Studies on the germination and emergence of Florida beggarweed (Desmodium tortuosum (SW) and its competition with soybea...

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Title:
Studies on the germination and emergence of Florida beggarweed (Desmodium tortuosum (SW) and its competition with soybeans (Glycine max (L.) Merr.) and peanuts (Arachis hypogaea L.)
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xiii, 128 leaves : graphs ; 28 cm.
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Hoopper, James Raymond, 1944-
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Beggar-weed   ( lcsh )
Soybean -- Diseases and pests -- Florida   ( lcsh )
Peanuts -- Diseases and pests -- Florida   ( lcsh )
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bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

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Thesis--University of Florida.
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Includes bibliographical references (leaves 114-127).
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by James R. Hoopper.
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Typescript.
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Vita.

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University of Florida
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STUDIES ON THE GERMINATION AND EMERGENCE OF FLORIDA
BEGGARWEED (Desmodium tortuosum (SW) AND ITS COMPETITION WITH
SOYBEANS (Glycine max (L.) Merr.) AND PEANUTS (Arachis hypogaea L.)








By

JAMES R. HOOPPER


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





UNIVERSITY OF FLORIDA


1978














ACKNOWLEDGMENTS

I express my sincerest thanks and gratitude to Dr. Ken Boote,

Dr. Jack Ewel, Dr. Kuell Hinson, Dr. Darell McCloud, and Dr. Wayne

Currey for their support of all phases of my graduate program. I

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

for his time, effort, and encouragement in sustaining the research

efforts of my graduate program.

This study could not have been accomplished without the tremendous

efforts of Mr. Kevin Conlin, who assisted in the arduous tasks of plot

management, data collection, and harvesting. I owe him a sincere expres-

sion of thanks.

Mssrs. Austen Tyre, Leroy Polk, John Swearingen, and Mrs. Mary Ann

Andrews assisted me directly or indirectly in effecting this study.

I thank Mr. Walter Offen, who patiently assisted me in statistically

analyzing the vast quantities of data collected.

I extend my gratitude to Dr. Earl Rodgers for his advice and gui-

dance during the graduate program.

Mr. Cyril G. Boyd graciously permitted me to sample his Florida

beggarweed infested fields. I thank him kindly.

Mrs. Carolyn Meyer typed the final manuscript, and I thank her for

her services.

My wife Carla, in addition to assisting in the planting and harvest-

ing of the field and greenhouse studies, stayed up until the wee hours

of the morning counting and measuring seedlings. They were not the best

ii








nights of our lives. Carla also typed the many drafts of this manu-

script and drew the figures herein. No number of thanks can express

my appreciation for her sacrifices. It is to Carla I dedicate this

work.














TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ............................................. ii

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

LIST OF FIGURES................................................ ix

ABSTRACT....................................................... xi

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

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

Botanical Description..................................... 3
Habitat ...................................... 3
Native Origin and Geographical Distribution................. 4
History of Desmodium tortuosum in Florida.................. 5
Seed Germination and Emergence.............................. 6
Temperature ......................................... 7
Moisture.............................................. 8
Light................................................ 9
Dormancy of Seeds.................................... 11
Seed Maturity......................................... 13
Herbicides and Seed Germination ....................... 14
Depth of Seeding.......... ......................... 16
Competition and Factors Affecting Crop-Weed Competition..... 17
Crop Species and Variety.................................. 19
Weed Species................................................ 20
Weed Densities and Economic Threshold Level................. 21
Critical Periods of Weed Competition ....................... 23
Row Spacing............................................... .. 25
Population of Crop Plants.................................. 26
Soil Water and Nutrient Relations .......................... 27
Light ....................................................... 28

METHODS AND MATERIALS......................... .................. 30

Experiment 1. Response of Florida Beggarweed Germination
and Seedling Elongation to Temperature .................. 30
Experiment 2. Effects of Simulated Drought on Germination
and Seedling Elongation of Florida Beggarweed and Seven
Other Species........................................... 30
Experiment 3. Effect of Seed Maturity and Shading on Seed
Weight, Percentage Germination, and Vigor................ 31








Page


\Experiment 4. Effects of Herbicide on Germination and
Seedling Growth.................................. ....... 32
Experiment 5. Effect of Depth of Seeding on Beggarweed
Emergence ............................................ 32
'Experiment 6. Periodicity of Beggarweed Emergence Under
Field Conditions................................... 34
Greenhouse Experiment: Competition of Soybeans and Beg-
garweed at Three Levels of Watering...................... 347
Field Experiment 1. Competition of Peanuts With Beggar-
weed at Five Population Densities...................... 36
Field Experiment 2. Competition of Florida Beggarweed With
Soybeans at Green Acres....................... .......... 38
Field Experiment 3. Competition of Florida Beggarweed With
Soybeans at Two Row Spacings............................. 39
Field Experiment 4. Competition of Florida Beggarweed With
Soybeansat Two Locations on Farmer's Fields, Newberry.... 41

RESULTS AND DISCUSSION......................... .... .............. 42

Experiment 1. Response of Florida Beggarweed Germination
and Seedling Elongation to Temperature .................. 42
Experiment 2. Effects of Simulated Drought on Germination
and Seedling Elongation Beggarweed and Seven Other
Species ............................... .................. 44
Experiment 3. Effect of Seed Maturity and Shading on Seed
Weight, Percentage Germination, and Vigor................ 48
Experiment 4. Effects of Herbicides on Germination and
Seedling Growth........................................ 54
Experiment 5. Effect of Depth of Seeding on Beggarweed
Emergence............................................... 61
Experiment 6. Periodicity of Beggarweed Emergence Under
Field Conditions...................................... 65
Greenhouse Experiment. Competition of Soybeans and Beg-
garweed at Three Levels of Moisture...................... 69
Field Experiment 1. Competition of Florida Beggarweed
With Peanuts.......................................... 75
Field Experiment 2. Competition of Beggarweed With Soy-
beans at Green Acres..................................... 87
Field Experiment 3. Competition of Florida Beggarweed
With Soybeans at Two Row Spacings ....................... 90
Field Experiment 4. Effects of Various Densities on
Soybean Yields on Farmer's Fields, Newberry............. 103

SUMMARY AND CONCLUSIONS........................... ........... 109

REFERENCES..................................................... 114

BIOGRAPHICAL SKETCH.......................................... 128














LIST OF TABLES


Table Page

1 Common and trade names, formulation, manufacturer, and
stock solution formulation of eight herbicides........... 33

2 Percentage germination of seven plant species at six
moisture tensions and at four time intervals............. 47

3 Analysis of variance, models, and individual sources
of variance for percentage germination, seed weight,
and germination vigor.................................. 49

4 Partial correlation coefficients of percentage germina-
tion, seed weight, and vigor............................. 53

5 Germination of Florida beggarweed as affected by eight
herbicides and eight herbicide concentrations............ 55

6 Mean, minimum, and maximum temperatures under bare soil
and sod, and rainfall amounts occurring between nine
observation dates of field germination of Florida beggar-
weed in 1976 and 1977.................................... 68

7 Mean and absolute weekly greenhouse temperatures ob-
served during the greenhouse study....................... 70

8 Means of growth characteristics of beggarweed and
soybean when grown alone and in competition at three
moisture levels ...................................... 71

9 Analysis of variance of soybean and beggarweed growth
characteristics grown alone and in competition at three
levels of water ...................................... 72

10 Regression of peanut yield on plant (P) and stem density
(S), weed dry weight (D), fresh weight (F), LAI (L), and
August (A) and harvest (H) height variables.............. 78

11 Linear correlation coefficients of characteristics of
beggarweed when grown in competition with peanuts........ 79

12 Regression of beggarweed dry weight on beggarweed stem
number (S), harvest height (H), and August height (A).... 80

vi








List of Tables Continued


Table Page

13 Correlation coefficients of weed variables on the
yield and growth characteristics of peanuts.............. 82

14 Means of yield, yield components, and growth charac-
teristics of peanuts and beggarweed growth characteris-
tics at different beggarweed densities................... 83

15 Regression of beggarweed stem and dry weight density
on LAI and beggarweed height, and regression of beggar-
weed stem density on beggarweed dry weight density at
Green Acres............................................ 85

16 Number of observations (N), means, minimum and maximum
values, and standard error of mean of soybeans and
beggarweed variables at Green Acres..................... 88

17 Regression of soybean yield on beggarweed stem density
(S), plant density (P), dry weight (D), and height (H)
variables at Green Acres................................ 89

18 Means of soybean plant characteristics when grown in
competition with beggarweed at 46 and 92 cm soybean
row spacing............................................ 92

19 Means of soybean yield and yield components when grown
in competition with beggarweed at 46 and 92 cm row
spacing .................................................. 94

20 Means of beggarweed and soybean plant characteristics
and soybean yield and yield components when various
densities of beggarweed were grown in competition with
soybeans in 46 cm rows.............................. .... 95

21 Means of beggarweed and soybean plant characteristics
and soybean yield and yield component when various
densities of beggarweed were grown in competition with
soybeans in 92 cm rows.................................. .. 96

22 Soybean yield and pod number per plant regressed on row
spacing (R), beggarweed dry weight (D), beggarweed stem
density (S), and soybean density (N) and interaction
effects with row spacing.... ...... ...... ............... 97

23 Regression of soybean yield and pods/plant over two
row spacings on beggarweed stem density (S), weed
dry weight (D), and August (A) and harvest (H) beggar-
weed height ............................. ................ 98








List of Tables Continued


Table Page

24 Regression of soybean yields at 46 and 92 cm row
spacing on beggarweed stem number (S), dry weight
(D), and August (A) and harvest (H) beggarweed
height................................................. 100

25 Linear correlation coefficients of growth character-
istics, yield, and yield components of soybean as
affected by various weed variables, soybean popula-
tion, and row spacing................................. 102

26 Mean, minimum, maximum, standard error of mean (Sx),
and number of observations of soybean and beggarweed
variables at two locations on farmer's field, Newberry.. 104

27 Regression of soybean yield on beggarweed plant
density (P), stem density (S), dry weight (D), beggar-
weed height (H), and soybean density (N) variables
at two fields on farmer's field, Newberry.............. 106


viii














LIST OF FIGURES


Figure Page

1 Relation of temperature to the percentage germina-
tion and seedling elongation of Florida beggarweed....... 43

2 Percentage germination of Florida beggarweed, soybean,
J. tamnifolia,and I. quamoclit at 0, 3, 5, 6, 8, and
10 -bars moisture tension after 24, 48, 72, and 96
hours incubation........................................ 45

3 Percentage germination of C. spectabilis, C. obtusi-
folia, and A. vaginalis at 0, 3, 5, 6, 8, and 10 -bars
moisture tension after 24, 48, 72, and 96 hours incuba-
tion .................................................... 46

4 Effect of seed maturity on the percentage germination
of Florida beggarweed seeds as predicted by regres-
sion analysis of actual and transformed data............. 50

5 Effect of seed maturity on seedling length (vigor)
after 48 hours incubation................................ 51

6 Effect of seed maturity on weight increase of seeds
from plants grown under 0 and 66% shade.................. 52

7 Root elongation of Florida beggarweed seedlings when
germinated at eight concentrations of dinitramine and
oryzalin and seven concentrations of vernolate and
alachlor .. ............................................. 56

8 Root elongation of Florida beggarweed seedlings when
germinated at eight concentrations of benefin, naptalam,
glyphosate, and metribuzin.............................. 57

9 Total length of Florida beggarweed seedlings when
germinated at eight concentrations of dinitramine,
vernolate, alachlor, and oryzalin........................ 59

10 Total length of Florida beggarweed seedlings when
germinated at eight concentrations of naptalam, benefin,
metribuzin, and glyphosate.............................. 60

11 Percentage emergence (Y) of Florida beggarweed seedlings
from a range of seeding depths (X) at two trial dates.... 62
ix








List of Figures Continued


Figure Page

12 Percentage emergence of Florida beggarweed seedlings
from six depths as a function of time................... 64

13 Emergence of beggarweed seedlings from cultivated
and non-cultivated soil at 17 observation dates and
total precipitation between observation dates............ 66

14 Emergence of beggarweed seedlings from cultivated and
non-cultivated soil at ten observation dates and pre-
cipitation between observation dates.................... 67

15 Height of soybean and beggarweed plants when grown
alone and in competition independent of water level...... 74

16 Precipitation observed during the 1976 study period
at Green Acres. (Average values for 7-day intervals.)... 76

17 Precipitation observed during the 1976 study period
at the Agronomy Farm. (Average values for 7-day
intervals.) ............................................ 91








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



STUDIES ON THE GERMINATION AND EMERGENCE OF FLORIDA
BEGGARWEED (Desmodium tortuosum (SW) AND ITS COMPETITION
WITH SOYBEANS (Glycine max (L.) Merr.) AND
PEANUTS (Arachis hypogaea L.)


By

James R. Hoopper

March 1978



Chairman: Wayne L. Currey
Major Department: Agronomy

Benefin, dinitramine, glyphosate, metribuzin, naptalam, oryzalin,

and vernolate had no effect on Florida beggarweed germination in

petri dishes. Dinitramine, alachlor, oryzalin, and naptalam demon-

strated the most inhibition of root and seedling elongation.

Florida beggarweed seedlings emerged from a depth of at least

8 cm in a sandy soil in the greenhouse. Between 10 and 15 days after

sowing, appreciable emergence continued from all seeding depths, but

after 15 days, from only the 6-8 cm depths.

On a temperature gradient bar, germination occurred between 21.5

and 44 C and maximum germination between 24.2 and 44 C. The compara-

tively high minimum temperature requirement for Florida beggarweed

most likely explains its emergence later in the spring than many other

warm season weed species.

The ranking of species according to ability to germinate under

simulated drought (0-10 bars osmotic pressure) was as follows: Florida

xi








beggarweed >Cassia obtusifolia >Crotolaria spectabilis Jacquemontia

tamnifolia >Alysicarpus vaginalis Ipomoea quamoclit >soybean>peanut.

Florida beggarweed's tolerance to simulated drought most likely

explains its widespread occurrence on dry, sandy fields.

Life history studies show that field emergence of Florida beggar-

weed seedlings occurred between March and September. Emergence was

earlier from tilled than no-tilled soil. The delayed emergence from

no-tilled soil most likely resulted from lower soil temperatures.

Frequency of beggarweed emergence was higher during the planting dates

of peanuts than soybeans. Florida beggarweed seeds began to germinate

10 days after flowering but vigor of seedlings was poor. At between

16 and 17 days, 50% vigor of immature seed was attained and germina-

tion was 75%. Complete germination occurred at 21 days.

Florida beggarweed and soybeans were grown in the greenhouse

alone and in competition at three levels of watering-150, 300, and

450 cc water per pot per day. Florida beggarweed competition reduced

soybean pod weight 35, 45, and 41%, respectively. As Florida beggar-

weed heights were approximately half those of soybeans and soil

fertility was not considered limiting, soybean pod weight reductions

most likely resulted from beggarweed's competition for limiting soil

moisture.

In field studies one beggarweed stem m-2 reduced yields of

soybeans 163 kg ha-I and peanuts83 kg ha-1. Peanuts were more

competitive than soybeans against Florida beggarweed. Soybean yield

reductions were more strongly correlated with weed dry weight than

stem density.








In 46 and 92 cm soybean rows with Florida beggarweed densities

between 0 and 8 plants m-2. Yield reduction per unit weed weight

or population density was found not to be significantly different

at the two row spacings. One beggarweed stem m-2 reduced soybean

yields 129 and 117 kg ha-1 in 92 and 46 cm rows, respectively.

Soybean yield reductions were more strongly correlated with weed dry

weight (r=-.56) than stem or plant density (r=-.48). As weed popula-

tion and dry weight increased, number of pods, number of seeds per

plant, and seed weight were reduced significantly. Narrow rows of

soybeans yielded significantly higher than wide rows.

In farmer's fields, soybean yield reductions were estimated better

by Florida beggarweed dry weight (r=-.58) than by stem density

(r=-.49) or plant density (r=-.37). One plant or stem m-2 reduced

yields 167 or 138 kg ha-1, respectively, and 2.2 kg weed dry weight

ha-I reduced yields 1.0 kg ha-1


xiii














INTRODUCTION

Florida beggarweed (Desmodium tortuosum (SW) DC) (henceforth,

beggarweed) is an important weed of field and vegetable crops in the

southeastern United States. In Florida it is considered to be the

third most costly weed irrespective of crop and one of the ten most

troublesome weeds in corn (Zea mays L.), cotton (Gossypium hirsutum

L.), peanuts (Arachis hypogaea L.), soybeans (Glycine max (L.) Merr.),

sorghum (Sorghum bicolor L. Moench), and tobacco (Nicotiana tobacum L.).

In Florida, Georgia, and Alabama, it is the most troublesome weed in

peanuts (116) and the most difficult to control broadleaf weed in

peanuts nationwide (144).

There are numerous reports (44, 51, 52, 63, 64, 81, 82, 141) on

the chemical control of beggarweed, but few studies have been conducted

on its germination (95), emergence and development (27, 89), and its

interference with crop plants (26, 80).

This study attempts to develop information on several different

aspects of beggarweed's life cycle with the objective of learning why

it is a problem. Areas receiving specific attention include:

a) effect of levels of temperature, moisture stress, and various

herbicides on the germination and seedling growth of beggarweed;

b) effect of seeding depth on seedling emergence;

c) growth of beggarweed at different moisture regimes with and

without competition from soybeans;

d) the effect of seed maturity on germinability.
1





2


In addition, competition trials under controlled conditions and

in farmer's fields were conducted to measure the yield response of

soybean and peanuts to beggarweed populations. These data may be used

to predict yield and economic losses. Although such procedures have

been developed to predict crop loss due to insects, no such procedures

have been developed for any given weed population. Such systems are

sorely needed.














REVIEW OF LITERATURE

Botanical Description

Desmodium tortuosom (SW) DC is an erect or strangling annual or

perennial herb of the family Leguminosae (subfamily Papilionatae).

The plant can grow to a height of 3-5 m. The stem, petioles, and

inflorescence have hooked and gland-tipped pubescence. The ovate to

ovate-elliptic leaflets are 2 to 8 cm long, about 4-5 cm broad, and

tomentose beneath and rough above. The corolla is about 4 mm long

and is purple to pale blue or pink in color and in long panicled

racemes. Pedicels exceed 1 cm and are mostly ascending in the fruit.

The fruit contain 2 to 7 net-veined rhomboidal lotates about 4-5 mm

long and 3-5 mm wide and possess minute, hooked hairs which readily

cling to feathers, hair, and clothing (hence, the common names "beggar-

weed," "beggarlice," "twisted tick tre-foil"). The lotates are

separately twisted (hence, tortuosum) and equally notched on both

margins-two features which distinguish it from other Desmodium species.

(1, 35, 139, 148, 158, 176).


Habitat

D. tortuosum is frequently found on roadside banks and waste areas

(1, 22, 35, 36, 108, 139, 148, 176),in fields and thickets(22, 74, 139,

148, 157, 159), in sourgrass (Bothriochloa pertusa (L.) A. Camus),

and in other pastures (65, 74). In Guatemala, Standley and Steyermark

(159) frequently found D. tortuosum growing along rocky or sandy soils.
3








In Florida it occurs naturally in pine flatwoods and hammocks (108,

148), as well as on sandy fields (76, 138, 148).


Native Origin and Geographical Distribution

The earliest collections of D. tortuosum were made from the West

Indies and Piper (138) states that it is native to that area. As

early as 1864, Grisebach (66) states that it had been found in Jamaica,

Dominican Republic, Cuba, Guadeloupe, Florida, Mexico, and New Granada.

In 1913 Small (147) stated that D. tortuosum occurred in Florida, Mexico

to Colombia, the West Indies, and South America. Fawcett and Rendle

(58) gave a similar distribution but restricted it to northern South

America. At about the same time Britton and Millspaugh (21) extended

its occurrence to Texas and the Bahamas. It was not known in the

Netherlands West Indies (17), Margarita, or Coche, Venezuela (94)

although it had been collected on the near-by islands of Trinidad

and Tobago (176).

Because of its reputation for being a potential ground cover,

soil improver, and green manure crop, beggarweed became widely dis-

tributed during the 1920's and 1930's throughout the subtropical and

tropical countries of the world. At the present time it is known to

occur in Hawaii, Italy, South Africa (190), southern Peru (109), Sao

Paulo (6), India, Java (29), Malaya (28, 29), and Australia (6). In

the United States it is found in all coastal states north to and

including North Carolina and all the Gulf states. Beggarweed can grow

in more northern latitudes, but it does not produce seed.





5


History of Desmodium tortuosum in Florida

Whence and when beggarweed came into the United States is unknown.

Both Piper (138) and Montgomery (121) state that it has been known in

Florida since 1832 or 1833 but offer no explanations or documentation.

A specimen of D. tortuosum collected from a cultivated field near

Tallahassee in September 1843, now resides at the Arnold Arboretum

of Harvard University (Bernice Schubert, personal communication). It

was not mentioned by Mohr (120) to be a recent introduction into the

Gulf states. In fact, no evidence suggests beggarweed has been

recently introduced into Florida.

Beggarweed reached a heyday of popularity between the turn of the

century and the 1930's. Originally known as "beggar's lice" or

"Florida clover," it was considered to be one of the most important

forage and green manure crops in Florida and Georgia (104, 126, 151).

Neal (126) in 1890 characterized beggarweed as by far the best green

forage plant available. It was also utilized as a hay, cover crop,

and as a soil renovator (126, 138, 151, 190). It usually did not

germinate until after rains in May or June (58), and hence, was

uniquely suitable for rotation with corn, cotton, melons (Citrullus

vulgaris Schrad.), peanuts, and sweet potatoes (Ipomoea batatas

(L.) Lam.) as it did not appear until the laying by of the crops

(104, 126); consequently, it was also thought never to become

a serious weed (151). Even without the addition of fertilizers,

beggarweed had a reputation for building up the production capacity of

the poorest land because of the addition of organic matter to the soil

and because it allegedly could not support nematode populations (138).








It was first recognized as a weed-but only reluctantly so-in 1890

(126).

The demise of beggarweed began when trials conducted in the 1930's

comparing beggarweed with other cover crops demonstrated that Crotolaria

mucronata Desv., velvet bean (Mucuna deeringiana (Bort) Merrill), and

cowpea (Vigna sinensis) fixed more nitrogen, produced more organic

matter, produced higher citrus (Citrus sp.) yields, and in rotation,

produced higher yields of corn and sweet potato (166, 167). Conse-

quently, these latter cover crops gradually supplanted beggarweed in

popularity as a soil improver. With the changing of weed control and

cropping practices and the increase in the use of fertilizer, mechaniza-

tion, and later maturity peanuts, beggarweed gradually emerged as one

of the most costly weeds in the southeastern United States (116, 170).


Seed Germination and Emergence

Germination of seeds consists of at least three stages: imbibi-

tion, development, and growth. Before germination of seeds occurs,

specific conditions are required, temperature and moisture being among

the most important. Other factors affecting germination include via-

bility and life span of seeds, gaseous composition of the ambient

atmosphere, intensity and quality of light, and innate dormancy of

seeds. Factors affecting the emergence of seedlings include seeding

depth, biotic factors, presence of herbicides, and soil crusting. Some

of the factors affecting germination and emergence are discussed below.

More exhaustive treatment of these subjects is given by Mayer and

Poljakoff-Mayber (113), Koller (103), Heydecker (84), and Wright (197).








Temperature

Where moisture is adequate, temperature is the most critical

environmental factor influencing germination of seeds. Every plant

species has a maximal and minimal temperature limit beyond which germi-

nation of its seeds cannot take place (143). In the range of tempera-

tures within which a seed germinates an optimal exists, below and

above which germination is delayed but not prevented. The optimal

temperature could be defined to be that at which the highest germina-

tion percentage is attained in the shortest time. The range of tem-

peratures within which seeds germinate and their optimum temperature

are determined by the source of the seeds and genetic differences within

a given species (for example, varietal differences) as well as the age

of the seed (113). These temperature parameters are also strongly

dependent on duration of the incubation period. As incubation is pro-

longed and more germination takes place, the optimum shifts to lower

temperatures, becoming less sharply defined, and the interval between

the minimal and maximal temperature becomes greater (103).

Many plant species germinate better under alternating temperatures

than under constant temperatures. Harrington (78) first explored this

phenomenon. For example, seeds of bermudagrass (Cynodon dactylon (L.)

Pers.) germinate poorly at constant temperature between 20 and 30 C

but do so readily when any temperature within this range alternates with

one that is several degrees lower, even though the latter itself does

not cause any germination when applied in a constant regime. On the

other hand Justice (95) found that D. tortuosum seeds germinated as

well at 30 C as at 20 to 30 C diurnal alternating temperatures.






The nature of the diurnal temperature alternation can provide

specific information which may be of value in characterizing the season,

the climate,and the micro-environment (103). Such information may

determine plant survival. Response of germination to diurnal thermo-

periodicity plays a role in the persistence of weeds in the field and

their periodicity of germination (181). Seeds of numerous weed species

germinate better under condition of diurnal thermoperiodicity than at

constant temperature (5). Seed buried at a depth beyond the reach of

the required diurnal fluctuation do not germinate but remain viable for

an extended period of time. They will germinate when brought closer

to the surface by future cultivation (103).


Moisture

Increasing moisture tension resulted in delayed and decreased

germination of seeds of crop species (50, 88, 145, 177) as well as

weed species (88). The rate of seed germination of Hieracium pratense

Tausch. decreased almost linearly with increasing osmotic pressure (129).

The amount of moisture for germination depends on species (48, 56, 90,

117) and variety (50, 165). Doneen and MacGillivray (48) tested 15

species of vegetable seeds and found that germination at low soil

moisture appears not to be correlated with size of seeds.

Studies comparing the ability of weed seeds to germinate at low

soil moisture are scarce. Evetts and Burnside (56) found that germina-

tion of Kochia scoparia (L.) Schrad., sorghum and sugar beets (Beta

vulgaris L.) occurred at -13 bars, but complete inhibition of germina-

tion occurred at -9 bars for hemp dogbane (Apocynum cannabinum L.),

-11 bars for honeyvine milkweed (Ampelamus albidus (Nutt.) Britt), and

-13 bars for common milkweed (Asclepias syrica L.). Comparing 17 weed








species with five crop species under simulated drought, Hoveland and

Buchanan (88) found that most weed species germinated better than

soybeans but poorer than pearl millet (Pennisetum americanum (L.) K.

Schum.) or sorghum-sudangrass. They concluded that weed seeds near

the surface of soil may have the competitive advantage in drier soils

by germinating earlier than the crop seed. On the other hand, large

seeded crops may be planted deeper where moisture is available for

germination while moisture at shallower levels is inadequate for weed

seed germination. According to Hunter and Erickson (90) the viability

of weed seed which lie in soil environments not sufficiently moist for

their germination may decline, and such seed are subject to damage or

destruction by fungi. Under field conditions, Wiese and Davis (192)

found that soil moisture may be sufficient for germination of weed

seeds, but as the soil surface dries out emergence may be reduced by

soil crusting.


Light

Numerous studies have demonstrated the role of light in stimulating

buried seed to germinate or preventing them from germinating. The

experiments of Wesson and Wareing (188, 189) present notable evidence

of this phenomenon. Light affects germination not only by its presence

or absence but also by its intensity, spectral composition, and dura-

tion. Seed may be divided into those which germinate only in the dark,

those which germinate only in continuous light, those which germinate

after being given a brief illumination, and those which are indifferent

to the presence or absence of light. Anderson (5) recognizes many weed

species whose germination is influenced by these factors.








The response of seeds to conditions of light during germination

may characterize strategies by which plants take advantage of or avoid

particular environments. Light stimulation may prove advantageous to

seeds low in storage materials where the seedling would have to start

photosynthesizing as soon as possible. Light is, in fact, the factor

which stimulates the germination of many small seeded pioneering species

which take advantage of disturbed habitats. Those seeds inhibited by

light are obliged to germinate beneath the ground guaranteeing a more

amenable environment for survival (102, 103). This also allows for

the periodic germination of batches of seed at irregular intervals

enabling the species to take advantage of favorable climatic and other

conditions (188). Other seeds require light but are unable to germi-

nate in the shade where the reduced light conditions or altered

spectral quality might severely restrict growth and prevent survival

(172).

It is known that seeds contain the plant pigment photochrome (P)

which explains the stimulation of germination by red light (transforma-

tion of PR to PFR) and its inhibition by far-red light (transformation

of PFR to PR). This effect is dependent on both the intensity of the
light and duration of the illumination. In general, germination is

determined by the amount of PFR as percent of the total P in the seeds.

However, the percentage required in order to reduce germination appears

to be quite variable, depending on the seed-species (113).

Spectral quality is a common phenomenon. Large changes in red/far

red energy levels do not occur in natural day light (150). However,

very large changes do occur under a vegetation canopy. Leaves filter

out much more of the incident red energy than the infrared (150, 172).







Taylorson and Borthwick (172) selected six weed species whose germination

was phytochrome controlled and gave the seeds a stimulatory irradiation

of red light. They found that leaf-filtered light inhibited germina-

tion of the seeds. Phytochrome probably serves to detect the shade

of other plants and to modulate growth and development accordingly

(150, 172).


Dormancy of Seeds

Dormancy refers to the failure of viable seeds to germinate when

supplied with water and oxygen at temperatures recognized as normally

favorable for plant growth. Dormancy is widespread in both temperate

and tropical plants. It is considered to be biologically advantageous

in adapting the growth cycles of the plant to favorable seasonable

variations in environmental conditions. One of the biological advan-

tages shown by weed seeds is the possession of efficient dormancy-

maintaining mechanisms which enable them to remain dormant but viable

in the soil for many years such that they are distributed in time as

well as in space, which for agriculturists, makes them more highly

resistant to measures of weed eradication (43, 168, 179). Hence, the

farmers' saying, "One year's seeding, seven years weeding" (174).

Several mechanisms of dormancy are recognized: immaturity of the

embryo, mechanical resistance to embryo growth, low permeability of the

seed coats to gases, endogenous dormancy of the embryo, impermeability

of the seed coats to water (hard seededness), or any combination of the

above (179).

The impermeability of the seed coat to water is most widespread in

the Leguminosae (77), but is also found in seeds of Malvaceae,








Chenopodiaceae, Convolvulaceae, Solanaceae, and other families (11,

179). The uptake of water by leguminous seeds is prevented by the

testa, and rupture of this layer is promptly followed by swelling due

to water uptake and germination begins almost immediately.

The seedcoat of most legumes consists of an acid insoluble

cuticle beneath which are macrosclerid cells containing lignin and

tannin deposits. Next in order are the osteosclerid cells, crushed

parenchymatous cells (or nutrient layer), inner integument, aleurone

layer, and the inner layer of endosperm (71, 118). Substantial dis-

agreement on the portion of seedcoat responsible for hard seedcoat

occurs and the arguments are discussed by Mckee et al. (118). In

summary, the cuticle, macrosclerid cells, and light line (a visible

demarcation tone between the outer rod-like structure and inner columnar

stratum of the macrosclerid cells) have all been alleged regions of

impermeability of legume seeds. The region of impermeability most

likely includes all of the cuticle and the macrosclerid cells.

Any treatment which effects rupturing of this subcuticular barrier

makes the seed permeable to water. Under natural conditions this sub-

cuticular is broken down by microbial attack, mechanical abrasion,

passage through the digestive tract of animals, or exposure to

alternating high or low temperature (11, 113). Strong alkalis or

acids, boiling water, very high or very low temperature treatments,

ethyl alcohol washes, and mechanical scarification have all been effec-

tive releasing hard seed from dormancy (11, 23, 43, 71). The most

widely used chemical method is treatment with concentrated sulfuric

acid. For example, Justice (95) increased D. tortuosum seed germina-

tion from 16 to 96% by soaking seed in concentrated sulfuric acid for

30 minutes.








Seed Maturity

In controlling weeds, many are cut down after their seeds have

started to develop and allowed to lie on the ground. Some of these

seeds may be viable and, thus, germinate (43). Gill (62) tested the

germination of a large number of weed species when cut down at various

stages of seed development. Some species within the Compositae family

produce viable seeds when cut down in the flowering condition. In

Papaver dubium L. and Datura stamonium L., the immature seed germinated

more readily than fully ripened seed, owing to the impermeability of

the seed coat of the latter. Certain species in other families produce

viable seeds when cut down at various stages of maturation following

fertilization.

Many studies have been conducted with crop plants on the effect of

seed maturity on germination capacity and vigor of seedlings produced.

Grunder and McGee (68) found that if alfalfa (Medicago sativa L.) seed

were one-half or more mature, even if the majority of the pods were still

green, a seed crop of fair quality could be secured. Testing several

western range and pasture grasses, McAlister (114) found immature seeds

were inferior to mature seeds in seedling emergence in field plantings;

however, at the end of the seedling year, the plants which were produced

from immature seeds were equal in survival and size to those produced

from mature seeds. Working with Desmodium intortum (Mill.) Urb. and

D. sandwiches E. Mey. under controlled conditions in the glasshouse,

Chow and Crowder (38) found that seeds reached a maximum weight at 28

days after pollination (DAP). Impermeability of the seeds showed only

a low percentage of viability about 20 DAP and then increased rapidly

to 98% at the time maximum weight was obtained.








Seed coats change from the permeable to the impermeable state

as a final step in the maturation process and even slightly immature

seeds may give high germination percentages (83). The intensity and

final percentage of hardseededness is influenced by growing season

(11), relative humidity prevailing during the maturation process (61),

altitude (47), sequence of flowering (91), and nutrient level (92).

Generally formation of hardseed is favored by high altitude, low rela-

tive humidity, harvesting late in the season, later flowers, and high

soil K levels.


Herbicides and Seed Germination

Herbicides are chemicals used to kill or inhibit growth of plants.

Many herbicides are applied to the soil where they are absorbed by the

roots, coleoptiles,or young shoots as they push through the soil follow-

ing germination of seeds (97). These chemicals may inhibit or even

promote the growth of seedlings. On the other hand they may kill the

seedling or have no influence at all. Situations in which herbicides

prevent germination or inhibit the growth of weed seedlings while not

affecting crop seedlings places the crop at a competitive advantage.

On the other hand, herbicides which may stimulate germination or pro-

mote weed seedling growth and emergence may give the weeds a more

competitive advantage over the crop. However, seeds stimulated to

germinate can be killed; fewer seeds would then be capable of germi-

nating later in the season after herbicides have degraded (57).

That herbicides can stimulate or inhibit germination of weed seeds

is well documented. For example, Fawcett and Slife (57) found that at

a rate of 0.1 kg a.i./ha the thiocarbamate herbicides butylate, EPTC,








vernolate, diallate, and CDEC and the carbamate chlorpropham increased

velvetleaf (Albutilon theophrasti Medic.) population in the field by

increasing the germination of "dormant" seeds. Butylate also increased

the germination of common lambsquarters (Chenopodium album L.) and

giant foxtail (Setaria faberii Herrm.) seeds. Biswas and Williams (13)

tested 15 herbicides at seven concentrations on two grass and two broad-

leaf weed species and found that low concentrations of herbicides had

either no effects or slightly promoted seed germination while at higher

concentrations these herbicides had variable effects on seed germina-

tion. The herbicides appeared to be specific in their actions, and

the degree of activity depended on the herbicide concentration used.

In the southeast U.S., preplant and preemergence herbicides are

used for controlling grasses and broadleaf weeds in peanuts and soy-

beans. These include alachlor, naptalam, vernolate, benefin, dinitra-

mine, oryzalin, trifluralin, dinoseb, metribuzin, and glyphosate. In

addition, glyphosate is used often in total vegetative control as part

of planting soybean in a no-tillage program.

The mode of action of some of these herbicides is suspected or

understood and have been summarized by the Weed Science Society of

America (187), Ashton and Crafts (7), and more detailed discussion is

provided by Crafts (41). Alachlor, applied preplant or preemergence

to control annual grass and certain broadleaf weeds, inhibits the

growth of shoots and roots, as well as lateral root development.

Naptalam, applied on peanut fields at the cracking stage, acts as anti-

geotropic agent; however, it has not been proved that this is associated

with its herbicide action. Naptalam acts primarily as an inhibitor of

seed germination, and also inhibits some growth responses induced by







IAA and gibberelic acid. Vernolate, a carbamate herbicide incorporated

preplant or preemergence, inhibits growth in the meristematic region

of the leaves of grasses. Oryzalin, dinitramine, benefin, and tri-

fluralin, which are dinitroaniline herbicides applied preplant and/or

preemergence, affect seed germination and associated growth processes,

usually by inhibiting secondary root development. Metribuzin is a

triazine herbicide which inhibits photosynthesis. Glyphosate is an

aliphatic compound applied foliarly and is essentially nonselective.

Jaworski (93) suggests that glyphosate interferes with the biosynthesis

of the essential amino acid phenylanine.


Depth of Seeding

If weak points in the life cycle can be identified and understood,

control of the weed may be enhanced. Depth of seeding studies identifies

optimal tillage, cultivation, and herbicide incorporation depths.

Numerous studies have been conducted to determine the maximum

depth of soil through which weed seedlings are capable of penetrating

and emerging (42, 87, 123, 140, 142, 155, 196).

Generally, percentage seedling emergence decreases and days-to-

emergence increases as seeding depth increases (9, 37, 46, 142, 155, 199).

Seeds of different plant species respond differentially to soil depth

(123, 191, 199), but such differential responses can be best explained

by differences in seed weight; that is, the heavier the seed, the

greater its emerging capabilities (46, 72, 123, 195) and greater its

penetrating power. Seedlings are able to penetrate better through a

coarse soil than through a fine-textured soil (9, 72, 140, 192, 196, 199),

because light soils have greater permeability and less compaction than






heavier soils (9). Seeds can emerge from greater depths on a more

finely prepared seedbed (87).

Under optimal field conditions, germination emergence of seed lay-

ing on the soil surface is often better than from deeper depths (123).

In the green house, however, unfavorable temperature-moisture relations

often prevents good germination from the soil surface (196). At

greater depths, seeds often fail to germinate (155) because of an

inadequate oxygen supply (72), the absence of light, too low tempera-

tures or too narrowly alternating temperatures (103), or for other

reasons. Many seeds do germinate but fail to emerge because the depth

exceeds maximum seedling growth capacity (199).



Competition and Factors Affecting Crop-Weed Competition

Weeds "interfere" with crops by competing for limited resources,

by producing chemical inhibitors which reduce crop growth, and by

clogging of harvesting machinery (124).

Competition can be defined as a struggle between organisms for

any limited resource. Intraspecific competition is the struggle be-

tween individuals of the same species while interspecific competition

is between two different species. According to Crafts (41) the most

limiting resources in the crop-weed environment are water, light, and

nutrients, in that order of importance. When competition becomes

severe for one or more of these limiting factors, one of the species

may be eliminated completely which is known as the competitive exclu-

sion principle (73); or the species may be able to persist together at

reduced density in some equilibrium by sharing the resources. Investi-

gating two species of clover competition in the same environment, Harper

and Clatworthy (75) concluded that two species of plants can coexist if








the populations are independently controlled by different nutritional

requirements (legume and non-legume), different causes of mortality

(differential sensitivity to grazing), sensitivity to different toxins,

and/or sensitivity to the same controlling factor (light, water,

nutrients) at different times in their growth periods.

The Lotka-volterra mathematical formulas describing relationships

between two species utilizing the same limiting resources are found in

most elementary ecology textbooks (106, 128). The basic growth curve

is involved:

dN K-N
dt H K

The above equation states that the rate of increase of a popula-

tion (dN/dt) is equal to the potential increase of a population

(unlimited specific growth rate Er] times number [N] in the popula-

tion) times the proportion of the carrying capacity [K] of the habitat

that is still unexploited (K-N/k). The above model is known as the

Verhulst-Pearl equation and describes a logistic curve. Where two

populations (N1 and N2)-each having its own K, or equilibrium level-

are competing, the simultaneous growth equations can be written in

the following form:

dN1 = rlN1 KI-Nl aN2
dt K1

dN2 = r2N2 K2-N2-bNI
dt K2

where "a" is the competition coefficient indicating the inhibiting

effect of species 2 on 1, and "b" is the corresponding competition

coefficients signifying the inhibition of 2 by 1. The smaller the

competition coefficients in relation to the ratios of saturation

densities (KI/K2 and K2/Kl) the more easily the two species may coexist.








Agriculturists are interested in competition between crops and

weeds for several important practical reasons. How late in the growth

of a weed-crop mixture can weed control be delayed before the weed has

produced an irreversible depression of yield? What degree of crop

recovery is possible if a weed population is removed after it has started

to reduce the growth of the crop? How long must the crop remain weed-

free before it becomes sufficiently competitive to resist weed encroach-

ment? How many weeds can a crop tolerate before an economic threshold

is reached? How can environment and crop factors be manipulated to give

crops the competitive advantage over weeds?

Because crop and weed growth often respond differentially to changes

in the soil or climate environment, agriculturists can manipulate crop

cultural practices or adjust planting times to take advantage of seasonal

temperature and rainfall patterns in order to increase the crop's com-

petitiveness. In fact, enhancing crop competition is one of the cheapest

and most useful methods of control available to farmers. Some of the

factors which determine crop-weed interaction are herein discussed.



Crop Species and Variety

Several studies have demonstrated that crop competitiveness against

particular weed species varies (18, 132). Knake and Slife (99, 100)

found that during the early stages of growth soybeans competed better

than corn against giant foxtail, but by maturity giant foxtail had

over-topped and reduced yields of soybean more than yields of the taller

growing corn. Alfalfa and soybeans are better competitors than cereal

crops against field bindweed (Convolvulus arvensis L.) because of the








legumes having less variation in shading effect and their capability

of orientating their shading values in a more definite shading pattern

(156).

Competitiveness of cultivars of soybeans (119, 162), sorghum (33),

potatoes (Solanum tuberosum L.) (171), and other crops (153, 161) is

known to vary. Superior competitiveness may be due to rapid germina-

tion, emergence, and root and shoot growth during the early stages of

growth as well as to rapid and dense canopy development which inter-

rupts the light for nearly the entire season (69, 171). Tall growth

habit, long or short growth duration, and large extinction coefficient

value are also important (105, 153).


Weed Species

Numerous studies (24, 25, 53, 70, 152) have demonstrated the wide

range of competitiveness of different weed species in cereals, corn,

soybeans, and other crops. The reasons one weed species may be more

competitive than another are similar to those mentioned above for crop

plants. Furthermore, many weed plants are not bound by strict photo-

period or temperature requirements but can be conditioned by an inter-

play of environmental factors which permit them to grow and reproduce

under a wide array of conditions (41).

Root development, dry weight production, height, and canopy develop-

ment are important factors determining a weed's competitiveness. Work-

ing in dry farm areas, Pavlychenko and Harrington (133) found that the

strong competitive force of wild oats (Avena fatua L.) was due to its

rapid and extensive root development. Soybean yield reductions have

been shown to be proportional to the amount of dry matter produced by








weeds (98, 101); however, weeds that overtop soybeans compete for light

as well as moisture and nutrients and reduce crop yields more per unit

of dry weight than weeds of short stature (180). Staniworth (163)

compared the competitiveness of yellow (Setaria glauca (L.) Beauv),

green (S. viridis (L.) Beauv), and giant foxtail against soybeans and

found soybean yield reductions were correlated with mature weed yields.

Because giant foxtail grew larger than the other two species, it demon-

strated a greater competitive effect. However, all three species

reduced yields equally with respect to soybean yield loss per unit

weight of weeds so no differences in competition effects were found.

Working with cotton, Buchanan and Burns (24) found tall morning-

glory (Ipomoea purpurea L.) Roth) more competitive than sicklepod

(Cassia obtusifolia L.) because of the former's smothering growth

habit. In a related study (25) they found common cocklebur (Xanthium

pensylvanicum Wallr.) was more competitive than redroot pigweed

(Amaranthus retroflexus L.) probably because of cocklebur's greater

height and shading effect. Eaton et al. (54) found that velvetleaf

produced more dry weight and more soybean yield reductions than prickly

sida (Sida spinosa L.) or Venice mallow (Hibiscus trionum L.). Velvet-

leaf height was almost twice that of the other two weeds.


Weed Densities and Economic Threshold Level

A number of studies have demonstrated that weed density is inversely

proportional to yield reduction of soybeans (10, 39, 173) and of other

crops (2, 25, 125, 184). For example, Naylor (125) found the depression

of wheat yield was linearly related to the log of the density of

Alopercurus myosuroides Huds. Each ten-fold increase in weed density








reduced grain yield by about 25%. Several studies (4, 10, 173) suggest

that crop yields decrease linearly with increasing weed density. De-

viation from linearity, however, would be expected to occur when weeds

began to compete intraspecifically as well as interspecifically. At

this point the competitive stresses would be shared between the crop

and within the weed population itself (70).

The weed densities needed to reduce crop yields are often quite

low. For example, Barrentine (10) reported soybean yields were reduced

10, 28, 43, and 52% from cocklebur densities of 3,300, 6,600, 13,000,

and 26,000 plants/ha when compared to cocklebur-free plots. The

proportional reduction in yield was approximately linear. Moolani et

al. (122) reported that one smooth pigweed (Amaranthus hybridus L.)

per square meter reduced yields 50% compared to weed-free plots.

The economic threshold level is defined as that minimum density

of pests at a given time which if left unchecked, would ultimately

cause damage equal in value to the treatment cost. The latter level

of crop damage is called the "economic injury level" (130). Several

researchers have expressed the concept that there is a minimum density

of weedsnecessary to reduce yields (24, 99, 122). Their concepts,

however, appear to have been derived from applying improper statistical

analysis. Using Duncan's multiple range test, they seek to show yields

of plots having a particular density of weeds significantly lower com-

pared to weed-free plots. Practically speaking, however, almost all

weed densities "significantly" reduce yields. Wherever the independent

variable is a quantitative continuum (as increasing weed densities),

a regression analysis is more appropriate and useful. Suggestive of

a regression approach, Smith (149) reports that each pigweed (Amaranthus








palmeri L.) in cotton costs 2, considering yield losses and

increased time to harvest the weedy crop. Anderson and McWhorter

(4) report 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 where cocklebur was controlled,

and $119/ha with 95% control. Greatest net returns were achieved

when cocklebur control was complete.


Critical Periods of Weed Competition

When crops and weeds germinate at the same time, the longer the

period of crop-weed competition, the greater the crop yield reduction.

Much research has been directed at determining the critical period of

weed competition, that is, that period of crop-weed competition which

results in an irreversible decline in crop yields.

The highest density of weeds usually emergesat about the same

time as the crop. The critical periodsfor soybeans and certain annual

weeds when emerging simultaneously are as follows: cocklebur (10) and

sicklepod (173),four weeks; smooth pigweed five weeks (164); Venice

mallow,six weeks (53); barnyardgrass,(Echinochloa crusgalli L. Beauv.)

four to seven weeks (112); morningglory,six to eight weeks (196); and

Pennsylvania smartweed (Polygonum pensylvanicum L.),six weeks (39).

Working with peanuts, Hauser et al. (80) found that Florida beggar-

weed and sicklepod had to compete for more than 10 weeks before peanut

yields were reduced. However, smooth pigweed and crabgrass (Digitaria

sanguinalis L. Scop.) reduced yields of Spanish peanuts after only four

to eight weeks of competition (86). According to York and Coble (198),

when Texas panicum (Panicum texanum Buckl.) competed with peanuts for








two or more weeks, peanut yields were significantly reduced. In Argen-

tina, peanuts could compete with weeds for 30 days before crop yields

were markedly reduced (111).

A review by Kasasian and Seeyave (96) shows that the yields of

many crops are only slightly reduced by weeds emerging more than 30

days after the crop. Hence, crops are most susceptible to competi-

tion from weeds establishing during the first 30 days of a crop's

100-135 days growth cycle. Such weeds will usually be bigger and more

aggressive than those starting growth 30 days after the crop.

If soybeans are kept free of weeds for the first two to four weeks,

later emerging weeds do not greatly reduce soybean yields. Coble (39)

found that if soybeans were kept free of Pennsylvania smartweed for

two to four weeks, subsequent emerging weeds did not significantly

reduce soybean yields. Knake and Slife (100) reported that giant fox-

tail which emerged after three weeks of weed-free maintenance did not

significantly reduce soybean yields.

Peanuts do not appear to be as competitive as soybeans. According

to York and Coble (198), when peanuts were kept free of fall panicum

(Panicum dichotomiflorum Michx.) for eight weeks, subsequent emerging

weeds still significantly reduced yields; however, control for the

first two weeks were the most critical. Hauser et al. (80) found that

if peanuts were maintained free of Florida beggarweed or sicklepod for

four or more weeks after crop emergence, yields were not significantly

reduced by subsequent weed infestation. Hill and Santelmann (86)

reported that if peanuts remained free of smooth pigweed and crabgrass

for six weeks after planting, subsequent invasions did not signifi-

cantly reduce yields. Marquina et al. (111) found that if peanuts








remained weed-free six weeks, yields were not appreciably reduced by

later weed growth.



Row Spacing

Many studies have demonstrated that soybeans grown in rows narrower

than standard 36 inches or 1 meter spacing produce significantly higher

yields (135). Narrow rows have been shown to give better weed control

by decreasing weed yields (30, 31, 32, 183) because the canopies of

soybeans grown in narrow rows shaded the ground more rapidly than soy-

beans grown in wide rows (183). For example, Burnside and Colville (30)

showed that soybeans planted in 10-, 20-, 30-, and 40-inch rows shaded

the ground in 36, 47, 58, and 64 days. As a result of earlier shading

with narrow rows, less weed control methods are needed (107) and are

needed for a shorter period of time (30). For effective postemergence

weed control in narrow soybean rows, fewer cultivations (137) and lower

rates of herbicides (107) are required.

Narrow rows of soybean offer other advantages. According to Man-

nering and Johnson (110) the earlier ground cover of narrow rows resulted

in 24% greater-water infiltration and 35% less soil loss. Comparing

20- and 40-inch rows, Peters and Johnson (136) found that 20-inch rows

shade the soil earlier than 40-inch rows resulting in a substantial

reduction in soil moisture evaporation loss. The higher soybean yields

in 20-inch rows could be explained by the fact that the root systems of

40-inch row soybeans do not make full use of available soil moisture

stored between rows. However, Timmons et al. (175) found no significant

differences in evapotranspiration rates between narrow and wide rows of

soybeans.








The main drawback to growing soybeans in narrow rows is that

should herbicides fail cultivation maybe difficult. McWhorter and

Barrentine (119) found that when the same herbicide practices were

applied to all treatments, cocklebur control was less in narrow (18

and 33 cm) rows without cultivation than in wide (100 cm) rows with

cultivation. In fact, cocklebur reduced the yield of 'Bragg' cultivar

7% when grown in 100 cm rows with cultivation and 39% when grown in 33

cm rows without cultivation. Fortunately, the recent availability of

metribuzin and bentazon for improved control of many troublesome broad-

leaf weeds greatly increases the potential for narrow-rowed soybean

production (182).



Population of Crop Plants

A certain minimum density of soybeans is required for optimum

yields and another minimum density is required for effective competi-

tion against weeds. The former is generally lower than the latter. Cartter

and Hartwig (34) summarized available information and concluded

highest yields in standard 0.9 to 1.0 meter rows were obtained from

seeding rates of 19.7 to 39.4 seeds per meter of row. Lower seed rates

resulted in reduced yields, shorter plants and pods too close to the

ground for efficient machine harvesting; higher seeding rates often

resulted in increased height, smaller stem diameters, increased lodging,

and thus, reduced yields. Hartwig (79) found less than 32-39 seeds

per meter of row yielded as well as thicker stands but the slower rate

of ground shading resulted in poor weed control. Similarly, Weber and

Staniworth (186) reported soybean densities less than 30 to 36 plants

per meter of row at standard spacing resulted in substantial yield







losses from weeds. Staniworth (162) reported that in standard rows

soybean stands between 43 to 50 plants per meter of row had less yield

reductions from weeds than soybeans planted 23 to 30 plants per meter

of row.

High rates of planting are not only justified for better weed

control but also for better emergence through encrusted soil (154).

But, high stand densities result in low water use efficiencies (175).



Soil Water and Nutrient Relations

Competition begins beneath the soil surface where the root systems

overlap in their utilization of water and nutrients and manifests it-

self in the retarded development of the top growth (134).

The quantity of rainfall and its distribution, as well as soil

moisture retention capability, influence crop and weed growth. Crops

and weeds vary in their water requirements, and soil moisture condi-

tions greatly influence their competitive ability. Wiese and Vandiver

(193) observed that weed species which grew well under wet soil condi-

tions competed poorly with a shortage of water. Weeds that produced

little growth under wet soil conditions were able to compete better

where soil moisture levels were low. Many studies have observed dif-

ferential crop-weed competition interaction associated with rainfall

distribution (99, 101, 160). Generally rainfall at the beginning of

the growing season germinates many weed seeds which brings heavy weed

pressure to bear on the crop (53, 122, 160, 191). When moisture is

favorable throughout the growing season, soybeans and other crops are

generally more competitive against weeds such as smooth pigweed and

giant foxtail (160, 186).








Weed plants are important competitors with crop plants for N and

K which are often limiting in crop production (15, 178). Even when

quantities of available soil P are low, weeds can accumulate high P

levels which indicate that weeds compete with crops for this element

(178). Crops and weeds may also compete for Ca and Mg but generally

a large difference in species response results from small soil N dif-

ference than Ca, Mg, and even P (19). Response of plants to different

levels of Ca per se is difficult to measure because of pH-dependency

(20). Studies comparing the growth response of D. tortuosum with that

of various crops and other weeds have shown it to produce poor growth

at low soil pH (4.8-5.1) compared to pH 6.5, and its growth responds

strongly to increases in soil K and P (27, 88).

Obviously the differential response of the crop and weed to any

change in soil nutrient level might influence the competitive relation-

ship. For example, the species with the most extensive and efficient

root system and which can produce the most height and leaf expansion

rate per unit of nutrient may become the more competitive species.

Hence, where different levels of nutrients are applied, either the crop

(115, 127) or weed (18, 67, 162) may benefit.


Light

Weeds compete with crops for light (134, 169). Competition for

light begins when leaves of crop and weeds shade one another. This

usually occurs after a period of root competition which may in itself

determine the intensity of competition for light. For example, crop

plants which produce early root growth at the expense of top growth

may be poor competitors against weeds which channel more of their

energy into dense canopy development.





29

Weed competition for light becomes important when the weed species

is tall growing and the density is high (14). Large yield reductions

are reported in soybeans (122) and other crops (45, 133) when weeds over-

top the crop. The reduction in yield is usually proportional to the

amount of light used by the weeds at the expense of the crop (132).














METHODS AND MATERIALS

Experiment 1. Response of Florida Beggarweed Germination and Seedling
Elongation to Temperature

D. tortuosum seeds were germinated on a temperature gradient bar as

described by Barbour and Racine (8). Either four or five holes occurred

at a particular gradient, and each hole was considered a replication.

Each hole was filled with vermiculite and planted with 10 seeds which

had been previously treated 20 minutes with conc. sulfuric acid to

break dormancy. Vermiculite was kept continuously moist and covered

during the experiment. After 98 hours of incubation, the number of

germinated, non-germinated, and hard seeds was recorded, and seedling

length was measured. Germination was considered accomplished when the

radicle attained a length of 1 mm. Hard seeds were excluded in the

calculation of percentage germination.


Experiment 2. Effects of Simulated Drought on Germination and Seedling
Elongation of Florida Beggarweed and Seven Other Species

Germination tests were conducted in germinators at 301 C with

seeds of Florida beggarweed (Bw), soybean ('Bragg') (Sb), Cassia

obtusifolia (Co), Ipomoea quamoclit L. (Iq), Alysicarpus vaginalis (L.)

DC (Av), and peanut ('Early Bunch') (Pn). Seeds were germinated in

water solutions having osmotic pressures (OP) of 0, 3, 5, 6, 8, and 10

bars which were prepared by mixing polyethylene glycol (mol. wt. 6000)

with distilled water by the method of Parmar and Moore (131). In order

to compare results, procedure,and presentation of data follow those of

Hoveland and Buchanan (88).








Thirty seeds were soaked for one minute in 3% sodium hypochlorite

solution, rinsed with distilled water, air dried for 15 minutes, and

placed on two sheets of Whatman No. 3 filter paper in 9 cm petri dishes.

Each treatment was replicated three times. Each petri dish received

6.6 cc of solution; but, because of larger seed, petri dishes contain-

ing soybeans and C. obtusifolia received 13.2 cc and peanuts, 19.8 cc.

Germination counts were made at the end of 24, 48, 72, and 96 hours.

An arc-sine transformation of data was made in order to secure

valid F tests.



Experiment 3. Effect of Seed Maturity and Shading on Seed Weight,
Percentage Germination, and Vigor

By means of shade frames, beggarweed plants were grown under 33,

66, and 100% sunlight. The desired shading was provided by stretching

strips of 2 cm wide, green, opaque, nylon cloth in a north-south direc-

tion between two sides of the frame. Shade frame height was adjusted

so that all leaves except those diminutive leaves on the panicle were

shaded. Final frame height was about one meter above ground level.

Between August 27 and September 28, 1976, flowers of shaded and un-

shaded plants were tagged, and lotates were harvested at regular inter-

vals. Fruits were air-dried. In March 1977, fruits were threshed

manually and seeds were collected, weighed, and scarified by pricking

the testa with a needle. Seeds were germinated in 9 cm petri dishes

at 301 C for 28 days using.two sheets of Whatman No. 3 paper beneath

the seeds and one sheet above. Germination was considered complete

when the radicle attained a length of 1.0 mm. Forty-eight hours after

germination, radicle length was measured. Hard seeds were rescarified

and allowed additional germination time.








In order to stabilize the variance, an arc-sine transformation

of the data was made before analysis.



Experiment 4. Effects of Herbicide on Germination and Seedling Growth

Commercially available formulations of herbicides (Table 1) were

made up to 1000 ppm stock solution, and the pH was adjusted to 7!.05.

Beggarweed seeds were germinated in petri dishes wetted with 6 ml of

alachlor, benefin, dinitramine, glyphosate, metribuzin, naptalam,

oryzaline, and vernolate at concentrations of 0.0, 0.01, 0.1, 0.5, 1.0,

5.0, 10.0, and 100.0 ppm a.i. In each petri dish thirty mechanically

scarified seeds were placed between filter papers as described in

Experiment 3 and germinated in the dark at 30t1 C. Three replications

were used.

After 48 hours of incubation, the number of germinated and non-

germinated seeds were recorded, and an ink mark was made 7 mm from the

radicle tip on ten randomly selected seedlings. After 72 hours of

incubation, seedling length and distance between the radicle tip and

the ink mark were recorded.



Experiment 5. Effect of Depth of Seeding on Beggarweed Emergence

Soil depth experiments were conducted in the greenhouse in November

1975 (Trial 1) and April 1977 (Trial 2). Soil was sifted through a No.

7 screen and placed in 35x50 cm flats. In Trial 1, seeds were placed

with forceps into moist soil at either 0, 2, 4, 8, 16, 32, or 64 mm

depth in a completely randomized design. Each treatment contained 32

seeds and was replicated four times. Trays were surface irrigated.
































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In Trial 2, fungicide-treated seeds were placed on the soil in rows

and covered to the desired depths of 0, 4, 8, 16, 32, 64, and 80 mm.

Each treatment was replicated three times in randomized complete block

design. Each experimental unit was sown with 40 seeds. Trays were

both sub- and surface-irrigated. Emergence was considered completed

when at least one cotyledon emerged horizontal with the soil surface.

Emergence counts were made daily and experiments were terminated after

28 days.


Experiment 6. Periodicity of Beggarweed Emergence Under Field Conditions


In April 1976 and March 1977, 0.5 m2 quadrats were staked out on

a beggarweed infested field on the Agronomy Farm, University of Florida.

In the 1976 trials, ten quadrats were randomly assigned within a 2x20 m

area which had been plowed and disked once in February. On an adjacent,

non-cultivated, 2x20 m strip where vegetation had been killed by para-

quat, ten more quadrats were randomly assigned. In 1977, the cultivated

and non-cultivated quadrats were paired on the cultivated strip used

in the 1976 trial. The soil of the cultivated quadrats was turned

under several times to a depth of 15 cm with a shovel. Non-cultivated

quadrats were not disturbed. Weekly, emerged seedlings were counted

and removed by hand or eradicated with paraquat or glyphosate.


Greenhouse Experiment: Competition of Soybeans and Beggarweed at
Three Levels of Watering

Plastic, 3.8 liter-capacity pots were filled to within 2 cm of

the brim with 3.2 kg of soil (Kendrick fine sand) which had been fer-

tilized at the equivalent rate of 20 kg N, 80 kg P205, and 100 kg K20








per hectare. About 8 weeks after soybean emergence, pots received a

K20 supplement at the rate of 120 kg per hectare.

A randomized complete block design with four blocks was used. In

each block, soybeans and beggarweed plants were grown in pots alone and

in competition at three daily water levels (X, 2X, and 3X). Tests were

conducted in an air-conditioned greenhouse where average minimum and

maximum temperatures ranged between 17 and 38 C.

Captan treated soybean seeds ('Bragg') weighing 1899 mg per seed

were used. On April 12, six soybean seeds, two per hill, were planted

three centimeters apart in a straight row, the middle hill at the center

of the pot. At the time of soybean planting, plump mechanically

scarified beggarweed seeds were germinated in petri dishes in the

greenhouse. After three days several seedlings were transplanted in

hills located at the corners of a six centimeter imaginary square posi-

tioned in the middle of the pot. In pots containing soybean, the sides

of the square were parallel to the soybean row. When soybeans and

beggarweed plants were well-established, seedlings were thinned to one

beggarweed or soybean plant per hill.

Between date of seeding and May 5 all pots received equal amounts

of water sufficient to establish the plants. Between May 5 and June 20,

X was equal to 100 cc per pot (equivalent to 21 mm rainfall per week)

for pots containing soybean with or without beggarweed in competition,

and X was equal to 50 cc per pot with beggarweed growing alone. After

June 20 when water demand appreciably increased because of increased

plant size, X was increased to 150 cc per pot containing soybean and

100 cc per pot containing only beggarweed. Water was metered daily as

long as wilting of either soybeans or beggarweed receiving X cc of

water was observed.








Soybean and beggarweed heights were recorded 12, 22, 35, 52, and

137 days after seeding. Height was the distance between ground level

and the terminal growing point. At 137 days after seeding, but while

pods were still filling, soybean and beggarweed plants were harvested

at ground level and dried at 70 C for two days, and the dry matter

weighed. Soybean pods were harvested and weighed to the nearest 0.01

gram.



Field Experiment 1. Competition of Peanuts With Beggarweed at Five
Population Densities

The experimental plots were located at Green Acres, Agronomy Farm,

19 km west of Grainesville and were established on an area where bermuda-

grass sod was previously growing. The soil is classified as a Kendrick

loamy sand (loamy, siliceous, hyperthermic arenic Palendult).

Land was disked in December 1975 and disked several more times

at biweekly intervals in order to decompose the sod. In April 1976

dolomitic limestone was applied at the rate of 2.8 MT/ha and incorporated

by disking. On May 26, parathion 10% G at the rate of 2.2 kg/ha a.i.

and 34 kg N, 68 kg P205, 108 kg K20, and 20 kg FTE 503 were applied per

hectare and incorporated. On June 1, 2.2 kg/ha a.i. vernolate was

applied as a spray and incorporated by disking for control of grass

weeds.

Plots were hand weeded during the third week of July to remove

Richardia scabra L. and Alysicarpus vaginalis DC infestations. Bravo

at the rate of 0.5 kg a.i./ha and Sevin at the rate of 0.9 kg a.i./ha

were applied for the control of Cercospora leaf spot and insects,

respectively.










On June 15, peanut seeds were drilled at recommended rates in 91

cm rows. A split plot design was used with weed-free maintenance (0,

3, 6, and 9 weed-free weeks) as main plots and weed density (0, 1, 2,

4, and 8 weeds per meter row) as sub-plots. Each treatment was repli-

cated three times. Beggarweed seeds were dibbled on both sides of a

20-cm band over the peanut rows. Plots consisted of six peanut rows

5.6 m long. The four middle rows were sown to beggarweed, but only

the middle two rows were used for collecting crop and weed data and

harvest samples. Harvest rows were 4.5 m long, and total harvest area

was 8.2 square meters.

On August 23 and September 2, heights of beggarweed and peanuts

were recorded in cm. On September 2, leaf area index (LAI) of beggar-

weed was estimated using a modification of the point quadrat method

(194). Twenty-two holes were drilled 15 cm apart in a board. The

board was placed diagonally across a peanut row such that when weighted

fish lines were suspended from the holes, the two end lines were located

in the middles between the rows. The number of contacts the line made

with the beggarweed leaves was recorded. This method was repeated for

the second harvest row such that 44 observations were recorded for

each plot.

At the time of harvest, beggarweed height was recorded in cm and

the number of beggarweed plants per sampling area was counted. Because

many of the plants had one or more secondary stems growing from the base

of the plants, stem number per sampling area was also recorded. Beggar-

weed plants were uprooted by hand and the fresh weight was recorded in

kg/ha. Plants were dried at 70 C to constant weight, and dry weight

was recorded in kg/ha.








Harvesting of peanuts occurred October 1. The soil around the

peanut plants was loosened with a shovel, and the plants were uprooted

by hand. Pods were mechanically removed from the peanut foliage, and

both pods and foliage were dried to constant weight at 35 and 70 C,

respectively. The yield of dry clean unshelled peanuts, shelling per-

centage, and seed moisture content was determined, and yields were

recorded in kg/ha. Seeds were sorted by size: large (seeds retained

by an 8.5 mm wide opening), medium (seeds retained by a 6.4 mm wide

opening), small (seeds retained by a 5.9 mm wide opening), and shriveled

(seeds which passed through a 5.9 mm wide opening). Peanut and beggar-

weed growth characteristics were statistically analyzed using single

and multiple regression analysis.


Field Experiment 2. Competition of Florida Beggarweed With Soybeans
at Green Acres

The experimental site was located adjacent to the peanut experi-

ment described above. All soil preparation and preplant chemical appli-

cations were identical with those of peanuts. On June 15, 1976, 'Bragg'

soybean seeds were drilled at recommended rates in 92 cm wide rows.

Beggarweed seed rates and planting pattern were the same as for peanuts.

Because of poor beggarweed emergence, however, the original experimental

design was abandoned, and four blocks having a wide range of beggarweed

plant densities were selected for sampling. Within each block, single

soybean rows between 4.0 and 4.4 m long were selected as sample plots.

Beggarweed stem and plant number and beggarweed height in cm were

recorded. Beggarweed plants were harvested by pulling out the plants








by hand. Plots were harvested on November 4. Fresh and dry weight of

beggarweed plants were determined as in Field Experiment 1, and were

recorded in kg/ha. Soybean plants were cut off at ground level; seeds

were mechanically threshed and dried to constant weight at 40 C. Soy-

bean seed yield was reported in kg/ha at 13% moisture content.



Field Experiment 3. Competition of Florida Beggarweed With Soybeans
at Two Row Spacings

The experimental plots were located in field S-13 on the Agronomy

Farm, University of Florida, Gainesville. The soil is classified as a

Kendrick fine sand.

Land was plowed and disked twice in March 1976. In April 1976

dolomitic limestone was applied at the rate of 2.8 MT/ha and incorporated

by disking. Nemagon at recommended rates was sprayed and incorporated

for the control of nematodes. In early June, 34 kg N, 68 kg P205, 108

kg K20, and 20 kg FTE 503 were applied per hectare and incorporated at

the same time. Vernolate at the rate of 2.2 kg/ha a.i. was applied and

incorporated for control of grassy weeds.

Plots were hand weeded during the second week of July to remove

grassy weeds and light infestations of Indigofera hirsuta L. Lannate

at recommended rates was applied during the season for the control of

leaf and pod feeding insects.

On June 10, 'Bragg' soybean seeds were drilled in 92 and 46 cm

wide rows. A split-split-block design was used with row spacing as

main plots, weed-free maintenance (0, 3, 6, and 9 weed-free weeks) as

sub-plots and weed density (0, 1, 2, 4, and 8 weeds per meter row) as sub-

sub-plots. Because of poor weed emergence in the sub-plots, however,








only 0 weed-free weeks were used. Each treatment was replicated three

times. Scarified beggarweed seed was dibbled on both sides of a 20 cm

band over the soybean rows. Rows were 5.6 m long. In the 46 and 92 cm

rows, the innermost eight and four rows, respectively, were over-seeded

with beggarweed but only the inner four and two rows, respectively, were

used for collecting crop and weed data and harvesting. Harvest rows

were 4.5 m long; total harvest area was 8.2 square meters.

On August 20, height of beggarweed and soybean was recorded. Beg-

garweed height was the distance in cm from ground level to the terminal

bud; soybean height was the distance in cm from ground level to the

horizontal height of the highest leaf. On September 1, LAI of beggar-

weed and soybean were measured using the point contact method described

in Field Experiment 1.

At the time of harvesting, soybean plant characteristics and

components of seed yields were determined from 10 randomly selected

plants. Soybean plant height was obtained by measuring in cm the

distance between the cotyledonary node to the terminal bud. The distance

to the first node-bearing pod was obtained by measuring in cm the dis-

tance between the cotyledonary node to the node bearing the first pod.

Stem diameter was measured in mm at the cotyledonery node.

Numbers of pods and seeds per plant were counted, and weight of

seeds per plant were determined. Number of seeds per pod was calculated

from the above information. Gram weight of 100 seed was calculated from

the number of seeds per plant and mean weight of seeds per plant.

Soybeans were harvested on October 21. Seed yield was obtained by

adding the weight of the soybean seeds obtained from the 10-plant sample

to the weight of the seeds from the remainder of the plot. Weights were

adjusted to 13% moisture content and recorded in kg/ha.








At soybean maturity, beggarweed plant heights were reduced in cm

and the population per 8.2 m2 counted. Plants were hand-pulled and

fresh and dry weight determined as in Field Experiment 1 and recorded

as kg/ha.



Field Experiment 4. Competition of Florida Beggarweed With Soybeans
at Two Locations on Farmer's Fields, Newberry

A series of plot samples were harvested from two beggarweed infested

soybean fields on a farm located 10 km southwest of Newberry, Alachua

County. Two fields were sampled. Field A had heavy infections in

a low-lying area. Field B had uniform but light infestation on the

plateau and slopes.

Soybeans were planted on beds in modified twin row planting pattern.

The distance between beds was 1.52 m and between twin rows on the bed,

0.5 m. Plot length measured 1.83 meters per twin row (2.78 m2). Areas

containing gaps in the rows or infested with other weeds were avoided.

In each plot, beggarweed height was recorded in cm and plant and

stem density were recorded in number per hectare. Beggarweed plants

were pulled up by hand and fresh and dry weight were determined as in

Field Experiment 1 and recorded in kg/ha. Soybean density was recorded

in number per square meter. Soybean plants were mechanically threshed

and weight of seed yields recorded in kg/ha, adjusted to 13% moisture

content.














RESULTS AND DISCUSSION

Experiment 1. Response of Florida Beggarweed Germination and Seedling
Elongation to Temperature

The temperature gradient bar subjected the seeds to temperatures

between 2 and 57 C (Figure 1). After four days incubation, germination

was linear between 20 and 24.2 C, was near maximum between 24.2 and 44

C, and abruptly ceased near 45 C. Elongation showed a quadratic re-

sponse to temperature and attained maximum elongation near 31 C. The

decrease in elongation at high temperature most likely resulted from a

dysfunction of enzymatic activity because of protein denaturization.

Beggarweed has a higher minimum temperature requirement for seed

germination than do many other species. The minimum/maximum tempera-

tures for three other species are as follows: corn, 9/42 C (40);

Echinochloa crusgalli, 10-15/45 C (5); and Amaranthus retroflexus,

10/46 C (55). Pendleton and Hartwig (135) reported 15 C to be the

minimum temperature requirement for soybean. All species, including

beggarweed, had similar optimal temperatures for radicle elongation.

The comparatively high minimum temperature for beggarweed could

be due to the relatively short time of incubation. Evans (55) reported

A. retroflexus germination at 9-10 C did not occur until after 100 hours

of incubation. It is known from studies using other species that longer

incubation times are needed for germination at critically low tempera-

tures (43).










- -me


S80


60


g 40


- 20


0




70


60


50
E
E

S40




10
0J
g1 30
*r--
--
S20


10


0


30 35
Temperature (C)


Relation of temperature to the percentage germination
and seedling elongation of Florida beggarweed.


100


25 30 35 40 45


Figure 1.




L4I+

The comparatively high minimum temperature requirement of beggarweed

explainswhy it does not emerge until May or June (58) and does not

appear until laying by of corn, cotton, and other crops (104, 126).



Experiment 2. Effects of Simulated Drought on Germination and Seedling
Elongation Beggarweed and Seven Other Species

Because of severe mold contamination, peanut data are not presented.

However, that peanut required lower moisture tensions and more time

than soybeans to germinate was readily apparent.

Figures 2 and 3 and Table 2 show that under low OP (0 and 3), all

species except soybean achieved rapid and complete germination. At OP

5 and 6 C. obtusifolia made the most rapid early germination but beggar-

weed attained the highest germination by the conclusion of the experi-

ment. The germination percentage of A. vaginalis and I. quamoclit was

low and soybean poor. At high OP beggarweed attained the highest germi-

nation followed by C. obtusifolia which was significantly lower. From

Table 2 the approximate ranking of the species according to drought

tolerance is: beggarweed (Bw)>C. obtusifolia (Co)> C. spectabilis

(Cs)LJ. tamnifolia (Jt)?A. vaginalis (Av) I. quamoclit (Iq) >soybeans

(Sb)>peanut (Pn).

Hoveland and Buchanan (88) showed C. spectabilis to have drought

tolerance intermediate between sicklepod and soybean. Furthermore, their

data demonstrated that I. lacunosa and I. hederacea to have more drought

tolerance than the convolvulaceous species tested in this experiment.

The drought tolerance of beggarweed and C. obtusifolia may explain

their widespread presence in dry sandy fields and the author's observa-

tion of their ability to germinate on moist soil surfaces under shade

or overcast conditions. This tolerance to drought permits these weed









Florida Beggarweed


100


80


60


40


20


0 24 48 72 96


Soybean
9 ---9o----9










0
0


0 24 48 72 96


J. tamnifolia


100


80



60


40


20


0 24 48 72 96
Hours


I. quamoclit


Q -8
o
I








R
oil


[0


0 24 48 72 92
Hours


a -5 bars

u -6 bars


v -8 bars

S-10 bars


Percentage germination of Florida beggarweed, soybean, J.
tamnifolia and I. quamoclit at 0, 3, 5, 6, 8, and 10 -bars
moisture tension after 24, 48, 72, and 96 hours incubation.


100


80


* 0 bars

, -3 bars


Figure 2.











C. spectabilis


0 24 48

Hours


100


S80
*r-

C(
*- 60
E
CD
- 40

(--
U


72 96


C. obtusifolia


0 24 48 72 96
Hours


Figure 3.


Percentage germination of
and A. vaginalis at 0, 3,
tension after 24, 48, 72,


A. vaginalis
-0 0
*- _o--o


0 24 48 72 96
Hours


C. spectabilis, C. obtusifolia,
5, 6, 8, and 10 -bars moisture
and 96 hours incubation.


* 0 bars


o -3 bars


* -5 bars


o -6 bars


v -8 bars


100


a 80
80


.~ 60
E
U3
a,
4, 40
U
S.-
0 20


0








Table 2. Percentage germination of seven plant species at six moisture
tensions and at four time intervals.


Moisture Spe
tension Spe
(-bars) Bw Co Cs Jt Av Iq Sb

24 hours


93.2ba
18.9b
O.Ob


79.6d
48.2a
8.3a


85.3cd
O.Oc
O.Ob


100.Oa
25.8b
O.Ob


97.7a
O.Oc
O.Ob


89.2bc
O.Oc
O.Ob


81.Id
O.Oc
O.Ob


48 hours


97.8ab
92.2a
6.7c
O.Ob
l.la


95.26
79.0b
68.3a
27.4a
O.Oa


93.2b
85.6ab
35.4b
3.7b
O.Oa


100.Oa
91.7ab
44.9b
5.0b
O.Oa


100.Oa
92.0ab
10.2c
2.3b
O.Oa


100.Oa
62.1c
13.3c
2.5b
O.Oa


100.Oa
14.4d
l.ld
O.Ob
O.Oa


72 hours


98.9a
96.7a
96.7a
81.2a
60.Oa


96.4a
97.5a
84.7b
57.2b
18.3b


94.3a
89.2a
73.1bc
17.9c
3.4c


100.Oa
95.2a
53.9c
12.4cd
3.5c


100.Oa
94.4a
24.9d
13.6cd
O.Oc


100.Oa
89.0a
22.5d
3.7dc
O.Oc


100.Oa
20.Ob
3.3c
O.Oc
O.Oc


96 hours


100.Oa
98.9a
97.8a
86.7a
60.Oa
l.la


100.Oa
97.5a
88.3b
70.9a
18.3b
O.Oa


94.3a
91.6a
73.1bc
33.2b
3.4c
O.Oa


100.Oa
95.2a
57.6c
22.0b
3.5c
O.Oa


100.Oa
95.5a
27.3d
18.2bc
0.Oc
O.Oa


100.Oa
98.9a
29.7d
9.2c
O.Oc
O.Oa


100.Oa
26.7b
4.5c
O.Od
O.Oc
O.Oa


a Within a row any two means having a letter in common are not signifi-
cantly different at the 5% level by Duncan's new multiple range test.








species to germinate over a wider range of soil moisture conditions

than other weed species and most crop species. Because of this tolerance,

a greater number of seeds may germinate throughout the soil moisture

profile bringing a large weed population pressure to bear on the crop.

The data also support the theory that weed species are generally capable

of germination at lower moisture tensions than crop species.



Experiment 3. Effect of Seed Maturity and Shading on Seed Weight,
Percentage Germination, and Vigor

Table 3 shows the results of analysis of vigor, germination percent-

age, and seed weight for time and shade treatments. Only seed weight

was significantly affected by shade, 66% shade being significantly dif-

ferent from 0% and 33% shading. Table 3 also indicates a cubic model

best described the effect of time on vigor, germination percentage, and

seed weight.

As predicted by models for both the actual and transformation of

the data, percentage germination is plotted in Figure 4. Germination

commenced 10 days after flowering (DAF) and 100% germination was attained

by 21 or 22 DAF; 50% germination occurred at 15 DAF. Vigor, however,

lagged behind germination attaining the 50% level between 16 and 17 DAF

and full vigor after 23 DAF (Figure 5). Seed weight is plotted in

Figure 6 for two shade levels, 0 and 67%. Maximum seed weight was

attained at 25 DAF and 50% seed weight at 13 DAF. By comparison, Chow

and Crowder (38) reported that seed weights of D. intortum and D.

sandwicense reached a maximum about 28 days after fertilization.

Table 4 gives the partial correlation coefficients between seed

weight, vigor, and germination percentage. Seed weight was significantly









Table 3. Analysis of variance, models, and individual sources of
variance for percentage germination, seed weight, and
germination vigor.


df PR>F R2


Germination percentage


model
days (d)
d2
d3
shadea


.0001
.0001
.0001
.050
.831


Seed weight


model
days (d)
d2
d3
shadea"


0 vs 67%
0 vs 33%
33 vs 67%


.0001
.0001
.0001
.137
.034

.012
.637
.071


Germination vigor


model
days (d)
d2
d3
shadea


.0001
.0001
.0005
.0001
.626


a No shade x day, shade x day2,or shade x day3
significant at the 5% level of probability.


interactions were




































I----j 95% Confidence Limits

f---e Transformed Data


Actual Data


8 10 12 14 16 18 20 22 24


Days After Flowering


Figure 4. Effect of seed maturity on the percentage germination
of Florida beggarweed seeds as predicted by regression
analysis of actual and transformed data.


100


90


80


70


























12-







*r-
C


4-- 6 -

sa,

C,



3


---- 95% Confidence Limits



10 15 20 25

Days After Flowering


Figure 5. Effect of seed maturity on seedling length (vigor) after
48 hours incubation.















36


32


28

S24


20 2
a'

S16

C 12
o /



/- 0% shade
4 -66% shade


oil I I I
0 12 16 20 24

Days After Flowering


Figure 6. Effect of seed maturity on weight increase of seeds from
plants grown under 0 and 66% shade.





53


Table 4. Partial correlation coefficients of percentage germination,
seed weight,and vigor.


Percent Vigor
germination


Seed weight .27* .25

Vigor .42* --


* Significant at the 5% level of probability.








and positively correlated with percentage germination but not signifi-

cantly with vigor.

Separation of the distal pods was observed between 15 and 17 DAF

when color of lotates was changing from green to dark olive brown.

These seeds would be germinable.

The prediction models indicate that germination begins at 10 DAF

but at this time seed vigor is poor. When lotate color darken and pods

begin to separate, 80% of the seeds are germinable, but vigor is still

less than 50% capacity. Whether these seeds are capable of over-

wintering in the soil is unknown. Chow and Crowder (38) reported that

hardseededness in two Desmodium species formed 17 days after fertiliza-

tion. Hardseededness would likely be an important factor in determining

survival of beggarweed seeds. Furthermore, whether these immature

but germinable seeds have sufficient vigor to establish and survive is

also unknown.

Two weeks after flowering would be the critical period before which

beggarweed plants would need to be removed in order to prevent germi-

nable seeds from forming.



Experiment 4. Effects of Herbicides on Germination and Seedling
Growth

The percentage germination of Florida beggarweed for eight herbi-

cides at eight concentrations is shown in Table 5. The data show no

consistent trends either between herbicides or along concentration

gradients. The data suggest that these herbicides do not affect germi-

nation.









Table 5. Germination of Florida beggarweed as affected by eight herbi-
cides and eight herbicide concentrations.


Herbicide Concentration, ppm a.i.
0 .01 .1 .5 1.0 5 10 100 xa

... .% germination . .

Alachlor 95 93 93 87 94 98 92 88 92.2

Naptalam 95 91 96 98 93 88 92 88 92.3

Benefin 98 93 86 96 94 93 -- -- 92.4

Dinitramine 95 93 91 93 93 96 95 92 93.3

Glyphosate 98 97 96 97 89 98 95 91 94.7

Metribuzon 98 93 96 90 94 -- 97 96 94.3

Oryzalin 95 99 91 94 94 96 93 88 93.6

Vernolate 95 96 93 95 96 93 97 94 94.9


a Does not include 0 ppm.


















Dinitramine


V n t






Vernolate


I I -. 0 I I- 1
.1 1 10 0 .01 .1 1 10 100


- Oryzalin









*---.


Alachlor


L 0I I I 1 0 1 I 1
0 .01 .1 1 10 0 .01 .1


Concentration (ppm)


1 10 100


Concentration (ppm)


Figure 7. Root elongation of Florida beggarweed seedlings when germi-
nated at eight concentrations of dinitramine and oryzalin
and seven concentrations of vernolate and alachlor.


0 .01

























Benefin


I I I I I n


"* \_


_ Naptalam

I I I I I


0 .01 .1 1 10 100 0 .01 .1 1 10 100


- "


Glyphosate


I I I I i


Metribuzin


I I I I I


.1 1 10 100 0 .01


Concentration (ppm)


.1 1 10 100


Concentration (ppm)


Root elongation of Florida beggarweed seedlings when germi-
nated at eight concentrations of benefin, naptalam, glyphosate,
and metribuzin.


0 .01


Figure 8.






Figures 7 and 8 show the effects of herbicides and herbicides

concentrations on root elongation over a 24-hour period. With increas-

ing concentration dinitramine, and to a lesser extent oryzalin and

alachlor, progressively inhibited root elongation. Dinitramine generally

showed the strongest inhibitory activity over all concentrations. Root

elongation was inhibited by benefin only at concentrations greater than

1.0 ppm. Vernolate showed either no activity or a slight promotion of

root elongation. Naptalam inhibited root elongation, but its activity

was not as strong as that of alachlor or oryzalin. Even at the highest

concentrations metribuzin showed no activity. This would be expected

as metribuzin chiefly affects the photosynthetic apparatus. Only at

concentrations greater than 5.0 ppm did glyphosate inhibit root elonga-

tion.

Figures 9 and 10 illustrate the activity of herbicides on total

seedling length. Dinitramine demonstrated the strongest inhibitory

activity followed by alachlor, naptalam, and oryzalin. Vernolate and

glyphosate showed no activity. Benefin inhibited seedling elongation

only at concentrations greater than 1.0 ppm. Metribuzin appeared to

stimulate seedling elongation at concentrations between 0.0 and 10.0

ppm.

The leveling off of activity of dinitramine and oryzalin at higher

concentrations was probably due to their low solubility, 1.0 ppm and

2.4 ppm, respectively. Other herbicides having low solubility include

benefin (1.0 ppm) and vernolate (90.0 ppm).

Based on these petri dish studies, dinitramine appeared to offer

the most promise for inhibiting or controlling Florida beggarweed, but

alachlor, oryzalin, and naptalam also demonstrated promising inhibitory

activity.











50
E
40

S30
-J
2 20
r-,

i 10
V)


0 .01 .1 1 10 100


*-. V.*a \.


Vernolate


0 .01 .1 1 10 100


Alachlor


I I 1 I I


0 .01 .1


1 10 100


Concentration (ppm)


0 .01 .1 1 10 100
Concentration (ppm)


Figure 9. Total length of Florida beggarweed seedlings when germinated
at eight concentrations of dinitramine, vernolate, alachlor,
and oryzalin.


'* Dinitramine




\*.
*-*-.O. O.


50

E 40

30
c
U,
-J 20
1-
I 10
C)
(y0


I


I-


~~cS


Oryzalin













E
E
- 40
4->
30
-J
S 20

,10
i lo


I


i


Naptalam


.01 .1 1.0 10 100
.01 .1 1.0 10 100


60 -

50 /

40


30

20-

10


0 .01 .1 1.0 10 100


30-

20 -

10 -


Metribuzin


01 1 1 I *
0 .01 .1 1.0 10 10
Concentration (ppm)


*o. '** o


Glyphosate


a I a a


0 .01 .1


1 10 100


Concentration (ppm)


Figure 10.


Total length of Florida beggarweed seedlings when germinated
at eight concentrations of naptalam, benefin, metribuzin,
and glyphosate.


f,








The inhibitory activity of alachlor and naptalam most likely ex-

plains why a preemergence dinoseb-alachlor-naptalam combination is

recommended for control or suppression of beggarweed in peanuts and

soybeans. Dinoseb acts primarily as a contact herbicide on emerged

beggarweed seedlings. Applied at recommended rates and shallowly

incorporated, soil concentrations of alachlor and naptalam would be

sufficiently high to severely retard growth of emerging seedlings

which would have escaped the contact activity of dinoseb. For example,

if a dinoseb-alachlor-naptalam combination was applied at recommended

rates and rains leached the herbicides into the surface 3 cm of soil,

alachlor and naptalam could be found in concentrations of approximately

10 and 30 ppm, respectively. Data presented in Figures 9 and 10 indi-

cate that these concentrations would be sufficiently high to result in

severe suppression of beggarweed seedling elongation. In combination

the effects of alachlor and naptalam may be additive or synergistic.

Even without complete kill, the herbicide combination could sufficiently

retard beggarweed growth to permit the crop a higher competitive edge.



Experiment 5. Effect of Depth of Seeding on Beggarweed Emergence

In trial 1, emergence was near complete between the 2 and 16 mm

depth of seeding, decreased slightly at 32 mm, and abruptly decreased

at 64 mm (Figure 11). Many of the seeds at 64 mm were observed not

to have germinated which may have resulted from the prevailing cool

temperatures during the December study. In trial 2, percentage emergence

was relatively constant between 4 and 64 mm seeding depth but abruptly

decreased at 80 mm. The lower percentage emergence from shallow depths










A- Trial 1
Y=93+0.508X-0.256X2
CV=10% R2=.93**
.--- Trial 2
Y=80.2+0.19X-0.0104X2
CV=25% R2=63**


A AII~

-


0\


I I I I I


Depth (cm)


Figure 11.


Percentage emergence (Y) of Florida beggarweed seedlings
from a range of seeding depths (X) at two trial dates.


100 -


20 -







of seeding in trial 2 compared to trial 1 probably resulted from the

lower soil moisture because of higher greenhouse temperatures.

Because of poor soil moisture, seeds planted on the soil surface

in trials 1 and 2 emerged poorly, 16 and 23%, respectively. In the

field under overcast skies, beggarweed seeds have been observed germinat-

ing on the soil surface.

Based on the total number of seedlings which emerged, the percentage

emerging at 5 day intervals from various depths is shown in Figure 12.

Generally time to emergence was positively correlated with depth of

seeding. From 0 to 15 days after sowing, appreciable emergence occurred

from all seeding depths. After 15 days, emergence was appreciable only

from 64 and 80 mm depths.

The data indicate that soil incorporation of root absorbed herbi-

cides would need to take place to a depth of at least 8 cm for effective

beggarweed control, and that cultivation should be no deeper than 8 cm

to prevent resurrection of additional weed seed populations. Further-

more a "flush" of germinating weed seeds may provide emerging seedlings

over a period of 3 to 15 days because of seedlings emerging from a range

of soil depths. This has particular implications for the application

of contact, early postemergence herbicides such as dinoseb and dinoseb

combinations which are used to control beggarweed in the seedling stage.

Two or three applications of dinoseb may be necessary for complete

control as seedlings are easily killed by this herbicide only before

the 2-3 leaf stage. A dinoseb-alachlor-naptalam combination would

control emerged seedlings as well as control or suppress those in the

process of emerging from greater soil depths and thereby reduce the

required number of sprays.







* 4 cm


o 8 cm


o 16 cm


* 32 cm


A 64 cm


* 80 cm


*_,



00
x *
o o



i








I i


5 10 15 20 25


Days


Figure 12.


Percentage emergence of Florida beggarweed seedlings
from six depths as a function of time (Trial 2).








Experiment 6. Periodicity of Beggarweed Emergence Under Field Conditions


Figures 13 and 14 show that beggarweed seedlings emerge through-

out the spring and summer months. Favorable temperature (>22 C)

permitting, emergence appears to be dictated by rainfall amounts in

both 1976 and 1977 trials. In 1977, temperatures generally favorable

for germination occurred during most sampling dates (Table 6), but

seedling emergence was poor because of insufficient rainfall. Heavy

rainfall during the week of May resulted in a flush of emergence.

When total emergence from cultivated and non-cultivated plots

was compared for 1977, no significant difference (t=.27) was found.

But seedling emergence was initially higher from cultivated plots than

from non-cultivated plots. Non-cultivated plots had characteristics

of a sod with 50% of the surface area covered by dead vegetation. Table

6 indicates that large temperature differences occur between a bare and

a sod covered soil. The initial lower number of seedlings emerging

from the non-cultivated plots is likely due to the unfavorable tempera-

ture for germination and seedling growth.

Because peanuts are planted earlier than soybeans, greater weed

population pressures would be expected in peanuts than soybeans. Peanuts

are usually planted between April 15 and May 15 and soybeans between May

15 and June 15. In both 1976 and 1977, more seedlings emerged between

April 15 and May 15 than between May 15 and June 15. Late planting of

peanuts have the advantage in that natural emergence or tillage-induced

emergence of seedlings can be permitted to reduce beggarweed seed popu-

lation. Late plantings also permit the soil to warm sufficiently to

permit favorable germination temperatures (>22 C) to at least an 8 cm

depth.








































































































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Table 6. Mean, minimum, and maximum temperatures under bare soil and
sod, and rainfall amounts occurring between nine observation
dates of field germination of Florida beggarweed in 1976
and 1977.


Temperature (C)
Observation Observation Bare Soil Sod Rainfall
number date (2.5 cm depth) (2.5 cm depth) (cm)
Max Min Max Min

1976

1 11 April 32.0 15.2 22.5 18.3 4.65
2 18 April 36.5 17.5 23.5 19.0 0
3 25 April 39.6 20.0 25.9 21.3 0
4 2 May 38.7 19.8 26.3 21.4 2.1
5 9 May 38.0 19.9 27.7 21.5 .7
6 16 May 37.5 22.5 28.1 23.0 4.2
7 23 May 37.9 20.3 29.0 22.9 4.2
8 30 May 32.7 21.2 27.2 23.2 7.7
9 6 June 39.0 21.9 29.9 23.9 1.0

1977

1 20 March 37.1 18.3 23.6 20.3 1.45
2 27 March 37.5 15.8 23.3 19.5 .41
3 3 April 40.8 18.3 24.9 20.2 0
4 10 April 41.5 15.2 24.8 19.4 .71
5 17 April 44.3 15.7 24.0 18.8 0
6 24 April 44.8 19.0 25.8 20.9 .2
7 1 May 41.4 15 25.3 19.4 1.6
8 17 May 35.2 18.2 29.1 21.8 .58
9 29 May 44.8 20.4 30.3 24.1 .58








Greenhouse Experiment. Competition of Soybeans and Beggarweed at Three
Levels of Moisture

Weekly maximum, minimum, and average temperatures are found in

Table 7. Frequent wilting of both soybean and beggarweed plants occurred.

Where soybean and beggarweed were growing together, soybean appeared to

suffer more severe wilting and a slower recovery than beggarweed.

Possibly this is because beggarweed was growing in the shade of the

soybean canopy where the heat load was lower.

Means of growth characteristics are given in Table 8. All soybean

growth characteristics except soybean height were significantly

depressed by beggarweed and soybean growing in competition compared to

growing alone.

Table 9 shows that soybean pod weight, dry weight, and height as

well as beggarweed height increased significantly and linearly with water

level. The model fit (R2) was not improved by using quadratic effects

of water, and effect of water showed no significant quadratic trends.

Furthermore, no interaction effects were indicated between weed and

water treatment on the growth of soybean dry weight and pod weight. At

water treatments x, 2x, and 3x, the presence of beggarweed significantly

reduced the pod weight of soybean by 35, 45, and 41%, respectively,

and significantly reduced dry weight by 32, 20, and 36%, respectively.

Soybean height was not significantly affected by the presence of

beggarweed, but it was significantly increased by water level (Table 9).

Beggarweed height, on the other hand, was significantly depressed by

the presence of soybean, but was not affected significantly by water

treatments. Apparently light was more limiting than moisture, and

soybean shading was more important in depressing weed height than compe-

tition for soil moisture.








Table 7. Mean and absolute weekly greenhouse temperatures observed
during the greenhouse study.


Week


May 19-25

May 26-June 1

June 2-8

June 9-15

June 16-22

June 23-29

June 30-July 6

July 7-13

July 14-20

July 21-27

July 28-August 3

August 4-10

August 11-17

August 18-24

August 25-27


Max

38

38

39

40

41

38

38

38

43

41

38

40

40

37

37


--------


Temperature
Mean

35

35

37

38

37

36

34

37

38

38

37

38

37

37

36


Mean

17

19

20

19

18

20

19

19

18

19

21

21

20

20

20








Table 8. Means
grown


of growth characteristics of beggarweed and soybean when
alone and in competition at three moisture levels.


Beggarweed Soybean
Dry wt. Height Dry wt. Pod wt. Height
(g/pot) (cm) (g/pot) (g/pot) (cm)

Water 1x

Soybean -- 40.5a 6.9a 129.9a

Beggarweed 15.2aa 131.6a

In competition 7.3b 69.9b 27.4b 4.5b 118.9a

Water 2x

Soybean -- -- 44.9a 9.5a 138.1a

Beggarweed 20.8a 179.7a

In competition 9.5b 63.6b 36.2b 5.2b 144.2a

Water 3x

Soybean -- 60.4a 11.4a 148.1a

Beggarweed 33.7a 201.6a

In competition 16.9b 89.2b 38.9b 6.8b 135.0a


a Within a column
a common letter
of probability.


of any one water treatment, any
are not significantly different


two means having
at the 5% level








Table 9. Analysis of variance of
acteristics grown alone
of water.


soybean and beggarweed growth char-
and in competition at three levels


df PR>F R2


Soybean Pod Weight
Model
Block
Water (W)
Beggarweed (B:
Wx B

Soybean Dry Wt.
Model
Block
Water (W)
Beggarweed (B:
Wx B

Soybean Height
Model
Block
Water (W)
Beggarweed (B'
Wx B

Beggarweed Dry Weight
Model
Block
Water (W)
Soybean (S)
Wx B

Beggarweed Height
Model
Block
Water (W)
Soybean (S)
Wx S


.0007
.0032
.0214
.0002
.5206


.0008
.0042
.0079
.0007
.3468


.051
.089
.0267
.2443
.3378


.0064
.1015
.0072
.0026
.4893


.0094
.0414
.2617
.0007
.5350


.789





.784






.583






.724






.706








Combined over water treatments, the height increases of soybean

and beggarweed at the early stages of growth are shown in Figure 15.

Soybean height increased rapidly between the time of seeding and 22

days after seeding and more rapidly between 22 and 52 days after seeding.

Compared to soybean, beggarweed height increases were small until 35

days after seeding, after which height increased rapidly. In growth

chamber experiments, Frazee and Stroller (60) found that soybean grew

more rapidly than seven weed species tested, but when soybeans reached

15 cm in height, the growth rate of weeds exceeded that of soybeans.

In this greenhouse study, the growth rate of beggarweed did not equal

that of soybean until soybeans were 40 cm high. The slow rate of

beggarweed growth during the first 35 days after seeding suggests that

soybeans can successfully compete if a dense canopy of leaves can be

attained by means of narrow rows and close plant spacing.

The heights of shaded beggarweed increased more slowly and dif-

ferences in height were evident as early as 22 days after seeding. The

height of soybeans growing alone were only slightly higher than those

grown in competition with beggarweed.

My data suggest that large soybean yield losses are caused by

competing beggarweed populations. As beggarweed plant heights were

only half those of soybeans (Table 8), shading was not considered a

significant factor in reducing soybean pod or dry weight. Beggarweed

and soybeans were competing for either moisture or nutrients. As pots

were liberally fertilized, soybean pod and dry weight reductions resulted

primarily from competition for soil moisture. That weed competition

for moisture is a significant factor in reducing crop yields is also

recognized by the studies of other investigators (14, 132, 164).









* Soybeans alone.

o Soybeans with beggarweed competition.

o Beggarweed alone

* Beggarweed with soybean competition.


100I


751-


E


50
4,
4,
0r


25 0





0


10 20 30 40

Days After Seeding


Figure 15. Height of soybean and beggarweed plants when grown alone
and in competition independent of water level.


50 60








Because of the physical restriction of the pots on root growth,

this experiment does not consider the possibility that under field

conditions soybean and beggarweed roots may feed at different soil

depths or at different lateral distances from the row. According to

Bohm et al. (16), soybean roots can reach depths up to 90 cm and can

extend laterally beyond 25 cm. The highest root density, however,

occurs in the upper 15 cm of the soil. Information concerning density

and distribution of Florida beggarweed roots is nonexistent.



Field Experiment 1. Competition of Florida Beggarweed with Peanuts

Figure 16 shows that 1976 rainfall was not uniformly distributed

during the period between seeding and harvesting. Rainfall amount was

low at five to six weeks after planting when pegging was initiating and

at ten weeks after planting during the peak pegging period. Rainfall

was also meager 13 weeks after planting during pod-filling.

An excellent stand of peanuts was established. A combination of

seeding and transplanting produced the desired stand of beggarweed.

Beggarweed seed sown three and six weeks after peanut establishment

emerged poorly, probably because of poor soil moisture near the soil

surface. Also, the mechanically scarified seeds may have been damaged.

Those seedlings which did emerge under the peanut canopy were later

observed to have died because of insect and disease damage or because

of the smothering effect of the peanut canopy. Consequently, mean-

ingful data are available only from plots in which beggarweed emerged

with peanuts. Beggarweed seedlings which emerge in the peanut rows

before three weeks after peanut emergence may not cause a significant

competition problem. Hauser et al. (80) found that when beggarweeds



















16


14


12


10 -


8 -
4-
6


4


2



0 14 28 42 56 70 84 98 112

Days After Planting




Figure 16. Precipitation observed during the 1976 study period
at Green Acres. (Average values for 7-day intervals.)








emerged two to four weeks after peanut emergence, near-normal yields

resulted, which indicated that the canopy of peanut leaves effectively

suppressed the weeds.

Table 10 shows that stem or plant density gave the best correla-

tion of the linear regression model examined. Other studies have shown

that dry weight was a better estimator than weed density of yield reduc-

tion (69, 173). Because of the strong correlation between yield reduc-

tion and weed density, there is the possibility of using plant or stem

density as a predictor of yield and economic losses from beggarweed

competition. The linear plant and stem density models indicate that

one plant or stem per square meter reduced peanut yields by 84 or 83

kg per hectare, respectively. No peanut-weed data are available for

comparison.

The linear dry weight model in Table 10 indicates that 5.4 kg of

dry weed weight reduced peanut yields 1.0 kg. This is a large ratio

and indicates that peanuts may tolerate weeds better than other crops.

Working with soybeans and sicklepod, Thurlow and Buchanan (173) reported

the weed/crop weight ratio was between 2.6 and 3.7. Other investigators

(98, 122) have reported ratios between 1.0 and 1.3. The large ratio

may also indicate that beggarweed is not a very competitive weed plant

compared to other weeds.

Interestingly, a stem x August height or stem x harvest height

model gave a slightly stronger correlation with yield reductions (Table

10). Table 11 shows that height, weed density, dry weight density,

and LAI are all positively and significantly correlated with one another.

Table 12 reveals that a stem number x height model gave a better esti-

mate of beggarweed dry weight than stem number alone.








Regression of peanut yield on plant (P) and stem density (S),
weed dry weight (D), fresh weight (F), LAI (L), and
August (A) and harvest (H) height variables.


Model

P

S

D

F

L

S+A

SxA

S**+H

SxH

S**+S2

S**+S2+A

D**+D2

D**+D2+A

S**+D+S2+D2+H


Equation

1830-0.00843P

1840-0.00828S

1815-0.1867D

1820-0.0653F

1770-739.8L

1870-0.0078S-1.06A

1840-0.03155SAa

1860-0.0079S-0.328H

1850-0.0459SH

1850-0.0092S+0.071S2

1870-0.0045S-0.074S2-2.28A

1910-0.4154D+0.0468D2

1820-0.6120+0.04998D2+5.86A


of probability,


Table 10.


--


--


--


* and ** denotes significance at the 5 and 1% levels
respectively. a0.03155 reads 0.000155.


R2

.67

.70

.58

.57

.41

.70

.74

.70

.75

.70

.71

.67

.73

.78


PR> F

.0001

.0001

.001

.001

.011

.0007

.0001

.0007

.0001

.0007

.003

.0014

.0019

.0081


CV

11.5

11.0

13.0

13.1

15.5

11.3

10.2

11.4

10.1

11.4

11.8

12.1

11.4

11.3







79












*r- 4-
(o
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u LA

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D* = -K K 4-
0 r-
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00 CD4-' > LO LO 'O C X- CO 4-
0 w .C Ln LOn N- N N CO 0)1

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Table 12. Regression of beggarweed dry weight on beggarweed
stem number (S), harvest height (H), and August
height (A).


Model PR>F R2 CV

S .0003 .64 56.3

S x H .0001 .76 45.6

S + S2 .0002 .75 48.3

S + S2 + H .001 .76 50.1

S + H .0004 .73 50.4

S x A .0001 .81 40.8

S + S2 + A .0005 .79 47.1








The height factor, therefore, may be a measure of shading effect

or greater competition for soil moisture because of both more dry weight

and leaf area, or both. It is known that transpiration is a function

of leaf area. However, part of the strength of the height factor is

probably derived from its significant and positive correlation with

stem number itself (Table 13); that is, the higher the density of

weeds, the taller the plants because of intraspecific competition for

light.

Table 10 shows that a regression model which includes multiple

weed variable gave the highest R2 value (.78) but accounted for only

3 or 4% more variation beyond that of stem number-height model. LAI

gave a comparatively weak correlation with crop yield reduction and

may have resulted from a lack of precision in measuring LAI.

Obtaining precise LAI values with the technique used was difficult.

Leaves were turgid and horizontal in the morning but as the sun ap-

proached its zenith the leaves became droopy and presented less hori-

zontal surface area. Hence, between taking the first and last

measurement, true LAI values were progressively underestimated. On the

other hand, the highest LAI measured was only .55 (Table 14) and the

highest value recorded was .80. This suggests that the beggarweed plant

does not shade the crop as much as one might expect from observing

the crop-weed height difference at crop maturity. Competition for

soil moisture may be equally, or more, important than competition for

light.

Increasing weed weight and density decreased percentage of large

and shriveled seed and increased percentage of small and medium seed

(Table 13). These changes were significantly correlated with dry weight







82










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Table 14.


Means of yield, yield components, and growth characteristics
of peanuts and beggarweed growth characteristics at different
beggarweed densities.


Characteristics Beggarweed density (plants/m2)
0 1.2 2.2 4.6 7.9 x


Peanuts

Unshelled yield (kg/ha)
Shelled yield (kg/ha)
Shelling %
% large seed
% medium seed
% small seed
% shrivelled seed
August height (cm)
September height (cm)
Foliage dry wt. (kg/ha)

Beggarweed

Stems/m2
Dry weight (kg/ha)
Fresh weight (kg/ha)
LAI
August height (cm)
September height (cm)
Harvest height (cm)


2295
1810
79.0
17.6
71.8
3.1
7.5
40.8
40.4
1370


2200
1730
79.2
13.7
74.3
4.6
7.5
42.6
45.9
1520


1.3
620
1880
.15
48.5
79.8
139


2210
1680
77.7
13.0
74.9
4.9
7.1
44.5
43.6
1530


2.3
950
2880
.22
47.8
78.6
135


1830
1440
78.3
13.4
73.8
5.0
7.3
50.1
51.3
1600


4.8
2620
7440
.55
61.7
94.7
153


1460
1150
78.8
12.6
75.5
5.0
6.9
48.0
50..3
1480


8.3
2590
7300
.49
52.0
84.4
142


2000
1560
78.6
14.1
74.1
4.5
7.2
45.2
46.3
1490


4.2
1700
4870
.35
52.5
84.4
142








per plant and all the height variables. The slightly stronger correla-

tion with the height variables suggests a shading effect. Possibly

shading was reducing photosynthetic rates and thereby decreasing seed

size. Using shade frames An (3) revealed that temporary shading of

peanuts during the pod-filling and maturing stages slightly decreased

the dry weight of mature pods but did not affect the rate of pod fil-

ling. However, moisture stress during the same stages would no doubt

have a similar effect on pod-filling.

Table 13 also indicates that increasing weed weight, density, and

LAI significantly increased the height of peanuts. An (3) found that

shading peanuts increased peanut height. Weed factors also increased

the weight of peanut foliage but had little or no effect on shelling

percentage.

Table 15 reveals that all weed and peanut variables regressed on

beggarweed dry weight were highly significant. Beggarweed LAI and

peanut height showed essentially linear responses to increasing weed

dry matter. Height of beggarweed demonstrated a strong cubic response

to dry weight of beggarweed. Peanut yield response was best character-

ized by a curvilinear or linear response. Possibly the curvilinear

responses were due to intraspecific competition at the higher weed

densities where the weeds were increasingly sharing the competitive

forces.

Cubic, quadratic, and linear response models relating stem density

to peanut and beggarweed characteristics are given in Table 10 and 15.

The relationship between peanut yield and stem density were described

equally well by all models. Peanut height and beggarweed dry weight

and LAI were described better by a curvilinear than linear model. Peanut

height responded more strongly to high weed densities than low densities.








Table 15.


Regression of beggarweed stem and dry weight density on LAI
and beggarweed height, and regression of beggarweed stem
density on beggarweed dry weight density at Green Acres.


Model Equation R2 PR>F CV

Harvest height


D

D**+D2**

D**+D2**+D3**

S
S**+S2**

S**+S2**+S3**


S
S**+S2*

S**+S2*+S3


S

S*+S2.07

S**+S.08+S3

D

D**+D2-09

D**+D2+D3


S
S**+S2


65.4+0.03550

39+0.1D-0.042D2
a
16+0.250-0.0313D2+0.063D3

75 +0.02115S

28.5+0.0057S-0.062S2

7.62+0.011S-0.062S2+0.061S3

Dry weight

282+0.0322S2

-205+0.0796S-0.0655S2

-90+0.0519S+0.0638S2-0.08S3

LAI

0.086+0.05587S

.024+0.04165S-0.08S2-0.08S2

-0.1+0.0413S-0.08S2-0.08S3

.01+032D

0.049+0.031D+0.073D2

0.04+0.03154D2-0.071D2+0.08D3

Peanut height

42+0.04946S

40+0.03316S-0.08S2


* and ** denote
respectively.


significance at the 5 and 1% levels of
a0.042 reads 0.00002.


probability,


.003

.001

.0001

.03

.0003

.0001


.0003

.0002

.0009


.005

.004

.01

.0001

.0001

.0001








As weed population density increased, dry weight per hectare and LAI

became asymptotic. Beggarweed height probably remained relatively

similar at low densities but at higher densities intraspecific compe-

tition caused height to increase. At the highest densities, height

tended to level off.

Comparing August and September stem heights of beggarweed and

peanut plants (Table 14) reveals beggarweed plants did not emerge

from the peanut canopy until about two months after peanut and beggar-

weed seedling emergence. "Late season broadleaf weeds in peanuts"

have been thought to be those beggarweed plants which emerged after

the crop is laid by. This is probably an illusion created by the slow

growth of beggarweed within the peanut foliage during the first half

of the season. During the ten days between August 23 and September 2,

beggarweed height increased 24 cm. If one assumes that high rate of

growth from the time of emergence, then the logical and false conclu-

sion is that these weeds emerged late in the season. Hauser et al.

(80) also explained that "late season weeds" probably emerge with and

shortly after the crop but go undetected in the dense foliage until

they emerge above the peanut canopy.

Not only were peanut yields decreased by Florida beggarweed compet-

ing for light and moisture, but the weed canopy appeared to interfere

with the application of fungicide for the control of Cercospora leaf

spot by screening the peanuts from the spray. The densest canopies of

beggarweed appeared to have the highest incidence of the disease. How-

ever, the severity of disease in these plots may also have resulted

from the weed canopies' modifying the peanut microclimate (lower tempera-

tures, higher humidity, etc.) which favored the disease.








Field Experiment 2. Competition of Beggarweed With Soybeans at Green
Acres

Soybeans were planted at the same time as peanuts so that Figure 16

also serves to illustrate the rainfall distribution and amount for the

soybean season.

Excellent stands of beggarweed were established; however, signifi-

cant soil variation was evident by the rolling appearance of the canopy

and was most likely because of residual effects from previous experi-

ments in the study area.

Mean, minimum, and maximum values of soybean and beggarweed growth

characteristics are given in Table 16. Generally weed populations were

low, the mean being less than one beggarweed stem per square meter.

Using various weed variables, different models were tested to

describe and predict the effects of beggarweed competition on soybean

yield reductions (Table 17). Little differences existed between linear

plant and stem number models because stem and plant density were similar.

A linear dry weight model accounted for more of the yield reduction

than a linear stem density model. Thurlow and Buchanan (173) have

also reported dry weight a better estimator than stem number of soybean

yield reductions. A stem density quadratic model was superior to the

linear dry weight model. The quadratic model indicates that decreases

in yield were less with each added weed unit.

A stem density + weed height model accounted for more of the soy-

bean loss than stem density alone or stem density height. The height

factor probably measures a shading stress on the plant. It may also

reflect greater LAI and, therefore, water use per plant, and hence,

greater competition for water when that resource is limited.




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