row spacing offers some competitive advantage to the crop over the weed allowing higher yields to be obtained. There were no significant yield differences between row spacings at the latest planting date.

No-till soybeans had significantly higher yields at both planting dates A and C than soybeans grown conventionally. There was not a significant reduction in sicklepod dry matter production by either row spacing or tillage system at planting date A or B (Tables 3.12 and 3.13), although a noticeable trend exists for increased sicklepod dry mater production as row spacing increases. The yield increase for the no-till soybeans in the earliest planting date may be a function of a more efficient water use rate which increased canopy closure and maximized light interception.






67




Table 3.11. Effect of row spacing, tillage system and
planting date on soybean grain yield, 1985. Planting Date May 15 June 1 June 15


------------Yield (kg/ha) -------------Row Spacing
(cm)

25 593a 850a 1302a 50 500ab 590b 1374a 75 424b 615b 1110a Tillage System

Conventional 367b 672a 1041b No-Tillage 644a 698a 1482a


+Means in a column within a row spacing or tillage system followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.









Table 3.12. Effect of row spacing and tillage system on sicklepod biomass
accumulation and soybean grain yield, 1985. Planting date A.


Soybean + Soybean Sicklepod Sicklepod Row Spacing Grain Yield Height Dry Weight Height
(cm) (kg/ha) (cm) (kg/ha) (cm) 25 593a 65a 12230a 94a 50 500ab 64a 14375a 95a 75 424b 69a 16118a 98a Tillage System

Conventional 367b 67a 15631a 98a No-Tillage 644a 64a 13589a 95a


Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.







Co









Table 3.13. Effect of row spacing and tillage system on sicklepod biomass
accumulation and soybean grain yield, 1985. Planting date B.


Soybean Soybean Sicklepod Sicklepod Row Spacing Grain Yield Height Dry Weight Height
(cm) (kg/ha) (cm) (kg/ha) (cm) 25 850a 68a 11780a 84a 50 590b 64a 13810a 90a 75 615b 60a 15630a 90a Tillage System

Conventional 672a 64a 14567a 92a No-Tillage 698a 60a 13853a 88a


+Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.







Oc'n






70

Sicklepod dry matter production did not vary between

row spacings or tillage systems at planting date B, although narrow-row soybeans outyielded the wider row spacings by approximately 30%. Row spacing did not significantly influence soybean yields at the latest planting date, although tillage system was significant. The added benefit of straw mulch and its capacity to increase soil moisture under the reduced growing season is reflected in a significant yield increase for no-till soybeans at planting date C. In addition the overall sicklepod dry matter production was 34% less under no-till production at the latest planting date (Table 3.14).

The practice of tillage for seedbed preparation ensures a more rapid uniform seedling emergence and therefore may result in a more aggressive sicklepod plant. Whereas the practice of planting into a stale seedbed in which all emerged sicklepod plants have been controlled by the application of a nonselective burn down herbicide, the subsequent seedling emergence is delayed and less uniform and may result in a less competitive plant. Plant height was not significantly effected by row spacing or tillage system at any of the three planting dates (Tables 3.12,

3.13, and 3.14).

Although statistical comparisons cannot be made within row spacings or tillage systems among planting dates, there are many recognizable trends. As the growing season is reduced by delaying planting, the plant height of both









Table 3.14. Effect of row spacing and tillage system on sicklepod biomass
accumulation and soybean grain yield, 1985. Planting date C.


Soybean Soybean Sicklepod Sicklepod Row Spacing Grain Yield Height Dry Weight Height
(cm) (kg/ha) (cm) (kg/ha) (cm) 25 1302a 44a 5420b 52a 50 1374a 48a 9304a 48a 75 1110a 50a 6785ab 54a Tillage System

Conventional 1041b 50a 8587a 54a No-Tillage 1482a 44a 5753b 48a


Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.






72

soybeans and sicklepod declines. There is also a decline in sicklepod dry matter production as the planting date is delayed from June 1 to June 15, which tends to be reflected in an overall higher soybean yield for planting date C. The mean sicklepod dry matter production, for all row spacings and tillage systems decreased by approximately 50% when the planting date was delayed from May 15 to June 15, resulting in a less competitive sicklepod plant. Planting date--1986

In 1986 sicklepod competition severly reduced seed

yields by as much as 60% when compared to typical soybean yields obtained in north central Florida (32). Soybean row spacing or tillage systems did not significantly influence soybean yield or sicklepod competitiveness at planting dates A or C (Table 3.15). Narrow-row soybeans resulted in 20% higher yields when compared with 50 or 75 cm rows, under similiar sicklepod densities, at planting date B. Tillage systems were significant only in the latest planting date. No-till soybeans had a 19% yield increase over soybeans grown conventionally. Plant height or sicklepod dry matter production did not vary significantly between row spacings or tillage systems on any of the three planting dates in 1986 (Tables 3.16, 3.17 and 3.18). However the same trends are evident as were observed in 1985, ie.; a general decline in plant height and sicklepod productivity and an increase in soybean yields occurs when the planting dates are delayed from May 15 to June 15.





73




Table 3.15. Effect of row spacing, tillage system and
planting date on soybean grain yield, 1986. Planting Date May 15 June 1 June 15


------------Yield (kg/ha)-------------Row Spacing
(cm)

25 796a 734a 1090a 50 640ab 562b 1175a 75 595b 580b 1067a Tillage System

Conventional 680a 619a 995b No-Tillage 671a 655a 1224a


+Means in a column within a row spacing or tillage system followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.










Table 3.16. Effect of row spacing and tillage system on sicklepod biomass
accumulation and soybean grain yield, 1986. Planting date A.


Soybean Soybean Sicklepod Sicklepod Row Spacing Grain Yield+ Height Dry Weight Height
(cm) (kg/ha) (cm) (kg/ha) (cm) 25 796a 63a 14880a 90a 50 640ab 69a 14560a 94a 75 595b 68a 15175a 92a Tillage System

Conventional 680a 68a 14387a 94a No-Tillage 671a 65a 15357a 90a +Means within columns within row spacings or within tillages systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.









Table 3.17. Effect of row spacing and tillage system on sicklepod biomass
accumulation and soybean grain yield, 1986. Planting date B.


Soybean + Soybean Sicklepod Sicklepod Row Spacing Grain Yield Height Dry Weight Height
(cm) (kg/ha) (cm) (kg/ha) (cm) 25 734a 67a 11805a 79a 50 562b 64a 11730a 76a 75 580b 65a 11905a 76a Tillage System

Conventional 619a 65a 12440a 83a No-Tillage 655a 66a 11187a 71a +Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.








-,









Table 3.18. Effect of row spacing and tillage system on sicklepod biomass
accumulation and soybean grain yield, 1986. Planting date C.


Soybean + Soybean Sicklepod Sicklepod Row Spacing Grain Yield Height Dry Weight Height
(cm) (kg/ha) (cm) (kg/ha) (cm) 25 1090a 49a 7805a 54a 50 1175a 52a 6815a 53a 75 1067a 49a 8330a 46a Tillage System

Conventional 995b 51a 8136a 56a No-Tillage 1224a 49a 7163a 46a Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.





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The effect of soybean row spacing, tillage system, and planting date on soybean canopy closure was evaluated in 1984 and 1985. The data for two years was combined and is presented in Tables 3.19, 3.20 and 3.21. Tillage system did not significantly effect percent canopy closure at any time in the growing season. By eight weeks after planting at all three planting dates the narrow-row soybeans were near full canopy closure and by ten weeks after planting 50 cm rows were at complete canopy closure at all three planting dates. The 75 cm row in planting date C never did obtain complete canopy closure. It is the ability of the narrow-row soybeans to rapidly close the row middles which makes them more competitive for light, aides in reducing evaporative soil water loss and better utilizes the soil moisture via a stronger source-sink relationship (45,50).






78




Table 3.19. Effect of soybean row spacing and tillage
system on percent canopy closure. Planting date
A (May 15). Average of 1984 and 1985.


Weeks After Planting+
Row Spacing 4 6 8 10
(cm) percent-------------25 64a 83a 95a 97a 50 52a 70ab 96a 96a 75 47a 54b 77b 88b Tillage System

Conventional 54a 71a 86a 91a No-Tillage 53a 68a 88a 93a


Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.






79




Table 3.20. Effect of soybean row spacing and tillage
system on percent canopy closure. Planting date
B, (June 1). Average of 1984 and 1985.


Weeks After Planting+
Row Spacing 4 6 8 10
(cm) percent-------------25 61a 86a 95a 98a 50 44b 70b 91a 96a 75 33c 59c 78b 90a Tillage System

Conventional 48a 68a 89a 96a No-Tillage 44a 72a 84a 94a


Means within columns with row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.






80




Table 3.21. Effect of soybean row spacing and tillage
system on percent canopy closure. Planting date
C (June 15). Average of 1984 and 1985.


Weeks After Planting+
Row Spacing 4 6 8 10
(cm) percent-------------25 58a 76a 94a 99a 50 37b 57b 82b 93a 75 21c 48c 63c 82b Tillage System

Conventional 38a 61a 78a 84a No-Tillage 40a 59a 84a 85a


+Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.














CHAPTER 4
SUMMARY AND CONCLUSIONS




The experiments described in this study have added

substantially to available knowledge concerning sicklepod competition in Florida grown soybeans. The following summarizes results from these experiments which may be applicable to the grower and which may provide a foundation for future research.

The technique used to define the area of influence of sicklepod in soybeans was a very effective and thorough method to provide a feasible alternative to the conventional method of studying weed competition. Results of this study show that weed-soybean interference is a complex and dynamic process that depends on each plant's response to the environment and to the plants closest to it. Use of the area of influence methodology to measure weed and soybean interference effects over time and distance by evaluating developmental and yield parameters allows a more complete determination and quantification of the dynamic relationship of weed-soybean interactions. These data, combined with data obtained from concurrent studies of weed competition in soybeans will allow the integration of weed interference


81






82

effects into existing soybean growth simulation models. Area of influence methodology will allow researchers to determine damage thresholds for many other weed species more efficiently and economically than currently possible with weed density studies.

Results of this study indicate that sicklepod

effectively competes for both soil moisture and light interception. Sicklepod sphere of influence ranges from 1.1 to 1.6 m depending on the degree of moisture stress or the developmental parameter monitored. It is extremely important when using the area of influence methodology that environmental variables are monitored and cultural parameters such as planting dates are recognized as influential. The sphere of influence of sicklepod in soybeans could also be dramatically effected by changing cultural factors such as row spacing or tillage systems in an effort to better capitalize on the crops inherant competitiveness.

Competition studies between soybeans and sicklepod were conducted from 1984-1986, in which the factors of row spacing, tillage systems, and planting dates were evaluated. In all three years of the research, soybean yields were significantly greater in the 25 cm rows than the 75 cm row, under similiar sicklepod densities. Treatment differences in 1983, were not clearly defined because of an overall low sicklepod pressure. Yield differences between row spacings were expressed in the first and second planting date of each





83

year. The narrow-row soybeans offered little competitive advantage over the wider row at the June 15th planting date primarily due to a reduction in sicklepod competitiveness at these late planting dates.

No-Till soybeans had higher seed yields than

conventionally grown soybeans by as much as 19% in 1986. The increased yield response was usually reflected only in the latest planting date and was positively correlated to a decline in sicklepod dry matter production. Plant heights or sicklepod dry matter production was not significantly influenced by row spacing or tillage system, although both parameters tended to decline as the planting date was delayed from May 15 to June 15.

Mean plant water use was significantly greater in the 25 cm rows in both 1985 and 1986. Mean water use between row spacings or tillage systems did not differ significantly in 1985 at the 15 cm depth. Higher soil water depletion at both the 15 and 30 cm depth by the 25 cm rows is primarily a function of increased plant population and also a more uniform arrangement of plants across the soil surface. Mean water use between tillage systems was not significantly different at either depth or in any year. Under stress conditions, the water use level was highest for the 25 cm rows and was followed by 75 and 50 cm rows in decreasing order. Increased yields with narrow-row soybeans in this experiment is a response of more efficient and often higher water use rates, whereby the narrow rows are utilizing more






84

water and therefore producing more photosynthate or transpiration as opposed to the soybeans grown in wider rows in which sicklepod was able to compete more effectively for soil water.

In addition to water, one of the essential growth

factors in plant competition is light. Narrow-row soybeans obtained 94% canopy closure by eight weeks after planting in all three planting dates. The more efficient canopy architecture of the narrow-row enables it to intercept a greater amount of light and therefore have the potential for higher productivity.

This research indicates that soybeans planted in narrow rows may be beneficial in the management of weeds such as sicklepod. Sicklepod competing in wide row soybeans was found to have an area of influence 2 times greater than its canopy width. However, possibly through the manipulation of cultural practices such as planting date, row spacing or tillage system, which are easily adjustible by the grower, the area of influence could effectively be reduced without greater costs. Also these results indicate that the addition of chemical weed control systems into reduced tillage narrow row culture would provide an important management tool.














APPENDIX A


GAINESVILLE, FLORIDA MAY TO NOVEMBER 1984 PRECIPITATION.


May Jun Jul Aug Sep Oct Nov Day
------------------------inches------1 0.12 T T
2 0.60 0.30
3 0.64 0.69
4 0.24 T
5 0.02 0.02 0.49
6 0.10 T
7 0.10 T 0.01 8 T 0.18
9 T T T 10 0.05 11 0.32 0.24 12 1.01 13 0.04 0.02 0.72 14 0.02 15
16 0.12 0.20 T 17 0.01 18 0.01 0.17 0.13 0.01
19 1.09 0.51 0.04 T 20 0.48 0.04 0.58 0.10 21 0.17 0.36 0.37 22 0.36 1.05 23 2.60 T 0.25 0.15 0.11 24 0.57 0.09 0.05 25 0.04 26 0.65 0.01 0.03 T 27 T 0.30 0.11 T 0.10
28 2.72 0.02 1.88 0.56 0.18 29 0.12 0.56 0.26 0.15 0.81 30 0.86 0.74 0.09 31 0.58 0.04 Tot. 6.92 2.88 6.15 3.36 2.40 1.79 2.86






85














APPENDIX B


GAINESVILLE, FLORIDA MAY TO NOVEMBER 1985 PRECIPITATION.


May Jun Jul Aug Sep Oct Nov Day
----------------------inches------------------------1 T 0.23 0.55 0.06 1.99 2 0.24 0.02 3 0.05 0.42 0.02 0.35 4 0.79 0.55 0.04 0.04
5 0.37 0.08 0.14
6 0.06
7 0.16 0.06 0.66 8 0.29
9 0.16 0.05
10 0.04 0.16 0.02 0.41 0.03 11 0.06 0.07 0.18 0.08 12 0.37 0.07 13 0.10 0.25 1.21 0.25 14 0.14 0.27 1.11 15 0.33 0.10 0.34 16 1.74 0.15 0.10 T 17 T 0.42 0.41 0.03 18 0.33 T 19 0.23 0.17 0.01 20 0.43 0.09 0.72 0.10 0.06 21 1.89 0.21 0.13 0.01 22 0.22 0.64 0.71 23 0.07 T 0.22 24 0.03 1.66 25 0.54 0.04 0.71 0.02 26 0.08 27 0.08 0.40 28 1.46 1.13 29 0.29 0.56 1.09 30 0.91 1.26 0.12 0.02 0.01 31 0.06 2.57 3.84 0.70 Tot. 3.43 6.46 5.39 13.43 3.29 4.34 3.30






86














APPENDIX C


GAINESVILLE, FLORIDA MAY TO NOVEMBER 1986 PRECIPITATION.


May Jun Jul Aug Sep Oct Nov
Day
----------------------inches----------1 0.02 T
2 0.40 0.10 T
3 0.39 0.39 0.03
4 0.17 T
5 0.01 0.33 0.12 1.17
6 0.01 0.02 0.03 T
7 0.01 1.04 0.81 0.41
8 0.22 9 0.83 0.63 10 0.95 0.34 1.43 11 0.06 0.03 0.53 0.18 T 12 0.69 0.71 13 0.36 0.03 0.43 0.22 T 14 1.74 1.06 T 15 0.30 0.10 T
16 0.02
17 0.02 T 18 0.41 0.11 19 0.18 0.91 0.03 0.05 20 0.78 0.11 2.01 21 0.02 0.02 0.22 0.53 0.06 22 2.05 0.16
23 0.13 24 0.62 0.20 25 0.06 0.10 26 0.11 0.04 27 0.97 0.04 0.52 28 0.25 0.09 0.17 0.02 29 0.84 0.24 2.12 30 0.16 0.30 1.00 31 0.14 T Tot. 0.98 5.65 6.18 8.54 3.10 3.89 3.96





87














LITERATURE CITED



1. Banks, P. A., T. N. Tripp, J. W. Wells, and J. E.
Hammel. 1986. Effects of tillage on sicklepod (Cassia obtusifolia) interference with soybeans (Glycine max) and soil water use. Weed Sci. 34:
143-149.

2. Barrentine, W. L. 1974. Common cocklebur competition
in soybeans. Weed Sci. 22:600-603.

3. Blevins, R. L., D. Cook, G. H. Phillips, and R. E.
Phillips. 1971. Influence of no-tillage on soil
moisture. Agron. J. 63:593-596.

4. Bridges, D. C., and R. H. Walker. 1985. Influence of
weed management and cropping systems on sicklepod (Cassia obtusifolia) seed in the soil. Weed Sci.
33:800-804.

5. Buchanan, G. A., and E. R. Burns. 1971. Weed
competition in cotton. I. Sicklepod and tall
morninglory. Weed Sci. 19:576-579.

6. Buchanan, G. A., E. W. Hauser, W. J. Ethredge, and S. R.
Cecil. 1976. Competition of Florida beggarweed
and sicklepod with peanuts. II. Effects of
cultivation, weeds, and SADH. Weed Sci. 24:29-39.

7. Buchanan, G. A., and C. S. Hoveland. 1971. Sicklepod-Success story of a weed and how to control it in
soybeans. Weeds Today. 2 (1):11-12.

8. Bunce, J. A. 1978. Effects of water stress on leaf
expansion, net photosynthesis and vegetative growth of soybeans and cotton. Can. J. Bot.
56:1492-1498.

9. Burnside, O. C., and W. L. Colville. 1964. Soybean and
weed yields as affected by irrigation, row
spacing, tillage, and amiben. Weeds 12:109-112.




88





89

10. Coble, H. D., and R. L. Ritter. 1978. Pennsylvania
smartweed (Polygonum pennsylvanicum) interference in soybeans (Glycine max). Weed Sci. 26:556-559.

11. Colvin, D. L., G. R. Wehtze, M. Patterson, and R. H.
Walker. 1985. Weed management in minimum tillage
peanuts (Arachis hypogaea) as influenced by
cultivar, row spacing, and herbicides. Weed Sci.
33:233-237.

12. Cooper, R. L. 1971. Influence of early lodging on
yield of soybeans. Agron. J. 63:449-450.

13. Creel, J. M., C. S. Hoveland, and G. A. Buchanan.
1968. Germination, growth, and ecology of
sicklepod. Weeds 16:396-400.

14. Currey, W. L., D. H. Teem, and J. H. Jordan. 1981.
Sicklepod competition and control programs in Florida soybeans. Proc. South. Weed Sci. Soc.
34:66.

15. Doss, D. B., R. W. Pearson, and H. T. Rogers. 1974.
Effect of soil water stress at various growth
stages on soybean yield. Agron. J. 66:297-299.

16. Dowler, C. C., N. C. Glaze, and A. W. Johnson. 1984.
The six year effect of weed management levels and
multiple-cropping sequences on weed populations.
Weed Sci. Soc. of America Abstracts. 142:54.

17. Dowler, C. C., and M. B. Parker. 1975. Soybean weed
control system in two southern coastal plain
soils. Weed Sci. 23:198-202.

18. Fleck, N. G. 1976. Competition of sicklepod (Cassia
obtusifolia) densities on soybeans (Glycine max) at variable row distances. Ph.D. Dissertation.
Univ. of Florida. 169 pp.

19. Flint, E. P., D. T. Patterson, G. H. Reichers, and
J. L. Beyers. 1984. Temperature effects on
growth and leaf production in sicklepod (Cassia obtusifolia); hemp sesbania (Sesbania exaltata),
and showy crotalaria (Crotalaria spectabilis).
Weed Sci. Soc. of America Abstracts. 156:60.

20. Geddes, R. D., H. D. Scott, and L. R. Oliver. 1979.
Growth and water use by common cocklebur (Xanthium
pensylvanicum) and soybeans (Glycine max) under
field conditions. Weed Sci. 27:206-211.





90

21. Godley, F. M., L. Thompson, and H. D. Coble. 1981.
Weed management in narrow and wide row soybeans.
Proc. South. Weed Sci. Soc. 34:57.

22. Gunsolus, J. L. 1986. Reciprocal interference effects
between weeds and soybeans (Glycine max). Ph.D.
Dissertation. North Carolina State Univ. 100p.

23. Gunsolus, J. L., and H. D. Coble. 1985. Interference
effects on weed and soybean growth and
development. Proc. South. Weed Sci. Soc. 38:82.

24. Hagood, E. S., Jr., T. T. Bauman, J. L. Williams, Jr.,
and M. M. Schreiber. 1980. Growth analysis of
soybeans (Glycine max) in competition with
velvetleaf (Abutilon theophrasti). Weed Sci. 28:
729-734.

25. Hagood, E. S., Jr., T. T. Bauman, J. L. Williams, Jr.,
and M. M. Schreiber. 1981. Growth analysis of
soybeans (Glycine max) in competition with jimsonweed (Datura stramonium). Weed Sci.
29:500-504.

26. Hanway, D. G. 1954. Growing soybeans in Nebraska.
The Soybean Digest. 14:19-20.

27. Hartwig, E. E. 1957. Row width and rates of planting
in the southern states. Soybean Dig.
17 (5):13-14, 16.

28. Hauser, E. W., G. A. Buchanan, R. L. Nichols, and R. M.
Patterson. 1982. Effects of Florida beggarweed
and sicklepod on peanut yield. Weed Sci.
30:602-604.

29. James, A. R., L. R. Oliver, and R. E. Talbert. 1974.
Distance of influence of common cocklebur on
soybeans. Proc. South. Weed Sci. Soc. 27:340.

30. Johnson, W. C., and H. D. Coble. 1981. A new method
to determine weed competition. Proc. South. Weed
Sci. Soc. 34:102.

31. Jones, J. N., J. E. Moody, and J. H. Lillard. 1969.
Effects of tillage, no tillage, and mulch on soil
water and plant growth. Agron. J. 61:719-721.

32. Jordan, J. H., Jr., 1983. Sicklepod (Cassia
obtusifolia L.) competition with soybeans as
influenced by row spacing, density, planting date,
and herbicides. Ph.D. Dissertation. Univ. of
Florida. 106p.




Full Text

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FACTORS AFFECTING SICKLEPOD ( Cassia obtusifolia L COMPETITION IN FLORIDA-GROWN SOYBEANS By kevin Mcdonald perry A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1987

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ACKNOWLEDGEMENTS I wish to express my sincere appreciation to Dr. Wayne Currey, Dr. Barry Brecke, Dr. David Hall, Dr. Cliff Hiebsch and Dr. Jerry Sartain for their support in preparing this manuscript. I especially acknowledge Dr. Wayne Currey, chairman of my committee, for allowing me to continue my education at the University of Florida and for providing of his time and experience. I would especially like to thank Mr. David W. Studstill, whose technical assistance in conducting this research was an essential ingredient. Thanks are extended to my fellow graduate students and co-workers for sharing with me their ideas, and fellowship and most importantly their friendships. I thank my parents, Mr. and Mrs. Ben C. Perry for giving of their time, love and understanding and for an upbringing which instilled the necessary character to reach this goal in my life. Finally I thank my loving, wife JoAnn, for her understanding and patience throughout my education. It is her love and kindness that has enabled me to pursue this degree and makes all these sacrifices worthwhile. ii

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TABLE OF CONTENTS PAGE ACKNOWLEDGEMENTS ii LIST OF TABLES V LIST OF FIGURES viii ABSTRACT ix CHAPTERS I INTRODUCTION 1 II SICKLEPOD ( Cassia obtusifolia L.) SPHERE OF INFLUENCE IN SOYBEANS 4 Introduction 4 Materials and Methods 6 Field and Planting Conditions 6 Soil Moisture Measurements 7 Experimental Design 8 Data Collection Procedures 9 Results and Discussion 11 Weed-Crop Biomass Accumulation 11 Sicklepod Interference Effects on Vegetative Growth 19 Sicklepod Interference Effects on Soybean Seed Yield 27 Sicklepod Interference Effects on Soil Water.. 33 III EFFECT OF ROW SPACING, TILLAGE SYSTEM AND PLANTING DATE ON SICKLEPOD COMPETITIION IN SOYBEANS 3 7 Introduction 37 Materials and Methods 40 Soybean-Sicklepod Competition Studies 40 Competition of sicklepod with soybeans — 1985 41 Competition of sicklepod with soybeans — 1986 42 Sicklepod Competition — Planting Date Study.... 43 Results and Discussion 45 Soybean-Sicklepod Competition Studies 45 Row spacing-tillage system 1985 45 Row spacing-tillage system 1986 53 iii

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Sicklepod biomass accumulation and soybean yield— 1985 59 Sicklepod biomass accumulation and soybean yield— 1986 61 Sicklepod Competition-Planting Date Study.... 63 Planting date — 1984 '. 63 Planting date — 1985 66 Planting date — 1986 72 IV SUMMARY AND CONCLUSIONS 81 APPENDICES A GAINESVILLE, FLORIDA MAY TO NOVEMBER 1984 PRECIPITATION 85 B GAINESVILLE, FLORIDA MAY TO NOVEMBER 1985 PRECIPITATION 86 C GAINESVILLE, FLORIDA MAY TO NOVEMBER 1986 PRECIPITATION 87 LITERATURE CITED 88 BIOGRAPHICAL SKETCH 95 iv

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LIST OF TABLES TABLE PAGE 2.1 Growth parameters for soybeans grown without weed interference 1984, 1985 and 1986 13 2.2 Growth parameters for sicklepod grown with soybeans 1984, 1985 and 1986 14 2.3 Relationship of soybean stem dry weight reduction to distance from sicklepod 20 2.4 Relationship of soybean leaf dry weight reduction to distance from sicklepod 21 2.5 Relationship of soybean leaf area reduction to distance from sicklepod 22 2.6 Relationship of soybean pod dry weight reduction to distance from sicklepod 28 2.7 Relationship of soybean seed weight reduction to distance from sicklepod 29 2.8 Relationship between sicklepod canopy width and sicklepod leaf area and its effect on soybean leaf area reduction, 1986 31 2.9 Relationship of soil moisture reduction to distance from sicklepod 34 3.1 Effect of soybean row spacing and tillage system on soil moisture (millibars) at 15 cm depth, 1985 46 3.2 Effect of soybean row spacing and tillage system on percent canopy closure 47 3.3 Effect of soybean row spacing and tillage system on soil moisture (millibars) at 30 cm depth, 1985 51 3.4 Effect of soybean row spacing and tillage system on soil moisture (millibars) at 15 cm depth, 1986 54 3.5 Effect of soybean row spacing and tillage system on soil moisture (millabars) at 15 cm depth. Sampling time 6, 1986 55 v

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3.6 Effect of soybean row spacing and tillage system on soil moisture (millibars) at 30 cm depth. Sampling time 6, 1986 55 3.7 Effect of soybean row spacing and tillage system on soil moisture (millibars) at 30 cm depth, 1986 58 3.8 Effect of soybean row spacing and tillage system on sicklepod biomass accumulation and soybean grain yield, 1985 60 3.9 Effect of soybean row spacing and tillage system on sicklepod biomass accumalation and soybean grain yield, 1986 62 3.10 Effect of row spacing, tillage system and planting date on soybean grain yield, 1984... 65 3.11 Effect of row spacing, tillage system and planting date on soybean grain yield, 1985... 67 3.12 Effect of row spacing and tillage system on sicklepod biomass accumulation and soybean grain yield, 1985. Planting date A 68 3.13 Effect of row spacing and tillage system on sicklepod biomass accumulation and soybean grain yield, 1985. Planting date B 69 3.14 Effect of row spacing and tillage system on sicklepod biomass accumulation and soybean grain yield, 1985. Planting date C 71 3.15 Effect of row spacing, tillage system and planting date on soybean grain yield, 1986... 73 3.16 Effect of row spacing and tillage system on sicklepod biomass accumulation and soybean grain yield, 1986. Planting date A 74 3.17 Effect of row spacing and tillage system on sicklepod biomass accumulation and soybean grain yield, 1986. Planting date B 75 3.18 Effect of row spacing and tillage system on sicklepod biomass accumalation and soybean grain yield, 1986. Planting date C 76 3.19 Effect of soybean row spacing and tillage system on percent canopy closure. Planting date A (May 15). Average of 1984 and 1985... 78 vi

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3.20 Effect of soybean system on percent date B (June 1) 3.21 Effect of soybean system on percent date C (June 15) row spacing and tillage canopy closure. Planting Average of 1984 and 1985... 79 row spacing and tillage canopy closure. Planting Average of 1984 and 1985.. 80 vii

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LIST OF FIGURES FIGURE PAGE 2.1 A digramatic representation of the experimental technique used for the sicklepod/soybean sphere of influence studies 10 2.1 Total dry weight and leaf area accumulation of soybean grown with sicklepod, 1986 14 2.2 Total dry weight and leaf area accumulation of sicklepod grown with soybeans, 1986 16 2.3 Total leaf accumulation of soybean grown at varying distances from sicklepod, 1986 23 2.4 Comparison of heights of competing sicklepod and soybean plants (average of 3 years) Gainesville 25 viii

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy FACTORS AFFECTING SICKLEPOD ( Cassia obtusifolia L.) COMPETITION IN FLORIDA-GROWN SOYBEANS By kevin Mcdonald perry May, 1987 Chairman: Dr. W. L. Currey Major Department: Agronomy Field studies were conducted to monitor the interference effects of one sicklepod ( Cassia obtusifolia L.) plant on soybean [ Glycine max (L.) Merr. 'Braxton'] development and seed yield from 1984-1986 to quantify the length and area of soybean row influenced by a single sicklepod plant. The sicklepod' s first measurable effect on soybean development occurred eight weeks after planting (WAP) in 1984 and 1985, and occurred at 10 WAP in 1986. Weed interference effects were greatest on soybean plants closest to the weed and decreased linearly with increasing distance from the weed. Sicklepod reduced soybean seed weight by 26, 12 and 19% when calculated for 2 m of row in 1984, 1985 and 1986, respectively, although the yield

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suppression was determined by regression analysis to be for a total length of 1.6, 1.26, and 1.3 m of row for the same periods. Sicklepod influenced available soil water at a distance Of 0.32 to 0.56 m of row from July 15 to October 2, 1986. It was determined that soybean leaf area was the most sensitive developmental parameter monitored, ie. reduced by an average of 21%. Sicklepod competition was not found to significantly reduce soybean height. Area of influence methodology can accurately monitor the effects of weed-soybean interference over the extent of a growing season as well as sicklepod effect on seed yield. Additional field studies were conducted to evaluate the effects of soybean row spacing, tillage system and planting date on sicklepod competition in soybeans. In all three years, soybeans grown in 25 cm rows yielded more highly than those grown in 50 or 75 cm rows, under similiar sicklepod densities. Soybeans grown in 25 and 75 cm rows had a significantly higher rate of soil water depletion in both years. Mean soil water use rate did not differ between tillage systems, although at individual sampling times during stress conditions water use was higher under no-tillage conditions. Narrow-row soybeans had higher yields when planted on two of the three planting dates tested, with 25 cm soybeans having a yield advantage when planted on the two early season planting dates. Narrowrow soybeans did not have a yield advantage when planted on June 15. x

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CHAPTER 1 INTRODUCTION Sicklepod ( Cassia obtusifolia L.) is considered to be the most troublesome weed in soybean [ Glycine max (L.) Merr.] production in several southeastern states and is a problem in many other important agronomic crops. In the absence of a reliable herbicide program that provides season-long control without crop injury many growers have relied on cultural management techniques such as cultivation, row spacing and planting dates to assist in managing heavy sicklepod infestations. In the coarse textured sandy soils of Florida, multiple tillage operations often lead to loss of soil structure resulting in water and wind erosion and reduced aeration and water infiltration. Selective herbicides have made possible the development of no-tillage cropping systems which considerably reduce soil erosion and increase soil water retention due to the organic mulch on the soil surface. In recent years the push to produce a higher profit margin per unit area has caused growers to initiate changes in production practices used by past generations. The advantages of reducing row spacings to better utilize essential growth factors such as water and light has been 1

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readily received by farmers in the northern region of soybean producing areas. The inability to use mechanical cultivation in narrow-row soybean production would lend itself readily to no-till production practices. Competition for water appears to be an important factor involved in the mechanism of interference between sicklepod and soybeans. Several researchers have investigated the effects of weeds on soil water depletion (20,49,58,65,66, 75) However, few have investigated the effects of tillage practices and row spacings on weed interference and soil water use in soybeans. The objectives of this research were to determine the effects of tillage and row spacing on the interference effects of a uniform sicklepod density, and to determine whether the added mulch and shading effect of reduced row spacing influenced soil water depletion. Determination of damage thresholds for sicklepod and other weeds is essential for the effective development of economical and reliable weed management systems. Several weed scientists have indicated that to increase accuracy in predicting weed-crop interactions, weed interference effects should be integrated into crop simulation growth models (39, 56). Density oriented competition studies have often been used to provide the necessary support data to validate existing prediction models. These types of competition studies, although very reliable, are also very laborious and difficult to maintain. A new research methodology (29) referred to as 'area of influence 1 in which the competitive

PAGE 13

3 effects of a single weed plant on a linear row of crop was evaluated as an alternative to the conventional method. A cooperative research project was initiated in hopes of providing a more feasible research technique and also to provide the necessary information to validate a weed-crop phenology model which is presently being implemented. Research objectives in which the application of grower production practices and the initiation of a unique research technique both serve to provide a better source of information to aid the farmer*.

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CHAPTER 2 SICKLEPOD ( Cassia obtusifolia L.) SPHERE OF INFLUENCE IN SOYBEANS. Introduction Weed competition is one of the primary limiting factors in soybean production throughout the world. McCormick (37,44,68) lists sicklepod as one of the most common and troublesome weeds in soybeans in the southeastern United States. Teem et al. (70) reported that sicklepod at 3, 5 and 7 weeds /m of row reduced soybean yields 19, 25 and 38%, respectively Many of the studies on weed competition such as work in soybeans by Coble and Ritter (10) and many others (5,6,14,18,28,32,46,60,71,73) have emphasized weed densities and duration of competition. Competition studies aid growers in determining the feasibility of controlling particular weeds at various densities. Determination of economic thresholds for sicklepod and other specific weeds and weed complexes is essential for the effective development of practical and reliable weed management systems (36) The development of models is one feasible approach to the study of weed biology and its application to crop 4

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5 production (56) Growth simulation models for weeds will be utilized to their fullest extent as we determine how the growth and development of weeds not only respond to their environment, but also how weeds change the environment of an associated crop (or weed) In a study conducted by Johnson and Coble in which the competition of five different broadleaf weeds in soybeans was evaluated using a replacement planting technique, sicklepod was found to be the least competitive species studied (30) Since there are numerous literature citations (7,14,18,32,70,71,73) detailing that sicklepod can be a very damaging weed in soybeans throughout the southeast, these results need further explanation. It has been shown by Patterson et al. (47) that the replacement technique using microplots in which only short term vegetative studies are done does provide a good indication of ultimate yield and/or crop yield reduction. Therefore a new method of determining crop-weed interactions as an alternative to conventional methods of weed competition research was evaluated. Experiments were designed to measure the effects of sicklepod on the vegetative and reproductive growth characteristics of soybeans at various intervals during the growing season. These data, combined with information obtained from concurrent studies of weed competition in soybeans, were used to develop a growth simulation model to predict crop response to varying levels of broadleaf weed

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6 competition. The data required for calibration of the model and validation of results include dry weight of individual soybean plants, leaf area, plant height, pod weight and seed weight. Objectives were to determine the effects of a single sicklepod plant on the growth and development of a soybean variety common to soybean production areas in the southeastern coastal plains of the United States. Materials and Methods Field and Planting Conditions Field experiments were conducted in 1984, 1985 and 1986 at the University of Florida Agronomy Farm in Gainesville, Florida. The soil at this site was a Bonneau fine sand (Loamy, Siliceous, Thermic, Arenic Paleudults) with a pH of 6.2. 'Braxton' soybeans were seeded in 75-cm wide rows which were planted in a north-south orientation, to a depth of 3 cm with a double-row planter on July 6, 1984, June 19, 1985, and June 5, 1986. On the same dates, local biotypes of sicklepod seed were hand planted 10 cm from every other soybean row to a depth of 3 cm and were spaced 2.5 m apart in 10 m long soybean rows to prevent interference effects between weeds. Sicklepod seed were randomly planted on either the east or west side of the soybean row. Prior to planting, sicklepod seed were scarified using a mechanical sandpaper scarifying device. To promote uniform soybean and sicklepod emergence, the area was irrigated with 2 to 3 cm

PAGE 17

7 of water after planting. Soybeans and sicklepod emerged 4 to 5 days after planting. Approximately 10 days after planting, soybeans were thinned to 25 plants/m. In all three years of this study, test plots were treated 4 preemergence with oryzalin (3, 5-dinitro-N N^-dipropylsulfanilamide) at a rate of 0.84 kg/ha to control annual grass species and small seeded broadleaves; other unwanted weed species were removed by hand weeding. Soil Moisture Measurements Relative soil moisture was recorded using single unit mercury manometers in 1985 and using tensiometer tubes in conjunction with a soil moisture TENSIMETER 1 in 1986. No soil moisture measurements were taken in 1984. Tensiometer tubes were placed at a depth of 25 cm in five replications at 5 different sicklepod plants in which only nondestructive data were obtained, on June 30 and June 20 for 1985 and 1986, respectively. 1985 The tensiometers consisted of a 50 cm tube with a 10 cm porous ceramic cup. The tube was placed to the 25 cm depth in the soil and connected to the above-ground manometer scale by a single, transparent, plastic tube that served as the manometer measuring tube as well as the connecting link between the manometer assembly and the tensiometric tube. The plastic tube was inserted into a reservoir containing 30 grams of mercury, with the opposite end in the water filled cup tube. The manometer scale was TENSIMETER, Soil Measurement Systems, Las Cruces, NM.

PAGE 18

8 graduated in millibars of soil tension, a standard unit of measurement for soil moisture. Tensiometers were placed 5 cm laterally from one of the soybean rows and adjacent to a sicklepod plant. Tensiometers were placed at a distance of 0, 15 and 30 cm from each of five different sicklepod plants which were growing within 10 cm of the soybean row. Tensiometric readings were recorded ten times from July 19 to October 20, 1985. 1986 A different technique was used for soil moisture measurements in 1986. Soil moisture tensiometer tubes were placed at the same depth and in an identical arrangement as in 1985. In 1986, a soil moisture TENSIMETER replaced the above ground manometer unit. The Soil Measurement Systems TENSIMETER is a hand-held meter that gives digital read-out for tensiometers. It includes a high quality pressure transducer with attached enclosed syringe needle, and a digital read-out. To operate the TENSIMETER the needle is inserted through the septum stopper of the tensiometer and the tension inside the tensiometer is read directly in millibars. Tensiometric readings were recorded nine times from July 3 to October 9, 1986. Experimental Design Each experiment consisted of two weed-soybean treatment combinations: sicklepod growing adjacent to soybeans and soybeans growing without w^ed interference. Weed interference effects on soybeans were tested in a completely randomized design. Treatment effects were analyzed using

PAGE 19

analysis of variance and regression procedures. Completely randomized design was employed using the assumption that the weed-free and sicklepod infested areas were uniform in response to growth characteristices of the soybeans. Eight subsamples were taken at each harvest interval and were used to generate the error term, no block effects were employed. Data Collection Procedures Destructive vegetative harvests of weeds and soybeans were conducted at 2-week intervals beginning 4 weeks after planting (WAP) and ending 10 WAP in 1984 and 1985 and 14 WAP in 1986. Soybeans were harvested in all three years at distances of 0-25, 25-50 and 50-100 cm from either side of the sicklepod plant (Figure 2.1). Soybeans grown without weed interference were sampled at the same distances from a central point. Plant height (ground to the most distal node) growth stage, leaf and stem dry weight and total leaf area (1986 only) were recorded for each soybean and sicklepod harvested. Weed canopy width in 1986 was determined by measuring across the widest point of the apex of the sicklepod plant. Plants were harvested at soil level and taken back to a laboratory work area where leaves were removed and stored in refrigeration until leaf area could be measured. Leaf area was measured with an automatic leaf area meter Leaves and stems were dried at 60-65C for 48 hours before dry weights were obtained. Soybean yield data Li-COR 3100, LI-COR, Inc., Lincoln, NE.

PAGE 20

10 Figure 2.1. A diagramatic representation of the experimental technique used for the sicklepod/soybean SDhere of influence studies.

PAGE 21

11 were collected at seed maturity (11-30-84, 11-27-85 and 11-13-86) The total pod and seed weights were recorded for each soybean plant harvested at the various specified distances from the weed. Rainfall data were collected at the experimental site for all three years. Data for soybeans sampled at identical distances from a weed combined to give an average value for each distance. Data were analyzed separately for each year. All soybean parameters were analyzed at each harvest date using analysis of variance and linear regresssion procedures. Each soybean parameter was regressed over distance from the weed, and a test of significance was performed on the slope of each regression line. Parameters found to be significant by the regression procedure were compared to the same parameters for soybeans grown without interference to determine the range of influence of an individual weed on a row of soybeans Results and Discussion Weed-Crop Biomass Accumulation Soybeans growing in competition with sicklepod did not show significant reductions in vegetative growth until eight WAP in 1984 and 1985, and ten WAP in 1986. These findings coincide with those of Gunsolus and Coble (22,23) and others (70,71,73)

PAGE 22

12 The ratio of dry matter partitioning between stems and leaves for soybean and sicklepod did not change significantly over the three years (Table 2.1 and 2.2). However, the overall dry matter production for these same parameters did differ between years and therefore the data for each year were analyzed separately. Differences in vegetative development over the three years appear to be a function of rainfall (Appendices A, B, C) but could also be due to differences in planting date as well as soil fertility. Soil water was very limmiting in the month of August in 1984, during the onset of the pod fill growth stage. Overall low dry matter production in 1986 may be due to limiting soil nutrients (Table 2.1). Leaching rains throughout the summer may have reduced the availability of potassium during critical periods of high requirement. For up to eight WAP sicklepod has a stem-to-leaf dry weight ratio of less than one. After eight weeks sicklepod directs a larger proportion of energy towards stem growth. It is during this period that sicklepod becomes a strong competitor Soybeans grown with sicklepod had a stem-to-leaf dry weight ratio of 0.7, while sicklepod grown in competition with soybean had a stem-to-leaf dry weight ratio of 2 at 12 WAP (Figures 2.2 and 2.3). Other workers have observed that normally soybean has a stem-to-leaf ratio much closer to two (22,76). These apparent discrepancies may be explained by the fact that in our study the soybean leaf petiole was

PAGE 23

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PAGE 27

17 included as a morphological and anatomical component of the leaf, while other researchers often include the petiole in the stem dry weight (24,25,76). The added weight of the petiole would have therefore adjusted the ratio in favor of the leaves to a value of less than one. Another explanation is that sicklepod competition alone altered the partitioning between leaf and stem growth of soybeans. Under limiting light conditions due to shading from sicklepod, the soybean stem may have become somewhat elongated and spindly in an attempt to garner more light, thus altering the stem-to-leaf ratio. However the stem to leaf ratio between soybeans grown alone and soybeans grown with sicklepod did not change significantly (Table 2.1 and Figure 2.2), thus indicating that the data collection technique may be a more determining factor in the stem-to-leaf ratio changes than was sicklepod interference. The high stem-to-leaf ratio of sicklepod is a primary component of the mechanism of competition. Soybean leaf area is positively correlated with leaf dry weight (Figure 2.2). Soybeans had an increase in leaf area and leaf dry weight of 64 and 63% respectively, from 8 to 14 WAP. In contrast, sicklepod leaf area was more positively correlated to stem dry weight (Figure 2.3). Sicklepod had an increase in leaf area and leaf dry weight of 77 and 64% respectively, from 8 to 14 WAP, while sicklepod stem dry weight had an 81% increase over the same period. Such a large increase in sicklepod leaf area without a corresponding increase in leaf weight may be due

PAGE 28

18 to increasing leaf size without a substantial increase in weight. As the sicklepod plant gets larger, the canopy at the top expands in diameter allowing more light penetration down to the main stem exposing previously shaded leaves to more intense light, the result being an expansion in area of the existing shaded leaves without a corresponding increase in weight gain. The recommended planting dates for 'Braxton' soybeans in Florida range from May 15 to June 15. In 1984, the initial year of this study, soybeans were planted on July 6, or approximately three weeks past the recommended planting date for this variety. The differences in sicklepod dry matter production between years is due to planting date (Table 2.2). Sicklepod biomass accumulation for stems and leaves is appoximately 68 and 79% less, respectively for sicklepod which emerges 3 to 4 weeks after June 1. Delayed emergence due to the later planting date in 1984 altered the stem to leaf ratio of sicklepod. In 1985 and 1986 the ratio was approximately two; however, in 1984 where sicklepod had less overall dry matter production the ratio was reduced to one (Table 2.2). Sicklepod is usually classified as a determinate, photoperiod sensitive plant (53) in which the onset of short days stimulates flower initiation. The onset of reproductive growth for both full season soybeans and sicklepod occurs about the first of August in Florida. It appears from data presented in (Table 2.2) that when sicklepod germinates at June 1 or earlier it has the

PAGE 29

19 characteristics of an indeterminate plant. Biomass accumulation for leaves and stems continues to increase even after the onset of reproductive growth which was approximately 10 WAP in 1985 and 1986. However, in 1984, when sicklepod germination and emergence did not occur until July 6, sicklepod was more of a determinate plant. Any cultural practice that reduces the vegetative growth phase of sicklepod before flowering appears to greatly reduce sicklepod competition. Therefore, the manipulation of the soybean planting date in order to reduce vegetative growth of sicklepod but still remain within the recommended crop planting period could reduce sicklepod competitiveness and therefore is a parameter which must be considered in determining the area of influence of sicklepod in Floridagrown soybeans. Oliver (42) reported that velvetleaf ( Abutilon theophrasti Medik.) competition was significantly reduced by delaying soybean planting date. Sicklepod Interference Effects on Vegetative Growth The effects of sicklepod interference on soybean leaf and stem dry weight and leaf area are presented in Tables 2.3 to 2.5. Weed interference effects were greatest on soybean plants closest to the weed and decreased linearly with increasing distance from the weed. These results coincide with work done by James et al. (29) on cocklebur and by Gunsolus and Coble (22) who evaluated both cocklebur and sicklepod interference on soybeans. The linear regression equation presented in Tables 2.3 to 2.5

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23 summarizes the responses of the various soybean vegetative growth parameters on each side of a sicklepod plant. These regression equations explain the area of influence up to a distance at which the regression equation is equal to an appropriate weed-free soybean parameter; beyond this distance the values for the vegetative parameters for soybeans grown with weeds were not significantly different from those for soybeans grown without sicklepod interference. The total length of soybean row affected by sicklepod was considered to be two times the distance explained by the regression equation; this accounts for the interference on opposite sides of the weed. The degree of influence of a weed on a row of soybeans was determined by multiplying half the total length of soybean row affected by the difference between the weed-free parameter and the y-intercept of the regression equation. The degree of influence was expressed in Tables 2.3 to 2.5 as a percent reduction per two meters of row as compared to the same 2 m of row without weed interference. The effects of sicklepod on soybean leaf and stem dry weight and leaf area differed over years and over harvest dates within a particular year. The three vegetative parameters did not show the same percent reduction as compared to 2 m of weed-free soybean row, indicating that different soybean plant parts were not equally affected by weed interference. Of the soybean measurements taken, sicklepod competition had the greatest effect on leaf area.

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24 Final harvest of soybean vegetation indicated a leaf area reduction of 42% in the 25 cm row segment closest to the sicklepod (Figure 2.4) Soybean leaf area was reduced proportionally to a measurable distance of 73 cm from the sicklepod. Final harvest of soybean vegetation indicated a leaf dry weight reduction of 33% and a stem dry weight reduction of 31% in the 25 cm of soybean row next to the sicklepod. Soybean leaf and stem weight was reduced proportionally to a measurable distance of 76 and 66 cm, respectively, from the sicklepod plant. When averaged over the three years of the study there was no height differential between sicklepod and soybeans for the first six weeks (Figure 2.5). After six weeks the sicklepod grew above the soybeans and was therefore competitive for light. This height differential continued to increase and reached 43 cm 10 WAP. In 1986 sicklepod height differential was 82 cm at 14 WAP (Figure 2.5). In all three years, sicklepod height surpassed the soybean height at or near the onset of reproductive growth. The time sicklepod first interfered with soybeans also coincided with the beginning of soybean reproductive development 8 or 10 WAP, for 1984 and 1986, respectively. Therefore sicklepod was able to compete for light during the reproductive phase of soybean development. Barrentine (2) and Oliver et al. (43) indicate that soybeans may be more susceptible to weed interference during the reproductive development stage than during their vegetative growth stage.

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27 Sicklepod competition did not have a significant impact on soybean height. Sicklepod Interference Effects on Soybean Seed Yield Differences in degree of influence on seed yield between years is a function of differences in available soil water and probably limiting soil nutrients. The effects of sicklepod on soybean yield components are presented in Tables 2.6 and 2.7. Weed interference effects reduced soybean pod weight (Table 2.6) per plant as well as seed weight (Table 2.7) per plant. Researchers (22,32) have reported that the number of pods per plant was the primary yield component most influenced by weed interference. The present study only dealt with pod and seed weight per plant, but it is fair to assume that pod number may have also been the yield component most affected. The linear regression equations presented in Tables 2.6 to 2.7 summarize the responses of the various soybean yield components on each side of a sicklepod plant. Total length of soybean row affected and the percent reduction in yield parameters as compared to the value for 2 m of weed-free soybean row was calculated as described for the vegetative parameters In 1984, sicklepod reduced seed weight by 26% per 2 m of row (Table 2.7). Yield reductions measured using this techique correspond positively to density work done by Jordan (32) who reported yield reductions of 25% with one sicklepod plant per 2 m of row. In 1985 and 1986, the

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30 reduction in soybean seed yield was 12 and 19%. Estimating from the report by Thurlow and Buchanan (71) one sicklepod plant per 2 m of row reduced soybean seed yield by 4%. In North Carolina (22) the seed yield of soybean was reduced by 3 and 9% for 2 different years. The higher percent yield reduction observed in this study occurred primarily because of differences in water-holding capacity between the soils at Gainesville and the other experimental sites Seed yield reductions (Table 2.7) in any given year are very similar to percent reductions in vegetative dry matter production (Tables 2.3 to 2.5) indicating that a reduction in vegetative dry matter is a good indicater of sicklepod interference effects on soybean yield. Late season reductions in leaf area and dry weight have been correlated to seed yield reductions by other researchers (20,23,24,43). Weed canopy width measurements were taken throughout the growing season in 1986 to estimate the length of soybean row shaded by a sicklepod plant (Table 2.8). Sicklepod canopy width as measured was positively correlated to sicklepod leaf area. Using canopy width to predict weed leaf area, cannot be clearly defined by one years work and is not a dependable prediction technique at this time. However, weed canopy width used as a parameter to predict the area of influence relative to partitioning the mechanism of competition is a very valid parameter. In 1986, mean canopy width increased from 52 to 63 cm from 10 to 14 WAP

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31 0 oo CD t-H i-H M u (1) H — 1 10 H H > C CO ro ai c 43 •rH +1 ro •-H 1 c >l o 0 c 0 ro 0 T3 u 0 a ro ai 0) rH M M ro o •H , 0 a to H c ta o c 0 +J •H 0 -P CD (0 4H tH m CD a) CO *J • •H CO CN C ro CD rH ro CD id u Eh ro *4H 0 o u J3 ai -p o CP 0) CD C £> P J O (C n CM -o ro O CD CD C7> H a) ro < rH M M CD >w o > ro •H < CD co J •P O CD -H & CPS •H < to o c ro u -P CO CO a; > CD cd U P CD ro (0 T3 00 O CO >0 ro c • • • •H ro rH rH rH ro oo V£> CO IT) o o CO a) — -H P > C co ro cd rH C ro u> e ro rH CO o LD o \o O ro 0^ CN co rH rH CO 00 CN] CN in in ro CM CO c ro Cu 0 C ro u "0 o CD rH 0 -H CO c CD CD £ -P CD a H £1 CO C 43 -P o c ro 0 O sx CD rH o •H CO c CD CD £ -P CD o CO -rH p -P CO ro •H rH X CD CD rH U •H •H £ -P CO ro C u 0 •H ro P • ro ro cr rH CD • CD rH ro ~ rH p ro CO o\ u m -H • ro ro O CD CD CD C rH U II •rH CD rH T3 J3N 0 Eh < a ro — XI CD rH CD rX E 0 •H •H -P CO CD > CD m o a CD rC -P -P ro -p c ro o •H UH •rH c tx> •H CO -P O c CO ro CD a o rH CO u

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32 (Table 2.8). During this time the distance of soybean row affected as related to soybean leaf area reduction (Table 2.5) was 1.38 and 1.46 m of row. Therefore, under these conditions sicklepod is competitive to a distance of 2.4 times its canopy width. Sicklepod as shown by McWhorter and Patterson (38) does not exhibit any allelopathic effects. Therefore the observed growth effects at a distance of 2.4 times its canopy width must be the result of competition for soil moisture, nutrients and/or light. It may be important to note that the soybean rows were planted in a north-south direction, indicating that during the early morning and late afternoon the sun's angle is such that considerable shading beyond the canopy diameter is possible. Researchers in North Carolina (22,23) have determined that sicklepod at 16 WAP was competitive to a distance equal to or below its canopy diameter. The large differences between research in North Carolina and the present results are primarily a result of a much more aggressive sicklepod biotype in Florida and a larger accumulation of vegetative dry matter before the start of reproductive growth. Research showing the differences between northern and southern biotypes revealed that the vegetative growth phase was 30 days shorter for the selections from Tennessee when compared to Florida biotypes (53) In the North Carolina study, sicklepod grown in competition with soybeans had a total leaf area of approximately 50 dm 2 /plant at 14 WAP, while for the same period sicklepod in Florida obtained a

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33 2 total leaf area of approximately 100 dm /plant when grown with soybeans. Sicklepod Interference Effects on Soil Water Soil moisture reduction as a function of distance away from a sicklepod plant is represented in (Table 2.9). Relative differences in available soil moisture were not detected until 7 and 12 WAP in 1985 and 1986, respectively. Determination of differences in available soil water using this technique can be variable relative to time, due to sporadic rainfall. Under high soil moisture conditions, water is not a limiting factor and therefore competition for water does not take place. Therefore, depending on reading date and its proximity to the last rainfall, the competition for soil water as it relates to increasing root growth with time may go undetected. This may explain the late date at which sicklepod was found to be competitive for soil water in 1986. Across both years sicklepod was found to be a moderate competitor for soil water. Shurtleff and Coble (61) in a greenhouse pot study reported that soybeans and sicklepod had a root: shoot ratio of 1.39 and 0.71, respectively at 11 WAP. They concluded that soybean was a better soil water competitor than sicklepod due to a more extensive root system. Banks et al. (1) reported that sicklepod density had minimal effect on early season water usage by soybeans. They reported that during soybean pod filling stage of growth, soybean water uptake from the soil profile decreased as sicklepod density increased.

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34 CO -p n •H •0 0 p c 0 •H -p u CD p cd P -p CO •H 0 B rH •H 0 co w 0 •H • x: 0) CO rH C H 0 -H •H > -p 00 fO rH a CD •H 0 a i i • 0 CN a CD cd rH rH M X) u £ •H Eh CO IP o 3 o Ih T3 c +> tn a) cu G X) 4h rl O HI CO CD U G •P CO •H o (D CD CO C (0 0 s -a xi •H o CO >H o CO CN c om •H C co o CO -H 0) -P M (CS tn 3 CD D 1 K CD cn G •H T3 CD CO +J a) CO CD c H (0 O X) 00 CD O in cn ro o O CN rH rH CN CN m CN rH rH rH rH rH rH rH rH CN I 00 r-~ o ro CN 00 00 CD CO in in 00 CD rjvo rH 1 • • • rH • • • rH o o o o rH o o o > to CD G H CO U X X X X X X CN rH rro o ro CO O o in lO • • • • • • • rH rH o CN o CN CN 1 I 1 1 o 1 1 in O 1 CO in CD • • • • S3 • • o 00 in rH in 00 o in ro ro 00 rj00 00 rH rH rH rH rH rH CN II II II II II II II >1 >1 >1 >1 >1 >1 CO m rH rH m CN >1 • -P • rH cn Cu -P G P CD 0 h3 i P Mi Cu — 1 X) *H •H -P rH i — 1 •H r— E p-rH *H rH CD P > P CO H -H n3 0 rr R — i C 1 — 1 • CD •H H o rH w > CD CD > II rH CD rH • >1 in CD o rH P • O i — 1 o • <0 £ o p> u II II c CD D-i CD CD CD -Q CD CD >i 3 P X? 0 •P CO t — 1 E P 0 CO 1 111 +J CD P (0 CD Uh P C c CO C 1 CO O co *o CD -H U CD ^2 >+H •H CD >i H UH u 0 C in rn cn G CD •H cn-P 4H to •H (0 0 CO -H -P M CD 0 cd a u c P 0 c CD P d to -p fO to CO to -H CD tJ CD fX CD o x; II 0 rH 4J rH cn X C/3 fO CO XI rH CO CD

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35 Sicklepod influenced available soil water from 0.32 to 0.56 m of soybean row from July 15 to October 2, 1985 (Table 2.9). Similiar areas of influence, disregarding time, were detected in 1986. In 1986 sicklepod' s ability to effectively compete for available soil moisture coincides with a exponential increase in total leaf area. Sicklepod was competitive for soil water to a distance almost equal to its canopy diameter at 14 WAP. Therefore, sicklepod competition was quantitatively proven to be additive to a distance equal to its canopy width and competitive for only light beyond this distance up to a factor of 2.4 times its canopy width. Results indicate that the area of influence methodology can accurately monitor the effects of weed-soybean interaction over a growing season as well as effects of sicklepod on soybean seed yield. This technique is a valid alternative to conventional additive competition studies and has the potential of developing new criteria for competitive indices for different weed species. Criteria for comparing the competitiveness between weed species could be established by determining the distance at which a 25% growth inhibition (1-25) occurs for a given weed and via comparison of 1-25 values a competitive index could be established. Area of influence methodology measures weed and soybean interference effects over time and distance. This technique will quantify the dynamic relationship of weed-soybean

PAGE 46

36 interference. In conjunction with environmental variables it will allow the integration of weed interference effects into existing soybean growth simulation models.

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CHAPTER 3 EFFECT OF ROW SPACING, TILLAGE SYSTEM AND PLANTING DATE ON SICKLEPOD COMPETITION IN SOYBEANS. Introduction Weed competition is one of the primary limiting factors in soybean [ Glycine max (L.) Merr.] production throughout the world (52). In the southeastern United States, sicklepod ( Cassia obtusifolia L.) is a major competitor in soybeans, cotton ( Gossypium hirsutum L.) and peanuts ( Arachis hypogaea L.) (5,6,17,18,21,28,60). Sicklepod is considered to be one of the ten most common and troublesome weeds in the major agronomic crops in the southeast (37, 44,68). Sicklepod is a summer annual broadleaf weed which reaches a maximum height of 2 to 2.5 m in the high rainfall conditions of the south. Its persistence as a weed can be attributed to its tolerance of a wide range of temperature, fertility and pH (13,19). Its prominance as a damaging weed is also characterized by abundant seed production (32) and the long persistance of these seeds in the soil profile (4, 13,19) Soybeans have traditionally been planted in 75 to 100 cm row width. The widespread use of 100 cm rows for soybeans developed because equipment for corn culture has 37

PAGE 48

38 been adapted for use on soybeans, and because of the need for cultivation for weed control (9) However, with the introduction of selective postemergence herbicides (11,14, 17,32,54,59,78) narrow-row soybeans are a feasible alternative (48) Many researchers both in the southern (9, 32) and northern United States (26,34,51) have reported increased yields for narrow-row soybeans. Research (12,41, 64) indicates that if weeds are controlled during early season in narrow-row soybeans, then shading from the crop will effectively reduce weed competition for the latter part of the season. If postemergence herbicides are needed in this system, lower use rates could be used. Increased crop yield observed with narrow-rows is partly due to better distribution of soybean plants over the soil surface and more efficient use of available water, light, and nutrients (9,69). Smith (63) and Hartwig (27) indicated that late planted soybeans are more likely to show a greater response to narrow rows than those planted in the spring, due to rapid canopy closure and better light utilization. Mannering and Johnson (35) reported that narrow-row soybeans provide advantages other than increased weed control and yields. Narrow-row soybeans provided significantly greater ground cover than wide row soybeans during the early growing season resulting in 24% greater water infiltration and 35% less soil loss to erosion. Jordan (32) reported that sicklepod dry weight, seed production and light interception were all decreased by

PAGE 49

39 soybeans in narrow-row spacings. He reported that mean water use was greater in the 25 and 50 cm rows than in the 75 cm rows. However, under water stress conditions, there was no difference in water use between row spacings when soil moisture was measured at a depth of 25 cm. Peters (49) reported that decreased row widths increased the overall water loss via transpiration. In the absence of weeds Banks et al. (1) reported no yield advantage for soybeans grown in non-tilled mulched conditions over that of conventionally tilled plots with no mulch. When sicklepod was present, however, higher soybean yields were obtained with the mulched plots than with the tilled plots at similiar sicklepod densities. He reported that soil water usage was greater in the no-till treatments during the soybean pod-filling growth stage. Blevins et al. (3) determined that the component primarily responsible for the increased yields under no-tillage conditions was the more efficient use of soil water. The decrease in evaporation and the greater ability to store moisture under no-tillage produces a greater water reserve in moderately well drained silt loam soils. This more efficient use of soil moisture under no-tillage conditions is reflected in increased yields. Reduced soil erosion (33) increased water intake (31) and reduced water loss through decreased evaporation (72) have all been suggested as bonus advantages of narrow-row spacings and reduced tillage systems. The following

PAGE 50

40 research was designed to monitor the effects of reduced row spacing, tillage systems and planting date on the competitiveness of sicklepod for light and soil moisture. Much research has been done detailing the added benefits of herbicidal weed control in reduced row spacings (9,11,32,40, 51) and tillage systems (9,11,54,78,79). These experiments are designed to determine whether changing the cultural parameters alone, without the added use of herbicides, would alter sicklepod competition in soybeans. Materials and Methods Soybean-Sicklepod Competition Studies Studies to evaluate the competition of sicklepod with soybeans planted in various row spacings and tillage systems were conducted during 1985 at the University of Florida Agronomy Farm (Green Acres) in Gainesville, Florida. Competition studies were continued at this location with modification during 1986. The soil type was a Bonneau fine sand (Loamy, Siliceous, Thermic, Arenic Palendult) with a pH of 6.2 and an organic matter content of less than one. A split-plot design with tillage systems as main plots and row spacings as subplots was utilized. Subplots were 3.1 m by 9.2 m and replicated four times. Oryzalin (3,5-dinitro-N 4 N 4 -dipropylsulfanilamide) was applied at 0.85 kg/ha over the entire experimental area for control of annual grasses and small seeded broadleaf weeds. Soybean

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41 row spacings were 25, 50, and 75 cm. Tillage systems used were conventional and no-tillage. Conventional tillage systems consisted of one mold board plowing followed by discing for seedbed preparation. No-Till soybeans were planted into a wheat straw stubble, with 2000 kg/ha of straw returned to the soil surface. No-Till plots were not subsoiled. Sicklepod density was 5 plants/m Analysis of variance, and Duncan's new multiple range test were utilized to analyze data. Competition of sicklepod with soybeans — 1985 'Braxton' soybeans, maturity group VII, was planted on June 1, 1985. Soybean seeding rates were 67, 84, and 100 kg/ha of soybean seed for the 75, 50, and 25 cm row spacings, respectively. Sicklepod populations were established after weed emergence and maintained by hand hoeing for the remainder of the growing season. Relative soil moisture content was monitored by placing mercury manometric tensiometers at a depth of 15 and 30 cm in each plot of all four replications on June 20, 1985. These depths were chosen so that season long competition for soil water could be monitored. Early season competition between soybeans and sicklepod was monitored at the 15 cm depth and late season competition for soil water monitored at the 30 cm depth. Soybeans possess a large fibrous root system which can penetrate to depths of 30 cm (57,67) and field observations reveal that sicklepod has a large tap root system with fibrous roots primarily concentrated near

PAGE 52

42 the soil surface. The tensiometers consist of a 50 cm plastic tube with a 10 cm porous ceramic cup on the distal end. The tube is placed to the desired depth in the soil and connected to the above ground manometer scale by a single, transparent, plastic tube that serves as the manometer measuring tube as well as the connecting link between the manometer assembly and the tensiometer tube. The plastic tube was inserted into a vial containing 30 grams of mercury with the opposite end in the water-filled tensiometer tube. The manometer scale was graduated in millibars of soil water tension, a standard unit of measurement for soil moisture. Tensiometer tubes were placed 15 cm from one of the middle soybean rows and within close proximity to a sicklepod plant. Tensiometer readings were recorded ten times at ten day intervals from June 30 to October 10, 1985. Soybean yields were hand harvested from a 9.0 m area on November 10, 1985 and yields recorded. Competition of sicklepod with soybeans — 1986 'Braxton' soybeans were planted on June 4, 1986. Soybean seeding rates were 67, 84, and 100 kg/ha of soybean seed for the 90, 60 and 30 cm row spacings, respectively. Sicklepod density was established after weed emergence and maintained by hand hoeing for the remainder of the growing season. A different technique was used for soil moisture measurements in 1986. Soil moisture tensiometer tubes were

PAGE 53

43 placed in an identical arrangement and at the same depths. The above ground manometer unit was not used in 1986, and in its place was used a soil moisture TENSIMETER*. The TENSIMETER consists of a hand held meter that gives a digital read out for tensiometers It includes a high quality pressure transducer with attached enclosed syringe needle, and a digital read out. To operate the TENSIMETER, the needle is inserted through the septum stopper of the tensiometer and the tension inside the tensiometer tube is read directly in millibars. Tensiometric readings were recorded nine times on ten day intervals from July 3 to October 9, 19 86. Soybean yields were hand harvested from a 2 9.0 m area on November 20, 1986, and yields recorded. Sicklepod Competition — Planting Date Study Studies to evaluate the effects of planting date, tillage system and row spacing on sicklepod competition in soybeans were conducted in Gainesville, Florida from 1984 to 1986. In all three years the planting dates used were May 15, June 1, and June 15, all of which are within recommended planting periods for 'Braxton' soybeans in Florida. Hereafter, these three planting dates will be referred to as A, B, and C. The row spacings and tillage systems were the same as those used in the sicklepod competition studies. A split-plot design with tillage systems as main plots and row spacings as subplots was utilized. In all three years of the experiment, each planting date was considered a TENSIMETER, Soil Measurement Systems, Las Cruces, NM.

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44 separate test; therefore under these conditions data from different planting dates as they relate to row spacing and tillage system cannot be statistically compared. Soybean canopy closure was evaluated using an integrating radiometer/photometer and line quantum sensor that measured photosynthetically active radiation (PAR) in -1 -2 microeinsteins sec m The line quantum sensor, has a sensing area of 1 m by 12.7 mm, and the light intercepted is integrated across the area of the sensor, eliminating the need for multiple measurements with small sensors. Canopy closure was determined progressively throughout the season for the various row spacings, tillage systems and planting dates, in 1984 and 1985. Determinations were made by placing the sensor above the soybean canopy to measure total incoming radiation at times ranging from one hour either side of solar noon. The sensor was then placed on the ground perpendicular to the soybean row and the output again recorded. Canopy closure was expressed as the percent of the difference between total incoming radiation and radiation at the ground divided by total incoming radiation. Sicklepod morphological data were obtained by randomly selecting plants from each plot and recording plant height and plant dry weights. Sicklepod biomass harvests were made using a harvest area of one square meter per plot.

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45 Results and Discussion Soybean-Sicklepod Competition Studies Row spacing-tillage system 1985 Water use did not differ between soybean row spacings or between tillage systems at sampling times 2, 4, 5, 7, and 8 (Table 3.1). Analysis of variance did not indicate a significant interaction between row spacing and tillage system at any time. Interactions were only analyzed when soil water tension across row spacing was greater than 100 millibars, thereby omitting sampling time 4, 5, and 7. This procedure was followed because at water reserves of less than 100 millibars there was no immediate competition occurring. For 70% of the sampling times there was no significant difference in soil moisture use among row spacings or tillage systems at the 15 cm depth (Table 3.1) At sampling time one, 25 cm soybean rows had a signicantly higher water use than soybeans grown in 75 cm rows. At sampling time three, soybeans grown in 25 cm row had soil tension levels of 32 and 26% greater than the 50 and 75 cm rows, respectively. Sampling time three corresponds to approximately six weeks after planting, when soybeans are still in the vegetative stage. At this growth stage soybean canopy closure was 86, 70, and 59% for 25, 50 and 75 cm row spacings, respectively (Table 3.2). The more rapid canopy closure by the narrow-row soybeans reduces direct light penetration to the soil surface (55) and therefore should

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48 measurably reduce soil water loss due to evaporation at soil depths of 15 cm when compared with wide-row soybeans. Therefore under these conditions, the appreciablely higher soil water deficits which are measured for 25 cm soybeans must be due to a higher soil water usage (Table 3.1). Increased water use may be due in part to a better distribution of soybean plants over the soil surface which results in a more rapid although more efficient water use rate. Tillage system had a highly significant impact on soil moisture at sampling time 3 (Table 3.1). Soybeans grown under no-till mulch conditions had 52% higher soil tension levels than those grown under conventional seedbed conditions. Results indicate that whenever tillage system was found to be significant, the no-till condition always had a higher soil moisture deficit than the conventional tillage method at the 15 cm depth. The addition of 2000 kg/ha of wheat straw back onto the soil surface in the no-till plots may have enhanced water infiltration into the soil profile as has been reported in the literature (31) Tne higher soil tension levels under no-till mulched conditions could therefore be attributed to higher soil moisture use by the soybeans or sicklepod. At sampling times 9 and 10, soil tension levels were approximately 23% higher under no-till conditions than for conventional tillage systems (Table 3.1). Under these conditions, soil moisture usage may indeed be higher for soybeans grown under mulched conditions. Banks et al. (1)

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49 reported that soil water depletion was greater in the no-till treatments during the pod filling stage when soybeans were grown in competition with sicklepod. The mean soil water use (Table 3.1) of all row spacings shows that the 25 cm rows did not significantly differ from the mean water use of the 50 and 75 cm spacings. Between tillage systems, mean water use did not differ significantly for the 15 cm depth. However, after examining each sampling date where stress occurred (average above 200 millibars for at least one row spacing) there was a significant difference between row spacings. Under stress conditions, the water use levels were highest for the 25 cm rows and was followed by 75 and 50 cm rows in decreasing order. This indicates that during conditions where moisture is limiting (sampling time 6) or not limiting (sampling time 3) narrow-row soybeans are using more water thus potentially producing more photosynthate Although this may or may not increase yields, it should increase early leaf area and therefore provide quicker canopy closure. In addition, when soil moisture is most critical, the period during flowering through pod fill (8,15,62,67) (sampling times 7-10), there were no significant differences between row spacings. Under stress conditions, the no-till mulched plots were not significantly different from the conventionally prepared plots. At the shallow depths (15 cm), soil moisture depletion was higher during the pod fill stage under the no-till conditions. The 15 cm depth in a coarse sandy soil

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50 is very much influenced by rainfall and evaporative water loss and therefore may not be a true indicator of soybean water usage. During the early vegetative growth stage of soybeans when both the crop and weed root systems are concentrated in this depth, a competitive advantage between row spacing or tillage system could be adequately monitored. Soybean row spacing and tillage system did not significantly effect soil moisture at the 30 cm depth at sampling times 4, 5, 7, 8, 9, or 10 (Table 3.3). During the early growing season (sampling times 1, 2 and 3), narrow-row soybeans consistently gave a lower soil moisture reading. Differences in soil moisture at these early sampling dates are similiar to differences measured at the 15 cm depth (Table 3.1). Since soybean or sicklepod roots are not present in large numbers at this depth this early in the growing season (57,77), differences in soil moisture may be in part due to reduced water infiltration or higher moisture usage. The higher soil water deficits measured at the 15 cm depth have a direct effect on the water content of the soil below. The mean water use at a depth of 30 cm (Table 3.3) of all row spacings shows that 25 cm rows used significantly more water than the 50 cm rows but was not different from the 75 cm rows. There were no measurable differences between row spacings during the reproductive growth stage of soybeans. During moisture stress conditions, soil moisture use was similiar to the mean water use, where 25 cm row spacings had a higher water demand than did the 50 cm rows.

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IX en oo in CM + e" CO o s •H Eh c •H O c (0 •H a rH CO I i (0 CO X 03 eo 03 rn o in 00 in ro i ) 1-1 rH rH H rH 03 03 03 03 03 CN m "3" 00 rH O CN O o rH rH rH rH rH rH 03 rd 03 CO o O r*> o 00 o |H rH rH 03 03 03 03 03 m O en frH CN i— 1 CO CN CN rH rH rH rH rH 03 03 03 03 03 rH 1 0 -P P CO •H •H (0 P rH > CD C rH P tn 0) H CO 0) cO En C -P rH c (0 -P rH 0 0 a> cu rH u Eh a) E (0 >i co Xi cu -a Xi 0) P c •H >i E •Q P 'a o o MH CO £ p CO CD P CD CO 03 P rH H X! iXl CO 0 p a) a tr> 03 4H rH 0 H P C •H P •H P o dp in CD X P CO P tn (0 C •H +J O C 03 OJ a P. co cu 4H £ <4H -p 0 -H CO M *0 0 P C >i •hh m XI -P <7> P c c -H (0 nj £ O P C 4H E 3 CD o U CO H rH H +J i CD C o B =3 a

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52 Higher soil water depletion by the 25 cm rows at both depths is a function of the increased plant population and also a more uniform arrangement of plants across the soil surface (74) Therefore the proportion of soil water used by the crop may be larger than that made available for use by sicklepod. These conclusions agree with Jordans (32), who concluded that soybeans in 25 cm rows effectively utilized the additional water while in 75 cm rows, sicklepod was able to increase its biomass dramatically at the expense of the crop. The wider row soybeans have a high moisture use rate because of increased water loss to evaporation, that results from slower canopy closure (Table 3.2) and may in fact be due to sicklepod being the more aggressive species and obtaining a larger share of the available soil water. Mean water use did not differ between tillage systems at the 30 cm depth (Table 3.3). However at sampling dates of approximately 6 and 9 weeks after planting soil water depletion was greatest under the no-till mulched conditions. During stress conditions at the 30 cm depth, soil water use was greater under no-till conditions at both sampling times 3 and 6. The reduction in soybean root growth expansion as reported by Geddes et al. (20) during the onset of the reproductive growth stage could be a possible explanation for no differences between row spacings during this period.

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53 Row spacing-tillage system 1986 Differences in soil moisture readings between years were primarily due to amounts and dates of summer rainfall. The soil moisture measuring technique used in 1986 reduced the variability between replications and was also a reason for analyzing the data for the two years separately. Water use did not differ at the 15 cm depth between soybean row spacing or tillage system for 50% of the sampling times (Table 3.4). Analysis of variance did indicate a significant interaction between row spacing and tillage system at sample time six. At sampling times 4, 7, and 8, the soil water depletion was greatest for soybeans grown in 25 cm row spacings. At these sampling times, the 25 cm rows were significantly higher in soil water usage than either the 50 or 75 cm rows. Under these conditions soil moisture usage was identical for the soybeans grown in 50 and 75 cm rows. A significant interaction between the row spacings and tillage systems exist at sampling time six (Table 3.5). Sample time six corresponds to approximately eight weeks after planting and is nearing the onset of the reproductive growth stage. Under conventional tillage, the soybeans grown in 25 cm rows have the highest soil water reserves while the 50 cm rows have the highest level of soil water depletion at the 15 cm depth (Table 3.5). The trend is reversed for no-tillage where the 25 cm rows have the highest water usage and the 50 and 75 cm rows are

PAGE 64

id o 'U IX CM o in CM in O to 00 CM — 1 — i — | (0 X! n 05 rCM m co o 00 00 r 00 CM I-I rH rH CM •0 m CM CO co CM rH rH rH r-j | (0 (0 05 05 r00 00 in o CM in CT. co o CM rH rH CM CM (0 03 03 03 ro VO in U0 r r-t 00 m 00 m cr, rH rH rH rH rH + E CO O CD £ •H CP Eh c •iH tP o C (13 -H a rH in | 0 in CM o in m S d) p tO c/3 a; ro (0 c o -iH P c (1) > c o u •H o 2 a) (0 XI CO CD 0) xi c P -H 6 >i p. X) 0/ p rQ (1) CD 0 w rH (0 rH o

1 p CO -H 6 rH CD -H P X) CQ (0 >iX! CO o u a) a &> ro im H 0 •H p C -H X! P H >H o m 0) x; P co p c •H + U (5 (0 a) Cu U CO cu C >i •iH rH x; p p •H p CO a) p 0) Cn C (13 P. C H p to c 03 a) c 03 U H m •H c a, CT>-H H P 0) to p o c 0) iH 03 M a; p p 0) rH c 10 03 U C a CO p CO H X 0) S 0) p to >i CO cu Cn 03 C ro tr c •H U 03 CO o M c 03 1 o to c o -H P U 03 u
PAGE 65

55 Table 3.5. Effect of soybean row spacing and tillage system on soil moisture (millibars) at 15 cm depth. Sampling time 6, 1986. Row Spacing Tillage System + (cm) Conventional No-Tillage 25 b 130b a 299a 50 a 189a a 233b 75 a 153ab a 223b Means within a row preceded by the same letter, or means within a column followed by the same letter, are not significantly different at the 5% level of probability, as determined by Duncans* s new multiple range test. Table 3.6. Effect of soybean row spacing and tillage system on soil moisture (millibars) at 30 cm depth Sampling time 6, 1986. Row Spacing Tillage System + (cm) Conventional No-Tillage 25 b 86b a 180a 50 a 130a a 142b 75 a llla a 152ab Means within a row preceded by the same letter, or means within a column followed by the same letter, are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.

PAGE 66

56 significantly lower. Narrow-row soybeans at eight weeks after planting had almost complete canopy closure under both no-tillage and conventional tillage systems. The 50 and 75 cm rows under conventional tillage are susceptible to moisture loss from solar evaporation as well as competitive moisture loss from sicklepod at the 15 cm depth. The higher soil water depletion in 25 cm rows versus 50 or 75 cm rows under no-tillage conditions could be due to higher soil moisture use rate. Soil water usage by soybeans in 25 cm rows is significantly different between tillage systems (Table 3.5). Soil water tension measurements are 57% higher under no-tillage conditions than conventional seedbed preparation. There are no differences in soil moisture between tillage systems at the 50 and 75 cm row spacings. It could be proposed that the increased row spacings tend to negate the advantages obtained by adding mulch back to the soil surface to reduce solar evaporative water losses. The mean water use (Table 3.4) between row spacings indicates that the 25 cm row was significantly higher than the 50 or 75 cm rows. These results coincide with Jordans (32) which reported that at the 15 cm depth narrow rows used more water in all conditions. He reported however that under stress conditions narrow-row offered no advantages over the wider rows in regards to soil moisture usage. During the reproductive growth stage the 25 cm rows had significantly higher soil water use rates than the 50 or 75 cm rows at the

PAGE 67

57 15 cm depth (Table 3.4). Mean water use did not differ between tillage systems at the 15 cm depth (Table 3.4). Water use at the 30 cm depths did not differ between soybean row spacings or between tillage systems at sampling times 1, 2, 3, 5 and 7 (Table 3.7). Soil moisture usage for narrow-row soybeans was significantly greater than soybeans grown with 50 or 75 cm spacings at sampling times 4 and 8. The mean soil water use for all row spacings indicates that soybeans grown in 25 cm rows have the capacity to use more soil water via transpiration than the wide rows. No significant difference exists at the 30 cm depth between tillage systems in relation to soil moisture. Therefore in 1986, the addition of 2000 kg/ha of wheat straw back onto the soil surface did not significantly effect the water holding capacity or the soil water use rate in this coarse sandy soil. In sampling time six, a significant interaction exists at the 30 cm depth between row spacings and tillage systems (Table 3.6). The response of these parameters are very similiar to those reported for 15 cm depths. Significant differences between tillage systems exists only for the 25 cm row spacings. This condition may be explained by the presence of a compaction layer or 'tractor pan 1 at or above this depth under no-tillage conditions. Conventional plots were plowed to a depth of 20-30 cm prior to planting, whereas no soil preparation was done to the no-till plots. Under these conditions, the soybean roots in the narrow-row

PAGE 68

+ 03 B H Eh tn C H H | cfl (0 CO •U *H to CU IX O 00 o to 00 ro -H i-H rH CN t— ( rH < — 1 (0 "J tti fU r\ M 00 CN CN 00 CN o CN CN o r-W (0 *u n CO tri CO iH IT) CO CN in IX) ro iH i o CO •H •P rH 0) CJ rH Cn 0) iH) cO > Eh rH c 1 rH 0 0 •H u Eh 03 | >i (0 XI CO o 4= C P -H E >i u XX 0) •p •O 0) £ O CO rH c0 rH O MH >, •P CO -H E H (1) -H P CO CO co o U 0) CU C7i (0 MH rH 0 in C •H -p •H r* o -P CO 4-1 (0 c •H 4J U C CO 0) a u co a) 4H • £ W +J O -H CO >H TJ 03 •P C >i •H H 1) +j tr> •use •H CO CO Sum •H C

-H •H +3 O CO CO C -H -p I 03 c 03 M CO CO >H C CO 03 (0 C -P O co -p C 0) 03 3 S H Q CO p 03 •H 03 E 0) -P 03 >i 0) 03 CP CO •H -P "0 c ro Cn C •H U CO 04 CO o M c CO 03 X! >1 O CO 4H 0 C o H -P U CO M 03 -P C

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59 soybeans would be more uniformly distributed within and limited to the upper soil profile and would have the capacity to deplete the water resevoirs from this area rapidly. Field observations indicated that no-till plots showed visual drought stress symptoms when the soybeans grown in conventional plots would not. However due to the abundance of rainfall (Appendix C) in August, these periodic drought stresses were not sustained long enough to decrease crop yields. Sicklepod biomass accumulation and soybean yield — 1985 The effect of soybean row spacing and tillage system on sicklepod biomass accumulation and soybean yield for 1985 is presented in (Table 3.8). Soybeans grown in 25 cm row spacings had significantly higher seed yields than the wider row soybeans. However there was no significant difference in sicklepod biomass accumulation between row spacings. Narrow-row soybeans must have used their essential growth factors such as light and water more efficiently to obtain increased yields under similiar weed pressures. Narrow-row soybeans had a higher soil water use rate early in the growing season which hastened development of the soybean canopy to better utilize the available light and possibly shade the competing sicklepod plants. Soybeans grown in 25 cm rows also responded with a higher water use rate under stress conditions which may have resulted in increased yields

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O ft-P OJ xt rH CP a; u •H ta p o CU-H 0) 0) -I 5 M o •H p CO Q (0 X! C (0 P i-H o i C o CO 03 P u c •H U 05 CO o 6 03 tD ro 0 o o CM 00 00 00 03 xi Cn id 0J 03 03 rH o o O r m iH 00 1— 1 m ID H 00 io LD 00 P H ro in m iH iH rH i—i rH C i 03 03 03 03 03 o 00 O o 03 Xi X! i CO X! § -P th a P Id-H CO io •H W rH "H 0) p rH 0) XI t7> c rH to x; oj 03 v •H G P XI rH > m o iH c 1 CU >, P H 0 i s xi a Eh u + (1) 0 0 W 4-1 O > P to 0) i 3 -P CO >1 tO e*> in ID tr> 0) d) Cn c 03 P a) rH p 03 Xi P •H

. c 0*>rH c p to •H C o id c id o OS tVP o tQ 14-1 •H o p c 3 C Q Cn •H >i to X) •H P T3 -COO) P c c •H -H > a> e p p (0 03 0) C P £ P
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61 There were no significant differences between row spacings for soybean or sicklepod plant heights, although there appears to be a trend for increased height at closer rows and higher populations. These results contradict those of Jordan (32) who reported a height increase for narrow-row soybeans as well as a height reduction for sicklepod grown in competition with narrow-row soybeans. Under present conditions in which no herbicides are used to slow early season competitiveness of sicklepod, the narrow-row soybeans alone do not have the ability to suppress weed growth as effectively. Neither soybean yield or sicklepod dry matter production was significantly different between tillage systems. Sicklepod biomass accumulation and soybean yield — 1986 The effect of soybean row spacing and tillage system on sicklepod biomass accumulation and soybean yield for 1986 is presented (in Table 3.9). Soybeans grown in 25 cm rows had a 23 and 21% higher yield than soybeans grown in 50 or 75 cm row, respectively. Sicklepod dry matter production did not differ among row spacings. As reported in Tables 3.4 and 3.7, the mean water use for all row spacings was greatest for the 25 cm rows in 1986 at both the 15 and 30 cm depth. In this case, the higher soil moisture use rate is positively reflected in a substantial yield increase. Even under similiar weed pressures the soybeans grown in 25 cm rows were able to more effectively utilize the limiting growth factors to obtain higher yields. There were no

PAGE 72

62 n co 03 e o •H X! O ft 0) H ^ 0 H co C • O vo oo B o> CU rH •P CO >1T3 03 rH 0 0) H Cr. >i (0 H C rH -H (0 P. Di -O C C (0 (0 CU trx C >i •H O V n (0 •H •P to :$ o M l+H o u CU 4H C (0 c o H -P 03 rH P E 0 <4H U W 03 CTl 0) 03 Eh O P ax: CP rH -H X 0) O S3 H W CT+B G p TS X O CP ft-H 03 m m 10 03 0) 0) id If) o in o rrH 2£ O rn o n00 OO r 0) cr rH CT 1 rH rH H CM rH H U rH rH rH rH rH CO Q c m cu x X tn| >vH O 0) CO K e cu m -H 0) Sh XI >i c O -H co m o 03 03 rcS (0 03 E IT) in u vo to kO CO o OS its x G •H U 03 ft E id 03 03 03 03 CTl VD CO rH r t> 00 03 X X 03 03 CM o in CO kC 00 rH LT) rin in to LD CM O LD in rE CU 4J to > CO 0 tji 03 •H Eh 03 C O H +J c cu > C o u 03 H Eh I O 2 13 cu o O **H 4H O (0 rH E CU Q) > -P CD CO r >1 tO <#> in CU cu Cn 0) 03 X rH -P -P CO cu -p c 03 M P 03 CU rH G -P ft •H G -H X CU -P P MrH •H 0> 3 O -O IS 0) c to >, tji rH 10 G -P •H C CO u us c mom ft-H to m •H O M X +J -H u c 3 G Q H >i to x P TJ O CU c •H E u CU M to m cu c E P •P u CU CU TJ p •P to o o cu m X -p •H 0) >i E -P m -h t0 rH •H CU X 00 X 03 C -P X m o CU >i M S X ft

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63 significant differences across row spacing or tillage systems in plant heights. It can therefore be concluded that by varying the soybean row spacing from 75 to 25 cm with a uniform sicklepod population and without the use of herbicides, an approximate 30% yield increase can be obtained. Sicklepod Competition — Planting Date Study The effects of row spacing and tillage systems on sicklepod competition in soybeans was evaluated in three planting dates starting at May 15 and occurring in two week intervals up until June 15, from 1984 through 1986. Data were analyzed separately by year due to differences in overall weed pressure and variable amounts and dates of rainfall Planting date — 1984 Overall yields were higher in 1984 due to a reduced and subsequently late competing sicklepod population. The area in which this experiment was established and maintained for the following three years had been in bahaigrass ( Paspalum notatum L.) pasture for the last three years and was essentially free of agronomic weed pressures. During the winter of 1984 prior to planting the wheat ( Triticum compactum Host) cover crop, sicklepod seed was spread evenly at the rate of 22 kg/ha over the experimental area. However even at this high seeding rate the sicklepod population was not sufficient, due to poor emergence, partial control by discing of conventional tilled plots and

PAGE 74

64 use of the burn down herbicide treatments in the no-till plots. After soybean emergence sicklepod was transplanted into this area, but was not as competitive as full season sicklepod would have been. This was especially evident in the later planting dates in which sicklepod competition was even more limited by the shorter growing season. Even though the weed pressure was low (less than 5 2 plants/m ) sicklepod still caused significant yield differences between row spacings in the intitial year of this experiment (Table 3.10). There are reports in the literature (21) that narrow-row soybeans are most beneficial in late planted soybeans. However these citations are indicative of soybean production to the north, in which the growing season is considerably shorter and maximizing early season canopy closure is more critical. Under Florida conditions narrow-row soybeans were significantly more productive at the earliest planting date, in which full season sicklepod competition was most severe. At the earliest planting date soybeans grown in 25 cm rows had seed yields approximately 30% higher than soybeans grown in either 50 or 75 cm rows. There were no significant differences in soybean yields between row spacings at planting dates B or C. Tillage system did not have a significant effect on soybean yield at any planting date in 1984.

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65 Table 3.10. Effect of row spacing, tillage system and planting date on soybean grain yield, 1984. Planting Date May 15 June 1 June 1 5 Row Spacing (cm) 25 50 75 Tillage System Conventional No-Tillage 1797a 1318b 1385b 1564a 1436a •Yield (kg/ha) 1594a 1575a 1762a 1658a 1630a 1511b 1824a 1675ab 1716a 1631a Means in a column within a row spacing or tillage system followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's multiple range test.

PAGE 76

66 Planting date — 1985 In 1985, when the sicklepod density and growth was adequate for full season competition there were significant differences in both row spacings and tillage systems (Table 3.11). At planting date A, 25 cm rows had significantly higher seed yields than the 75 cm rows, but had yields similiar to the soybeans grown in 50 cm rows. The narrow-row soybeans at planting date B had significantly higher yields than than either the 50 or 75 cm rows. Thus indicating that under conditions of full season sicklepod competition the reduction in row spacing offers some competitive advantage to the crop over the weed allowing higher yields to be obtained. There were no significant yield differences between row spacings at the latest planting date. No-till soybeans had significantly higher yields at both planting dates A and C than soybeans grown conventionally. There was not a significant reduction in sicklepod dry matter production by either row spacing or tillage system at planting date A or B (Tables 3.12 and 3.13), although a noticeable trend exists for increased sicklepod dry mater production as row spacing increases. The yield increase for the no-till soybeans in the earliest planting date may be a function of a more efficient water use rate which increased canopy closure and maximized light interception.

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67 Table 3.11. Effect of row spacing, tillage system and planting date on soybean grain yield, 1985. Planting Date May 15 June 1 June 15 Roy Spacing (cm) 25 50 75 Tillage System Conventional No-Tillage 593a 500ab 424b 367b 644a Yield (kg/ha) 850a 590b 615b 672a 698a 1302a 1374a 1110a 1041b 1482a Means in a column within a row spacing or tillage system followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.

PAGE 78

T3 O P 0) X CD O K •H co On e •P o X l-l lH rH c (0 -P V X X O CD CO EC trie o T3 C 0) (0 -H CD >h XI >i C O CO (C M O Cn c H U 03 CO en 05 to in 00 00 LO (0 o X co CM in r* m in X 05 o o in LD CM o in 05 oo in 05 o> oo m 05 05 03 CD in >x> X! XI (0 in IE 0) -P to CO c o u 0) 03 O 2 O O *w U-i o CO e CD CD > (1) CO iH >1 10 <#> in CD Cr> 0) (0 x H -P •H P C •H -P -H o co >, C -P H C U (0 (0 u CVH CO M-l •H o u c -H x o u c H -p -p to CD -P CD Cr> C 05 P, 0 I — I •H P i-H E (1) C to c 05 o c Q H >, CO XI 0) c H E u CD p P. as p o c 0) 03 to c 03 a) S CD >i E P (d -h CO rH -H XI 03 X o >1 p x a a) x -p

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T3 O P O 53 H w +J O 0.-H d) 0} rH S O H P OS— <0 Xi c c0 -P 0) X3 Xi cn| O d) C/> 55 C CD (0 -H CD 5h XI >i c o W (0 P u i rG (0 R) (0 u oo O O U3 c •H U (0 C^E w I o (0 id r0 id (0 *f o O CN 00 00 CO id id id o o o 00 rH ro r00 vo cn in iH rH rH id in (0 \ o Cn m X oo XI o in XI in iH (0 CN rm CN O m in m m oo m rd oo E rH 9 c -p rH •P CD >i 05 (0 x! e -p >1 C +> m -h CO o tr> •H H •H (0 -H Eh co o H 1 a) >i p •H 0 O S xi a E-t u ss + O o Q) > 4J d) 1H +J >i co cn <*> 0) in -p d) Did) o CO £ rH P C co p R •rH Xi p p CO d) P rH c & d) -H P -P HUH s w g MH E p -p O TJ £ V to >, C <7>rH c +> m •H C O CO C CO O <0 tVH CO 4-1 •H o p c p •H o c C Q •H >i CO Xi •P T3 O d) c H e p 0 p CO CO d) C -P E P d) 3 0) "O -P P (0 d) (0 O u

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70 Sicklepod dry matter production did not vary between row spacings or tillage systems at planting date B, although narrow-row soybeans outyielded the wider row spacings by approximately 30%. Row spacing did not significantly influence soybean yields at the latest planting date, although tillage system was significant. The added benefit of straw mulch and its capacity to increase soil moisture under the reduced growing season is reflected in a significant yield increase for no-till soybeans at planting date C. In addition the overall sicklepod dry matter production was 34% less under no-till production at the latest planting date (Table 3.14). The practice of tillage for seedbed preparation ensures a more rapid uniform seedling emergence and therefore may result in a more aggressive sicklepod plant. Whereas the practice of planting into a stale seedbed in which all emerged sicklepod plants have been controlled by the application of a nonselective burn down herbicide, the subsequent seedling emergence is delayed and less uniform and may result in a less competitive plant. Plant height was not significantly effected by row spacing or tillage system at any of the three planting dates (Tables 3.12, 3.13, and 3.14) Although statistical comparisons cannot be made within row spacings or tillage systems among planting dates, there are many recognizable trends. As the growing season is reduced by delaying planting, the plant height of both

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71 BO • co U 0 tO rH E CD *u rH 0 •p 0 -P 0 m •H <0 0ux 4h o XI T3 E 03 fO cd <0 rH "H U cm oc CO (0 rH X) D> a: a) k ~" LT) LD LO E u 0 C 0 X CD > Cb-H •H P CD • CD P CO CO H -P rH C >1 CO <0 CO <*> CD V rH ID -P H •p CD 01 C7> CD CD O CT X (0 x: c • CU-H CO XI c0 X rH •P c 0 (1) 0) O LD ro rH (0 00 rH JS i LD CO LD CD -p •H P, C •P rH CO Q rH C CU >i-0 X CD "H cn el it k -P CD rH 0) •H Uh 3 MH E rC C M •H rH C fO -P 0 T3 > rH •H CD £ CD H (0 43 CTi E c0 rO co rO fO CO >i C -P >i-H u •"3CO O o Q-> rH O (D ^j" lo LD ^ C P CO T3 W ED •H C C G 0 (0 c id (0 (0 O <0 0) CnX (0 4H C c >1 •H S H 0 C Q U CO 'O 0 C7 1 CO rH M H >i c i C CP O rrH CO -P c c u 0 O -H ro n rH o •H •H -rl w to rH rH rH rH rH ? CD 6 MH •P u tH M 0 la C9 ns (0 CD P -p E U CD u i CD T3 CD 0 rH •P IH o 0 •P CO 4H o O CD (0 W id tem rH in rH CD >i • CJ w (0 e -p c >i C CD •P (0 -H rH CO 0 c^ •H CO rH • o •H fO •H <0 cd •P rH CD X) a 9 LO o LD c rH CO x: ro (1) (0 o tN LT) rc0 CD •H c •P X rH rH > EH (0 0 X :* rH a CD >1 IH 0 -H 0 0 s X Cu Eh u Z +

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72 soybeans and sicklepod declines. There is also a decline in sicklepod dry matter production as the planting date is delayed from June 1 to June 15, which tends to be reflected in an overall higher soybean yield for planting date C. The mean sicklepod dry matter production, for all row spacings and tillage systems decreased by approximately 50% when the planting date was delayed from May 15 to June 15, resulting in a less competitive sicklepod plant. Planting date — 1986 In 1986 sicklepod competition severly reduced seed yields by as much as 60% when compared to typical soybean yields obtained in north central Florida (32) Soybean row spacing or tillage systems did not significantly influence soybean yield or sicklepod competitiveness at planting dates A or C (Table 3.15). Narrow-row soybeans resulted in 20% higher yields when compared with 50 or 75 cm rows, under similiar sicklepod densities, at planting date B. Tillage systems were significant only in the latest planting date. No-till soybeans had a 19% yield increase over soybeans grown conventionally. Plant height or sicklepod dry matter production did not vary significantly between row spacings or tillage systems on any of the three planting dates in 1986 (Tables 3.16, 3.17 and 3.18). However the same trends are evident as were observed in 1985, ie.; a general decline in plant height and sicklepod productivity and an increase in soybean yields occurs when the planting dates are delayed from May 15 to June 15.

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73 Table 3.15. Effect of row spacing, tillage system and planting date on soybean grain yield, 1986. Planting Date May 15 June 1 June 1 5 Row Spacing (cm) 25 50 75 Tillage System Conventional No-Tillage 796a 640ab 595b 680a 671a •Yield (kg/ha) 734a 562b 580b 619a 655a 1090a 1175a 1067a 995b 1224a Means in a column within a row spacing or tillage system followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.

PAGE 84

O +J ax 0) rH -H 0) o a •H CO -p -o x o di fV-H H £ X u •H Ih CO Q CP c (0 -P i-H o cu CP E rC o ro — VD C CU (0 -h 0) JH X >i C O -H CO A3 M U c H U rC OjE CO o n3 x id x CP M id ro re o at id d o CO 00 id id o in vo r in h •"3" in re r00 rn id a! CO VD r0 CO VD ro 16 X <0 X o in t cri vo in u in o in m E a) *J co to cu ro rH rH H Eh id c o H -P c a) > c o CJ id o on r> in ro in id in VO CO (0 O rH oo vo vo CP (0 •H Ei I O S2 d) o o U-l Uh O CO E rH 0) 0) p > 10 0) >irH CO OP co in 0) Cn 0) rd H -P C •H X •p •H M O CO >i CTlrH C -P -H C u ro Cu co O u c -H X p •H in c E rH O u -p CO 0) 4J 0) CP c rO >H 0) rH Cu •H 4-> rH EJ E ? 0) c CO c r0 CJ C 3 Q C CP H >i co X) c cu E 1H >H (0 CD P in a) CU T3 P 4-> CO 0) rO X 4-1 •H CO c r0 0) CU >i E *J (0 -H CO rH •H CU X) X! r0 P X O >i U x a

PAGE 85

O -P cux: H -H .* 0) u ac •H CO u> e o O CVH 0) (1) ,— i 5 o •rH >H C/2 Q 03 C 03 -P OJ £ a &i >i-H o a> T5 rH C 0) 05 -H 0> >H £) >i C O -H CO (0 M o o c u (0 a e 03 05 03 03 id lO O in O ro o 00 ra> i-i rH t-H rH rH rH E ro os 03 0 rLO LD IX) IX (0 JS as &> ro M rX! CN LO o oo LO LO CN O LO LO r0) O rH rH 0 HH 03 03 <4H o ro iH 00 0) rH e a) Q) > P 0) • 03 rH +J >1 0J in <*> a> in -P 0) tn a) 0 (0 £ cr> 03 03 H+l C o r rH fO CO •H +J U I-I p m CM E rH 0) c p iH •H 03 >i 03 03 X! £ -P >l C +) 03 -H W 0 Cr> •H 03 rH •rl 03 -H 0) P rH a) jq OH c rH 03 x; to 03 •H C -P Xl rH > Eh 03 0 rH c 0) >iH H 0 0 Eh u 55 + 0) rH rH C -PH •H C H X! 1 C LO VD tjl rH VD LO G -P 03 •H C O fl c 03 O 03 Cl,-h cj 03 IW C •H S IS C Q 0 03 Xl c 03 03 •H +J TJ LO rH in +> c c IX vo •H -H £ 0) E 3H M (0 (0 0) C -P E >H 0) nu rH -P 0 -P to O 03 03

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O 4-1 (D cn| rH u K •H to 4-> •O X\ O CVH (1) d) H £ U •H H CO Q — rd Cn Ha; c (0 4-> Q) £ XI >i-H o 0) c d) (0 -H 0) >H >i c O •H rd ro rd rd X) X> in in m ro ro id rH in in O >£> CO rH o i—i n CO vo •H 00 00 ro i — i i-H -P r CO 00 ro (0 Cn a; C •H U rd cue CO o u rd ro ro ro rO 01 CM cn in in rO ro (0 X ro O in P> in r>x> cn CN o rH o cn CM rH rH r-H rH in CNJ o in in B -p to >i co D Cn (0 -H Eh rH rO C o •H -P c > C o u Cn ro O 52 -a o O 4h m o CO rH e cu 0) > 4-> Q) W rH >1 CO <#> in a> cn a> ro C •H x •H M o 4-1 P ro 4J C 0) u 0) 4-1 4-1 H CD >, Cn rH C -P •H C O fO rd O CVH CO UH •H 5 O P c •H x; p •H oo c e rH O U c •H x: 4-> •H CO c (0 a) c Cn H to 4J to 0) 4-> 0 Cn c ro U a> rH a H 4-> rH p E > 0) c to ro O C Q >i £5 -P O C u ro 0) C •H e p 0) 4-1 )h a> 0) T3 4-> 4-> CO 0) ro 0) >i 6 4J (0 -rl C0 rH •H a) X) X! rd 4-> X) O >i H XI ft

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77 The effect of soybean row spacing, tillage system, and planting date on soybean canopy closure was evaluated in 1984 and 1985. The data for two years was combined and is presented in Tables 3.19, 3.20 and 3.21. Tillage system did not significantly effect percent canopy closure at any time in the growing season. By eight weeks after planting at all three planting dates the narrow-row soybeans were near full canopy closure and by ten weeks after planting 50 cm rows were at complete canopy closure at all three planting dates. The 75 cm row in planting date C never did obtain complete canopy closure. It is the ability of the narrow-row soybeans to rapidly close the row middles which makes them more competitive for light, aides in reducing evaporative soil water loss and better utilizes the soil moisture via a stronger source-sink relationship (45,50).

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78 Table 3.19. Effect of soybean row spacing and tillage system on percent canopy closure. Planting date A (May 15). Average of 1984 and 1985. Row Spacing (cm) Weeks After Planting + 8 10 percent25 64a 83a 95a 97a 50 52a 70ab 96a 96a 75 47a 54b 77b 88b Tillage System Conventional 54a 71a 86a 91a No-Tillage 53a 68a 88a 93a Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.

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79 Table 3.20. Effect of soybean row spacing and tillage system on percent canopy closure. Planting date B, (June 1). Average of 1984 and 1985. Weeks After Planting Row Spacing 4 6 8 10 (cm) percent 25 61a 86a 95a 98a 50 44b 70b 91a 96a 75 33c 59c 78b 90a Tillage System Conventional 48a 68a 89a 96a No-Tillage 44a 72a 84a 94a Means within columns with row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.

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80 Table 3.21. Effect of soybean row spacing and tillage system on percent canopy closure. Planting date C (June 15) Average of 1984 and 1985. Weeks After Planting Row Spacing 4 6 8 1£ (cm) percent 25 58a 76a 94a 99a 50 37b 57b 82b 93a 75 21c 48c 63c 82b Tillage System Conventional 38a 61a 78a 84a No-Tillage 40a 59a 84a 85a Means within columns within row spacings or within tillage systems followed by the same letter are not significantly different at the 5% level of probability, as determined by Duncan's new multiple range test.

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CHAPTER 4 SUMMARY AND CONCLUSIONS The experiments described in this study have added substantially to available knowledge concerning sicklepod competition in Florida grown soybeans. The following summarizes results from these experiments which may be applicable to the grower and which may provide a foundation for future research. The technique used to define the area of influence of sicklepod in soybeans was a very effective and thorough method to provide a feasible alternative to the conventional method of studying weed competition. Results of this study show that weed-soybean interference is a complex and dynamic process that depends on each plant's response to the environment and to the plants closest to it. Use of the area of influence methodology to measure weed and soybean interference effects over time and distance by evaluating developmental and yield parameters allows a more complete determination and quantification of the dynamic relationship of weed-soybean interactions. These data, combined with data obtained from concurrent studies of weed competition in soybeans will allow the integration of weed interference 81

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82 effects into existing soybean growth simulation models. Area of influence methodology will allow researchers to determine damage thresholds for many other weed species more efficiently and economically than currently possible with weed density studies. Results of this study indicate that sicklepod effectively competes for both soil moisture and light interception. Sicklepod sphere of influence ranges from 1.1 to 1.6 m depending on the degree of moisture stress or the developmental parameter monitored. It is extremely important when using the area of influence methodology that environmental variables are monitored and cultural parameters such as planting dates are recognized as influential. The sphere of influence of sicklepod in soybeans could also be dramatically effected by changing cultural factors such as row spacing or tillage systems in an effort to better capitalize on the crops inherant competitiveness. Competition studies between soybeans and sicklepod were conducted from 1984-1986, in which the factors of row spacing, tillage systems, and planting dates were evaluated. In all three years of the research, soybean yields were significantly greater in the 25 cm rows than the 75 cm row, under similiar sicklepod densities. Treatment differences in 1983, were not clearly defined because of an overall low sicklepod pressure. Yield differences between row spacings were expressed in the first and second planting date of each

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83 year. The narrow-row soybeans offered little competitive advantage over the wider row at the June 15th planting date primarily due to a reduction in sicklepod competitiveness at these late planting dates. No-Till soybeans had higher seed yields than conventionally grown soybeans by as much as 19% in 1986. The increased yield response was usually reflected only in the latest planting date and was positively correlated to a decline in sicklepod dry matter production. Plant heights or sicklepod dry matter production was not significantly influenced by row spacing or tillage system, although both parameters tended to decline as the planting date was delayed from May 15 to June 15. Mean plant water use was significantly greater in the 25 cm rows in both 1985 and 1986. Mean water use between row spacings or tillage systems did not differ significantly in 1985 at the 15 cm depth. Higher soil water depletion at both the 15 and 30 cm depth by the 25 cm rows is primarily a function of increased plant population and also a more uniform arrangement of plants across the soil surface. Mean water use between tillage systems was not significantly different at either depth or in any year. Under stress conditions, the water use level was highest for the 25 cm rows and was followed by 75 and 50 cm rows in decreasing order. Increased yields with narrow-row soybeans in this experiment is a response of more efficient and often higher water use rates, whereby the narrow rows are utilizing more

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84 water and therefore producing more photosynthate or transpiration as opposed to the soybeans grown in wider rows in which sicklepod was able to compete more effectively for soil water. In addition to water, one of the essential growth factors in plant competition is light. Narrow-row soybeans obtained 94% canopy closure by eight weeks after planting in all three planting dates. The more efficient canopy architecture of the narrow-row enables it to intercept a greater amount of light and therefore have the potential for higher productivity. This research indicates that soybeans planted in narrow rows may be beneficial in the management of weeds such as sicklepod. Sicklepod competing in wide row soybeans was found to have an area of influence 2 times greater than its canopy width. However, possibly through the manipulation of cultural practices such as planting date, row spacing or tillage system, which are easily adjustible by the grower, the area of influence could effectively be reduced without greater costs. Also these results indicate that the addition of chemical weed control systems into reduced tillage narrow row culture would provide an important management tool.

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APPENDIX A GAINESVILLE, FLORIDA MAY TO NOVEMBER 19 84 PRECIPITATION. Day May Jun Jul Aug Sep uct Nov 1 0.12 T T 2 0.60 0.30 3 0.64 0.69 4 0.24 m T 5 0.02 0.02 A A Ci o 4 y 6 0.10 T 7 0.10 T 0.01 8 T 0.18 9 T T T 10 0.05 11 0.32 0.24 12 1.01 13 0.04 0.02 0.72 14 0.02 15 16 0.12 0.20 T 17 0.01 18 0.01 0.17 0.13 0.01 19 1.09 0.51 0.04 T 20 0.48 0.04 0.58 0.10 21 0.17 0.36 0.37 22 0.36 1.05 23 2.60 T 0.25 0.15 0.11 24 0.57 0.09 0.05 25 0.04 26 0.65 0.01 0.03 T 27 T 0.30 0.11 T 0.10 28 2.72 0.02 1.88 0.56 0.18 29 0.12 0.56 0.26 0.15 0.81 30 0.86 0.74 0.09 31 0.58 0.04 Tot. 6.92 2.88 6.15 3.36 2.40 1.79 2.86 85

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APPENDIX B GAINESVILLE, FLORIDA MAY TO NOVEMBER 1985 PRECIPITATION. Day May Jun T„ 1 Jul Aug Oct Nov 1 T A O 1 U. jj 1 99 2 A OA 0.02 3 0.05 a A o 0 4z n no u.uz 0 35 4 0.79 0 04 5 0.37 u u 0 U • X H 6 a r\ c U U b 7 0.16 a a c U U b U D D 8 A OO 9 A 1 C 0.1b 10 0 04 A 1C U I b u u ^ fl 41 0.03 11 A A £ 0 Ub A A "7 u u / n 1 ft U X o 0 08 12 0.37 n, n 7 13 0. 10 0.25 u / _> 14 0.14 0.27 1 11 1*11 15 0 .33 0.10 A O A U J4 16 1.74 A 1 C 0.1b n in U • X v T J. 17 T 0.42 0.41 0.03 18 0.33 T 0.01 19 0.23 0.17 20 0.43 0.09 0.72 0.10 0.06 21 1.89 0.21 0.13 0.01 22 0.22 0.64 0.71 23 0.07 T 0.22 24 0.03 1.66 25 0.54 0.04 0.71 0.02 26 0.08 27 0.08 0.40 28 1.46 1.13 29 0.29 0.56 1.09 30 0.91 1.26 0.12 0.02 0.01 31 0.06 2.57 3.84 0.70 Tot. 3.43 6.46 5.39 13.43 3.29 4.34 3.30 86

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APPENDIX C GAINESVILLE, FLORIDA MAY TO NOVEMBER 1986 PRECIPITATION. Day May Jun Jul Aug Sep Oct Nov -inches — 1 0.02 T 2 0.40 0.10 T 3 0. 39 0.39 0. 03 4 0.17 T 5 0.01 0.33 0. 12 1.17 6 0.01 0.02 0.03 T 7 0.01 1.04 0.81 0.41 8 0.22 9 0. 83 0. 63 10 0.95 0.34 1.43 11 0.06 0.03 0.53 0.18 T 12 0.69 0.71 13 0 .36 0.03 0. 43 0. 22 T 14 1.74 1.06 T 15 0. 30 0.10 T 16 0.02 17 0.02 T 18 0.41 0.11 19 0.18 0.91 0.03 0.05 20 0.78 0.11 2.01 21 0.02 0.02 0.22 0.53 0.06 22 2.05 0.16 23 0.13 24 0.62 0.20 25 0.06 0.10 26 0.11 0.04 27 0.97 0.04 0.52 28 0.25 0.09 0.17 0.02 29 0.84 0.24 2.12 30 0.16 0.30 1.00 31 0.14 T Tot. 0.98 5.65 6.18 8.54 3.10 3.89 3.96 87

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LITERATURE CITED Banks, P. A., T. N. Tripp, J. W. Wells, and J. E. H amine 1. 1986. Effects of tillage on sicklepod (C assia obtusif olia ) interference with soybeans ( Glycine~max ) and soil water use. Weed Sci. 34: 143-149. Barrentine, W. L. 1974. Common cocklebur competition in soybeans. Weed Sci. 22:600-603. Blevins, R. L., D. Cook, G. H. Phillips, and R. E. Phillips. 1971. Influence of no-tillage on soil moisture. Agron. J. 63:593-596. Bridges, D. C, and R. H. Walker. 1985. Influence of weed management and cropping systems on sicklepod ( Cassia obtusifolia ) seed in the soil. Weed Sci. 33:800-804. Buchanan, G. A., and E. R. Burns. 1971. Weed competition in cotton. I. Sicklepod and tall morninglory. Weed Sci. 19:576-579. Buchanan, G. A., E. W. Hauser, W. J. Ethredge, and S. R. Cecil. 1976. Competition of Florida beggarweed and sicklepod with peanuts. II. Effects of cultivation, weeds, and SADH. Weed Sci. 24:29-39. Buchanan, G. A., and C. S. Hoveland. 1971. Sicklepod — Success story of a weed and how to control it in soybeans. Weeds Today. 2 (1):11-12. Bunce, J. A. 1978. Effects of water stress on leaf expansion, net photosynthesis and vegetative growth of soybeans and cotton. Can. J. Bot. 56: 1492-1498. Burnside, O. C, and W. L. Colville. 1964. Soybean and weed yields as affected by irrigation, row spacing, tillage, and amiben. Weeds 12:109-112. 88

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89 10. Coble, H. D., and R. L. Ritter. 1978. Pennsylvania smartweed ( Polygonum pennsylvanicuni ) interference in soybeans ( Glycine max ) Weed Sci. 26:556-559. 11. Colvin, D. L., G. R. Wehtze, M. Patterson, and R. H. Walker. 1985. Weed management in minimum tillage peanuts ( Arachis hypogaea ) as influenced by cultivar, row spacing, and herbicides. Weed Sci. 33:233-237. 12. Cooper, R. L. 1971. Influence of early lodging on yield of soybeans. Agron. J. 63:449-450. 13. Creel, J. M. C. S. Hoveland, and G. A. Buchanan. 1968. Germination, growth, and ecology of sicklepod. Weeds 16:396-400. 14. Currey, W. L. D. H. Teem, and J. H. Jordan. 1981. Sicklepod competition and control programs in Florida soybeans. Proc. South. Weed Sci. Soc 34:66. 15. Doss, D. B., R. W. Pearson, and H. T. Rogers. 1974. Effect of soil water stress at various growth stages on soybean yield. Agron. J. 66:297-299. 16. Dowler, C. C. N. C. Glaze, and A. W. Johnson. 1984. The six year effect of weed management levels and multiple-cropping sequences on weed populations. Weed Sci. Soc. of America Abstracts. 142:54. 17. Dowler, C. C, and M. B. Parker. 1975. Soybean weed control system in two southern coastal plain soils. Weed Sci. 23:198-202. 18. Fleck, N. G. 1976. Competition of sicklepod ( Cassia obtusifolia ) densities on soybeans ( Glycine max ) at variable row distances. Ph.D. Dissertation. Univ. of Florida. 169 pp. 19. Flint, E. P., D. T. Patterson, G. H. Reichers, and J. L. Beyers. 1984. Temperature effects on growth and leaf production in sicklepod ( Cassia obtusifolia ) ; hemp sesbania ( Sesbania exaltata ) and showy crotalaria ( Crotalaria spectabilis ) 7 Weed Sci. Soc. of America Abstracts. 156:60. 20. Geddes, R. D. H. D. Scott, and L. R. Oliver. 1979. Growth and water use by common cocklebur ( Xanthium pensylvanicum ) and soybeans ( Glycine max ) under field conditions. Weed Sci. 27:206-211.

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90 Godley, F. M. L. Thompson, and H. D. Coble. 1981. Weed management in narrow and wide row soybeans. Proc. South. Weed Sci. Soc. 34:57. Gunsolus, J. L. 1986. Reciprocal interference effects between weeds and soybeans ( Glycine max ) Ph.D. Dissertation. North Carolina State Univ. lOOp. Gunsolus, J. L., and H. D. Coble. 1985. Interference effects on weed and soybean growth and development. Proc. South. Weed Sci. Soc. 38:82. Hagood, E. S., Jr., T. T. Bauman, J. L. Williams, Jr., and M. M. Schreiber. 1980. Growth analysis of soybeans ( Glycine max ) in competition with velvetleaf ( Abutilon theophrasti ) Weed Sci. 28: 729-734. Hagood, E. S., Jr., T. T. Bauman, J. L. Williams, Jr., and M. M. Schreiber. 1981. Growth analysis of soybeans ( Glycine max ) in competition with jimsonweed ( Datura stramonium ) Weed Sci. 29:500-504. Hanway, D. G. 1954. Growing soybeans in Nebraska. The Soybean Digest. 14:19-20. Hartwig, E. E. 1957. Row width and rates of planting in the southern states. Soybean Dig. 17 (5):13-14, 16. Hauser, E. W. G. A. Buchanan, R. L. Nichols, and R. M. Patterson. 1982. Effects of Florida beggarweed and sicklepod on peanut yield. Weed Sci. 30: 602-604. James, A. R. L. R. Oliver, and R. E. Talbert. 1974. Distance of influence of common cocklebur on soybeans. Proc. South. Weed Sci. Soc. 27:340. Johnson, W. C, and H. D. Coble. 1981. A new method to determine weed competition. Proc. South. Weed Sci. Soc. 34:102. Jones, J. N., J. E. Moody, and J. H. Lillard. 1969. Effects of tillage, no tillage, and mulch on soil water and plant growth. Agron. J. 61:719-721. Jordan, J. H. Jr., 1983. Sicklepod ( Cassia obtusif olia L.) competition with soybeans as influenced by row spacing, density, planting date, and herbicides. Ph.D. Dissertation. Univ. of Florida. 106p.

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91 Larson, W. E., and G. J. Osborne. 1982. Tillage accomplishments and potential. Pages 1-12 in D. M. Krai, ed. Predicting Tillage Effects on Soil Physical Properties and Processes. Am. Soc. Agron., Madison, WI. Lehman, W. F. and J. W. Lambert. 1960. Effects of spacing of soybean plants between and within rows on yield and its components. Agron. J. 52:84-86. Mannering, J. V., and C. B. Johnson. 1969. Effect of crop row spacing on erosion and infiltration. Agron. J. 52:84-86. Marra, M. C, and G. A. Carlson. 1983. An economic threshold model for weeds in soybeans ( Glycine max). Weed Sci. 31:604-609. McCormick, L. L. 1977. Weed survey — Southern states. Res. Rep. So. Weed Sci. Soc. 38:184-215. McWhorter, C. G., and D. T. Patterson. 1980. Ecological factors affecting weed competition in soybeans. Pages 371-372 in F. T. Corbin, ed. World Soybean Res. Conf. II Proc. Westview Press, Boulder, CO. McWhorter, C. G. and W. C. Shaw. 1982. Research needs for integrated weed management systems. Weed Sci. 30 (Suppl. l):40-45. Miller, J. H., L. M. Carter and C. Carter. 1983. Weed management in cotton ( Gossypium hirsutum ) grown in two row spacings. Weed Sci. 31:236-241. Murphy, T. R. and B. J. Gossett. 1981. Influence of shading by soybeans ( Glycine max) on weed suppression. Weed Sci. 29:610-615. Oliver, L. R. 1979. Influence of soybean ( Glycine max) planting date on velvetleaf ( Abutilon theophrasti ) competition. Weed Sci. 27:183-188. Oliver, L. R. R. E. Frans, and R. E. Talbert. 1976. Field competition between tall morningglory and soybean. I. Growth analysis. Weed Sci. 24:482-487. Palmer, R. D. 1980. Weed survey — Southern states. South. Weed Sci. Soc. Res. Rep. 33:186.

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92 45. Patterson, D. T. and E. P. Flint. 1983. Comparative water relations, photosynthesis, and growth of soybeans ( Glycine max ) and seven associated weeds Weed Sci. 31:318-323. 46. Patterson, M. G. R. H. Walker, D. J. Isenhour, E. W. Hauser, and J. Slaughter. 1983. Effect on row pattern and weed interference periods on soybean-sicklepod competition. Proc. South. Weed Sci. Soc. 36:419. 47. Patterson, M. G., R. H. Walker, J. A. McGuire, and D. L. Colvin. 1985. Comparison of small plot and large plot techniques for soybean-weed competition studies. Proc. South. Weed Sci. Soc. 38:458. 48. Pendleton, J. F., and F. W. Slife. 1953. Hill planting vs. drilling of soybeans. Agron. J. 45: 451-452. 49. Peters, D. B. 1960. Relative magnitude of evaporation and transpiration. Agron. J. 52:536-539. 50. Peters, D. B. and L. C. Johnson. 1960. Soil moisture use by soybeans. J. Amer. Soc. Agron. 52:687-689. 51. Peters, E. J., M. R. Gebhardt, and J. F. Strityke. 1965. Interrelations of row spacings, cultivations, and herbicides for weed control in soybeans. Weeds 13:285-289. 52. Quakerbush, L. S., and R. N. Anderson. 1984. Effect of soybean ( Glycine max ) interference on eastern black nightshade [Solanum ptycanthum ) Weed Sci. 13:638-644. 53. Retzinger, J. E. 1984. Growth and development of sicklepod ( Cassia obtusifolia ) selections. Weed Sci. 32:608-611. 54. Robinson, E. L. G. W. Langdale, and J. A. Stuedeman. 1984. Effect of three weed control regimes on no-till and tilled soybeans ( Glycine max) Weed Sci. 32:17-19. 55. Sakamoto, C. M. and R. H. Shaw. 1967. Light distribution in soybean canopies. Agron. J. 59:7-9. 56. Schreiber, M. M. 1982. Modeling the biology of weeds for integrated weed management. Weed Sci. 30 (Suppl. 1):13-16.

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93 57. Scott, H. D., and L. R. Oliver. 1976. Field competition between tall morningglory and soybeans. II. Development and distribution of root systems. Weed Sci. 24:454-459. 58. Scott, H. D., and R. D. Geddes. 1979. Plant water stress of soybean ( Glycine max) and common cocklebur ( Xanthium strumarium ) : A comparison under field condition. Weed Sci. 27:285-289. 59. Sherman, M. E. L. Thompson, Jr., and R. E. Wilkinson. 1983. Sicklepod ( Cassia obtusifolia ) management in soybeans ( Glycine max). Weed Sci. 31:622-627. 60. Shurtleff, J. L., and H. D. Coble. 1985. The interaction of soybean ( Glycine max ) and five weed species in the greenhouse. Weed Sci. 33:669-672. 61. Shurtleff, J. L., and H. D. Coble. 1985. Interference of certain broadleaf weed species in soybean ( Glycine max ) Weed Sci. 33:654-657. 62. Siomit, N and P. J. Kramer. 1977. Effects of water stress during different stages of growth of soybeans. Agron. J. 69:274-278. 63. Smith, R. L. 1968. Effect of date of planting and row width on yield of soybeans. Soil Crop Sci. Soc. Fla. Proc. 28:130-133. 64. Stoller, E. W. and J. T. Woolley. 1985. Competition for light by broadleaf weeds in soybeans ( Glycine max). Weed Sci. 33:199-202. 65. Stuart, B. L. S. K. Harrison, J. R. Abernathy, D. R. Krieg, and C. W. Wendt 1984. The response of cotton ( Gossypium hirsutum ) water relations to smooth pigweed ( Amaranthus hybridus ) competition. Weed Sci. 32:126-131. 66. Stuart, B. L., S. K. Harrison, J. R. Abernathy, and C. W. Wendt. 1982. Water relations of cotton-pigweed competition. Proc. South. Weed Sci. Soc. 35:313-314 67. Swan, J. B. 1978. Water use by soybeans. M.S. Thesis. Univ. of Illinois, Urbana. 98p. 68. Talbert, R. E. 1984. Weed survey — Southern states. Research Report of South. Weed Sci. Soc. 37:196-197.

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94 Teasdale, J. R. and J. R. Frank. 1983. Effect of row spacing on weed competition with snap beans ( Phaseolus vulgaris ) Weed Sci. 31:81-85. Teem, D. H., C. S. Hoveland, and G. A. Buchanan. 1980. Sicklepod and coffee senna: Geographic distribution, germination, and emergence. Weed Sci. 28:68-71. Thurlow, D. L., and G. A. Buchanan. 1972. Competition of sicklepod with soybeans. Weed Sci. 20:379-384. Timmons, D. W. R. F. Holt, and R. L. Thompson. 1967. Effect of plant population and row spacing on evaporation and water-use efficiency by soybeans. Agron. J. 59:262-265. Walker, R. H., M. G. Patterson, E. Hauser, D. J. Isenhour, J. W. Todd, and G. A. Buchanan. 1984. Effects of insecticide, weed-free period and row spacing on soybean ( Glycine max ) and sicklepod ( Cassia obtusifolia ) growth. Weed Sci. 32:702-706. Weber, C. R. R. M. Shibles, and D. E. Bryth. 1966. Effect of plant population and row spacing on soybean development and production. Agron. J. 58:99-102. Wiese, A. F., and C. W. Vandiver. 1970. Soil moisture effects on competitive ability of weeds. Weed Sci. 18:518-519. Wilkerson, G. G. J. W. Jones, K. J. Boote, K. T. Ingram, and J. W. Mishoe. 1983. Modeling soybean growth for crop management. Trans. Amer. Soc. Agric. Eng. 26:63-73. Willatt, S. T., and H. M. Taylor. 1978. Water uptake by soybean roots as affected by their depth and by soil water content. J. Agric. Sci. 90:205-213. Wilson, H. P., M. P. Mascianica, T. E. Hines, and R. F. Walden. 1986. Influence of tillage and herbicides on weed control in a wheat ( Triticum a estivum ) -soybean ( Glycine max ) rotation. Weed Sci. 34:590-594. Wrucke, M. A., and W. W. Arnold. 1985. Weed species distribution as influenced by tillage and herbicides. Weed Sci. 33:853-856.

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BIOGRAPHICAL SKETCH Kevin McDonald Perry was born in Louisville, Kentucky, on September 14, 1959. He is one of seven children born to Mr. and Mrs. Ben C. Perry. He received his primary and secondary education in the Jefferson and Todd County, Kentucky, school systems and was graduated from Todd County Central High School in June 1977. In the fall of 1977 he entered Murray State University in Murray, Kentucky. In August 1979, he enrolled at the University of Kentucky, Lexington, and in August 1981 he received a Bachelor of Science degree with a major in entomology. On August 29, 1981, he wed Miss JoAnn Miller of Herman, Kentucky. In September of 1981, he entered the Graduate School of the University of Tennessee at Knoxville as a graduate research assistant in the Plant and Soil Science Department. He received the Master of Science degree in plant and soil science with emphasis in weed science in August 1983. Since that time he has been enrolled in the Graduate School of the University of Florida to pursue the degree of Doctor of Philosophy with a major in agronomy (weed science) His hobbies include camping, gardening, photography and golf. 95

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. W. L.^Currey, Chairman Associate Professor of/ Agronomy I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. B. J. Bredke/ Associate Professor of Agronomy I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. D. W. Hall Associate in Botany

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. C. K. Hiebsch Associate Professor of Agronomy I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. This dissertation was submitted to the Graduate Faculty of the College of Agriculture and to the Graduate School, and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. May, 1987 leqe of Aqricu Dean, College of Agriculture Dean, Graduate School


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