Sicklepod (Cassia obtusifolia L.) competition with soybeans as influenced by row spacing, density, planting date, and he...

MISSING IMAGE

Material Information

Title:
Sicklepod (Cassia obtusifolia L.) competition with soybeans as influenced by row spacing, density, planting date, and herbicides
Uncontrolled:
Cassia obtusifolia
Physical Description:
x, 106 leaves : ill. ; 28 cm.
Language:
English
Creator:
Jordan, J. H ( Jerry Holland ), 1956-
Publication Date:

Subjects

Subjects / Keywords:
Soybean -- Weed control   ( lcsh )
Genre:
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1983.
Bibliography:
Includes bibliographical references (leaves 98-105).
Statement of Responsibility:
by Jerry Holland Jordan, Jr.
General Note:
Typescript.
General Note:
Vita.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 000419152
notis - ACG6855
oclc - 10955901
System ID:
AA00003838:00001

Full Text









SICKLEPOD (Cassia obtusifolia L.) COMPETITION
WITH SOYBEANS AS INFLUENCED BY ROW SPACING,
DENSITY, PLANTING DATE, AND HERBICIDES









BY

JERRY HOLLAND JORDAN, JR.


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


UNIVERSITY OF FLORIDA


1983













ACKNOWLEDGMENTS


I wish to express my sincere appreciation to Dr. Wayne Currey,

Dr. David Teem, Dr. Barry Brecke, Dr. Don Herzog, Dr. David Hall,

and Dr. Cliff Hiebsch for their support in preparing this manuscript.

I especially acknowledge Dr. Wayne Currey, chairman of my committee,

and Dr. David Teem for allowing me to continue my education at the

University of Florida and for giving of their time and experience,

thus providing me an education not obtainable from classwork and

textbook studies.

Thanks are extended to Dr. Ken Quesenberry, Dr. Ken Boote, and

Dr. Jerry Bennett for their helpful suggestions during my research.

My upmost appreciation is extended to Mrs. Mary Ann Andrews for

her professional assistance in preparing this manuscript.

To my parents, Mr. and Mrs. Jerry Jordan, thank you for preparing

me for life in a way only a loving environment can provide.

To Dot Bailey and Jeffie Filgas, thank you for all of your support

without which this manuscript would not be possible.

To my loving wife Lisa, thank you for your patience and under-

standing throughout my education. Your love and devotion have made

any hardships seems insignificant.

To April Michelle and Brett Holland, thank you for showing me

that children are God's assurance that life is worthwhile.

Finally, I would like to thank God for opening doors only he

could open to allow me to accomplish this goal in my life.









TABLE OF CONTENTS


Page
ACKNOWLEDGMENTS............................................... ii

LIST OF TABLES............................ ................... .

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

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

INTRODUCTION.... .............................. ............... 1

LITERATURE REVIEW....................... ......................... 3

Classification and Description of Sicklepod................. 3

Weed-Crop Competition..................................... 4

Definition........................ .......... ......... 4
Nutrient Competition.................................. 5
Water Competition..................................... 8
Light Competition...................................... 9
Weed Density.................................. ... ...... 12
Row Spacing.......................... ................ 16

Sicklepod Control Programs................................ 20

MATERIALS AND METHODS.......................................... 23

Soybean-Sicklepod Competition Studies...................... 23

Competition of Sicklepod with Soybeans--1980......... 23
Competition of Sicklepod with Soybeans--1981 .......... 24
Competition of Sicklepod with Soybeans--1982........... 27

Herbicide Programs and Planting Date Studies................ 28

RESULTS AND DISCUSSION......................................... 32

Soybean-Sicklepod Competition Studies...................... 32

Competition of Sicklepod with Soybeans--1980........... 32
Competition of Sicklepod with Soybeans--1981........... 35
Competition of Sicklepod with Soybeans--1982........... 60

Herbicide Programs and Planting Date Studies................ 78

SUMMARY AND CONCLUSIONS....................... ..... .............. 90


iii








APPENDICES

A RAINFALL DATA, QUINCY, FLORIDA, 1980................ 94

B RAINFALL DATA, GAINESVILLE, FLORIDA, 1980........... 95

C RAINFALL DATA, GAINESVILLE, FLORIDA, 1981........... 96

D RAINFALL DATA, GAINESVILLE, FLORIDA, 1982........... 97

LITERATURE CITED ............................................. 98

BIOGRAPHICAL SKETCH.......................................... 106








LIST OF TABLES


TABLE PAGE

1 Treatment Variables for Competition Studies............ 30

2 Treatments for Herbicide Programs and Planting Date
Studies............................................ 31

3 Soybean Yield and Sicklepod Dry Weight as Affected by
Soybean Row Spacing and Sicklepod Densities, 1980...... 33

4 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Dry Weight (kg/ha). Quincy, 1980......... 34

5 Soybean Yield and Heights as Influenced by Soybean Row
Spacing and Sicklepod Density in 1981. Gainesville.... 44

6 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Dry Weight (kg/ha), 1981. Gainesville.... 46

7 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 25 cm depth, 1981.
Gainesville........................................... 47

8 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 25 cm Depth. Sampling
Time 2, 1981. Gainesville............................ 53

9 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 25 cm Depth. Sampling
Time 3, 1981. Gainesville............................. 53

10 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 25 cm Depth. Sampling
Time 5, 1981 .......................................... 55

11 Effect of Soybean Row Spacing on Percent Canopy Closure,
1981. Gainesville.................................. 55

12 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Height (cm), 1981. Gainesville........... 56

13 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Width (cm), 1981. Gainesville............ 58

14 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod 1st Branch Height (cm), 1981. Gainesville. 58

15 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Seed (per ha), 1981. Gainesville......... 59








16 Effect of Soybean Row Spacing and Sicklepod Densities
on Percent Sicklepod Light Interception, 1981.
Gainesville...................... ...................... 59

17 Soybean Yield and Characteristics as Influenced by
Soybean Row Spacing and Sicklepod Density, 1982.
Gainesville............... ............................ 61

18 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Dry Weight (kg/ha), 1982................. 62

19 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 25 cm Depth, 1982.
Gainesville........................................... 68

20 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 15 cm Depth, 1982.
Gainesville......................................... 71

21 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 15 cm Depth. Sampling
Time 1, 1982. Gainesville............................ 73

22 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 15 cm Depth. Sampling
Time 3, 1982. Gainesville............................. 73

23 Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 15 cm Depth. Sampling
Time 17, 1982. Gainesville............................ 75

24 Effect of Soybean Row Spacing on Percent Canopy Closure,
1982. Gainesville.................................... 75

25 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Height (cm), 1982. Gainesville........... 76

26 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Width (cm), 1982. Gainesville............ 76

27 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod 1st Branch Height (cm), 1982. Gainesville. 77

28 Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Seed (per ha), 1982. Gainesville......... 77

29 Effect of Soybean Row Spacing and Sicklepod Densities
on Percent Light Interception by Sicklepod, 1982.
Gainesville ........................................... 79

30 Effect of Soybean Row Spacing and Herbicide Programs
on Soybean Yield, 1981. Gainesville.................. 80









31 Effect of Soybean Row Spacing and Herbicide Programs
on Sicklepod Control, 1981. Gainesville.............. 83

32 Effect of Soybean Row Spacing on Percent Canopy Closure,
1981. Gainesville.................................. 83

33 Effect of Soybean Row Spacing, Herbicide Programs,
and Planting Date on Soybean Yield and Sicklepod
Control, 1982. Gainesville........................... 84

34 Effect of Soybean Row Spacing, Herbicide Programs,
and Planting Date on Sicklepod Weed Control. Planting
Date A (May 3), 1982. Gainesville.................... 85

35 Effect of Soybean Row Spacing, Herbicide Programs,
and Planting Date on Sicklepod Weed Control. Planting
Date B (May 17), 1982. Gainesville.................... 85

36 Effect of Soybean Row Spacing on Percent Canopy Closure
Planting Date A (May 3), 1982. Gainesville............ 87

37 Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date B (May 17), 1982. Gainesville........... 87

38 Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date C (May 31), 1982. Gainesville........... 88

39 Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date D (June 14), 1982. Gainesville.......... 88

40 Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date E (June 28), 1982. Gainesville........... 89








LIST OF FIGURES


FIGURE PAGE

1 Soybean yield as influenced by sicklepod density.
Quincy (1980)....................................... 37

2 Soybean yield as influenced by sicklepod density.
Gainesville (1980)................................. 39

3 Soybean yield as influenced by sicklepod dry weight.
Quincy (1980)......................................... 41

4- Soybean yield as influenced by sicklepod dry weight.
Gainesville (1980) .................................. 43

5 Soybean yield as influenced by sicklepod dry weight.
1981 ................ .......... .................... 49

6 Soybean yield as influenced by sicklepod density.
1981 ............... ............................. 51

7 Soybean yield as influenced by sicklepod dry weight.
1982................................................. 65

8 Soybean yield as influenced by sicklepod density.
1982................ ....... ........ ................... 67


viii













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


SICKLEPOD (Cassia obtusifolia L.) COMPETITION
WITH SOYBEANS AS INFLUENCED BY ROW SPACING,
DENSITY, PLANTING DATE, AND HERBICIDES

By

JERRY HOLLAND JORDAN, JR.

April, 1983

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


Sicklepod (Cassia obtusifolia L.) competition with soybeans

(Glycine max (L.) Merr.), as influenced by soybean row spacing,

sicklepod density, and planting date, was studied from 1980 to 1982

at Quincy and Gainesville, Florida. Soybean row spacings were 30,

60 and 90 cm in 1980 and 25, 50 and 75 cm in 1981 and 1982. Sickle-

pod densities evaluated were 0, 1, 5 and 15 per m2 in 1980 and 0,

0.5, 1.0 and 5.0 per m2 in 1981 and 1982.

Soybean yields in 1980 were significantly greater in the 30 cm

rows than in the 60 and 90 cm rows, while in 1981 there was no differ-

ence in row spacings and in 1982 both the 25 and 50 cm row yields were

significantly higher than the 75 cm rows. Soybean yield decreased as

sicklepod density increased in 1980. In 1981, yields were similar at

low and medium densities. In 1982 yields at the zero and low densities

were not significantly different from each other. Soybean yield responded








curvilinearly to both sicklepod density and dry weight. Sicklepod dry

weight, seed production and light interception were all decreased by

soybeans in narrow row spacings.

Mean water use was greater in the 25 and 50 cm rows than in the

75 cm rows in both 1981 and 1982. However, under water stress con-

ditions there was no difference between the narrow and wide rows at

25 cm depth. At 15 cm depth, the narrow rows used more water under

all conditions.

When herbicide treatments were evaluated, weed control increased

as row spacing decreased. Excellent sicklepod control was obtained

with both trifluralin (a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-

toluidine) + metribuzin (4-amino-6-tert-butyl-3-(methylthio)-as-

triazin-5(4H)-one) preplant incorporated and alachlor (2-chloro-2',

6'-diethyl-N-(methoxymethyl)acetanilide) + metribuzin preemergence.

Poor sicklepod control resulted with the same herbicides in wide row

spacings. Two applications of toxaphene (chlorinated camphene) gave

100% control at all row spacings. Planting date did not affect sickle-

pod control. Soybean yields were greatest in the recommended planting

dates. The narrow rows outyielded the wide rows in the late planting

date and ine one of the three recommended planting dates.














INTRODUCTION


Weed competition is one of the primary limiting factors in soybean

(Glycine max (L.) Merr.) production throughout the world. In the south-

eastern United States, sicklepod (Cassia obtusifolia L.) is a major

competitor in soybeans, cotton (Gossypium hirsutum L.) and peanuts

(Arachis hypoqaea L.) (9, 11, 13, 15, 30, 35, 38, 39, 44). McCormick

(62) lists sicklepod as one of the ten most troublesome weeds of soybeans

in Alabama, Florida, Georgia, Mississippi, North Carolina, and Tennessee.

It is also in the top ten weeds of peanuts, cotton and corn in Alabama,

Florida and Georgia.

Sicklepod is a non-nodulating legume whose persistence can be at-

tributed to its tolerance of a wide range of soil fertility, pH, and

temperatures (25, 27, 92). Also contributing to this success as a

major pest is sicklepod's prolific seed production and tolerance to

many commonly used herbicides.

Farmers have been reluctant to use narrow rows in their soybean

production mainly because cultivation is no longer an option in their

weed control program. Recent advances in chemical weed control now

make narrow row soybeans an alternative program. Decreasing row spa-

cing in soybeans has advantages and disadvantages. The impact upon

various soybean weed pests, such as sicklepod, to this cultural prac-

tice has not been completely determined. Numerous studies have inves-

tigated the competitiveness of weeds in soybeans. However, most of

these investigations involve weeds per meter of linear row in soybeans

1









of 85 cm row spacing or greater. Many researchers and consultants are

now advising growers to decrease row spacing to achieve more rapid

canopy closure; however, therearelittle data available evaluating the

interaction between weed density and crop row spacing.

Investigations were initiated to evaluate the effect of decreasing

soybean row spacing on sicklepod growth. The major objective of this

study was to determine whether improved control of sicklepod could be

achieved by increasing crop competition through modification of cul-

tural practices. Variables involved include row spacing, sicklepod

densities, planting dates, and chemical control.













LITERATURE REVIEW


Classification and Description of Sicklepod


Cassia obtusifolia L. is an erect annual herb of the family

Leguminosae (subfamily Caesalpinioideae). The Weed Science Society of

America approved common name is sicklepod although it is often referred

to as coffeeweed by growers in the South. This species has pinnately

compound leaves with leaflets obovate, 2-7 cm long and numbering 4-6.

Sicklepod's petiolar gland is elongate. This gland is about 2 mm long.

It is located between, or just above, the petiolules of the lowest pair

of leaflets. The sepals are unequal and are 5-10 mm long and 2-5 mm

wide. The petals are yellow and are 8-17 mm long. There are 6-7 fer-

tile stamens and 3-4 staminodes. The legume is narrowly linear, strongly

curved, tetragonal, 1-2 mm long and 3-5 mm broad. Sicklepod often

reaches heights of 2 meters or more under conditions of good moisture

and fertility. Coffee senna (Cassia occidentalis L.), a closely related

species found commonly throughout the southeast, is separated from

C. obtusifolia L. by the number and shape of the leaflets and the

petiolar gland which is larger and bulb-shaped at the base of the

petiole. Both species are commonly found in Virginia, North and South

Carolina, Georgia, Florida, Alabama, Missisippi, and Tennessee (14, 17,

44, 50, 66, 77, 102, 104).








Weed-Crop Competition

Definition

Competition, as a comprehensive term, is defined by Smith (89) as

common use of a resource, regardless of supply, resulting in harm to

one organism by another seeking the resource. Smith (89) considers

competition to be one of two types, exploitative or contest. Exploi-

tation competition is where organisms have equal access to a limited

resource. Contest or interference competition is where a competitor

is denied access to a resource. However, these theories were developed

from animal populations.

Competition between plants has been defined by many authors. One

of the first researchers to investigate the subject of plant competition

was Pavlychenko (71) who in 1949 defined competition as a natural force

exerted by each living organism, tending to attain maximum advantage at

the expense of other living organisms occupying the same space. Donald

states "competition occurs when each of two or more organisms seeks the

measure it wants of any particular factor or thing and when the immediate

supply of the factor or thing is below the combined demand of the organ-

isms" (30:356). Although competition theories are well documented, even

the term "competition" is somewhat controversial. Harper (42) dislikes the

word "competition" because of its connotations to games and sports and its

unscientific meaning. He prefers the term "interference" which includes

both competition and allelopathy. Allelopathy describes the addition of

a chemical substance to the environment, while competition involves the

removal or reduction of a necessary factor from the environment (108).








Allelopathic effects will not be addressed in this manuscript and the

term "competition" used. Plants usually compete for water, nutrients,

light, oxygen, carbon dioxide and agents of pollination and dispersal.

Water, nutrients, and light are the most commonly deficient (24, 30,

108).


Nutrient Competition

Competition for nutrients constitutes an important factor in crop

production (7, 56). Vengris et al. (98) determined that weeds are

important competitors with crop plants for nitrogen and potassium.

High phosphorus content of weeds, even when available soil phosphorus

is low, indicates weeds compete effectively for this element. However,

this may be due to a more efficient utilization of soil phosphates. In

competition between leguminous crops and weeds, nitrogen may not be as

important a factor as phosphorus or potassium. On a soil acutely de-

ficient in nitrogen and phosphorus, Donald (30) determined that grass

weed growth was significantly depressed by gramineous cereal crops but

not by peas (Pisum sp.). Conversly the growth of clovers was depressed

by the peas but was unaffected by barley (Hordeum vulgare L.) or oats

(Avena sativa L.). Donald concluded that the main factor of competition

between the gramineous species was nitrogen, whereas between the leg-

uminous species with an independent nitrogen supply,competition was

primarily for phosphorus.

Blackman and Templeman (6) found that annual weeds in cereal crops

resulted in lower nitrogen and potassium content of the cereal but did

not alter the phosphorus content. Hoveland et al. (48) showed that

weeds respond to added potassium and phosphorus. Of the weed species









they studied, redroot pigweed (Amaranthus retroflexus L.), jimsonweed

(Datura stramonium L.) and Florida beggarweed (Desmodium tortuosum L.)

were the most responsive warm-season weeds to potassium and phosphorus

levels. Showy crotalaria (Crotalaria spectabilis Roth), tall morning-

glory (Ipomoea purpurea (L.) Roth), sicklepod, and coffee senna were

the most tolerant to low soil phosphorus. Generally, weeds were more

sensitive to low soil-test phosphorus than to low levels of potassium.

Chambers and Holm (21) found phosphorus competition by weeds to be

minimal. In their tests, weeds were found to have less effect on

phsophorus uptake than bean plants. Weeds may also compete with crops

for Ca and Mg, but nitrogen is usually the nutrient most subject to

competitive uptake (10). Response of plants to different levels of

Ca is difficult to measure because of its pH-dependent properties (11).

Increased fertility levels may not remedy nutrient competition by

weeds but may instead enhance it through increased weed growth thus

reducing crop yield. Vengris et al. (98) in a later study concluded

that weeds compete for essential nutrients and decrease crop yield

even at high fertility levels. Nakoneshny and Friesen (64) determined

that wheat (Triticum aestiuum L.) yield increases due to fertilizer

applications were not different from increases due to weed removal.

Staniforth (91) stated that soybean yield reductions due to weeds

were greater in tests where 75 or 150 kg/ha of nitrogen was applied

the previous year. In studying competition with wild buckwheat

(Polygonum convolvulus L.) Nalewaja (65) reported greater yield re-

ductions of wheat and flax (Linum usitatissimum L.) occurredin fertilized

treatments. Zimdahl (108) in his review on weed-crop competition stated

that weed control cannot be achieved with fertilizer. Maximum benefits








from fertilizer occur only to crops with relatively few weeds. The

solution to nutrient deficiencies in weed infested areas is obviously

not solely increased fertilizer, but depends on removal or reduction

of the weeds.

It is difficult to separate nutrient competition from other con-

founding factors such as light, moisture and pH since they are all

interrelated and a change in one may result in a correspodning change

in the other. Numerous investigators have attempted to separate light

from nutrient competition in the field by using weeds which are low-

growing and have smaller leaf areas (30, 37). Donald, in studying

nutrient competition, concluded that secondary effects must be consid-

ered. He states "success in gaining a larger share of available nu-

trients may stimulate growth increases resulting in dominance as much

from competition for light as for nutrients" (30:356). However, the

ability to compete for nutrients is an important aspect of the success

of weeds (103).

Moolani et al. (63) concluded that smooth pigweed (Amaranthus

hybridus L.) competition was greatest when early season rainfall was

high. Knake and Slife (52) found that giant foxtail (Setaria faberi

Herrm.) reduced corn (Zea mays L.) and soybean yields more in years of

high rainfall than in years of low initial rainfall followed by ade-

quate midseason moisture. In studying Venice mallow (Hibiscus trionum

L.) competition in soybeans, Eaton et al. (36) discovered that compet-

ition was greatest when moisture conditions were good initially and

poor after midseason than when moisture was limited early but adequate

from midseason on. Sicklepod competition in soybeans was found by

Thurlow and Buchanan (95) to be less severe in years of high moisture.









Size, distribution, and developmental rate of a plant's root

system influence the competitiveness of a plant for water. Scott and

Oliver (80) found that, compared with the soybean root system, tall

morningglory roots were found deeper and had greater root densities.

Little expansion of the soybean root system was found after initiation

of reproductive growth; however, during this phase the tall morning-

glory root system was still increasing.

The quantity and distribution of seasonal rainfall can alter the

competitive ability of various weeds as well as crops. Wiese and

Vandiver (104) showed that soil conditions can affect the competitive

ability of weeds. Barnyardgrass (Echinochloa crus-galli (L.) Beauv.)

and large crabgrass (Digitaria sanguinalis (L.) Scop.) were found to

be serious competitors when adequate soil moisture was available.

However, in semi-arid or arid areas, these weeds are not problems.

Kochia (Kochia scoparia (L.) Schrad), Russian thistle (Salsola kali

L.), buffalobur (Solanum rostratum Dunal) and tumblegrass (Schedonn-

ardus paniculatus (Nutt.) Trel.) were more competitive under dry con-

ditions, while Palmer amaranth (Amaranthus palmeri S. Wats.) grew

equally well under high and moderate soil moisture. Of the species

studied, cocklebur (Xanthium pensylvanicum Wallr.) was the only weed

that did not survive extreme drought.


Water Competition

Water is often considered to be the primary limiting factor in

soybean production throughout the world (33, 72). The effects of

water stress on growth and yield of soybeans aredependent on the degree

of stress and the stage of growth when the stress occurs. Burnside and








Colville (18) found that soybeans irrigated during the late bloom stage

outyielded non-irrigated soybeans by 685 kg/ha. Sionit and Kramer (87)

found that of all growth stages, water stress during early pod formation

resulted in the greatest decrease in number of pods and seed at harvest.

However, stress applied during pod formation or during pod fill re-

sulted in greater yield reductions than stress applied during flower

induction or flowering. This phenomenon has been confirmed by numerous

investigators (32, 78, 82, 94). Peters and Johnson (75) found that

from July 1 to September 20, 2.5 cm of water was required to produce

134 kg/ha of soybeans. Pendleton and Hartwig (72) state that at this

rate it would take 625 cm of water during the late growing season to

produce a high-yielding soybean crop of 3,369 kg/ha.

It is evident from the above investigations that soybeans require

sufficient moisture, especially during the late growing season, in

order to produce high yields. With the dependence upon natural rain-

fall in non-irrigated soybeans, adequate moisture is not always pres-

ent. When weeds exist the amount of soil moisture available to the

soybeans is further depleted. Staniforth (90) found that soybean

yields were only slightly reduced by yellow foxtail (Setaria lutescens

(Weigel) Hubb.) competition when water was limited in early season,

but was sufficient later in the season. However, when moisture was

plentiful early but limited in late season, there was a 15% reduction

in soybean yield.


Light Competition

Competition for light is unique among the factors for which plants

compete. With water or nutrients there is a fluctuating reservoir from








which to draw; however, light is available from a constant source.

Light energy must be intercepted and utilized instantaneously or be

lost. Donald (29) points out that this is particularly evident in

the young crop where most of the energy passes to the soil surface

due to lack of leaf area. Competition for light also differs from

other factors in that the competition is not necessarily between spe-

cies or plants butrather among leaves. This intra-plant competition

also occurs for water and nutrients but it is not as intense since

water and nutrients are translocated throughout the plant. The com-

petition along leaves for radiant energy is among individual units

within the plant canopy (30)..

The role which light plays in photosynthetic reactions has been

well documented (40, 41). The photosynthetic rate of a whole canopy

usually shows a greater response to light intensity than that of a

single leaf which may reach light saturation at less than full sun-

light. This is due to canopy architecture which includes leaf angle,

leaf distribution and leaf area index which allows more leaves to

intercept light at an average lower light intensity where they are

more efficient. Although single leaves may reach their maximum

photosynthetic rate at near full sun intensities, they are not very

efficient at utilizing light at that intensity (46). Bowes et al. (9).

states that the light saturation intensity of field grown soybeans is

approximately equal to the maximum light intensity under which they

are grown thus indicating that soybeans acclimate to the light avail-

able. Therefore, soybeans appear to develop sufficient, but not ex-

cessive, photosynthetic capacity to utilize the maximum available light.

In view of these facts the question arises as to how much depletion of








soybean yield potential is due to the interception of solar radiation

caused by a low or intermediate population of weeds arising above

the soybean canopy.

Light interception and subsequent photosynthesis depend largely

on foliage density and percent ground cover. Leaf area indes (LAI),

the units of leaf area per unit of land area, is the best criterion

for analyzing this. Brougham (12) defined the critical LAI as that

leaf area required to result in 95% interception at local noon. This

value for soybeans is approximately 3.2 (85). The relative light in-

tensity through the plant profile is reported to follow the relation-

ship I/I = eKA, where I = radiant energy received at the bottom of

an increment LAI, A, I = incident energy at the canopy top, K =

extinction coefficient. Sakamoto and Shaw (79) demonstrated by using

the above equation that light interception occurred primarily at the

periphery of the soybean canopy. When the open space between rows

closed, interception was primarily at the top of the canopy. Several

investigators using Beer's Law (30, 79) have shown that light inter-

ception by a canopy of leaves is exponential. Light intensity, there-

fore, decreases sharply as it penetrates into the canopy. Sinclair's

(86) equation of light attentuation, derived from Beer's Law, states

that the fraction of photon flux density (Pz) reaching any depth in

the canopy is an exponential function of cumulative leaf area from the

top (Lz), extinction coefficient (K), and the solar elevation (sin e)-

I/0 = Pz = e( ). The majority of the light is therefore found to

be intercepted by the upper unit of LAI (57). The effect that weeds

extending above the soybean canopy have on this relationship has not

been adequately investigated.








Perhaps the single most important factor in weed-crop competition

for light is plant height. Blackman and Templeman (6) stated that

light competition existed only when the weed species is tall growing

and the density is high. Shadbolt and Holm (81) reported that high

populations of redroot pigweed and ladysthumb (Polvgonum persicaria L.)

populations reduced light penetration 85%. Weber and Staniforth (101)

found that soybean yields were reduced more by overshadowing weeds

than by weeds of approximately the same height as the crop. Moolani

et al. (63) found that smooth pigweed had a greater effect on soybeans

than on corn due to the height differential between soybean and pigweed.

Knake and Slife (53) reported that giant foxtail had the greatest

effect on soybeans after reproductive growth had begun or after the

weeds began shading the soybeans.

It is evident that light competition is interrelated with water

and nutrients and that individual effects are difficult to separate.

A shaded plant, for example, suffers reduced photosynthesis leading

to poorer growth, and a smaller root system, and ultimately reduced

capacity for water or nutrient uptake (30). However, if water and

nutrients are in adequate supply, then light will become the major

limiting factor controlling rate of growth and the production of dry

matter (28).


Weed Density

Crop yield reductions due to weeds have been demonstrated by

numerous investigators (11, 22, 26, 39, 51, 81) with much of this

work involving soybeans. Wilson and Cole (107) reported that soy-

bean yields were reduced 12 and 44% by tall and ivyleaf morningglory








(Ipomoea hederacea (L.) Jacq.) at densities of one plant per 51 and

4 cm or row, respectively. Removal of morningglories at six to eight

weeks after planting permitted maximum soybean yields. Oliver et al.

(69) found that at one weed per 61, 30, or 15 cm of row, tall morning-

glory could remain for ten, eight, and six weeks, respectively, without

soybean yield reduction. Barrentine (3) indicated that tall morning-

glory and cocklebur were similar in competitive ability with soybeans.

Buchanan and Burns (13) found that one tall morningglory plant per

30 cm of row reduced cotton yields 10 to 40% on Norfolk sandy loam

soil and 50 to 75% on Lucedale sandy clay soil. Tall morningglory

was found to be more competitive than sicklepod in this study.

In 1976, Bloomberg and Wax (8) stated that common cocklebur ranked

as the most important and detrimental weed in soybeans. In a three

year study involving common cocklebur in soybeans, Barrentine (3)

found that full season competition by cocklebur at 3,300, 6,600,

13,000, and 26,000 plants/ha reduced soybean yields by 10, 28, 43,

and 52%, respectively. When controlled for the first four weeks,

further cocklebur removal was not necessary in order to obtain max-

imum yields. In studying the economics of common cocklebur control,

Anderson and McWhorter (1) reported that soybean yields were increased

about 6% for each 10% increase in cocklebur control. Net returns to

land, management, and general farm overhead were $63/ha with 0% con-

trol and $119/ha with 95% control of cocklebur. A 70% weed control

level was required to escape losses caused by excessive seed moisture.

Hauser et al. (45) stated that common cocklebur as well as yellow nut-

sedge (Cyperus esculentus L.) reduced yields 75% in experiments con-

ducted in Georgia. The above data clearly give evidence to support








Bloomberg's and Wax's initial statement. Cotton is also adversely

affected by cocklebur competition. Buchanan and Burns (14) found

that 8 weeds/7.3 m of row reduced yields more than 20% and 48 reduced

yield more than 80%.

Sicklepod competitiveness in soybeans is well documented, although

it is reportedly less competitive than cocklebur in some regions of

the country. Thurlow and Buchanan (95) found that soybean yields were
2
reduced linearly between 0 and 15 weeds/m2. In two separate locations,

yield was reduced 19 to 32% and 34 to 35%, respectively by densities
2
of 7.7 sicklepod/m2. Weeds allowed to compete for the first four weeks

of crop growth failed to reduce yields. Competition for six weeks

reduced yields in two of five experiments. Although early season

sicklepod competition may not adversely affect soybean yield at the

present, there is currently no reliable means of sicklepod control

after the early seedling stage until a weed-crop height differential

has been established (27). Teem (92 ) reported that sicklepod at

3, 5, and 7 weeds/m of row reduced soybean yields 19, 25, and 38%,

respectively. A regression coefficient of -.40 was obtained in these

studies indicating a reduction in soybean yield of .40 for each kg/ha

increase in dry weight of sicklepod. In studies evaluating soybean

yield as affected by sicklepod dry weight or density, Teem indicated

the coefficient of determination was .93 for the former and .38 for

the latter, thus indicating the weed weight is a more precise measure

of sicklepod competitiveness than weed density.

Buchanan and Burns (13) reported that cotton yields were reduced

10 to 40% at 8 plants/7.3 m of row, depending on soil type. A density

of 48 plants/7.3 m of row resulted in a 45 to 80% yield reduction.









Thurlow and Buchanan (95) stated that in general sicklepod is more

effective in reducing cotton yield than soybean yield, probably due

to soybean's shorter life cycle.

Numerous other weeds have been found to be deleterious to soybean

yields. Moolani et al. (63) reported that soybean yield reductions

from the highest population of smooth pigweed averaged 55% during a

three year study. Coble and Ritter (22) found that soybean yield

was reduced 13% by a density of 8 Pennsylvania smartweed (Polygonum

pensylvanicum L.) plants/!O m of row. Further yields reductions of

21, 37, and 62% resulted from full-season competition by densities

of 16, 32, and 240 weeds/l0 m of row, respectively. Berglund and

Nalewaja (5) found that soybean yields were reduced 21% by one wild

mustard (Brassica kaber (D.C.) L. C. Wheeler) plant/0.3 m of row.

Velvetleaf (Abutilon theophrasti Medic.), Venice mallow, and prickly

sida (Sida spinosa L.) were found to decrease soybean yields 720, 250,

and 230 kg/ha, respectively.

Monocots are also reported to be competitive with soybeans. Nave

and Wax (67) reported giant foxtail at 1 plant/0.3 m of row reduced

soybean yields by 13%. Knake and Slife (52) reported that 54 giant

foxtail/0.3 m of row caused a 28% soybean yield reduction.

Competition studies aid growers in determining the feasibility

of controlling particular weeds at various densities. Thus the term

economic threshold has been coined. This concept may have some prac-

ticality on a short term basis; however, for a long term weed control

program the economic threshold principle may not apply. When even a

low population of weeds is allowed to mature and produce seed each

year, the seed reserve in the soil can become enormous. A herbicide








program giving 95% control may not be adequate if the weed population

is extremely high. One pigweed plant has been reported to produce

over 117,000 seed (2). At this rate, only a few plants are needed to

result in a high weed population for many years. If weeds are not

permitted to produce seed, then a long term weed control program may

eventually be less costly.


Row Spacing

Soybeans have traditionally been planted in 75 to 100 cm row widths.

These widths are also used for cotton, corn, and grain sorghum. Wide

rows were necessary to accommodate conventional cultivation equipment;

however, with the development of selective herbicides, narrow-row soy-

beans are becoming a viable alternative (20).

Several studies with indeterminate soybean varieties have shown

increased yields with rows narrower than 100 cm (31, 66, 76, 105).

Wax and Pendleton (100) noted an increase in soybean yields of 10,

18, and 20% for 76, 51, and 25 cm rows, respectively, when compared

to 102 cm rows. Lehman and Lambert (55) found that seed yields ob-

tained from the 50 cm spacing were approximately 15% greater than

yields from the 100 cm spacing. Lovely et al. (58) achieved highest

yields from 30 cm rows. Hammerton (41) found in competition studies

with mixed weed stands that soybeans in 30 cm rows gave higher yields

than 60 cm rows.

Weed control is the most important consideration in the production

of soybeans in rows too narrow to allow cultivation (99). Studies on

the interaction of weed control methods and row spacing have shown that

soybeans in narrow rows provided more shade between the rows; therefore








fewer cultivations were needed after initial weed control had been

obtained in the row with herbicides (73). Wax and Pendleton (100)

demonstrated an increased weed growth in the wider soybean row spac-

ings. They stated that weed control by either trifluralin (a,a,a-

trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine) or cultivation.was

more effective in 50 cm rows than in 100 cm rows. Burnside and

Colville (18) showed that soybeans planted in 25, 50, 75, and 100 cm

rows obtained canopy closure in 36, 47, 58, and 64 days, respectively.

They also noted a yield increase of 39% from the 25 to 100 cm rows.

Lower rates of chloramben (3-amino-2,5-dichlorobenzoic acid) could

be used and still obtain adequate weed control. Kust and Smith (54)

found that soybeans planted in narrow rows were much more effective

than those planted in wide rows in suppressing the growth of yellow

foxtail and barnyardgrass. In addition, they found that lower rates

of linuron (3-(3,4-dichlorophenyl)-l-methoxy-l-methylurea) were re-

quired for comparable weed control as row spacing decreased. Cooper

(23) noted that if weeds are controlled with herbicides for four to

six weeks after planting, narrow row soybeans may provide more shading

than would wide row soybeans and lower rates of herbicide may be re-

quired in narrow rows because of the greater shading by soybeans.

Weed control may be increased in narrow row soybeans with effec-

tive herbicides, but a serious problem may arise if the herbicides are

unsuccessful. At this point, cultivation or a postemergence directed

spray could salvage a wide row soybean crop; however, this is not an

option with narrow row soybeans. One alternative early in the growing

season would be the rotary hoe. Lovely et al. (58) noted 70 to 80%

weed control by rotary hoeing when weeds had germinated but not emerged.








Peters and Johnson (75) found the rotary hoe effective on emerged weeds

less than 1 cm in height.

Another problem that may arise in narrow rows is the presence of

perennial weeds. Perennial weed species that normally do not persist

in cultivated row crops may increase if cultivation is eliminated (99).

Narrow row soybeans offer advantages other than increased weed

control. Mannering and Johnson (61) found that narrow row soybeans

provided significantly greater ground cover than wide row soybeans as

early as three to four weeks after planting resulting in 24% greater

water infiltration and 35% less soil loss to erosion. Hicks et al.

(47) noted that a greater leaf area index was provided earlier in the

season when plants were spaced in 25 cm rows. One further advantage

of narrow rows is the height of the lowest pods on the stems is raised,

which can help reduce harvest losses (51). Burnside and Colville (19)

found that height to lowest pod decreased with increasing row spacing.

Basnet et al. (4) found that in narrow rows the first pods were pro-

duced 3 to 9 cm higher above ground level than those in wider soybean

row spacings.

One disadvantage is that soybeans planted in narrow rows tend to

be lodged more than wide row soybeans (4, 19, 66). Hartwig (43) states

that planting more than 10 to 12 soybean seeds per 0.3 m of row gives

better early season weed control but a greater amount of loding usually

results. Most research has shown that when rows are narrowed to less

than 50 cm the seeding rate should be increased 10 to 30% to help assure

an adequate stand (51). Water use is supposedly another disadvantage of

narrow rows; however, current data do not support this. Johnson et al.

(51) state that if narrow rows do use more water it is being used to








produce more crop, not just lost to evaporation. Narrow rows may not

yield more than wide rows during a dry year, nor should they yield

less. Doss and Thurlow (33) reported that average daily water use

differed little between row widths except for a period early in the

season when plants were 25 to 60 cm high. At this stage more water

was used by the 90 cm than the 60 cm rows. The lower water use rate

on 60 cm rows probably resulted from less evaporation from the soil

surface due to more shading effect by the narrow rows during the early

season. Timmons et al. (96) stated that neither row spacing nor plant

population significantly altered evapotranspiration.

In contrast to the yield increases found in the northern U.S., row

width studies conducted in the major soybean producing areas of the

South indicate that there is no yield advantage to planting in rows

narrower than 30 to 40 cm. Hartwig (43) in Mississippi and Smith (88)

in Florida found that planting narrow rows had no effect on soybean

yields. Hartwig (43) explained that adapted southern varieties have

more foliage than northern indeterminate varieties and normally will

completely fill the row middles in 80 to 100 cm rows. Johnson et al.

(51) reported that row spacing will not have an effect on yield as long

as the rows are sufficiently narrow to close the soybean canopy by the

time the plants have begun flowering. Generally the farther north, the

narrower the optimum row width because the plants are smaller at flow-

ering. However, late planted soybeans are more likely to show a greater

response to narrow rows than those planted in the spring.








Sicklepod Control Programs

Sicklepod remains one of the South's most troublesome weeds in

soybean production despite numerous investigations on herbicide effi-

cacy. In corn and pastures, sicklepod can be easily controlled with

phenoxy herbicides, while in soybeans this is not an option (49).

Soil applied herbicides such as trifluralin (a,a,a-trifluro-2,6-dinitro-

N,N-dipropyl-p-toluidine) and nitralin (4-(methylsulfonyl)-2,6-dinitro-

N,N-dipropylaniline).have essentially no activity at normal use rates

(16); however, alachlor (2-chloro-2',6'-diethyl-N-(methoxymethyl)

acetanilide), metribuzin (4-amino-6-tert-butyl-3-(methylthio)-as-

triazin-5(4H)-one), and vernolate (S-propyl dipropylthio-carbamate)

have been reported to result in fair to good suppression of sicklepod.

Oliver et al. (70) reported that alachlor, metribuzin, and vernolate

gave 79, 56, and 49% control, respectively, of sicklepod thus providing

a height differential adequate for successful directed postemergence

herbicide applications. However, Currey et al. (27) stated that

metribuzin was the single most effective herbicide to control sickle-

pod. They also stated that sicklepod control and soybean tolerance

were consistently better for preplant incorporated than for preemer-

gence treatments containing metribuzin. Teem (92) also reported that

metribuzin gave greater control of sicklepod than either alachlor or

vernolate at normal use rates. While vernolate has provided good

control of sicklepod in some cases, soybean injury and stand reduction

have also occurred (26, 83).

The most effective method of controlling sicklepod involves the

use of postemergence applications in addition to preplant or preemergence








treatments. Toxaphene (chlorinated camphene) has been the most success-

ful of these chemicals both in terms of efficacy and low crop injury;

however, it was never granted a federal use label and will not be

available in the future. Sherman et al. (84) reported that toxaphene

at 2.24 kg/ha plus an oil concentrate controlled 89 and 95% of the

sicklepod in the cotyledon or first true-leaf stage, respectively.

Lunsford (59) found that toxaphene at 2.24 kg/ha plus an oil concen-

trate applied 11 days after planting to soybeans in the V2 stage when

sicklepod was in the cotyledon stage provided excellent control. A

subsequent application was needed seven to ten days later to control

newly emerging sicklepod. Once sicklepod passes the cotyledonary

stage, tolerance to toxaphene increased (60, 106). Acifluorfen (sodium

5-[2-chloro-4-trifluoromethyl)-phenoxy]-2-nitrobenzoate) has been shown

to provide some control of sicklepod in the cotyledonary stage with a

single application; however, split applications resulted in severe

soybean injury (34, 59, 68). A single application of acifluorfen at

0.56 kg/ha gave only 30% control of sicklepod while causing 20% soy-

bean injury on the V2 and V4 growth stage. A split application of

acifluorfen at 0.56 kg/ha at V2 and V4 stage soybeans resulted in 70%

sicklepod control and 60% crop injury; however, a split application

of toxaphene at 2.24 kg/ha at the V2 and V4 soybean growth stage gave

95% sicklepod control and no crop injury (59).

Buchanan and Hoveland (16) reported that postemergence appli-

cations of chloroxuron (3-[p-(p-chlorophenoxy)phenyl]-1,1-dimethy-

lurea) resulted in adequate control of seedling sicklepod. The add-

ition of a crop oil concentrate improved control; however, some crop

injury occurred. Oliver et al. (70) found that chloroxuron at 1.12









kg/ha plus a surfactant (0.5%) resulted in 92% control of sicklepod

while experiments in Florida showed that 97% control could be obtained

with the chloroxuron treatment (26).

Post-directed spray treatments have been one of the most success-

ful sicklepod control programs according to recent investigations;

unfortunately, grower acceptance of this practice has been slow (93).

Preemergence treatments of metribuzin, alachlor, alachlor plus metri-

buzin, or vernolate applied preplant, may provide sufficient suppress-

ion of sicklepod to provide the height differential necessary for a

post-directed spray (70). Sherman et al. (84) reported that either

linuron (3-(3,4-dichloro phenyl)-l-methoxy-l-methylurea) at 0.56 kg/ha

or metribuzin at 0.28 and 0.56 kg/ha applied alone or as tank mixes

with 2,4-DB (4-(2,4-dichlorophenoxy) butric acid) at 0.22 kg/ha pro-

vided greater than 90% sicklepod control when applied to the lower

7 to 10 cm or 20 to 25 cm tall soybeans. Other successful post-directed

treatments are paraquat (l,l'-dimethyl-4,4'-bipyridinium ion) alone or

in combination with either metribuzin or 2,4-DB (27, 60, 70).

From these results, it is clear that a complete herbicide program

is necessary for adequate sicklepod control. Currey et al. (27) found

that consistently acceptable control of sicklepod was obtained with use

of a dinitroaniline herbicide plus metribuzin preplant incorporated or

alachlor plus metribuzin preemergence, toxaphene early postemergence

to seedling sicklepod, and a post-directed spray of metribuzin plus

either 2,4-DB or paraquat. Oliver et al. (70) stated that by combining

the best preemergence and postemergence treatments, excellent control

of sicklepod was obtained resulting in a 37% increase in yield over

the check.













MATERIALS AND METHODS


Soybean-Sicklepod Competition Studies


Studies to evaluate the competition of various densities of

sicklepod with soybeans planted in various row spacings were conducted

during 1980 at the University of Florida Agricultural Research and

Education Center in Quincy and at Agronomy Farm (Green Acres) in

Gainesville. Competition studies were continued with modification

during 1981-1982 at the Gainesville location only. The soil at Quincy

was a Norfolk loamy fine sand (Fine-loamy, Siliceous, Thermic, Typic,

Paleudults) whereas the soil at Gainesville was a Bonneau fine sand

(Loamy, Siliceous, Theremic, Arenic Paleudults).

A split-plot design with row spacings as main plots and sickle-

pod populations as subplots, was utilized. Plots were 3.6 m by 6.0 m

and replicated four times. Trifluralin was applied at 0.56 kg/ha over

the entire experimental area for annual grass and small seeded broad-

leaf weed control. Soybean row spacings and sicklepod densities used

in these studies are listed in Table 1. Analysis of variance, Duncan's

New Multiple Range Test and regression equations were utilized to

analyze the data.


Competition of Sicklepod with Soybeans--1980

'Bragg' variety, maturity group VII, was planted on June 12 and

June 24, 1980, in Gainesville and Quincy, respectively. Soybean seeding








rates were 67, 84 and 100 kg/ha of soybean seed for the 90, 60 and

30 cm row spacings, respectively. Sicklepod populations were estab-

lished approximately two weeks after planting and maintained by hand

weeding. The middle two rows were harvested from each plot on October

28 and November 6, 1980, at Gainesville and Quincy, respectively.

Sicklepod plants were harvested from a 1 m2 area in each plot and its

green and dry weights were recorded. Regression analysis was utilized

to determine the influence of sicklepod density and dry weight on soy-

bean yield.


Competition of Sicklepod with Soybean--1981

'Bragg' soybeans were planted on May 27, 1981, at Gainesville.

After emergence the soybean stand was thinned to densities of 67,

84, and 100 plants/ha for the 75, 50, and 25 cm row spacings, respec-

tively. Weed populations were established approximately two weeks

after planting and maintained by hand weeding throughout the test.

Mercury manometric tensiometers were placed at a depth of 25 cm

in each plot of the first three replications on June 22, 1981. These

tensiometers consist of a 50 cm tube with a 10 cm porous ceramic cup.

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 served both as the manometer measuring tube as well as the

connecting link between the manometer assembly and the tensiometric

cup 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 graduated in millibars of soil tension

and is a standard unit of measurement for soil moisture. Tensiometers








were placed 15 cm laterally from one of the middle soybean rows and

10 to 15 cm from the nearest sicklepod plant. Tensiometric readings

were recorded nine times from June 30 to September 25, 1981.

Soybean canopy closure and sicklepod light interception were

evaluated utilizing a intergrating radiometer/photometer and line

quantum sensor that measured photosynthetically active radiation (PAR).

The preferred measurement for PAR is Photosynthetic Photon Flux Density

(PPFD) (85), the number of photons in the 400 to 700 nm waveband inci-

dent per unit time on a unit surface, and is recorded in microeinsteins
-1 -2
sec m The line quantum sensor, which has a sensing area of 1 m

by 12.7 mm, effectively averages PPFD over its 1 m length thus elimin-

ating the need for averaging measurements from numerous small diameter

sensors. Canopy closure was determined by placing the sensor on the

ground perpendicular to the soybean rows and recording the output.

The sensor was then placed over the canopy and the output again recorded.

The difference was calculated as a percent and recorded as amount of

canopy closure. This procedure was replicated four times with two

observations per replication.

Sicklepod light interception was determined with a light attenuation

equation derived from the Beer-Lambert law of light absorbance.

(-K; LAI)
I sin e
0

where-- = fraction of radiation penetrating to depth 0
o
K = transmissivity constant (extinction coefficient)
LAI = cumulative leaf area index
Sin o = solar elevation








The fraction of photon flux density is an exponential function of

cumulative leaf area, extinction coefficient, and the altitude of

the sun (sin e). The extinction coefficient, K, depends upon leaf

angle relative to horizontal and to the solar angle. The K value used

was derived from results obtained by Shibles and Weber (85). The leaf

area index of the canopy can be determined from a derivation of the

above equation, LAI = -In(frac.) x Sin e
K

The solar elevation was obtained by the equation,

Sin a = Sin L Sin 6 + Cos L Cos 6 Cos t

where a = altitude
L = latitude
6 = declination of the sun

t = time before local apparent noon 1 hr = 150

In the solar elevation equation the only unknown is the declination of

the sun which was derived from the equation,

6 =23.45T~- Cos [3-2 (172-D)]

where 6 = declination in radians
D = day of year


In calculating the LAI, the fraction of light penetrating the

canopy was determined. The line quantum sensor was placed on the

ground under the soybeans at a specified sicklepod density. Light

penetrating the total (soybean + sicklepod) canopy was recorded, the

sicklepod plants were removed, and the light recorded again. The

sensor was then placed above the canopy to record the total PAR

available to the plants. This fraction of light with both weed and

crop was inserted into the formula resulting in the cumulative LAI









of the canopy. The process was duplicated utilizing the weed removal

reading resulting in soybean LAI. Sicklepod LAI was obtained by sub-

tracting the soybean LAI from the total LAI. A ratio was then estab-

lished between total LAI and sicklepod LAI, thus percent sicklepod light

interception can be obtained. These equations assume that leaves are

horizontal and the soybean and sicklepod leaves are in the same plane.

All light measurements were taken between 11:00 a.m. and 1:00 p.m.

Sicklepod morphological data were obtained by randomly selecting

plants from each plot and recording the number of pods per plant, seeds

per pod, seeds per plant, plant height, plant width, distance from

ground to first lateral branch, and plant dry weight. Seeds per hectare

were also calculated. Soybean height measurements were recorded both

from soil to first pod and from soil to top pod. Soybeans were hand

harvested on November 4, 1981, and yields recorded. Regression analyses

were used to determine the influence of sicklepod density and dry

weight on soybean yield.


Competition of Sicklepod With Soybeans--1982

'Bragg' soybeans were planted on May 21, 1982. After emergence

soybean stand was thinned to densities of 67, 84, and 100 plant/ha

for the 75, 50, and 25 cm row spacings, respectively. Weed populations

were established two weeks after planting and maintained by hand weed-

ing throughout the test.

Mercury manometric tensiometers were placed in the first two rep-

lications. Two tensiometers at 15 cm depth and one tensiometer at

25 cm depth were placed in each plot in the first replication. In the

second replication, one at each depth was placed in each plot. Readings






28

were recorded 17 times from June 16 to September 15, 1982. Soybean

canopy closure, sicklepod light interception, and morphological data

were measured by the same procedures used in 1981. Soybeans were

hand harvested on November 12, 1982, and yields recorded. Regression

analyses were again used to measure the influence of sicklepod density

and dry weight on soybean yield.


Herbicide Programs and Planting Date Studies


In 1981 and 1982, field studies were conducted at the University

of Florida's Agronomy Research Farm (Green Acres) in Gainesville, to

determine the effect of soybean row spacing, herbicide programs and

planting date on sicklepod control. In 1981, one planting date (June 5)

and in 1982 five planting dates (May 3, May 17, May 31, June 14, and

June 28) were utilized. Hereafter, these five planting dates will be

referred to as A, B, C, D, and E. The recommended planting dates for

'Bragg' soybeans in Florida are May 15 through June 15. These planting

dates are approximately two weeks apart, thus representing one planting

date two weeks prior to, three planting dates during, and one planting

date two weeks after the recommended period.

A split-plot design was utilized with three replications in which

row spacings were the main plots and herbicide treatments were subplots.

All herbicide treatments were applied in 187 liters of water/hectare at

3.08 kg/cm2 with a CO2 backpack sprayer. In 1982, each planting date

was considered a separate test; therefore data from different planting

dates cannot be statistically compared. Plot diminsions were 3.6 m by

4.5 m. Table 2 lists herbicide treatments for 1981 and 1982. Analysis

of variance, Duncan's New Multiple Range Test, and regression equations

were used to analyze the data.






29

Canopy closure was determined according to the procedure used in

the competition studies. Readings were initiated four weeks after

planting and were repeated at weekly intervals until canopy closure.

Weed control ratings were recorded once. A scale of 0 to 10 was used,

where 0 = no control and 10 = total control. In 1982, the last three

planting dates did not contain sufficient uniform sicklepod stands to

warrant weed control ratings. Soybeans were hand harvested on November

18, 1981, and November 11 and 12, 1982.



















TABLE 1. Treatment Variables for Competition Studies

1980 1980 1981 1982
Quincy Gainesville Gainesville Gainesville
Row Spacing: Narrow-30cm Narrow-30cm Narrow-25cm Narrow-25cm
Medium-60cm Medium-60cm Medium-50cm Medium-50cm
Wide-90cm Wide-90cm Wide-75cm Wide-75cm

Weed Population: 0 0 0 0
Low-I/m2 Low-I/m2 Low-0.5/m2 Low-0.5/m2
Medium-5/m2 Medium-5/m2 Medium-1/m2 Medium-1/m2
High-15/m2 High-15/m2 High-5/m2 High-5/m2



















to NI O N N

01 0 0 00

+ + +*

0c-







O
0
C



I
--r


r- *r- C

4-


+M I 4-)
0 r- I
*r- E W r4-

3I 0 I 'c-
-O:)O I

E *r-- *- E


I I X
E S .- 0o u
0 Z r- 0
r0 Q -C -4
U I 34--
C *r- 0 I E .
*- EE LE 4-

t r I Z





Oo-- --
SM -- *i Q. -

So0 a
E 5- a- u a c-











+ o

(n 043' C- 4-
4E 3 CD *U
5- -- 4- a
C4- 4 C
*r- 0 | r-
5- S- 1 0 C

S-- to -
IC 0 *r- t7

4 -E Ec o C
*r S- U E

o i .

m t
C + 0
c. 5)
F- CC C (- (
0 *r- *r- 0 4 *r-
-.- N N U 0) U 3
C 3 C U <0 4-'
In .0 *r- .0 *r- C 4-
4 *r-N *r- C -U
C S- 3 4 r r-
0) 4-) .0 4-> C 5- CO *r-
E O *- a rIt C 0) r-
4- E 5- E r- E -- 0.
i0 4-3 0. CU 0.
0 -4- 0 + 0 0) C r
5- E 5-. S *-
H 0 C C 0. n Q. 0
E *- + **- -o C
it -- )- U "0 0) 1)
* 2S o 5- rL C 0) 0) t) .
CM S.- o 5- 0 *-r Tr 3 D.
C 3 r- 3J r- r- to
Lii 0 r- -C r- Q. .V 0. Q. 4-) X
-I E 4- U 1- lC U 0. 0. 0 o
Co E **- t *7- X ) < 2
C 0 5- ra- S- 0
H 0 4-' (0 4- 4-' U 4- ion e.














RESULTS AND DISCUSSION


Soybean-Sicklepod Competition Studies


Competition of Sicklepod with Soybeans-1980

Soybean yields at both Quincy and Gainesville locations were

significantly higher in the 30 cm rows than in the 50 and 75 cm rows

(Table 3). Previous research (43, 51) in the southern U.S. has not

shown a yield advantage due to narrow row spacing. The late planting

may provide an explanation since higher soybean populations of narrow

row spacings have been shown to produce higher yields than wide rows

at late planting dates (20). Soybean yields showed a significant

decrease with increasing sicklepod density at both locations. Sick-

lepod dry weight was not affected by row spacing in Gainesville but

weed dry weight increased with increasing sicklepod density.

In Quincy, there was a significant interaction between row spac-

ing and sicklepod density. At the high weed density, the narrow row

soybeans produced the greatest sicklepod dry matter. At the low and

medium densities, weed dry weight was not affected by row spacing

(Table 4). Competition from the 50 cm row soybeans resulted in a

decrease in sicklepod dry weight from that in the 90 cm rows. However,

an expected decrease in sicklepod dry weight in the 30 cm rows did not

occur. Since the sicklepod density of 15/m2 is so great, the lower

populated soybeans in the 60 and 90 cm rows did not produce a yield

increase over the 30 cm rows even with a lower sicklepod biomass.






























to




U

0B
CV)


0






>0
o



4o
*I-









4.
0)


4-)






_I-
10

r





S.
C)
,a






0






r-
M c
*r-
in
So
LI-
O



5- 0



.-
e0

r
001
>
O











w
r- 0-


* i-
--1 V




i-- 1


)
0 r-

5-
O > C
Q. VI w


-K *T--
w o

*- CD







r- LO

*0,- -
> *r- (L
> .C


0) C -e






0
o tc
C')



































S-
03
*r






--

.- +-





-0
uC o





E



P- 1




r




D. n:


Lu .0 Li



co Cj r-.


r.r.
CO CO











Ld .0 U


LO- C- C
in cn- Nt o

r-
















Cv O C) r
Ni om LO
1- c3- In












ca -0 Li -O
- %~- -- -


O- (0 0 -
0C1 o C) LO
- r- I-


CM
E

-,

C) C) C) CD C) CD


o O


CO


L LC LO


CD (0
r- CO to













rL .0 .0

(0 In (0
r- (0 *st-
CO Rd L-



















cD- d













Lu .0 .0



d- C.- ,-
5-* 5


00)


C tn
5- -
1o C
rs
LC
Lu
0)C


0)
r->)

0)
E -

tV C

0 5
-C 1-



.0



ar
C)
0 *

5-)




r- .
r- S.
*r-


4-5
S).0

*.-.0
4m- 0

) 0.











to U'
C
S0






4-,-
C C,
c r-

ro n
Q.







4-
0
a. 4-)
*1


c c
*r-


*.- -. *
0 0) 4.
4- to
C CI- 0)
E *r- 4-
3 *0
-- 0E





C U 0)
*r- *I- 5-
4- Q.


C C4,-
Lu *O- 3
S: U) E
4-





















TABLE 4. Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Dry Weight (kg/ha). Quincy, 1980.

Row Spacing Sicklepod Density/m2 t
(cm) 1.0 5.0 15.0
30 a558e b2257e c16650e

60 a340e b2720e c9525g

90 a371e b2492e C10879f

Means within a row preceded by the same letter, or means with-
in 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.








With all row spacings, sicklepod dry weight increased as density

increased.

Increasing sicklepod density and sicklepod dry weight decreased

soybean yield curvilinearly at both locations (Figs. 1-4). Yield

decrease was rapid initially but was similar at the medium and high

densities. At Quincy, the coefficient of determination was slightly

higher for sicklepod density than for sicklepod dry weight, 0.78 vs.

0.72, respectively. This was a result of the large sicklepod dry

weight value in the 30 cm rows at the high sicklepod density. In

Gainesville, the coefficients of determination were similar at 0.64

and 0.63, for density and dry weight, respectively. Yields and cor-

relations were lower at the Gainesville location.


Competition of Sicklepod with Soybeans-1981

Soybean yields in 1981 (Table 5) were not significantly different

between row spacings. This experiment, unlike 1980 tests, was planted

during the recommended planting dates for the 'Bragg' variety. In-

creasing sicklepod density reduced yield across all row spacings except

the low and medium densities. The suboptimal rainfall in 1981 resulted

in a decrease of 401 kg/ha with the low sicklepod density, whereas in

1980 the decrease for the same densities was 249 and 313 kg/ha at Quincy

and Gainesville, respectively. The yields were higher over all row

spacing and sicklepod densities in 1981 compared to 1980 probably due

to a more favorable planting date in 1981.

Other soybean morphological characteristics evaluated were soy-

bean height and height of first pod (Table 5). Plants were signifi-

cantly taller in the 25 cm rows than in either the 50 or 75 cm rows,



































**

W V EI
C 0
-0 4r-
0()0
CL


Q. r--

u


o
0 3
) O


U- 0)
U


C- C
0U Q


r- a>
4- >





o o
'r--











) u
0 CO 0)
VI












L L
r-0 -



C VI
0)U 0











*r
L0)


















CM


t-
'C
0
05

<0
cM
0d


>-


* A


EEE
[00
0 U 0o


NV38AOS


CJ
E
o >-
-I--

z


0

O
0c
w.
-J
Iog
CO


(D4/-b) CI731A






































0
.-
*r- VI




o C
C O







0 U




ti-

-

Or- -







>co C
Q. w-













r Li


S-



*
toC C
-- t


)r- -












*r-

C1)
10 .. i


















Co
01
rci)

x c
o E
to -


.0 ,, W




a.





EE E
4010





0
00 0
i,0
/Q-J

to 0 -O













(Dql/6) (9131A NV38AOS


-r



































.-
Ca




*-




o
0

a-

U
r-
tn










-I-
94-














a u
C
*-

VI



r- O
) CO
*,- oC


re






41






0
0
-
O


0
0
0


8
0

\ i^

b 0



+o


d
co







CO
0
000 0

E EE0
0 l/o 0




000
C\j LO c\j
COO / C


0 O 0 I
N -
(Dq/6() a131A NV38AOS


































en
.C










-o
0-


Q.







.0


U
L

4o








C)

*-
*o





V CD


-r


r -
0)

u/)





















000

EEE

ccr i.
C o n-


CMJ
(ec
b

C0
x


Dro


-0
x
ro
0




11 U >
>- cco~


0 /


(Dq/bj) G1i31A NV3aAOS


0




t-









0



-J

0
CO
(1




















TABLE 5. Soybean Yield and Heights as Influenced by Soybean Row
Spacing and Sicklepod Density in 1981. Gainesville.

t Soybean 1st
Soybean Yield Pod Ht. Soybean Ht.
Parameters (kg/ha) (cm) (cm)

Row Spacing (cm)y

25 1537 a 19.1 a 73.6 a

50 1768 a 15.9 b 65.9 b

75 1670 a 14.3 b 67.5 b

Sicklepod/m2

0.0 2144 (a) 16.3 (a) 70.9 (a)

0.5 1743 (b) 15.7 (a) 68.9 (a)

1.0 1572 (b) 16.7 (a) 67.1 (a)

5.0 1175 (c) 17.2 (a) 69.0 (a)


Means within columns within row
followed by the same letter are
at the 5% level of probability,
multiple range test.


spacings or within densities
not significantly different
as determined by Duncan's new


VInteraction of soybean row spacing x sicklepod population present.








while sicklepod densities did not significantly alter soybean height.

The 25 cm rows also resulted in the soybean's first pod being signif-

icantly higher from the ground than the other rows, thus resulting in

the possibility of less harvest loss associated with reduced row spacing.

Sicklepod dry weight (Table 6) was significantly lower in the 25 cm

rows than in the 50 and 75 cm rows at each density. Again,.however,

the 50 cm rows resulted in the greatest dry weight which was not ex-

pected. Although not significant, the 50 cm rows on the average out-

yielded the 25 and 75 cm rows. Growing conditions for both the crop

and the weed seemed to be at an optimum at this row spacing. On a

per hectare basis, sicklepod dry weight increased as sicklepod density

increased for each row spacing. This increase was 32 and 27% from low

to intermediate density for the 25 and 50 cm rows, respectively. How-

ever, a 63% increase in sicklepod biomass resulted in the 75 cm rows.

When yield was correlated to sicklepod dry weight, a curvilinear re-

sponse was obtained with a coefficient of determination of only 0.24

(Fig. 5). Similar results were obtained between soybean yield and

sicklepod density giving a R2 of 0.43 (Fig. 6). These low correlations

are indicative of the unfavorable growing conditions in 1981.

Water use did not differ between soybean row spacings or between

weed densities at sampling times 4, 6, 7, 8, and 9 (Table 7). Water

use was also not significant at these times between densities. Analy-

sis of variance indicated a significant interaction between row spacing

and density at sampling times 1, 2, 3, and 5. Interactions were only

analyzed when water use across row spacing was greater than 100 milli-

bars, thereby omitting sampling time 1. This procedure was followed

because water reservoirs were sampledat readings below 100 millibars,


























TABLE 6. Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Dry Weight (kg/ha), 1981. Gainesville.

Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0
25 a401 e a592e b2300e

50 a861 f a2577f b78639

75 a653e b1975f c5013f

tMeans within a row preceded by the same letter, or means with-
in 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.




















E





in
'a




03





E



Lor

to-
3

Ctn
*r-
o


r-



O0




*r-






















4-)
CL


*r-
C











ULJ




0-
.-



Ur






r-







3>
Cn





Cr
f f







o *
(r-






aC )





4 r-

o .c
a







L'J3
N-




CO
a r-
4-


n3 o .0-


O F r- r-






Cr to (a

to to r-
40 40 N-
C r- i







'0 (a 'a








CO CO
a- CO








O CM C O
-) r- rO








co to- ri





CM a- CM






o to N-





C C) U)
CY) to r'

C)O ) C)
0) z- Ct
ac O- a-
N r-- CM
























n N- N0
CO~ 't U


N
E

*O
0
LO C LO C. oC Ln o o
2 LA N- C *.


r-
V)O


-> .0 -
f0 ra 10


CV v
cO 1 M





to ro ta


o0 CO
O C( I









r- -- -
(\j i- cc >






'a a o ra


d- 00 0C)








m )- N-


C- CO C-


CO CM U)-






tn to LO



r- C0 cn


N O CO





- M r--
I- U) CM


4-3
0


C-
a
S- m.

4-)






t-
I- C)
-1 3







.C



0
a-








< 0
r-
,--- E


U >
Cto
c: o







or"
C 4->3
4J -o








3 J
O C






r-

r- E

0 O










O

4- O)
C) 0


















*I-
c -) c)

VIn


or-
C,



Cr-



r.- J.
30

L


0 Q
"4-
0)

*r- C)
0 >

Qa,-
10






*r- <


-) 4'a

3+)

9- 4-

IX)
( -



C C--
4-, (0 10





'a 01 C


*--


t1


X




0
r-




I--
O




0.




0
0
C)
O
s-







a
r-
U

0.












3.
0
*O-





ro
r-

.0





In
C
o


OU
L





4--


C
o






C
I-
c,








































-0

0
0
L)

(o








L)

r_
(U
4-



00
-0
0J



r--



0a)
o-
r-










on0
i- C
o)cr
-i- i
>i5



















CM
ci





'C
X
ro







00
2 i
3_CDJ
N21
ci


d
co






4c


ooo
r cc cIr
EEE
000
0 U
ej O '


(D4/b6) 0131A NV38AOS


CL
-c
1-




03
0
0
0r


w


C-)






































*r-
*r





10
*o
0.






-n





r
a)
r






I,,


-o
9--
a)
u
c
=J

c

M


(10

TO




'01
*r-

>>



o r>
<^ i




















CV


a)
ro

0

Srr-


=0
NJ
>L


oo
00

EE
io
Co 0
* *


O 0 0
0 to 0 N
(Dq4/bM) C-1I3A NV39AOS


cn
'I

z

Q



w
-J


- O
cn









thus no immediate competition for water was occurring. At sampling

time 2 the 0.0 and 1.0 sicklepod densities showed no significant diff-

erences in water use among row spacings (Table 8). At 0.5 density,

the 75 cm rows used more water than the narrow rows while at the 5.0

density the 25 cm rows used the most water. With 25 cm rows, water

use was significantly greater only at the high weed density. There

was no significant difference in water use between densities in the

50 cm rows while the 75 cm rows.had increased water use only at the

low sicklepod density. At sampling time 3 (Table 9) increasing sick-

lepod density did not increase water use except in the 25 cm rows where

the medium and high densities are similar. Water use in the 25 cm rows

was significantly higher than in the 75 cm rows at 0.0 and 5.0 densities.

No difference occurred at the 0.5 and 1.0 densities. At sampling time

5 (Table 10) no significant difference among densities was found in the

50 cm rows. Water use in the 25 cm rows at 0.5 and 5.0 densities was

greater than at 0.0 and 1.0 densities, whereas 0.0 and 5.0 density water

use was not significantly different in the 75 cm rows. At this row

spacing however, the greatest water use was at the 0.5 density. The

mean water use (Table 7) of all row spacings shows that the 25 and 50

cm rows used significantly more water than the 75 cm rows. Across sick-

lepod densities, mean water use indicates that the high weed population

used more water than the zero or medium density. However, after exam-

ining each sampling date where stress occurred (average above 200 mill-

ibars for at least one row spacing), there was no significant difference

between the 25 and 75 cm rows. This indicates that during conditions

where moisture is limited, narrow rows do not use more water but when

water is plentiful the narrow rows are using water thus producing more











TABLE 8. Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 25 cm depth. Sampling
Time 2, 1981. Gainesville.

Row Spacing Sicklepod Densities/m2 t
(cm) 0.0 0.5 1.0 5.0
25 a95c al37c al28c b359c

50 al1llc al20c all5c a115d

.75 al31c b231d a120c a156d

Means within a row preceded by the same letter, or means with-
in 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.


TABLE 9. Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 25 cm Depth. Sampling
Time 3, 1981. Gainesville.

Row Spacing Sicklepod Density/m2 t
(cm) 0.0 0.5 1.0 5.0

25 a141c a209cd a166c b405c

50 a11 a 153c a173c a169d

75 a247d a257d a215c a81 d

Means within a row preceded by the same letter, or means with-
in 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.









photosynthate. Although this may or may not increase yields, it

should increase early leaf area and therefore quicker canopy closure.

In addition, when soil moisture is most critical, the period during

flowering through pod fill (sample times 5-8), there were no signifi-

cant differences between the 25 and 75 cm rows.

No significant correlation existed between mean water use and

soybean yield. This is expected since high water use at field ca-

pacity would increase production while stress situations would de-

crease production if water use were.high.

A parameter that could alter soybean competitiveness with sickle-

pod is canopy closure. The progression of canopy closure is shown

in Table 11. At four weeks after planting, the 25 cm row canopy had

a canopy closure of 83.7% which was significantly higher than the 50

and 75 cm rows which were 56.5 and 39%, respectively. Five weeks after

planting, canopy closuresin the 25 cm rows were still significantly

higher than the other spacings. Canopy closure did not equalize be-

tween row spacings until ten weeks after planting. Five weeks were

required for the 25 cm rows to reach 90% closure while the 75 cm rows

required nine weeks.

Other sicklepod characteristics evaluated include sicklepod height,

width, first branch.height, seed per plant, and seed per hectare.

Sicklepod height was not altered by row spacing or sicklepod density

(Table 12). A significant interaction was detected between both row

spacing and sicklepod density relative to sicklepod plant width

(Table 13). A decrease in plant width occurred as density increased

at all row spacings. Sicklepod width was also less in the 25 cm rows

due to the competitive ability of the soybeans. The soybeans in the










TABLE 10.


Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 25 cm Depth. Sampling
Time 5, 1981. Gainesville.


Row Spacing Sicklepod Density/m2 t
(cm) 0.0 0.5 1.0 5.0

25 a 67d b226d a 93d b407d

50 al99d a167e a72d a84f
75 b290e c397d a95e b277e

*Means within a row preceded by the same letter, or means with-
in 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.


Effect of Soybean Row Spacing on
1981. Gainesville.


Percent Canopy Closure,


Row Spacing Weeks After Planting
(cm) 4 5 6 7 8' 9 10

25 83 a 92 a 96 a 98 a 97 a 96 a 97 a

50 56 b 74 b 90 a 93 b 96 a 97 a 95 a

75 39 c 57 c 76 b 84 c 89 b 95 a 95 a

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.


TABLE 11.



















TABLE 12.


Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Height (cm) 1981. Gainesville


Sicklepod Height
Parameters (cm)

Row Spacing (cm)

25 118 a

50 135 b

75 128 b

Sicklepod/m2

0.5 169 (b)

1.0 172 (b)

5.0 167 (b)

tMeans in a column within row spacings or within densities
followed by the same letter are not significantly different
at the 5% level of probability, as determined by Duncan's
new multiple range test.
TNo interaction exists between row spacings and sicklepod
densities.






57

50 cm rows had the greatest plant width, the same row width which

produced the most sicklepod dry weight. Sicklepod plants delayed

lateral branch initiation in the 25 cm rows until breakthrough of

the soybean canopy occurred (Table 14).

The effect of row spacing and sicklepod density on sicklepod

seed production is shown in Table 15. In the 25 cm rows only the.high

density produced significantly greater weed seed. In the 50 cm rows

the weed seed production in low and medium dersities was not signif-

icantly different but both did produce significantly less seed than

the high density. As sicklepod density increased in the 75 cm rows,

weed seed production also increased. At all densities, sicklepod

seed per hectare was least in the 25 cm rows and greatest in the

50 cm rows. The 25 cm rows reduced sicklepod seed production an

average of 76% compared to the 50 cm rows and 60% compared to the

75 cm rows. This reduction was due to soybean competition with the

sicklepod.

One important competition variable is light. No significant

difference was measured between light intercepted at the low and

medium density in all row spacings (Table 16). Light interception

was greatest at the high density of sicklepod in each row spacing.

At each density the sicklepod intercepted the least light in the 25 cm

rows. Over all densities the sicklepod in the 75 cm rows intercepted

75% more light than those in the 25 cm rows, while those in the 50 cm

rows intercepted 67% more light than those in the 25 cm rows. A re-

duction in sicklepod biomass by soybean competition caused the re-

duction of light interception by sicklepod in the narrow row spacing.










TABLE 13.


Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Width (cm). 1981. Gainesville.


2 t
Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0

25 b97e ab84e a73e

50 b76f ab168g al529

75 b147g b 45f a19f

Means within a row preceded by the same letter, or means with-
in 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.


TABLE 14.


Effect of Soybean Row Spacing and Sicklepod Densities
on SickleDod 1st Branch Heiaht (cm), 1981. Gainesville.


Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 .5.0

25 a65.3e a61.0e b89.0e

50 a9.0f a20.3f al2.0f

75 a6.0f a4.8g al7.0f

Means within a row preceded by the same letter, or means with-
in 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.











TABLE 15.


Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Seed (per ha), 1981. Gainesville.


Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0

25 a5165e a8550e b31606e

50 a26469f a34413f b1221939

75 al0437e b29212f c79800f

Means within a row preceded by the same letter, or means with-
in 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.


TABLE 16.


Effect of Soybean Row Spacing and Sicklepod Densities
on Percent Sicklepod Light Interception, 1981.
Gainesville.


Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0

25 a3.9e a5.5e b9.3e

50 a4.1 f a17.2f b24.4f

75 a20.6g a18.8f b35.9g

tMeans within a row preceded by the same letter, or means with-
in 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.









Competition of Sicklepod with Soybeans--1982

Unlike the 1981 results, significant differences between soybean

yields across row spacings were observed (Table 17). Yields in the

25 and 50 cm rows were significantly greater than those from the 75 cm

rows. Growing conditions were excellent in 1982; therefore moisture

stress was much less than in 1981. Yields increased 25% from 1981 to

1982 in the 25 cm rows while increasing only 12% in the 50 cm rows and

no increase in the 75 cm rows. Soybean yield did not decrease sig-

nificantly from the zero to low weed density. This was probably due to

the increased rainfall in 1982. There was a 19% decrease in yield when

sicklepod density increased from 0 to 0.5 weed/m2 in 1981 and 13% de-

crease in 1980. Soybean height, as in 1981, was greatest in the 25 cm

rows. Although taller, soybeans in the 25 cm rows were not subject to

increased lodging. Sicklepod densities did not affect soybean height.

Soybean first pod height was also greatest in the 25 cm rows and lowest

in the 75 cm rows. This could increase harvesting efficiency. Sickle-

pod densities had no effect on first pod height.

Sicklepod dry weight was greatest at the high weed density for

each soybean row spacing (Table 18). No significant difference in

sicklepod dry weight between row spacing was observed at the low weed

density. Sicklepod dry weight in the 75 cm rows was significantly

greater than that in the 25 cm rows at the intermediate and high weed

densities. Soybean yields were therefore greatest where sicklepod dry

weight was the least. From 1981 (a dry year) to 1982 (a wet year),

sicklepod dry weight increased 1% in the 25 cm rows while the yield

increased 25%. For the 75 cm rows, sicklepod dry weight increased



















TABLE 17.


Soybean Yield and Characteristics as Influenced by
Soybean Row Spacing and Sicklepod Density, 1982.
Gainesville.


S Soybean 1st
Soybean Yield Pod Ht. Soybean Ht.
Parameters (kg/ha) (cm) (cm)

Row Spacing (cm)

25 2054 a 17.6 a 68 a

50 2013 a 14.2 b 59 b

75 1663 b 11.6 c 59 b

Sicklepod/m2

0.0 2458 (a) 13.9 (a) 65 (a)

0.5 2264 (a) 14.9 (a) 62 (a)

1.0 1905 (b) 14.6 (a) 61 (a)

5.0 1012 (c) 14.6 (a) 60 (a)

tMeans within a column within row spacings or sicklepod densi-
ties followed by the same letter do not differ significantly
at the 5% level of probability, as determined by Duncan's new
multiple range test.
























TABLE 18. Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Dry Weight (kg/ha), 1982. Gainesville.

Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0
25 a285e a660e b2600e

50 a1129e a1383ef b7688f

75 a811e a2295f b 00889

tMeans within a row preceded by the same letter, or means with-
in 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.









42% while yield did not increase. Thus in the 25 cm rows, the addition-

al water would appear to be utilized more efficiently by the soybeans

while in the 75 cm rows, the sicklepod was using the additional water.

These results indicate that narrow row soybeans in a dry growing season

yield similarly to the wide rows. However, in a wet growing season,

narrow rows may be more beneficial.

A curvilinear response was obtained when yield was correlated to

sicklepod dry weight (Fig. 7), and sicklepod density in 1982 (Fig. 8).

A coefficient of determination of 0.81 and 0.74 was calculated for dry

weight and density, respectively. These results indicate that dry

weight of weeds is a more precise indicator of soybean yield than weed

density. Teem (92) reported similar results with dry weight and density

correlations.

Mean water use at the 25 cm depth was significantly greater in the

25 cm rows than in the 75 cm rows (Table 19). However, no difference

was observed between 25 and 50 cm rows. When examining individual

sampling time where stress was approached (average above 200 millibars

for at least one row spacing) there was no significant difference be-

tween 25 and 75 cm rows 80% of the time. Thus indicating that even

though the mean tensiometer value is greater during the 17 readings,

in stress situations the narrow rows may not be depleting more soil

moisture than the wide rows. This trend was also shown in the 1981

data (Table 7). Mean water use across sicklepod densities revealed

that only the high weed population used significantly more water.

This is not surprising considering the heavy rainfall in the 1982

growing season.







































D-


O










-0 C
0

'r




o-


* -
0


C
'U



r-

4-

C
*r-
or


rd


r- CO




c. *
.0 r.D


>.r-
0O(
tn a


































- r

7ci


000

EE


o* 0
00t0
-t O


0
0

(131A


00

r0

8 F
0
0
0
o 0


0
o


0]
0 C)
0
8..
0 O


0
i n
o
0
0
C\J


NV38AOS


(Dq /5bM)




































I,
e-







0
CL






















to
X>


r-
















0-
U


I-










'-0








C
I-









oai


















CM








I-
CO





IIt
0
CM













-* o *o I -I0
CD
0 0


















SI I ,
CO 0

o C oj






0 0 0 0


0 0 0 0O
0 0 0 0N


(Df/bj) C131A NV38AOS






















Sr-


to to o (CM to to
Sro- O 0. L co C) Ln L- -
CM-- CM-- -CM








0 Ur- 4-






E S-
L~ -t to 0 CO t 0
t- 1- 0 0 -- t -o S.. a


0 r
o e co CM in 4-
0 cC rc O 0 co CM ( V E

R0 *i- r4--
toE I
LA n LO CM c L c) 0J
A O I 1 0 L U C -~ L W (
-I C' C' r- CM CM CM CO LOr-
0- Q.
- r- -r





S.0 .0 -- -- 4- ir-
0o to0 10 a0 = (
o -0 ,0 0 *- -" 0U E








o rca co o i
w c
(D 3 D Nm CC) A LA 3




LO C- I,- to LID c \ --0 0
*)- 9 4 CM LA Ec





o 3
to U)










0) n- 0 C) C CO Co CM >n
L C L LA L CM E0Co











0,- '-0 Q
Um q) "
S3 0 (0 -I C
00 t10 .0 *-- -.- .-
r a- ED
a) k 0 C n mo to O ) i U)




m- r- OL- to r- cn 14- S *-
*r- 1- -

*- >-. C- Ur U)







U to C .
e d 0 ) Co
0O Q.0 3--- *- *0)
EC c t (a to r 0o *.



Soo go 5 to




0 0 ) LA C C -- C ) CO LA CL S-
O- D A L o C )O OD m 4-
c F- F- C F- 00 F- 0 0 .r-




*- > 3 4-
v) ol C: a)
C0 Qa 0 0a > 0
('- 0 0 0d *' Q- -u 00

>-, co r i- O O a).
S CM cC) C cO C m) CO CO

0 a0
C CM E-a



> co r o N r-- e r*~ rr > : u
W4-) 5 = C


0 0" r -- iC



a)CI E C L
4- n U in

=C E 0 E
* V- r- C-4-'

E 4-'
L-e C -*. CC
* *- O *-- 0)

I- C 0 LA O LO C) LA o o U o>
*r- C. M LA N- 0) C 4-
U r- / F- 0 0 -- LA 03 4-
. 1 D. .: 0) *r-
Co E O
I- 1t Qt t 4-+









69









.0 LCO Kj- cO -
x C\M Ln CO om co L -




0










,- ,- Cr O0 C) O 1,

r-



Or
C 4-3
N- C) C'o O'n tN CM.J iN-. m l o-













L- ,O C- .- m CM CO m-,
1 cl (a U+ r'














( ,- ,-- 1--h- ur U4-~- -
(0 10 10 0 4 0
to to ,- -- OJ-


I- I- 0 I 0 *r-




























0- .r
-4)
A LA 10 (V) L CO L E4
o- u L 0f ,- co 0
m'4 CN g- i- m- Cn CN W
VU)













M -














OO
Ct CO
10
d in on -r o co Id
m n3 ooo ci ( n


0































t LO C) C) N C CO C
S0)
-S C Cs O -

00O O 0 (0 c



CQl


































(0 tm (0 0 S-
N O C) *: U- 4-
U )


M 0 tO in .- CM ->

C) Cr) CJ (N Co) C) I *r-



10 '0 0 .0 U .Q

D- S.-
r- CM CM C3 C) '. O (



s .0 .0 ~ CU)





3 C )


*r U~ U4-)


o 0) 10
U E CM 10



a- c ta i o n C o m o o InO)

Lii a- U) O n

I- U) CO-








At the 15 cm depth there was no significant difference in mean

water use between 25 and 50 cm rows (Table 20), but the water use was

greater than that for the 75 cm rows. Only 37% of the time was there

no significant difference between the 25 and 75 cm rows during periods

of stress as compared to 82% at the 25 cm depth. Difference in row

spacing water use is greater at the 15 cm depth than at the 25 cm depth.

Drying of the soil is faster at the shallow depth and the higher popu-

lations of soybeans in the 25 cm rows use the soil moisture at this depth

more rapidly. However, at the growing season where soybean roots can

utilize water in the 25 cm depth range, water use difference is not as

great between the row spacings.

A significant interaction with the tensiometer measurements was

present at sampling times 1, 2, 3, 13, 14, and 16 (Table 20). At sam-

pling time 1 (Table 21) there was no significant difference in water

use between row spacings at 0 and 0.5 densities. Density 1.0 showed

higher tensiometer readings for the 25 and 50 cm rows than for the

75 cm rows and at 5.0 density the 25 cm row readings were greater than

those from the 50 and 75 cm rows. Within row spacing, there was no

significant difference between densities at 75 cm while only the 5.0

sicklepod/m2 density was higher in the 50 cm rows. In the 25 cm rows

the 0.0 and 0.5 densities were similar. At sample time 3 the 25 cm

rows were greater than the 75 cm rows at 0.0 and 1.0 densities (Table 22).

Within row spacing, there was no significant difference between densi-

ties in the 50 and 75 cm rows. In the 25 cm rows 0.5 and 5.0 sicklepod

densities were greater than the 0.0 and 1.0 densities. At sampling

time 17, as sicklepod density increased, water use did not increase

for the 50 and 75 cm rows, but in the 25 cm rows water use in the low

























4-3


s-


0
.0
4r-

r-
E


0u

CL
r4-
L)









"
t-
0



*r-
O
/)


0


Ur)


































to 0
0)
0






r-)
.M
0-


a)
c





00)
o










a-
01a-
C r-


0)j
Q. C






ca
0 o



.0<-

o -

cl
0cn

0

UE

4--



*
La)




- c
Ir-




O
(/I E
^-*
0 <
Q:


NC
E

"O
0
LO o LO CL.
(M LO r 0)


*--
/)


o o r- Ln
C)U- C C

C) C>U


o tIn o C OC
0 0 c c

Cf) LO 0 tfl 0' 0 C C)
Nr LA O 0' O 0 0



(0j IC Cs C sQ Cs

to> ca oa .0







t to ro-


CO m < CO O- O -N
1 r. LA .O N
0-









o0 0 (a .0





to I co I -



c CO CM M Cm C0) C


cis (0 co cs


co cc $ 2- 10 0' c' r-

Ln 00n 1-0 m
rC MO- Ot C C C
3) C\ O CO Ci CO cO





toC r( ca ( 0
to .0 o






fl co oe eo ro 0C 10
L .0 0 m -C ) )








%d- ro CD CM co
C i-- r- r- CM j e-






cn i- co LO MI to co
O1 co r-_ co co r- 0








co co c r-- r- r- C
10 CM) m CM) 0 Rd- '-


U
r_



*r- *





o c'
O 3
CCn





0)L
0 0


Lc
1-

r-
S-

4-+ +-
0) 3

-- E


E )

tl


.C C

I 0






r-
4-Q
i- C)









C0)
E)


0)0
U4




to +o
ci .0



4-
rU c




0



0-




c Q-
o 4-







a0)




O
r -E



C C




--


4--


*

4-3

*r-
x
0

I)
to
C

4,.
0

c.
0




0.
o
0
o-


0.
Q.
Ua
0






r-




o
Sc.

C
0



c0





o
U-
C
*S.
0




ta

o






*r




at
4-
r-


CO















CL

4-






--,
01
E
*r-

C-



ca
)r
r--

E

C/)
























ro to

CO co
C r) -
CMJ .-.


ON CM Co r- Co r- C)
-I) Cr-) C) c cj CM
in~- ~ p.-. 5-- I


CO 0 Co O C)O
co 0n .> 0 r C 0>)


co Co

C' -O


co r-. '0 o0 C -
r l_ o %D U r) t0 Fr


COj I-O





co- r- LO









o to .0X
LO t -O





O I- 0--
C to C' r-








o co to
r-- CC CC)
CO CO 0
U- r- c d







So N
E





r-


O t CO COc
10 CO CO r-


0 .0


o o
'0 -


C) r-- Lo


4-
1,,





*r- .
04) (



O 01
0 c
(U

a)
O-







E







S,a
0 S


0E














c E





















t--
o-)









t0o


















O
S-
413
E


*r- 4

*r- 0







o 4-

4- r
c .0








03.0
0


r- O0



0- U)





EOO

:,-
*r- 0
>









0 J4-
*1I
r-r-
*r-



U J
0 0C





CC

*r- >
L-



0
UE 4->


r--

O
P.--






En


0)




C
E
-V)
E-

01
r-


4->

*r-

a)

En















S-
0
*r-

4-




0






0)
T-








O
-,
r-
*0


C











c-
EU
O0
Q.


0

r-

.U



0
aE




4-
0
C





r-
0




S.-
a)
c
*-4
3











TABLE 21. Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 15 cm Depth. Sampling
Time 1, 1982. Gainesville.

Row Spacing Sicklepod Density/m2 t
(cm) 0.0 0.5 1.0 5.0

25 al17e a113e b200e b240e

50 30e ab117e a178e b87f

75 a117e a92e a63f a100f

Means within a row preceded by the same letter, or means with-
in 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.


TABLE 22.


Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 15 cm Depth. Sampling
Time 3. 1982. Gainesville.


Row Spacing Sicklepod Density/m2 t
(cm) 0.0 0.5 1.0 5.0

25 a220e b387e a47e a 243e

50 a203e a170f al40e al57f

75 al13f a93g a73f al57f

tMeans within a row preceded by the same letter, or means with-
in 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.









and high sicklepod densities was significantly greater than water use

at the zero and medium densities (Table 23). Water use at this time

was significantly greater in the 25 cm rows compared to the 75 cm rows

at the zero and medium sicklepod densities. The mean tensiometer read-

ing across sicklepod densities was significantly higher only at the

high density which is similar to the 25 cm depth.

The effect of soybean row spacing and densities on canopy closure

for 1982 is shown in Table 24. At four weeks after planting, canopy

closure was 74% in the 25 cm rows which was significantly greater than

the 50 cm rows at 55% and the 75 cm rows at 38%. At five weeks the

25 cm rows had greater canopy closure than the 50 and 75 cm rows.

Greater than 90% canopy closure occurred five weeks after planting in

the 25 cm rows and not until seven and nine weeks for the 50 and 75 cm

rows, respectively.

As in 1981, sicklepod height, width, first branch height, seed per

plant and seed per hectare were measured. Sicklepod height increased

as density increased in the 25 and 75 cm rows (Table 25). Sicklepod

height in the 75 cm rows was greater than in the 25 cm rows at all

densities. Sicklepod width was unaffected by density in each row

spacing (Table 26). In the 25 cm rows, sicklepod width was signifi-

canly lower than the 50 and 75 cm rows at all densities. Sicklepod

first branch height was greatest in the 25 cm rows indicating lateral

branching was delayed until the soybean canopy was penetrated by the

plant (Table 27).

Sicklepod seed per hectare increased significantly at the high

sicklepod density for all row spacings (Table 28). At the medium and

high densities, seed per hectare was significantly lower in the 25 cm










TABLE 23. Effect of Soybean Row Spacing and Sicklepod Densities
on Soil Moisture (millibars) at 15 cm Depth. Sampling
Time 17, 1982. Gainesville.

Row Spacing Sicklepod Density/m2 t
(cm) 0.0 0.5 1.0 5.0

25 a197e b127e a212e b 00e

50 al17f a120e a123f a167f

75 a97f a137e a107f a93e

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


Effect of Soybean Row Spacing on Percent
1982. Gainesville.


Canopy Closure,


Row Spacing Weeks After Plantingt
(cm) 4 5 6 7 8 9 10

25 74 a 91 a 97 a 99 a 97 a 98 a 97 a

50 55 b 72 b 88 a 95 a 95 a 98 a 96 a

75 38 c 49 c 70 b 83 b 89 b 94 b 96 a

Means within a column followed by the same letter are not signifi-
cantly different at the 5% level of probability, as determined by
Duncan's new multiple range test.


TABLE 24.











TABLE 25. Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Height (cm), 1982. Gainesville.

Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0
25 a126e b147e c156e

50 b 55g a138e b163e

75 a142f bl60f c176f

Means within a row preceded by the same letter, or means with-
in 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.


TABLE 26.


Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Width (cm). 1982. Gainesville.


Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0
25 59e a82e a61 e

50 al40f a114f a22f

75 a129f a143g al44g

tMeans within a row preceded by the same letter, or means with-
in 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.











TABLE 27. Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod 1st Branch Height (cm), 1982. Gainesville.

Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0

25 a70e a68e a71e
50 a29f al5f b47f

75 al3g al8f al59

Means within a row preceded' by the same letter, or means with-
in 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.


TABLE 28.


Effect of Soybean Row Spacing and Sicklepod Densities
on Sicklepod Seed (per ha), 1982. Gainesville.


Row Spacing Sicklepod Density/m2 t
(cm) 0.5 1.0 5.0

25 a6021e a12184e b52725e

50 al5426e a21 090ef bl29675f

75 a 5532e a38333f bl681509

Means within a row preceded by the same letter, or means with-
in 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.






78


rows than in the 75 cm rows. There was no significant difference be-

tween row spacings at the low density The 25 cm rows reduced sicklepod

seed per hectare an average of 54% from the 50 cm rows and 66% from the

75 cm rows. The greatest sicklepod seed production was 168.15 million

seeds/ha produced in the 75 cm rows at the.high density.

The effect of soybean row spacing and sicklepod densities on per-

cent light interception by sicklepod is shown in Table 29. In the 25 cm

rows, light interception was significantly greater in the medium and

high densities than in the low sicklepod density. In the 50 and 75 cm

rows, the low and medium densities are similar but significantly lower

than the high density. Sicklepod light interception was least in the

25 cm rows at the low and high densities and lower than the 75 cm rows

at all densities. The sicklepod in the 75 cm rows on the average of

all densities intercepted 78% more light than those in the 25 cm rows.

The sicklepod in the 50 cm rows intercepted 42% more light than those

in the 25 cm rows.


Herbicide Programs and Planting Date Studies


Effects of soybean row spacing and herbicide programs on soybean

yield in 1981 are shown in Table 30. There was no significant differ-

ence in soybean yield among row spacings which is similar to the re-

sults obtained from the competition studies in 1981. For all herbicide

treatments, no difference in soybean yield was found between triflur-

alin + metribuzin and trifluralin + metribuzin + toxaphene which were

both greater than the check. Excellent weed control in the narrow rows

(Table 31) seemed to mask the effect of the two treatments on soybean

yield. Although the toxaphene treatment resulted in significantly



























TABLE 29.


Effect of Soybean Row Spacing and Sicklepod Densities on
Percent Light Interception by Sicklepod. Gainesville.


Row Spacing Sicklepod Density/m2t
(cm) 0.5 1.0 5.0

25 a2.4e b12.7e b13.7e
50 al0.8f a13.7e b26.6f

75 a32.1g a39.1f b53.39

Means within a row preceded by the same letter, or means with-
in 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.




















TABLE 30.


Effect of Soybean Row Spacing and Herbicide Programs
on Soybean Yield. 1981. Gainesville.


Soybean Yieldt
Parameters (kg/ha)

Row Spacing (cm)

25 1610 a

50 1761 a

75 1623 a

Treatments

Trifluralin + Metribuzin 2114 (a)

Trifluralin + Metribuzin + Toxaphene 2222 (a)

Check 658 (b)

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









greater weed control ratings than the preplant treatment alone at all

row spacings, the actual differences were not great enough to decrease

yield significantly. The 25 cm rows at all densities resulted in the

best weed control. The trifluralin + metribuzin + toxaphene treatment

provided excellent sicklepod control in both the 25 and 50 cm rows.

An unexpected result was 39% reduction of sicklepod in the 25 cm rows

receiving no herbicide application due to competition of the soybeans

with sicklepod.

Directly affecting weed control is the rate of soybean canopy

closure. At four weeks after planting the 25 cm rows had a canopy

closure of 85% which is significantly greater than both 50 cm rows at

60% and the 75 cm rows at 34% (Table 32). The 25 cm rows were over

90% closed after only five weeks, whereas the 50 cm rows required seven

weeks and the 75 cm rows needed nine weeks. The early canopy closure

of the 25 cm rows is responsible for the excellent weed control with

only a preplant herbicide treatment which gave early suppression of

sicklepod. However, in the wide rows when additional sicklepod plants

emerged,therewere adequate light and space for the weed to become es-

tablished.

In 1982, five planting dates were used and an additional herbicide

treatment of alachlor + metribuzin. Soybean yield from different row

spacings was affected at planting dates A and C while at planting date

D no difference was found between the 25 and 75 cm rows. The 1982

competition test showed that the 25 and 50 cm rows produced significantly

greater yields than the 75 cm rows. Nonuniform and low sicklepod pop-

ulations resulted in the overall yields to be similar in planting dates

C and D. It was expected that in both the early planting date A and









late planting date E, narrow rows would result in a yield increase over

the wide rows. However, this held true for only the late planting date

E. In planting date B, where weed population was dense and uniform,

the 25 and 50 cm yields were significantly greater than the 75 cm rows.

There were no significant differences between herbicide treatments ex-

cept when compared to the check in planting dates A, B, and E. In

planting dates C and D the check was not significantly different from

the preplant and preemergence treatments. This is indicative of the

low weed density present in planting dates C and D areas (Table 33).

Weed control ratings for planting date A are shown in Table 34.

There was no significant difference between the preplant and preemer-

gence treatments at the 50 and 75 cm row spacings. Alachlor + metri-

buzin at a 9.9 rating was significantly greater than trifluralin +

metribuzin at a 9.0 rating. Control with trifluralin + metribuzin +

toxaphene was significantly better than that obtained with the preplant

and preemergence treatments at all row spacings. Weed control in the

25 cm rows was excellent for all treatments. The 25 cm row check re-

duced sicklepod density by 40% due to soybean competition. Weed con-

trol in planting date B was similar to A except in the trifluralin +

metribuzin treatment where the rating in the 75 cm rows was signifi-

cantly lower than the 50 cm row in planting date B (Table 35). In

general, as row spacing decreased, weed control increased with the

exception of the toxaphene applications, where 100% control was ob-

tained for all row spacing. However, toxaphene may not be available

for use in soybeans in the future due to its banning by EPA.










TABLE 31.


Effect of Soybean Row Spacing and Herbicide Programs on
Sicklepod Control, 1981. Gainesville.


Treatmentst
Row Spacing Trifluralin + Trifluralin+Metribuzin+
(cm) Metribuzin Toxaphene Check

25 a9.0d b10.0d c3.9d

50 a7.1e b9.4e cO.0e

75 a5.6f b7.7f C0.0e

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.


TABLE 32.


Effect of Soybean Row Spacing on Percent Canopy Closure,
1981 rainPevile


Row Spacing Weeks After Plantingi
(cm) 4 5 6 7 8 9 10

25 85 a 91 a 96 a 97 a 97 a 96 a 97 a

50 60 b 72 b 88 b 94 a 97 a 97 a 97 a

75 34 c 45 c 60 c 80 b 88 b 96 a 96 a

Means within a column followed by the same letter are not signifi-
cantly different at the 5% level of probability, as determined by
Duncan's new multiple range test.






















0- C0
,C tro to to 0
.C
C O-)- J. 1M ,n O LO 0,1 O L to
to LU r- CT co C) Ln LO Ln OLD
=3 C)- _li co C r co rN Co cr)

o0
.0- >- CO- 0 ,0 4

o C-)

c -Q 0 .0 --- .-. Lo



ro QC *- CT L m L. r- Cro LA 3 44 E
-,> C' C C CN C) C" CO C
V) Q)






C .a .0 co
(0 r- o (0 10 .0 u 0c t
)- -C *- O
?. o cC Ci

C S >- .! r- CN CO O O i rO N. C)

0 0)-
SN
o -0 t
E C o


o r- 0-- .C ,-
to- n CD-. C m -- C 4 o
) o 0o 0-o C C m- o





CF- o 4- .0
.0 0 N m LO 0 4-.



SC > .o O tD r.. ro C- -t -
o- >- mJ Ch ro r0 (n cli -
*u > Gor



c 0U0
t t 0 4) S-





*0r- CM







CC + 0
r 0D C C C R-




J rr-. *- Oo K


; CN N CM
00 .0 *r- .0 .CC
u 0 a M a )






tE 1- N *r- 4-4 )


V3Gr a. L) 1i-Q
Co L 0) O









4 -C C to- .5 S- '0 r- C


*- 44 4- 0 +- 3E.) CO )
C0 U C r- o *- *r- I-- r--
C o LA LA ) 5 -- B .C 4- 0.
c N LU N E 0 *r- -








Co ro 0 r-)
000 0 5- an
*- .- S- 4
uo X 1- M: c: 4- a)

(L) tE M: L W
4--a CA + C4
*J (a 4r) o 4-3CCO
cQ to c S-






m l I cuin E C-- c- cn r -










TABLE 34. Effect of Soybean Row Spacing, Herbicide
Planting Date on Sicklepod Weed Control.
Date A (Mav 3). 1982. Gfinpsvillp


Programs, and
Planting


Treatments
Trifluralin +
Row Spacing Trifluralin + Alachlor + Metribuzin +
(cm) Metribuzin Metribuzin Toxaphene Check

25 a9.0d b 99d b0.d c4.0d

50 a6.3e a6.5e b10.0d c0.Oe

75 a5.7e a4.3f bl0.0d CO.Oe

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


Effect of Soybean Row Spacing, Herbicide
Planting Date on Sicklepod Weed Control.
Date B (May 17), 1982. Gainesville.


Programs, and
Planting


Treatments
Trifluralin +
Row Spacing Trifluralin + Alachlor + Metribuzin +
(cm) Metribuzin Metribuzin Toxaphene Check

25 a9.0d bl0.0d b10.0d c3.8d

50 a5.3e a5.6e bl0.0d C0.0e

75 a4.3f a4.3f b0.O0d 0.0e

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


TABLE 35.









The effect of soybean row spacing on percent canopy closure for

planting date A is shown in Table 36. There was no significant differ-

ence in canopy closure among row spacings at four weeks. At five weeks

the canopy closure in the 25 and 50 cm rows was significantly greater

than that of the 75 cm rows. The 25 cm rows reached 90% closure in

six weeks while the 50 and 75 cm rows required seven and nine weeks,

respectively. Planting date B (Table 37) canopy closure was more rapid

than A. At four weeks the 25 cm rows had an 81% closure compared to

only 47 and 35% for the 50 and 75 cm rows, respectively. The 25 cm

rows obtained 90% closure at five weeks and the 75 cm rows required

over ten weeks. Ninety percent closure was not achieved in planting

date C until six weeks for the 25 cm rows, eight weeks for the 50 cm

rows and nine weeks for the 75 cm rows (Table 38). Planting date D

soybean canopy closure was further delayed for all row spacings

(Table 39). The 25 cm rows did not achieve 90% canopy closure for

seven weeks. At planting date E (Table 40) the narrow rows had 90%

canopy closure at seven weeks. The 75 cm rows at ten weeks had only

81% closure. The narrow row canopy closure advantage was greatest at

planting dates B and E which also is where the narrow rows yields are

significantly greater than the yields of the wide rows.











TABLE 36. Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date A (May 3), 1982. Gainesville.

Row Spacing Weeks After Plantingt
(cm) 4 5 6 7 8 9 10

25 59 a 74 a 90 a 97 a 98 a 98 a 99 a

50 48 a 62 ab 77 ab 91 a 97 a 98 a 98 a

75 45 a 56 b 64 b 79 b 85 b 93 a 97 a

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.











TABLE 37. Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date B (May 17), 1982. Gainesville.

Row Spacing Weeks After Plantingt
(cm) 4 5 6 7 8 9 10

25 81 a 95 a 93 a 96 a 98 a 97 a 98 a

50 47 b 66 b 83 b 81 b 89 b 96 a 98 a

75 35 c 41 c 69 c 66 c 74 c 83 b 87 b

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.











TABLE 38. Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date C (May 31), 1982. Gainesville.

Row Spacing Weeks After Plantingt
(cm) 4 5 6 7 8 9 10

25 67 a 69 a 92 a 89 a 98 a 98 a 98 a

50 56 b 47 b 70 b 82 b 94 a 97 a 99 a

75 36 c 43 b 65 c 67 c 74 b 94 b 97 a

tMeans 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.











TABLE 39. Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date D (June 14), 1982. Gainesville.

Row Spacing Weeks After Plantingt
(cm) 4 5 6 7 8 9 10

25 49 a 85 a 75 a 92 a 97 a 99 a 99 a

50 23 b 39 b 58 b 72 b 82 b 87 b 98 a

75 13 c 18 c 42 c 66 b 69 c 73 c 93 b

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.






















TABLE 40.


Effect of Soybean Row Spacing on Percent Canopy Closure.
Planting Date E (June 28), 1982. Gainesville.


Row Spacing Weeks After Plantingt
(cm) 4 5 6 7 8 9 10

25 59 a 63 a 67 a 95 a 95 a 96 a 95 a

50 44 ab 53 a 56 a 87 b 93 a 93 a 94 a

75 31 b 30 b 38 b 62 c 73 b 76 b 81 b

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.














SUMMARY AND CONCLUSIONS


Competition studies between soybeans and sicklepod were conducted

from 1980-1982. In 1980 and 1982 soybean yields were significantly

greater in the 25 cm rows than the 75 cm rows. There was no signifi-

cant difference in yield between row spacings in 1981. Soybeans in

1980 were planted late; therefore the higher soybean populations of

the narrow rows were advantageous. Soybeans in 1981 were planted at

the appropriate time; however, low rainfall suppressed yields and

treatment differences. While narrow row soybeans did not provide a

yield advantage neither did they decrease yields. Growing conditions

in 1982 were excellent. In this situation, yields from the 25 and 50

cm rows were significantly.higher than from 75 cm rows. From 1981 to

1982, soybean yield increased 25, 12 and 0% for the 25, 50, and 75 cm

rows, respectively.

Corresponding to these yield increases, sicklepod dry weight in-

creased only 1% for the 25 cm rows, decreased 17% for the 50 cm rows,

and increased 42% in the 75 cm rows. Conceding that these values

cannot be statistically compared, a trend seems apparent. With ample

moisture, the soybeans in the 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. Soybean yield

correlated well to sicklepod dry weight in 1982 giving a curvilinear

response with a coefficient of determination of 0.81. Soybean yield

significantly decreased as sicklepod density increased in 1980. In