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This is to certify that the
thesis entitled

Horseweed [Conyza canadensis (L.) Cronq.] Management

 

in No-Tillage Soybean Production

presented by

Joseph Alan Bruce

has been accepted towards fulfillment
of the requirements for

 

 

M.S. degree in Crop and Soil Sciences

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HORSEWEED [Comm madam} (L.) Cman MANAGEMENT

IN NO-TILLAGE SOYBEAN PRODUCTION

By

Joseph A. Bruce

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Crop and Soil Sciences

1989

(900l4b4'

ABSTRACT

HORSEWEED [Conym oanademir (L.) Croan MANAGEMENT
IN NO-TILLAGE SOYBEAN PRODUCTION
By

Joseph A. Bruce

Research was conducted in Michigan in 1986, 1987 and 1988 to identify consistent
and effective herbicide programs for control of horseweed in no-tillage soybean production.
Horseweed densities as low as 13 plants/m2 significantly reduced soybean yield by 29% as
compared to a weed-free environment. Preemergence application of paraquat (0.56 kg/ ha)
plus metolachlor (2.24 kg/ ha) plus linuron (0.84 kg/ ha) plus nonlionic surfactant (0.25%
v/v) provided less than 60% horseweed control. Early pre-plant applications of glyphosate
(0.84 kg/ha), 2,4-D ester (0.56 kg/ha), HOE-39866 (0.84 kg/ha) or BAS-514 (0.07 kg/ha)
provided greater than 95% horseweed control when followed by the preemergence
application of paraquat plus linuron plus metolachlor plus surfactant. The substitution of
glyphosate or HOE-39866 for paraquat in the above preemergence treatment significantly
improved horseweed control. Horseweed control improved significantly when metribuzin,
metribuzin plus chlorimuron (10:1) or linuron plus chlorimuron (16:1) was substituted for
linuron in the preemergence treatment containing paraquat. Postemergence application of
selective foliar herbicides and ropewick application of glyphosate did not provide consistent
and effective horseweed control. Nomenclature: glyphosate, N-(phosphonomethyl)glycine;
2,4-D, (2,4-dichlorophenoxy)acetic acid; HOE-39866, ammonium-(3-amino-3-carboxy-
propyl)-methyl-phosphinate; BAS—S 14, 3,7-dichloro-8-quinoline carboxylic acid; paraquat, 1-

1’-dimethyl—4-4’-bipyridinium ion;linuron,N-(3,4-dichlorophenyl)~N-methoxy-N-methylurea;
metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl-N—(2-methoxy-l-methylethyl)acetamide;
metribuzin, 4-amino-6-(1,1-dimethyethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one;
chlorimuron, 2-[[[[[4-chloro-6-methoxy-2-pyrimidinyl]amino]mrbonyl]amino]sulfonyl] ben-
zoic acid; horseweed, Corryza canadensis (I...) Cronq.; soybean, Glycine max (L.) Merr.

ACKNOWLEDGEMENTS

I would like to thank my graduate committee, Dr. Jim Kells, Dr. Don Penner, Dr.
Alan Putnam and Dr. Stephen Stephenson for their contributions to this research. I wish
to sincerely thank my major advisor, Jim Kells, for his valuable ideas and assistance during
this project. A very special thanks to Geoff List for his valuable assistance in the field.
Without his help, this project would not have been nearly as successful. Thanks to Jim, Rich
Zollinger and especially Geoff and Jay Schmidt for their friendship and ability to make the
long days a little shorter.

A very special thank you goes to my wife, Kathy, for her assistance, patience and

love which made life more enjoyable.

iv

TABLE OF CONTENTS

PAGE

LIST OF TABLES ................................................. vii
CHAPTER 1: REVIEW OF LITERATURE

NO-‘TILLAGE SOYBEAN PRODUCTION ........................... 1

Introduction .............................................. 1

Adaptions for No-tillage Soybeans .............................. 3

Weed Control in N o-tillage Soybean Production ................... 5

HORSEWEED BIOLOGY ....................................... 9

Introduction .............................................. 9

Life Cycle .............................................. 10

Allelopathy .............................................. 12

Ecology and Competition ................................... 12

Horseweed in No-tillage Production ........................... 14

CHEMICAL CONTROL OF HORSEWEED IN NO-TILLAGE SOYBEANS 15

Introduction ............................................. 15

Early Preplant Herbicide Applications .......................... 16

Preemergence Herbicide Applications .......................... 18

Postemergence Horseweed Control ............................ 19

BIBLIOGRAPHY ............................................. 20

CHAPTER 2: HORSEWEED [Carma canadensis (L.) Cronq.]
INTERFERENCE IN NO-TILLAGE SOYBEAN PRODUCTION

ABSTRACT ................................................. 25
INTRODUCTION ............................................ 27
MATERIALS AND METHODS .................................. 28
RESULTS AND DISCUSSION ................................... 32
BIBLIOGRAPHY ............................................. 39

TABLE OF CONTENTS (cont’d)

PAGE
CHAPTER 3: EARLY PREPLANT AND PREEMERGENCE CONTROL
OF HORSEWEED [Conyza canadensis (L.) Cronq.] IN
NO-TILLAGE SOYBEANS
ABSTRACT ................................................. 41
INTRODUCTION ............................................ 43
MATERIALS AND METHODS .................................. 44
RESULTS AND DISCUSSION ................................... 47
Early Preplant Applications ................................. 47
Preemergence Applications .................................. 52
Sequential Applications .................................... 56
BIBLIOGRAPHY ............................................. 59
CHAPTER 4: HORSEWEED [Conyza canadensis (L.) Cronq.] CONTROL
WITH FOLIAR APPLIED HERBICIDES
ABSTRACT ................................................. 61
INTRODUCTION ............................................ 63
MATERIALS AND METHODS .................................. 64
Nonselective Herbicide Applications ........................... 64
Selective Herbicide Applications .............................. 65
Ropewick Application of Glyphosate ........................... 68
RESULTS AND DISCUSSION ................................... 69
Nonselective Herbicide Applications ........................... 69
Selective Herbicide Application ................................ 71
Ropewick Application of Glyphosate ........................... 77
BIBLIOGRAPHY ............................................. 80

vi

TABLE

CHAPTER 2:

CHAPTER 3:

1.

LIST OF TABLES

PAGE
Soybean plant populations for each of three soybean
row spacings ..................................... 29
Herbicide programs and corresponding herbicide rates ...... 31
Total and average monthly rainfall for Cass County,
Michigan for 1986, 1987 and 1988 ..................... 33
Results from 1987 horseweed interference research
conducted in Cass County, Michigan ................... 34
Results from 1988 horseweed interference research
conducted in Cass County, Michigan ................... 35
Environmental conditions for early preplant and
preemergence application of herbicides ................. 46
Total and average monthly rainfall in Cass County,
Michigan for 1986, 1987 and 1988. ..................... 48
Early season horseweed control, horseweed density,
soybean injury and late season horseweed control as
affected by early preplant herbicide application ........... 49
Early season horseweed control, horseweed density,
soybean injury and late season horseweed control as
affected by preemergence herbicide application ........... 53
Early season horseweed control, horseweed density,
soybean injury and late season horseweed control as
affected by preemergence application of soil active
herbicides ....................................... 55
Comparison of single preemergence and sequential
application of soil active herbicides for control
of horseweed ..................................... 58

vii

LIST OF TABLES (cont’d)

TABLE

CHAPTER 4:

1.

Environmental conditions for nonselective foliar

herbicide applications in 1987 and 1988 ...........

Environmental conditions for selective foliar

herbicide applications in 1987 and 1988 ...........

Horseweed control provided by nonselective foliar
applied herbicides as affected by horseweed height

at application ..............................

Total and average monthly rainfall for the
nonselective herbicide research conducted in

1987 and 1988 ..............................

Total and average monthly rainfall for selective
foliar herbicide research conducted in 1987 and

1988, Shiawassee County ......................

Horseweed control provided by selective foliar
applied herbicides as affected by horseweed height

at application ..............................

Ropewick application of glyphosate for controlling

horseweed and reducing flower production in 1987 . . .

viii

PAGE

...... 66

...... 67

...... 70

...... 72

...... 73

...... 74

...... 78

CHAPTER 1
REVIEW OF LITERATURE
NO-TILLAGE SOYBEAN PRODUCTION

Soil tillage has played an important role in crop production. The primary reason
for its use has been weed control. Spring tillage will destroy existing vegetation and create
an even-start condition for both crop and weed seeds (Staniforth and Wiese, 1985). Tillage
has also been important for incorporation of herbicides, fertilizers and previous crop
residue, control of insects and diseases, as well as soil aeration and removal of previous crop
residue (Phillips, 1984). Crop producers generally believe that a well prepared seedbed is
necessary to promote rapid crop seed germination.

No-tillage crop production, often called no-till or zero-till, is the production of crops
without the use of tillage prior to planting. This production method has become increasing
popular during the past decade. Michigan full season no-tillage soybean production
increased from 405 hectares in 1978 to 27,900 hectares in 1988 according to Quisenberry
(1988a). National no-tillage soybean production was 4.6 x 10‘ hectares in 1988, a 16.9%
increase since 1987 (Conservation Technology Information Center, 1988).

No-tillage crop production has many advantages and disadvantages as compared to
conventional tillage systems. Phillips and Phillips (1984) noted these advantages:

1. Reduced soil erosion

2. Ability to crop erosive soils

3. Decreased labor requirements (up to 50%)

4. Decreased fuel consumption

5. Decreased equipment costs
Reducing both soil erosion and crop production inputs have become increasingly important
to soybean producers. The United States government recently passed a farm bill, effective
1990, which will require producers to reduce erosion of highly erodible soils to acceptable
levels in order to qualify for governmental financial assistance (Quisenberry, 1988b).
Producers are also faced with lower commodity prices which have forced them to decrease
production inputs to maintain profit levels. These factors are a few of the many reasons
for the increase in no-tillage soybean production.

There are several disadvantages to no-tillage soybean production (Phillips and
Phillips, 1984). No-tillage planting operations are sometimes delayed due to higher soil
moisture content and lower soil temperatures than experienced in conventional tillage
systems. Delayed planting dates can often result in decreased soybean yields. In Michigan,
soybean yields decline approximately 63 kg/ ha per day if planted after May 10 according
to Hesterman er a1. (1987). Incidence of disease, insect and rodent damage are also more
prevalent in no-tillage crop production (Phillips and Phillips, 1984). The large quantity of
crop residue remaining on the soil surface in no-tillage production favors the incidence of
insects and diseases which overwinter in these residues. The use of tillage will bury residues
thus reducing the incidence of insect and disease problems. According to Crosson (1981),
the incidence of soybean insects is not influenced by tillage, however, soybean disease
incidence increases as the amount of tillage is reduced. Weed control is the most important
disadvantage in no-tillage systems. Sanford et al. (1973) has reported weed control as being
the most common deterrent to successful no-tillage soybean production. In the absence of
tillage, producers generally rely entirely upon herbicides to control all weeds. Should

herbicides fail, mechanical row cultivation often provides ineffective weed control due to the

3
firm soil conditions (Richey et al., 1977). Advances in agriculture are helping to solve these
problems. Changes in cultural practices and crop management combined with an increasing
number of herbicides available to the producer are making no-tillage soybean production

a good alternative to conventional tillage production.
Adaptations for No-Tillage Soybeans

N o-tillage soybean production has become possible through modification of planting
and cultural practices to overcome previous crop residue and soil conditions. According to
Sprague and Triplett (1986), conventional tillage systems may have only 2-5% soil surface
coverage by crop residue the spring following soybeans or corn. In no—tillage, however, crop
residues may cover 60-80% of the soil surface the following spring. The absence of tillage
also increases the soil water content (Thomas, 1986; Phillips and Phillips, 1984; Unger and
McCalla, 1980). This is attributed in part to increased water infiltration due to improved
soil structure and increased soil porosity (Triplett et at. 1968). Thomas (1986) reported
that increased plant residue acts as a barrier which prevents diffusion of water vapor from
the soil. This residue also reflects more incoming light than bare soil, resulting in decreased
soil temperatures and reduced evaporation of water. Through different planting and
cultural methods, several of these obstacles have been overcome.

Planting equipment has been modified by adding coulters which effectively cut
through crop residue. The coulters slice through residue and at the same time loosen the
soil. The planting unit follows in the same path and places the soybean seed in the loosened
soil. The seeds are then covered and packed firmly by specially designed press wheels to

obtain good seed to soil contact.

4

The incidence of disease damage is more prevalent in no-tillage soybean
production. The use of crop rotation combined with disease resistant soybean varieties can
help overcome these problems. In a 20 year comparison of no-tillage and conventional till
soybeans, Dick and Van Doren (1977) found a higher incidence of phytophthora root rot
and lower grain yields in no-tillage soybeans than in conventional fill The use of resistant
soybean varieties combined with crop rotation resulted in equivalent soybean grain yields
for the two tillage systems.

A common adaptation in no-tillage soybean production is the shift to narrower
soybean row spacings. Research has proven that reducing row spacing from 102 cm to 51
cm or less will improve soybean grain yields (Wax and Pendleton, 1968; Lehman and
Lambert, 1960; Peters et al., 1965; Burnside and Colville, 1963). When compared to 102 cm
rows, Burnside and Colville (1963) found a 39, 17 and 5% increase in soybean grain yield
for 25, 51 and 76-cm row spacings, respectively. Wax and Pendleton (1968) reported a
similar soybean yield trend. Wax and Pendleton (1968) as well as Peters et al. (1965)
observed greater weed control when soybeans were planted in narrow row spacing.
Increased soybean grain yield and weed control provides very strong incentives for no-tillage
soybean producers to switch to narrow row soybean production.

Yield potential is a very important factor influencing the adoption of no-tillage
soybean production. Producers will not utilize no-tillage production if soybean yields are
inferior to those obtained in conventional tillage systems. Current research has shown that
no-tillage soybean production will produce equivalent or greater soybean yields under
certain conditions. Soil type appears to be very critical. According to Dick and Van Doren
(1985) and Unger and McCalla (1980), no-tillage soybean generally have lower yields than
conventional tillage soybeans when planted in soils with high water holding capacity. No-

tillage has produced equal or greater soybean yield than conventional till in soils with low

5

to moderate water holding capacity and during years of low rainfall (Unger and McCalla,
1980; Dick and Van Doren, 1985; Tyler and Overton, 1982; Edwards et al., 1988). Tyler
and Overton ( 1982) also reported greater soybean seed quality from no-tillage than
conventional tillage systems during a hot dry year.

The most common deterrent to successful no-tillage soybean production is weed
control (Sanford et al. 1973). Kapusta (1979) found that in two out of three years, method
of tillage did not influence soybean yields. However, this was only true when similar

acceptable weed control was obtained.

Weed Control in No-tillage Soybean Production

Weed control is an essential component of any production system. Conventional
tillage utilizes soil disturbance to control existing spring vegetation prior to crop planting.
After crop emergence, row cultivation is often used to remove weeds not effectively
controlled by herbicides. In no-tillage production, producers must rely upon a combination
of cultural methods and herbicides to eflectively control weeds.

According to Triplett et al. (1964), an effective no-tillage herbicide program must
incorporate the following concepts or elements:

1. Obtain complete control of all existing vegetation prior to soybean planting
Exhibit growth suppression of annual and perennial weed seedings
Induce no injury to the present crop

Induce no injury to the succeeding crop

9:559!"

Be competitive in cost with alternative weed control techniques
Through the use of cultural and chemical weed control, no-tillage soybean weed control has

become increasingly successful.

6

Cultural weed control in no-tillage consists of planting optimum soybean populations
and utilizing narrow row spacings to maximize soybean competition with weeds. McWhorter
and Barrentine (1975) observed increased weed control as soybean populations increased
from 80,000 to 350,000 plants/ ha. Reducing soybean row spacing reduces weed dry weight.
Burnside and Colville (1963) reported average total weed dry weights of 190, 190, 314 and
347 kg/ha in 25, 51, 76 and 102-cm soybean row spacings. The time period for canopy
closure was 36, 47, 58 and 67 days after planting for ‘Ford’ soybeans in 25, 51, 76 and 102-
cm rows, respectively. Wax and Pendleton (1968) reported canopy closure for ‘Wayne’
soybeans in 35, 50, 65 and 80 days when planted in 25, 51, 76 and 102-cm row spacings,
respectively. Rapid canopy development shades the soil surface and thus inhibits weed
germination and growth of established weed seedlings. The use of these cultural practices
can further enhance herbicidal effectiveness.

Effective season-long weed control in no-tillage soybean production requires the use
of several herbicide types. Fawcett (1983) and Fawcett et al. (1983) stated that control of
all established vegetation prior to soybean emergence is essential. A nonselective foliar
active herbicide such as glyphosate [N-(phOsphonomethyl) glycine] or paraquat [1-1’-
dimethyl-4-4’-bipyridinium ion] is most commonly used. Nonselective herbicides are usually
applied after planting but prior to soybean emergence. This preemergence (PRE) approach
generally utilizes soil active herbicides in combination with the nonselective herbicide. Soil
active herbicides are used to provide control of germinating weed seedlings for an extended
time period. The PRE herbicide program provides effective season-long weed control.
Since the soil active herbicides are applied to the soil surface, rainfall is required for moving
the herbicide into the soil for effective weed control (Fawcett, 1983; Fawcett et al., 1983).
Despite the risk of inadequate rainfall, PRE herbicide programs are the standard weed
control programs used by no-tillage crop producers (Kapusta, 1979).

7

The early preplant (EPP) herbicide progam was developed to overcome the
disadvantages of preemergence applications (Fawcett, 1983; F awcett et al., 1983). In this
progam (EPP), soil active herbicides are applied prior to weed seed germination. Early
applications reduce the risk of herbicide failure due to dry weather since there is a geater
probability of rain prior to weed seed germination. This approach usually eliminates the
need for a nonselective postemergence herbicide. Possible causes of reduced weed control
in this approach are: (1) herbicide degadation prior to crop planting, and (2) disruption
of the herbicide layer by planting operations. , To overcome these shortcomings, sequential
or split herbicide applications were used. In this approach a portion of the total herbicide
rate is applied EPP followed by a PRE application of the remaining portion of herbicide
after crop planting (Fawcett, 1983; Fawcett et al., 1983).

Postemergence herbicide programs are also utilized to provide effective season-
long weed control. A total postemergence weed control program utilizes a nonselective
herbicide to destroy all existing vegetation prior to planting. Approximately four to six
weeks after planting, selective postemergence herbicides are applied for control of existing
broadleaf and grassy weed species. Selective postemergence herbicides are commonly used
to control weeds when soil active herbicides fail to provide effective weed control.

Following several years of no-tillage production, researchers have seen changes in
the weed species composition of a field. Conventional tillage systems contain predominately
annual gasses and broadleaves. However, in the absence of tillage, populations of annual
gasses and perennial weeds often increase (Phillips and Phillips, 1984; Buhler and Oplinger,
1989; Staniforth and Wiese, 1985; Triplett and Lytle, 1972). No-tillage soybean production
has its own unique shift in weed composition. Horseweed [Conyza canadensis' (L.) Cronq.]
has become a serious problem in no-tillage soybean production. Kapusta (1979) reported

horseweed in no-tillage soybeans at populations of 24,000, 12,000 and 96,000 plants/ ha over

8
a three-year period compared to no horseweed in areas receiving tillage. He also observed
a high density of horseweed in a first year no-tillage field following 20 consecutive years of
conventional tillage cropping. Brown and Whitwell (1988) and Elmore and Heatherly (1983)
have also reported horseweed populations in the absence of tillage but a shallow disking
eliminated the weed.

HORSEWEED BIOLOGY

Horseweed [Conyza canadensir (L.) Cronq.] is commonly referred to as marestaiL
This species, originally identified by Linnaeus as Efigeron canadensis (L.), is referred to by
both names in the literature. This species has been identified as having three different
varieties, ‘canadensis’ (most common), ‘pusilla’ and ‘glabrata’ according to Cronquist (1947).

This annual composite normally has a stout unbranched erect stem reaching heights
of 0.3-1.8 m. The stem is also covered with bristly hairs. Horseweed plants have linear
leaves which lack petioles. The leaves have toothed or entire margins with coarse white
bristly hair on the leaf surface. The plant produces numerous small geenish or pinkish
white flowers in axillary panicles with a narrow pointed bract at the base of each head.
Flowers produce very small (1.5 mm) seeds which are attached to a pappus (Anonymous,
1981). Horseweed is commonly found in pastures, roadsides, waste areas and undisturbed
or abandoned agricultural fields (Regehr and Bazzaz, 1979; Hopkins and Wilson, 1974;
Anonymous, 1981; Cronquist, 1980; Brown and Whitwell, 1988; Kapusta, 1979). The
horseweed variety ‘canadensis’ has a hairy stem and geen involucre bracts (Gleason and
Cronquist, 1963). This common variety is found in all portions of the United States
according to Cronquist (1980). The variety ‘pusilla’ is found along the eastern coastal area
from Connecticut to tropical America. The variety has a nearly glabrous stem and purple-

tipped involucre bracts (Gleason and Cronquist, 1963).

Life Cycle

The horseweed life cycle is that of a winter and summer annual. Keever (1950)
reported fall germination of horseweed rosettes. These rosettes overwintered and then
"bolted" the following spring as temperatures and daylength increased. This life cycle made
horseweed very competitive since rosettes would bolt prior to the establishment of other
weeds.

Regehr and Bazzaz ( 1979) observed horseweed germination from August to October
in Illinois. In Massachusetts, Bekech (1988) reported horseweed germination from August
to September beneath crop canopies. Shontz and Costing (1970) reported no difference in
fall germination with horseweed in sandy, heavy or peat soils. In North Carolina, he found
horseweed most prevalent in fields with a low sand content. Hanf, (unknown) however,
reported horseweed that prefer stony, sandy or loamy soils.

Apparently, newly produced seed are viable at the time of dissemination (Shontz
and Oosting, 1970). Germination rates are higher after periods with high soil moisture
(Regehr and Bazzaz, 1979). Seed placement in the soil profile also geatly effects
germination. Tremmel and Peterson (1983) saw a 94% decline in germination from seeds
planted at a 1 cm depth compared to surface planting. Shontz and Oosting (1979) reported
similar results. Eighty percent of the horseweed seeds that can germinate were located in
the top 2 cm of the soil profile according to Bekech (1988). Field plots fumigated in the
spring to destroy all seed reserves had significantly fewer horseweed rosettes in December
than non-fumigated plots (Regehr and Bazzaz, 1979). Since the soil seed reserve is a major
contributor to a horseweed population, a stand of horseweed could be established in an area

despite the absence of horseweed for several years.

10

11

Horseweed rosettes have a very low mortality rate (1%) prior to frost according to
Regehr and Bazzaz (1979). Regehr and Bazzaz (1976) observed rosettes accumulating
energy reserves by photosynthesis in cool temperatures which enable rapid spring growth.
Rosette winter mortality rates are quite variable (14 to 84%). The primary cause of
mortality is frost heaving. Mortality is higher among smaller rosette sizes. Plants which
survive the winter experience very low mortality (2.4 to 5.5%) due to partial uprooting from
the winter or competition from other plant species (Regehr and Bazzaz, 1979).

Spring germination of horseweed has also been reported. Regehr and Bazzaz
(1979) observed an average population of 10 plants/m2 germinating in April and May. Of
these, only 36% reached maturity and produced seed without passing through the rosette
stage. Bekech (1988) also noted spring germination; however, residue cover delayed this
germination by four weeks and reduced germination to only 20% of that in bare soil.

Horseweed flowering and seed production occur between July and October,
depending upon location (Hanf, unknown; Shontz and Costing, 1970). A single horseweed
plant may produce as many as 200,000 seeds according to Bekech (1988). Regehr and
Bazzaz (1979) also examined seed production. They found total seed production was
proportional to mature plant height. Reproductive effort, however, was inversely
proportional to plant height. They suggested that maximum plant height is more important
than maximum energy allocation to seed production. Horseweed height is very important
for seed dissemination. The small, light seed and pappus allow it to utilize the wind to
move large distances. Regehr and Bazzaz (1979) reported densities of 126 seeds/m2 at a

distance 122 m downwind of the source.

Allelopathy

Allelopathic substances have been identified in horseweed. Kobayashi et al. (1980)
discovered three Clo-polyacetylene compounds in horseweed. Concentrations of trans- and
cis-matricaria esters and cis-lachnophyllum ester were found in horseweed roots and shoots.
Crude extracts from horseweed showed strong gowth inhibitory effects on common ragweed
Ambrosia anemiriifolia (L.). Raynal and Bazzaz (1975) observed insignificant suppression
of A. artemtrizfolia gowth in the presence of horseweed leachates. Keever (1950) and
Shontz and Oosting (1970) reported reduced horseweed germination or gowth in soil with
horseweed residue present. Shontz and Costing (1970) suspect horseweed allelopathy to
cause reduced seed germination of Haplopappus divalicatus (Nutt.). In a field crop situation,
horseweed residue could potentially reduce crop germination, gowth and yield due to the

presence of these allelopathic substances.
Ecology and Competition

Horseweed has a specific light requirement for germination and gowth.
Horseweed seed germination is minimal in the absence of light (Shontz and Oosting, 1970).
Gorski et al. (1977) observed that light quality or intensity did not affect horseweed seed
germination, therefore shading does not account for seasonal variations in germination rate.
Full horseweed germination in the gowth chamber was obtained by either continuous light
exposure or ten minute irradiation one to two days after the onset of dark incubation
(Zinzolker et al. 1985). Four to six days after onset of dark incubation, horseweed seeds
were unresponsive to the short irradiation but full germination was obtained by continuous

irradiation.

12

13

Little information is available on the cause of stem elongation or flowering.
Zinzolker et al. (1985) reported earlier stem elongation and flowering of horseweed rosettes
exposed to long day (16 hr) compared to short day (8 hr) irradiation.

Bekech (1988) examined the effect of light intensity on horseweed gowth and seed
production. She found total dry weight (wt ‘ m‘z) increased significantly from 0.44 to 3.53
kg as light intensity increased from 25% to 100% of full sunlight. Increasing light intensity
also increased horseweed plant height from 92 to 192 cm. Seed production at 25% of full
sunlight was only 4% of the 19 x 10‘ seeds/m2 produced by plants gown in full sunlight.

The effects of competition on horseweed gowth and development have been
addressed. Keever (1950) reported horseweed as a poor fall competitor. Plant residue
combined with germination of other winter annuals decreased horseweed germination and
competitiveness. In an intraspecific environment, Palmblad (1967) reported delayed
flowering and decreased seed production as horseweed densities increased. Horseweed also
exhibited "controlled germination" where germination is inhibited as seed density increased.
This phenomenon appears to be an adaptive mechanism to regulate horseweed density.
Similar results were observed by Bekech (1988). As horseweed density increased, total seed
production per plant decreased significantly from a maximum total seed production of 19
x 10‘ seeds/m2. She also found that inflorescence dry weight decreased from 27% to 12%
and dry weight allocation to stems increased from 51% to 58% as horseweed density
increased from 100 to 400 plants/m2 respectively. Plant density did not affect plant height.

Density dependant mortality has also been examined by Bekech (1988). In naturally
occurring pOpulations, a peak horseweed density of 5960 plants/m2 declined to an average

of 587 plants/m2 at flowering. Mortality was geater in more fertile areas.

Horseweed in No-Tillage Production

Bekech (1988) summarized several reasons for the increased frequency of
horseweed population in no-tillage crop production. The natural environment to which
horseweed is adapted closely resembles the conditions created by no-tillage crop production.
Horseweed germinates in the fall under a crop canopy since germination is not effected by
light intensity. Without tillage, overwintering rosettes are not destroyed. These plants are
then able to compete successfully with spring weeds and crops.

Horseweed is seldom a problem with the use of tillage for two reasons according
to Bekech (1988). First, tillage effectively destroys overwintering rosettes. Secondly, spring
germination of horseweed still occurs, however the low light conditions generated by crop
canopy and weeds effectively reduces horseweed gowth reducing its competitiveness.

Horseweed will persist as a no-tillage crop production problem. No-tillage provides
undisturbed bare soil surfaces which remain plant-free throughout the entire gowing season.
These soil conditions provide a non-competitive environment which is the ideal gowth
environment for horseweed. Continued regeneration of these conditions will lead to the

persistence of horseweed in no-tillage crop production.

14

CHEMICAL CONTROL OF HORSEWEED IN NO-TILLAGE SOYBEANS

In no-tillage crop production, herbicides are utilized to provide effective weed
control. Horseweed has become a severe weed control problem in no-tillage soybeans. This
is due, in part, to ineffective horseweed control from commonly used soybean herbicide
progams. This theory is reinforced by observations that horseweed is not reported as a
serious weed control problem in no-tillage corn production. The commonly used herbicides
in corn, atrazine [6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine], cyanazine
[2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile], sirnazine [6-
chloro-NJV’-diethyl-1,3,5-triazine-2,4-diamine], 2,4-D [2,4-dichlorophenoxy)acetic acid] and
dicamba [3,6-dichloro-2-methoxybenzoic acid] are each reported to effectively control
horseweed (Wilson et al., 1985; Triplett and Lytle, 1972; Keeling and Abernathy, 1988;
Kaufman and Ritter, 1986).

Herbicide resistant biotypes of horseweed have been reported. Atrazine resistant
horseweed were found in Switzerland (Lebaron and Gressel, 1982) and in Hungarain
vineyards (Hartman, 1981; Mikulas and Polos, 1983). Paraquat resistant horseweed has
been reported in Japan by Watanabe et al. (1982) and Kato and Okuda (1983). In all cases,

the herbicides had been applied for several consecutive years.

15

Early Preplant Herbicide Applications

Early preplant (EPP) herbicide applications are made prior to crop planting. This
approach is desigied to control existing vegetation for soil water conservation. EPP
applications reduce the risk of soil active herbicide failure from insufficient rainfall by
geater likelihood of early season rainfall. Generally, an EPP approach will utilize a
nonselective herbicide alone or in combination with soil active herbicides.

Hagood and Davis (1986) obtained excellent horseweed control from EPP
applications of cyanazine in Virginia. Combinations of cyanazine with 2,4-D or paraquat
applied two weeks prior to planting provided geater than 90% horseweed control without
soybean injury in studies conducted by Kaufman and Ritter (1986). Stougaard et al. (1984)
applied cyanazine at 2.2 kg/ ha or geater, four weeks prior to planting and observed geater
than 95% midseason horseweed control without soybean injury in southern Illinois.

Kells (1985) observed significant soybean injury in Michigan when cyanazine (2.2
kg/ha) was applied up to four weeks prior to planting. In Michigan, Kells and List (1986)
examined the use of 2,4-D ester for horseweed control. Rates as low as 0.28 kg/ha when
followed by a preemergence application of paraquat in combination with soil active
herbicides provided geater than 90% control. A similar treatment by McCutchen and
Hayes (1983) obtained excellent horseweed control with 2,4-D ester at 1.1 kg/ ha. In Texas,
Henniger et al. (1989) obtained geater than 80% control of horseweed rosettes with 2,4-
D ester (0.6 kg/ha) and 2,4-D amine (1.12 kg/ha). Effective control of 10-cm tall
horseweed required 1.1 and 2.2 kg/ha of the ester and amine formulations of 2,4-D,
respectively. At heights of 30 cm, 2,4-D did not provide effective horseweed control.

Henniger et al. (1989) also examined several nonselective foliar herbicides.

Paraquat did not provide adequate horseweed control. Glyphosate (0.4 kg/ ha) or

16

17
glyphosate (0.3 kg/ ha) plus 2,4-D (0.5 kg/ ha) provided geater than 80% control of
horseweed rosettes. HOE-39866 [ammonium-(3-amino-3-carboxypropyl)~methyl-phosinate]
(1.1 kg/ ha) was the only herbicide to provide effective control from rosette to 30-cm tall
horseweed.

Based on the observations of Hagood and Davis (1986), horseweed control from
paraquat, glyphosate, HOE-39866 and SC-0224 [trimethylsulfoniumcarboxymethylamino
methylphosphonate] ranged from good to excellent. Kells and List (1986) obtained geater
than 90% midseason horseweed control from glyphosate (0.42 kg/ ha) when followed by a
preemergence application of paraquat in combination with metolachlor [2-chloro-N-(2-ethyl-
6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], linuron [N ’-(3,4-dichlorophenyl)-
N-methoxy-N-methylurea] and surfactant.

In southern Illinois, Kapusta and Krausz (1988) observed pendirnethalin [N-( 1-
ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] in combination with either irnazaquin [2-
[4,5-dihydro—4-methyl-4-(1-methylethyl)-5-oxo-lH-imidazol-Z-yl]-3-quinolinecarboxylicacid]
or imazethapyr [(i)2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol—2-yl]-5-
ethyl-3-pyridinecarboxylic acid] provide some early season control, however by mid-June no
horseweed control was observed. The addition of glyphosate to these combination provided
99% horseweed control. Kapusta (1981) obtained 32% control of 38 to 64 cm tall
horseweed from alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] plus
metribuzin [4-amino—6-(1,1—dimethylethyl)-3-(methylthio)-l,2,4,-triazin-5(4I)-one] (0.42
kg/ha). By combining this mixture with either paraquat (0.28 kg/ha), glyphosate (0.86
kg/ha), HOE-39866 (0.56 kg/ha) horseweed control was at least 96%.

Excellent horseweed control (100%) was obtained from BAS 514 [3,7-dichloro
linecarboxylic acid] (0.28 kg/ ha) applied EPP followed by a tank-mix combination of

paraquat, metolachlor, linuron and surfactant applied preemergence (Kells and List, 1986).

Preemergence Herbicide Applications

Preemergence (PRE) herbicide applications are soil applied after planting but prior
to crop emergence. A PRE herbicide progam most generally consists of a nonselective
foliar herbicide in combination with soil active herbicides. A herbicide progam such as this
is designed to control all weeds in one herbicide application.

Considerable research has been on the use of nonselective herbicides for control
of horseweed. Wilson et al. ( 1985) found HOE-39866 (0.6 kg/ha) or glyphosate (1.7 kg/ha)
provided significantly geater control of 20 to 35-cm and 35 to 90-cm horseweed than
paraquat (0.6 kg/ ha). Considerable horseweed regowth was observed in the paraquat
treatment. Greater horseweed control was observed from HOE-39866 when applied to
taller plants. Application of 800224 (0.6 kg/ ha) provided geater horseweed control than
0.6 kg/ha of glyphosate. Bellinder and Wilson (1983) observed that HOE-39866 provided
horseweed control superior to that obtained from glyphosate or 800224. Horseweed
control decreased with time in plots treated with paraquat or HOE-39866.

Glyphosate (1.1 kg/ha) provided significantly geater control (94%) of 15-46 cm
horseweed than paraquat (58%) at the 0.6 kg/ha rate (Wilson and Worsham, 1988).
According to Kaufman and Ritter (1988), a glyphosate application rate of 1.1 or 1.7 kg/ ha
- was required to provide horseweed control equivalent to 0.84 kg/ ha of HOE-39866.

Kaufman and Ritter (1988) obtained effective control of horseweed with paraquat
when a sequential application of 0.28 kg/ ha applied EPP was followed by another 0.28
kg/ ha applied PRE in combination with linuron.

According to Kells and List (1986), the substitution of HOE-39866 (0.84 kg/ ha) for
paraquat (0.56 kg/ha) as part of a PRE combination with metolachlor and linuron provided

significantly geater horseweed control (84%) than paraquat (38%). When metribuzin

18

19
replaced linuron in combination with paraquat horseweed control improved to 83%.
Kapusta (1979) observed similar results. Kells and List (1986) also reported significantly
less horseweed control from imazaquin or, irnazethapyr (0.14 kg/ ha) as compared to
metribuzin. Slack et al. (1988) observed less than 40% horseweed control from imazaquin
or irnazethapyr in combination with paraquat; however, substitution of glyphosate for
paraquat resulted in at least 90% control. Combinations of chlorimuron [2-[[[[[4-chloro-6-
methoxy-Z-pyrirnidinyl)amino]carbonyl]amino]sulfonyl]benzoic acid] ethyl ester with

glyphosate or paraquat provided geater than 90% horseweed control.
Postemergence Horseweed Control

Information on herbicides for postemergence horseweed control in soybeans is very
limited. Hagood and Davis (1986) report unsatisfactory control from bentazon [3-(1-
methylethyl)-( 1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] or acifluorfen [5-[2-chloro-
4-(trifluoromethyl)phenoxyl-2-nitrobenzoic acid]. Kells and List (1986) also found
inadequate control with acifluorfen (0.56 kg/ha), however, bentazon (1.1 kg/ ha) plus crop
oil concentrate (2.3 L/ha) provided 95 % control of 3 to 30-cm horseweed. Postemergence
applications of metribuzin in carrots [Daucus carota (L.)] provided 98% control of 2.5 to

5 cm tall horseweed at the 0.56 kg/ha rate (Henne, 1978).

BIBLIOGRAPHY

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Publication No. 281, Bulletin 772. University of Illinois at Urbana-Champaign.

Bekech, M. M. 1988. The biology of horseweed [Conyza canadensis (L.) Cronq.].
University of Massachusetts, Amherst. Thesis for the degee of M. S. p. 18-70.

Bellinder, R. R. and H. P. Wilson. 1983. Comparison of several nonselective herbicides
in reduced tillage systems. Proc. Northeast Weed Sci. Soc. 36:51.

Brown, S. M. and T. Whitwell. 1988. Influence of tillage on horseweed, Conyza canadensis.
Weed Tech. 2:269-270.

Buhler, D. D. and E. S. Oplinger. 1989. Influence of tillage systems on weed populations
and control in soybean production. Abst. Weed Sci. Soc. Am. 29:17.

Burnside, O. C. and W. L. Colville. 1963. Soybean and weed yield as affected by irrigation,
row spacing, tillage and amiben. Weeds 12:109-112.

Conservation Technology Information Center. 1988. 1988 National Survey Conservation
Tillage Practices. Conserv. Tech. Info. Ctr., W. Lafayette, IN.

Cronquist, A. 1947. Notes on the compositae of northeastern United States. Bull. Torrey
Bot. Club 74:142-150.

Cronquist, A. 1980. Vagtlag E1913 Qf the Southeastern United Sthtgs. University of North
Carolina Press, Chapel Hill, NC. p. 166.

Crosson. Pierre R 1981 WWW
assessment. Soil Conservation Society of America, Ankey, IA. p. 10- 12.

Dick, W. A. and D. M. Van Doren Jr. 1985. Continuous tillage and rotation combinations
effects on corn, soybean and oat yields. Agon. J. 77:459-465.

Duffy, M. 1983. Pesticide use and practices. 1982. Agricultural Information Bull. No. 462,
Economic Res. Service. 14pp.

Edwards, J. H., D. L. Thurlow and J. T. Eason. 1988. Influence of tillage and crop rotation
on yields of corn, soybeans and wheat. Agon. J. 80:76-80.

Elmore, C. D. and L. G. Heatherly. 1983. Preplant tillage effects on the weed flora in
soybeans. Abst. Weed Sci. Soc. Am. 23:66-67.

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21

Fawcett, R. S. 1983. Weed control in conservation tillage systems. Proc. North Central
Weed Control Conf. 38:67.

Fawcett, R. S., M. D. K. Owen and P. C. Kassel. 1983. Early preplant treatments for weed
control in no-till corn and soybeans. Proc. North Central Weed Control Conf. 38:112-
117.

Gleason, H. A. and A. Cronquist. Manug of Vasculn; Eta anhs gt Ngflhenstegn United States
W. Willard Grant Press, Boston, MA. 1963. p. 734-735.

Gorski, T., K. Gorska and J. Rybicke. 1977. Studies on the germination of seeds under leaf
canopy. Flora 166:249-259.

Hagood, E. S. Jr. and P. H. Davis. 1986. Horseweed control in no-till corn and soybeans.
Proc. Northeast Weed Sci. Soc. 40:25.

Hanf, M. (year unknown). Weed a t e' eed ' . BASF United Kingdom Limited,
Agricultural Division Ipswich, England. p. 237.

Hartman, F. 1981. Resistance of Erigeron canadensir L. against atrazine and spread of
the weed in department Komarom. Novenyvedelem 17:133-137.

Henne, R. C. 1978. Control of horseweed, nutsedge, and other weeds in carrots. Proc.
Northeast Weed Sci. Soc. 32Dz234-238.

Henniger, C. G., J. W. Keeling and J. R. Abernathy. 1989. Horseweed [Conyza canadensis
(L.) Cronq.] control in conservation tillage systems. Abst. Weed Sci. Soc. Am. 19:12.

Hesterman, O.B., J.J. Kells and ML Vitosh. 1987. Producing soybeans in narrow rows.
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Lansing, MI.

Hopkins, W. E. and R. E. Wilson. 1974. Early Oldfield succession on bottomlands of
southeastern Indiana. Castanea 39:57-71.

Kapusta, G. 1979. Seedbed tillage and herbicide influence on soybean (Glycine max) weed
control and yield. Weed Sci. 27:520-526.

Kapusta, G. 1981. Evaluation of HOE 661 for no-till corn and soybean weed control. Res.
Rept. North Central Weed Control Conf. 38:76-77.

Kapusta, G. and R. F. Krausz. 1987. Soybean pendimethalin plus imazaquin or
irnazethapyr no-till EPP. Res. Rept. North Central Weed Control Conf. 44:355-357.

Kato, A. and Y. Okuda. 1983. Paraquat resistance in Erigeron canadensis. Weed Res.
(Japan) 28:54-56.

Kaufman, L. M. and R. L. Ritter. 1986. Early preplant applications of cyanazine in full
season no-tillage soybeans. Proc. Northeast Weed Sci. Soc. 40:25.

22

Kaufman, L. M. and R. L. Ritter. 1988. Use of HOE-039866 in no-till soybeans. Abst.
Weed Sci. Soc. Am. 28:19.

Keeling J. W. and J. R. Abernathy. 1988. Weed control systems for conservation tillage
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Kells, J. J. 1985. Weed control strategies for no-tillage corn and soybean production. Proc.
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Kells, J. J. and G. A. List. 1986. Horseweed [Cornea canadensis (L.) Cronq.] control in
no-tillage soybeans. Proc. North Central Weed Control Conf. 41:44.

Kobayashi, A., S. Morirnoto, Y. Shibata, K. Yamashita, and M. Numata. 1980. 10 Carbon
polyacetylenes as allelopathic substances in dominants in early stages of secondary
succession. J. Chem. Ecol. 6:119-132.

Lebaron, H. M. and J. Gressel. 1982. Herbicide Resistance Ln' Plants. John Wiley and
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Lehman, W. F. and J. W. Lambert. 1960. Effects of spacing of soybean plants between and
within rows on yield and its components. Agon. J. 52:84-86.

McCutchen, T. C. and R. M. Hayes. 1983. Control of horseweed, cocklebur, and
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McWhorter, C. G. and W. L. Barrentine. 1975. Cocklebur control in soybeans as affected
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Mikulas, J. and E. Polos. 1983. The spreading of Erigeron canadensir L. in gapevine
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Quisenberry, D. 1988b. Personal communication.

Raynal, D. J. and F. A. Bazzaz. 1975. Interference of winter annuals with Ambrosia
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23

Regehr, D. L. and F. A. Bazzaz. 1976. Low temperature photosynthesis in successional
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Shontz, J. P. and H. J. Costing. 1970. Factors affecting interaction and distribution of
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Slack, I... H., R. B. Wells and W. W. Witt. 1988. Chlorirnuron, imazaquin, and irnazethapyr
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System; A. F. Wiese, ed. Weed Sci. Soc. Am., Champaign, Illinois.

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Tremmel, D. C. and K. M. Peterson. 1983. Competitive subordination of a piedmont old
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58.

24

Watanabe, Y., T. Honma, K. Itoh and M. Miyrahara. 1982. Paraquat resistance in Erigeron
_ philadelphicus L. Weed Res. (Japan) 27 :49-54.

Wax, L. M. and J. W. Pendleton. 1968. Effect of row spacing on weed control in soybeans.
Weed Sci. 25:462-464.

Wilson, H. P., T. E. Hines, R. R Bellinder and J. A. Grande. 1985. Comparisons of HOE-
39866, SC-0224, paraquat, and glyphosate in no-till corn (Zea mays). Weed Science
33:531-536.

Zinzolker, A., J. Kigel and B. Rubin. 1985. Effects of environmental factors on the
germination and flowering of Conyza albida, C. bonariensis and C. canadensis.
Phytoparasitica 13(3/4):229-230.

CHAPTER2

HORSEWEED [Com madam} (L.) Cronq.] INTERFERENCE
IN NO-TILLAGE SOYBEAN PRODUCTION

ABSTRACT

Research was conducted in 1987 and 1988 to: 1) study the impact of horseweed
populations on soybean yield and 2) examine the effect of soybean row spacing on soybean
competition with horseweed. The herbicides glyphosate, paraquat and linuron were applied
to horseweed to provide a range of horseweed population densities.

Horseweed densities of 0, 13, 16 and 165 plants/m2 resulted in soybean yields of 1837,
1065, 1289 and 170 kg/ ha, respectively when averaged over row spacing in 1987. Horseweed
densities of 16, 13 and 165 plants/m2 significantly reduced soybean yield 29, 42 and 91%,
respectively as compared to the weed-free environment. Soybean yields averaged 541 kg/ ha
in the weed-free areas during the extremely dry 1988 gowing season. Significant soybean
yield reductions of 87 to 94% were observed in horseweed densities of 79, 94 and 208
plants/m2. Increasing horseweed density significantly reduced soybean plant height in 1987
as well as the number of soybean pods per plant and seed weight in 1987 and 1988.

Soybean yields were significantly geater in soybean rows spaced 18 and 36 cm than
71 cm in 1987. Due to the low yields and drought related variability in 1988, this yield

advantage was not observed for narrow row spacing. No significant differences in

26
horseweed population were observed with soybean row spacing averaged over herbicide
progam in either year. Nomenclature: glyphosate, N-(phosphonomethyl) glycine; paraquat,
1-1’-dimethyl-4-4’-bipyridinium ion; linuron, N’-(3,4-dichlorophenyl)-N-methoxy-M-
methylurea; horseweed, Conyza canadensis (L.) Cronq. #1 ERICA; soybean, Glycine max
(L.) Merr. # GLXMA.

 

1Letters following this symbol are WSSA-approved computer code from Composite List of
Weeds, Weed Sci. 32, Suppl. 2. Available from WSSA, 309 W. Clark St., Champaign, IL 61820.

INTRODUCTION

Horseweed is a winter or summer annual plant commonly found in pastures, roadsides
and fallow areas (Anonymous, 1981). Kapusta (1979), Elmore and Heatherly (1983) and
Brown and Whitwell (1988) have reported horseweed populations in the absence of tillage
but a shallow disking eliminated the weed. Horseweed has become a severe weed control
problem in no-tillage soybean production due to lack of tillage and inadequate horseweed
control provided by commonly used herbicides.

Only limited studies have examined the impact of horseweed control on soybean yield
due to the recent severity of the weed. McCutchen and Hayes (1983) reported soybean
yields of 2345 and 1512 kg/ ha when horseweed control ratings were 91 and 68%,
respectively. Wilson and Worsham (1988) also reported soybean yields, however, their
results were affected by other annual weed species as well as horseweed. No results
examining the effect of horseweed population on soybean yield have been reported at this
time.

Narrow soybean row spacing is commonly used in no-tillage soybean production.
Burnside and Colville (1963) reported decreased total weed dry weight / unit area, reduced
time for soybean canopy closure and increased soybean gain yields as row spacings were
reduced from 102 to 25-cm. By reducing soybean row spacing, the crop becomes a stronger
competitor with weeds.

Horseweed has been reported as a poor competitor in sub-optimal conditions. Fall

germinating horseweed fared poorly in severe winter annual competition according to

27

28
Keever (1950). Bekech (1988) reported significant decreases in total horseweed dry
weight/m2 and plant height as light intensity decreased from full sunlight. Soybeans may
compete more effectively with horseweed when planted in narrow rows compared to wide
rows.
The objectives of this study were to: 1) exannine the impact of several horseweed
populations on soybean yield, and 2) study the effect of soybean row spacing on soybean

competition with horseweed.
MATERIALS AND METHODS

Research was conducted in adjacent experimental areas during 1987 and 1988 in Cass
County MI. The soil type for each location was a Kalamazoo loam (Fine-loamy, mixed,
mesic T‘ypic Hapludalfs). The soils contained 1.9 and 1.2% organic matter with soil surface
pH of 6.0 and 6.6 for 1987 and 1988, respectively.

The study was conducted in a split plot design with four and three replications in 1987
and 1988, respectively. Main plots were row spacings of 16, 38, and 72 cm row spacings.
Sub-plots consisted of four herbicide progams designed to provide a range of horseweed
population densities. Plots were 3.4 meters wide and 11 meters in length.

The soybean variety ‘Corsoy 79’ was planted without tillage into soybean residue on
May 15, 1987 and into corn residue on May 17, 1988. A no-tillage drill was used to plant
soybeans into the desired row spacings by preventing seed flow to specific planting units.
The drill was calibrated to deliver 40,470 seeds/ ha. Actual soybean plant populations for

each row spacing are shown in Table 1.

29

13mg. Soybean plant populations for each of three soybean row spacings.

 

 

 

 

 

Soybean population
Row spacing 1987 1988
(cm) (Plants/ ha) --
18 43,215 37,020
36 52,420 42,297

71 36,846 29,314

 

30

Herbicides were applied preemergence with a tractor mounted compressed air sprayer.
Applications utilized 80015 or 8003 flat fan nozzles’ which delivered 98 and 207 L/ha
respectively at a spray pressure of 207 kPa. Glyphosate treatments were applied with a
spray volume of 98 L/ ha spray volume. All other treatments were applied at 207 L/ha.
At the time of application, horseweed height averaged 7.5 cm and 6 cm with populations
of 139 and 166 plants/m2 for 1987 and 1988, respectively.

Four herbicide progams were selected to provide a range of horseweed control. These
progams are summarized in Table 2. The glyphosate plus linuron progam was designed
to provide complete horseweed control. The two paraquat treatments, with and without
linuron, were applied to provide intermediate horseweed control and population densities.
The untreated progam is included to examine the effect of a natural horseweed population
on soybeans. All herbicide plots received a postemergence application of sethoxydirn plus
crop oil concentrate at the rates of 0.17 kg/ha and 2.3 L/ ha, respectively for control of
gassy weed species. Weed species other than horseweed were removed by handweeding.

The horseweed populations were estimated bi-weekly beginning four weeks after
soybean planting through nnid-July. Horseweed population in low density plots was
estimated by counting horseweed plants in an area 0.5 by 11 m from the center of each plot.
In high density plots, horseweed populations was estimated by counting horseweed plants
in three randomly selected 0.09-m2 areas within each plot. Evaluation of horseweed control
was taken every bi-weekly throughout the gowing season beginning 4 weeks after soybean
planting. Visual horseweed control evaluations were based on a scale of O to 100 where O
was no visible horseweed injury and 100 represented complete horseweed control.

At harvest, 10 soybean plants were randomly selected from each plot for

determination of soybean height, number of pods per plant and weight of 100 seeds.

 

3Spraying Systems Co., North Ave. and Schmale Road, Wheaton, IL 60188.

31

jIhble 2,. Herbicide progams and corresponding herbicide rates.

 

 

 

 

Herbicide Herbicide
progam component and rate
(kg/ha)
1 Glyphosate (1.7) + linuron (0.8) + surfactant2 (0.5% v/v)
2 Paraquat (0.6) + linuron (0.8) + surfactant (0.25% v/v)
3 Paraquat (0.6) + surfactant (0.25% v/v)
4 Untreated

 

¥

2X-77, Valent U.S.A. Corp., 1333 N California Blvd., Walnut Creek, CA 94596.

32
Soybeans were harvested with a mechanical harvester and soybean yields were corrected to
13% moisture. All data were subjected to analysis of variance and means separated by least

significant difference at either the 0.1 or 0.05 level of significance.
RESULTS AND DISCUSSION

The environmental conditions experienced in 1987 were relatively normal. However,
the gowing season in 1988 was very dry with below-normal rainfall occurring between mid-
May and mid-July as well as above normal temperatures (Table 3). The horseweed control
and soybean yields reflect these conditions.

The analysis of variance indicated a _ significant treatment effect from herbicide
progam but no row spacing by herbicide progam interaction. Therefore data for the
herbicide progam are averaged over soybean row spacing and data for soybean row spacing
are averaged over the herbicide progam.

An increase in horseweed population decreased soybean gain yields in 1987 (Table
4). The weed-free environment of herbicide progam 1 (glyphosate plus linuron) resulted
in soybean yield of 1837 kg/ ha when averaged over soybean row spacing. Herbicide
progams 2 and 3 which utilized paraquat with and without linuron, respectively, resulted
in horseweed populations of 16 and 13 plants/m2. These populations significantly reduced
soybean yields by 29% and 42% as compared to the weed-free environment. The untreated
horseweed population had 165 plants/m2 and a soybean gain yield of 170 kg/ha or 9% of
the weed-free yield.

The weed-free areas produced an average soybean yield of 541 kg/ ha during the
extremely dry 1988 gowing season (Table 5). These weed-free yields were considerably less

than the 1987 weed-free yields. Herbicide progams 2 and 3 had horseweed densities of 79

33

Table S. Total and average monthly rainfall for Cass County, Michigan for 1986, 1987 and

 

 

 

 

 

1988.
Total monthly rainfall
Year April May June July
(cm)
1987 7.2 ' 8.8 5.8 7.1
1988 9.3 4.6 1.3 5.4
30 year ave . 9.5 7.9 10.1 9.4

 

34

Ighle_4. Results from 1987 horseweed interference research conducted in Cass County,
Michigan. '

 

Hotseweed ' te
Row Herbicide Density antgnl Average Pod Seed

gpacing progam 4 WAT ' 4 WAT 10 WAT height number weight Yield

 

 

(cm) (PltS/mz) (%) (cm) (if/Plant) (8/100) (kg/ha)
18 1 0 100 100 32 58 10.9 2099

2 12 69 38 31 42 11.2 1483

3 12 70 28 28 28 10.9 1088

4 180 0 0 13 3 9.1 138

36 1 0 100 100 32 32 10.6 1738
2 8 74 51 31 26 11.9 1442

3 11 69 38 28 25 12.3 1172

4 135 0 0 17 6 11.5 273

71 1 0 100 100 34 49 11.2 1673
2 29 66 21 32 34 11.6 968

3 15 74 34 30 28 11.9 933

4 179 0 0 14 5 10.3 100

LSDam)b 57 7 20 4 10 1.6 408

0 av d ve w

1 0 100 100 33 46 10.9 1837

2 16 70 37 31 34 11.5 1298

3 13 71 33 29 27 11.7 1065

4 165 0 0 15 5 10.3 170

LSDM,” 33 4 12 2 6 1.0 235

w ' aver ed over erbic' r

18 60 41 51 26 33 10.5 1202

36 61 47 39 27 22 11.6 1156
71 60 39 56 27 29 11.2 918
LSDGW)b 4 8 3O 2 6 0.9 203‘

 

'Weeks after treatment.
bComparisons valid within columns.
°Least significant difference at the 0.1 level.

35

 

 

fIfablg S. Results from 1988 horseweed interference research conducted in Cass County,
Michigan.
Horgeweed Snyhean yi 1;; parameters
Row Herbicide Density Control Average Pod Seed

gacing prngam 4 WAT ' 4 WAT 10 WAT height number weight Yield

(cm) (PltS/mz) ----(%) (CHI) (it/Plant) (8/ 100) (kg/ha)

 

 

18 1 0 100 100 18 43 12.5 332
2 60 37 32 12 14 8.7 41
3 47 30 25 11 10 10.7 57
4 277 0 0 2 3 3.7 4
36 1 0 100 100 21 45 13.4 712
2 86 27 18 12 9 8.0 50
3 107 25 0 9 8 9.0 37
4 178 0 0 8 1 5.4 7
71 1 0 100 100 19 45 13.3 577
2 92 27 13 10 4 6.7 114
3 130 17 7 9 3 7.1 11
4 167 0 0 9 4 8.0 20
ISBN,” 95 11 3o 3 9 3.5 186
av ed ove a r w s c'
1 0 100 100 -- 45 13.1 541
2 79 30 21 - 9 7.8 68
3 94 24 11 -- 7 8.9 35
4 1208 0 0 - 3 5.7 10
148D(o_05)b 55 6 13 " 5 2 . 0 107
Snybean row snacnn' g averaged over herhicide progam
18 42 39 96 -- 18 8.9 108
36 38 30 93 -- 16 9.0 201
71 36 30 97 -- 14 8.8 180
I.SD(O_05)" 8 12 78 -- 5 3.2 58‘

 

'Weeks after treatment.
l’Comparisons valid within columns.
‘Least significant difference at the 0.1 level.

36
and 94 plants / m2, respectively. Soybean yields for these densities were 68 and 35 kg/ ha
which represent 13% and 6% of the yield potential in a weed-free culture.

Horseweed is a very competitive weed in no-tillage soybean production. Populations
as low as 13 plants / m2 significantly reduced soybean yield by 29% as compared to a weed-
free environment. The horseweed population obtained from application of paraquat and
linuron, two commonly used no-tillage soybean herbicides, significantly reduced soybean
yield as compared to a weed-free environment.

The lower weed control ratings and higher horseweed densities experienced in 1988
may be due in part to the very dry gowing conditions. These conditions prevented the
incorporation of soil active herbicides and possibly reduced the foliar absorption of foliar
active nonselective herbicides compared to 1987. Ahmadi et al. (1980) observed reduced
bamyardgass (Echinochloa cms-galli) phytotoxicity from foliar applications of glyphosate
and paraquat plus terbutryn as soil water potential decreased from 1/8 to 37 bar tension.
Further examination revealed decreased absorption and translocation of 14C-glyphosate as
soil moisture decreased. Sherrick et al. (1986) observed significantly geater deposition of
epicuticular wax on plants gown in a high light, low humidity environment than plants
gown in a low light, high humidity. They speculate that differential cuticle development
may be responsible for reduced glyphosate absorption in drought stressed plants.

The effect of horseweed density on soybean yield parameters is also reported in
Tables 4 and 5. As horseweed density increased, soybean height at harvest decreased.
Soybean heights could not be averaged over row spacing in 1988 (Table 5) due to a
significant row spacing by herbicide interaction for this variable. However, soybean heights
decreased as horseweed density increased in the 18 and 36 cm row spacings. The number
of soybean pods per plant averaged over row spacing decreased significantly in the presence

of 13 to 16 horseweed plants/m2 in 1987. A soybean plant in the weed-free environment

37
averaged 46 pods compared to 27 and 34 pods in the 13 and 16 horseweed densities,
respectively. The combined effects of dry conditions and geater weed densities in 1988
caused pod number reductions of even geater magnitude than in 1987. Soybean plants
gown among 79 horseweed plants/m2 averaged only 9 pods per plant compared to 45 pods
per plant on soybeans gown weed-free.

Soybean seed weights were not significantly different when horseweed populations
were 16 plants/m2 or less in 1987. Soybean seed produced under weed-free conditions in
1988 had significantly geater weight than seed produced in the presence of 79 or 94
horseweed plants / m2.

Soybean yield data from 1987 indicate that soybeans planted in narrow row spacings
have a yield advantage compared to wide row spacings. Soybean yields, averaged over
herbicide progam in 1987, were 1202 and 1156 kg/ha in 18 and 36 cm row spacings,
respectively. These yields were significantly geater at the 0.1 level than soybean yields from
71 cm row spacings. This observation is consistent with data reported by Lehman and
Lambert (1960), Burnside and Colville (1963), Peters et al. (1965) and Wax and Pendleton
(1968) who observed soybean yields improved when row spacings were reduced from 102
to 52 cm or less. Soybean yields averaged over row spacing in 1988 were only 17% of yields
obtained in 1987 due to the droughty conditions. These yields are not representative of
typical Michigan no-tillage soybean production. Due the low yields and variability caused
by the drought, the effect of soybean row spacing on yield will not be addressed.

The effects of soybean row spacing on horseweed control is reported in Tables 4 and
5 for 1987 and 1988, respectively. No significant differences were observed with soybean
row spacings averaged over herbicide progams. Horseweed densities for 18, 36 and 71-
cm soybean row spacings were 51, 39 and 56 plants/m2 respectively in 1987 and 96, 93 and

97 plants/m2 in 1988. Soybean inability to effectively compete with horseweed, regardless

38
of row spacing, may be due in part to the rapid rate of horseweed recovery following
herbicide application. Horseweed regowth initiated from apical and axillary meristems
near the top of the plant within 2 weeks after herbicide application. At the time of
application, horseweed plants averaged 6 to 8 cm in height. Horseweed regowth initiating
from these heights was taller than newly emerged soybean plants, thus giving the horseweed

a competitive advantage over the soybean.

BIBLIOGRAPHY

Ahmadi, M. 8., L. C. Haderlie and G. A. Wicks. 1980. Effect of gowth stage and water
stress on bamyardgass (Echinochloa crus-galli) control and on glyphosate absorption
and translocation. Weed Sci. 28:277-282.

Anonymous. 1981. We e t St t . North Central Region Research
Publication 286, Bulletin 772. Univ. of Illinois at Urbana-Champaign. p.204.

Bekech, M. M. 1988. The biology of horseweed [Conyza canadensis (L.) Cronq.].
University of Massachusetts, Amherst, Thesis for the degee of M. S. p. 18-70.

Brown, S. M. and T. Whitwell. 1988. Influence of tillage on horseweed, Conyza canadensis.
Weed Tech. 2:269-270.

Burnside, O. C. and W. L. Colville. 1963. Soybean and weed yield as affected by irrigation,
row spacing, tillage and anniben. Weeds 12:109-112.

Elmore, C. D. and L. G. Heatherly. 1983. Preplant tillage effects on the weed flora in
soybeans. Abst. Weed Sci. Soc. Am. 23:66-67.

Kapusta, G. 1979. Seedbed tillage and herbicide influence on soybean (Glycine max) weed
control and yield. Weed Sci. 27 :520-526.

Keever, C. 1950. Causes of succession on old fields of the Piedmont, North Carolina.
Ecol. Monogaphs 20:229-250.

Lehman, W. F. and J. W. Lambert. 1960. Effects of spacing of soybean plants between and
within rows on yield and its components. Agon. J. 52:84-86.

McCutchen, T. C. and R. M. Hayes. 1983. Control of horseweed, cocklebur, and
smartweed in no-till soybeans. Proc. So. Weed Sci. Soc. 37:20-26.

Peters, E. J ., M. R. Gebhardt and J. F. Stritzke. 1965. Interrelations of row spacings,
cultivations and herbicides for weed control. Weed Sci. 23:386-390.

Sherrick, S. L., H. A. Holt and F. D. Hess. 1986. Effects of adjuvants and environment
during plant development on glyphosate absorption and translocation in field bindweed
(Convolvulus arvensir). Weed Sci. 34:811-816.

Wax, L. M. and J. W. Pendleton. 1968. Effect of row spacing on weed control in soybeans.
Weed Sci. 25:462-464.

39

40

Wilson, J. S. and A. D. Worsham. 1988. Combinations of nonselective herbicides for
difficult to control weeds in no-till corn (Zea mays), and soybeans (Glycine max). Weed
Sci. 36:648-652.

CHAPTER 3

EARLY PREPLANT AND PREEMERGENCE CONTROL OF HORSEWEED
[Conym madam} (L.) Cronq.] IN NO-TILLAGE SOYBEANS

ABSTRACT

A three year study was initiated in 1986 to identify effective and consistent horseweed
control strategies in no-tillage soybean production. Preemergence (PRE) applications of
paraquat (0.56 kg/ ha) plus metolachlor (2.2 kg/ ha) plus linuron (0.84 kg/ ha) plus non-ionic
surfactant (0.25 % v/v) provided less that 60% horseweed control. Early preplant (EPP)
applications of either glyphosate (0.84 kg/ha), 2,4-D ester (0.56 kg/ha), HOE-39866 (0.84
kg/ha) or BAS-514 (0.07 kg/ha) provided geater than 95% horseweed control when
followed by the above preemergence treatment. Horseweed control and soybean yield from
these EPP treatments were significantly geater than the PRE application alone.

The substitution of 'either glyphosate (0.84 kg/ha) or HOE-39866 (0.84 kg/ha) for
paraquat in the above PRE treatment significantly improved horseweed control. The
addition of BAS-514 (0.07 kg/ha) to the above PRE treatment containing paraquat
significantly improved horseweed control in all years and soybean yield in 1987, however,
soybean injury was observed at all rates tested. Horseweed control improved when either
metribuzin or linuron plus chlorimuron (16:1) were substituted for linuron in the PRE

treatment containing paraquat. The substitution of metribuzin plus chlorimuron (10:1) for

41

42
linuron in the paraquat containing PRE treatment significantly improved horseweed control.
The herbicides imazaquin or irnazethapyr did not provide adequate horseweed control
( _<_ 65 %) when applied PRE with paraquat plus metolachlor plus surfactant.

Sequential application of soil active herbicides provided equal or geater horseweed
control in 1987 than a single preemergence application of the same total rate. In the dry
1988 gowing season, the soil active herbicides linuron, linuron plus chlorimuron (16: 1) and
metribuzin plus chlorimuron (10: 1) provided significantly geater horseweed control than
the same total herbicide rate applied PRE. Nomenclature: glyphosate, N-(phosphono-
methyl)glycine; 2,4-D, (2,4-dichlorophenoxy)acetic acid; HOE-39866, ammonium-(3-amino-
3—carboxypropyl)-methyl-phosphinate; BAS-514, 3,7-dichloro-8-quinolinecarboxylic acid;
paraquat, 1-1’-dimethyl-4-4’-bipyridinium ion; linuron, N-(3,4-dichlorophenyl)-N-methoxy-
N-methylurea; metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methyl
ethyl)acetamide; chlorimuron, 2-[[[[[4-chloro-6-methoxy-2-pyrimidinyl]amino]carbonyl]
amino]sulfonyl]benzoic acid; metribuzin, 4-amino-6-(1, 1-dimethyethyl)-3-(methylthio)-1,2,4-
triazin-5(4H)-one;imazaquin,2-[4,5-dihydro-4-methyl-4-( l-methylethyl)-5-oxo- lH-irnidazol-
2-yl]-3-quinolinecarboxylicacid;imazethapyr,(jg-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-
5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid; horseweed, Conyza canadensir
(L.) Cronq. 11f1 ERICA; soybean, Glycine max (1..) Merr. # GLXMA.

 

1Letters following this symbol are WSSA-approved computer code from Composite List of
Weeds, Weed Sci. 32, Suppl. 2. Available from WSSA, 309 W. Clark St., Champaign, IL 61820.

INTRODUCTION

Successful no-tillage weed control requires complete control of existing vegetation prior
to crop emergence as well as control of emerging weeds after planting. Early preplant
(EPP) and preemergence (PRE) herbicide applications provide effective weed control in no-
tillage crop production (Stougaard et al., 1984; Fawcett et al., 1983). In an EPP herbicide
progam, soil active herbicides are applied prior to weed emergence to control weeds
through the entire gowing season. PRE applications are made following crop planting but
prior to crop emergence. An effective PRE herbicide progam consists of a nonselective
foliar active herbicide, to control existing vegetation, in combination with soil active
herbicides. A combination of both systems is referred to as sequential herbicide application.
In this system, a portion of soil active herbicide is applied EPP followed by an additional
application PRE (Fawcett et al., 1983). The PRE application progam is most commonly
used in no-tillage crop production aocordirng to Kapusta (1979).

The presence of horseweed is frequently observed in the absence of tillage. The use
of shallow tillage eliminated this weed according to Kapusta (1979), Elmore and Heatherly
(1983) and Brown and Whitwell (1988). Horseweed is a serious weed control problem in
no-tillage soybean production. The commonly used herbicides, paraquat and linuron, fail
to provide effective horseweed control in no-tillage soybeans (McCutchen and Hayes 1983;
Wilson et al., 1985; Wilson and Worsham, 1988). Researchers have observed significantly
geater horseweed control when paraquat was substituted with either glyphosate (Wilson et

al., 1985; Wilson and Worsham, 1988) or HOE-39866 (Wilson et al., 1985). The substitution

43

44
of metribuzin for linuron improved horseweed control (Kapusta, 1979).
Presently, only linnited research has been conducted to specifically address horseweed
control strategies in no-tillage soybeans. In 1986, a three year study was initiated to identify

consistent and effective herbicide progams for the control of horseweed in no-tillage

soybean production.

MATERIAL AND METHODS

Research was conducted in Cass county Michigan. The location of the study was
rotated yearly between two adjacent fields. Soil at all experimental locations was a
Kalamazoo loam soil (Fine-loamy, mixed, mesic Typic Hapludalfs) with naturally high
horseweed populations. Soybeans were planted without tillage into com (1986 and 1988)
and soybean (1987) residue, two weeks after EPP applications. Herbicide treatments were
arranged as a randomized complete block with three replications. All herbicide applications
were made in plots at least 3 m wide by 11 m long with a tractor mounted compressed air
sprayer. The sprayer was calibrated to deliver 98 L/ ha and 206 L/ ha at 207 kPa. In all
years, glyphosate treatments, which included non-ionic surfactant2 (0.5% v/v), were applied
with a spray volume of 98 L/ ha. All other herbicide applications were applied in a spray
volume of 206 L/ha.

Visual evaluations of horseweed control and soybean injury were conducted bi-weekly
throughout the gowing season. Evaluations were based on a scale with zero being no
visible injury and 100 representing complete plant death. Horseweed density was estimated
bi-weekly beginning four weeks after soybean planting and continued through mid-July.

The horseweed population in low density plots was estimated by counting horseweed plants

 

2x-77. Valent 0.3.6.. Corp., 1333 N. California Blvd., Walnut Creek, CA 94596.

45
in an area 0.5 m wide by 11 m long from the center of each plot. In high density plots,
population was estimated by counting horseWeed plants in three randomly selected 0.09-
m2 areas from each plot. Soybeans were harvested with a mechanical harvester. Soybean
yields were corrected to 13% moisture. All data were subjected to analysis of variance with
means separated by least significant difference at the 0.05 level of significance.

The first year of research was conducted on a sandy loam soil containing 1.2% organic
matter with soil surface (0 to 5 cm) pH of 6.6. The site had a uniform 'horseweed
population averaging 94 plants / m2. EPP herbicide applications were made to horseweed
averaging 8 cm in height on May 13, 1986. Table 1 shows the environmental conditions at
the time of herbicide application. Approximately two weeks later, the soybean variety
‘Northrup King 2596’ was planted into corn residue in 48-cm row spacings. Herbicides were
applied preemergence on May 29, the same day as planting, to horseweed at an average
height of 18 cm.

The 1987 and 1988 studies were located in adjacent fields with the 1988 study conducted
in the same location as the 1986 study. These soils had surface pH of 6.0 and 6.6 and
contained 1.9 and 1.5% organic matter in 1987 and 1988, respectively. Horseweed
populations averaged 139 plants/m2 in 1987 and 166 plants/m2 in 1988. In both years, very
few weed species other than horseweed were observed. The soybean variety ‘Corsoy 79’ was
planted in 18-cm row spacings in both years. EPP herbicide applications were made to
horseweed at average heights on 1 and 6 cm on May 1, 1987 and April 29, 1988,
respectively. Herbicides were applied PRE on May 15, 1987 and May 17, 1988, the same
day as soybean planting to horseweed at average heights of 8 and 6 cm, respectively. Table

1 summarizes the environmental conditions at the time of all herbicide applications.

46

1312111. Environmental conditions for early preplant and preemergence application of

 

 

 

 

herbicides.
.1986 1(2311 1933
Planting date 5 /29 5/ 15 5/17-
Horseweed density (plants/m2) 94 139 166
EPP application

Date 5/13 5/1 4/29
Cloud cover (%) 10 95 Clear
Air temperature (°F) 65 48 52
Relative humidity (%) 43 50 39
Soil temperature (°F) 63 50 46
Leaf surface moisture

(1 =wet, 5 = dry) 4 3 5
Horseweed height (cm)

ave (range) 8(3-13) 5(0.5-9) 1(0.5-3)

PRE application

Date 5/29 5/ 15 5/17
Cloud cover (%) 100 5 Clear
Air temperature (°F) 63 62 65
Relative humidity (%) 80 50 45
Soil temperature (°F) 60 78 71
Leaf surface moisture

(1 =wet, 5 = dry) 3 3 5
Horseweed height (cm)

ave (range) 18(5-23) 8(1-15) 6(2-13)

 

RESULTS AND DISCUSSION

Early preplant applications. Above average, normal and below average rainfall occurred
irn 1986, 1987 and 1988, respectively (Table 2). Despite the very different environmental
conditions, horseweed control from the EPP herbicide applications remained consistent
between years (Table 3). Fawcett et al. (1983) reported that EPP herbicide progams
reduce the risk of herbicide failure due to inadequate rainfall.

Early season horseweed control and early season horseweed densities for the herbicide
treatments are correlated negatively as indicated by the data presented in Tables 3, 4 and
5. Horseweed control ratings declined as the gowing season progessed. This decrease in
weed control is probably due, in part, to (1) increased number of horseweed axillary shoots
per plant late in the season, and (2) geater visibility of existing horseweed plants became
more apparent as they gew above the soybean canopy. Since horseweed control decreased
with time, only late season control will be discussed.

All herbicides applied EPP were followed by a PRE application of paraquat (0.56
kg/ha) plus linuron (0.84 kg/ha) plus metolachlor (2.2 kg/ha) plus non-ionic surfactant
(0.25% v/v). This commonly used PRE herbicide progam provided less than 56% late
season horseweed control in any of the three years. For simplicity, this PRE herbicide
progam shall be referred to as the standard PRE.

EPP application of glyphosate at 0.86 kg/ha with low carrier volume (98 L/ ha) when
followed by the standard PRE, provided significantly geater horseweed control (> 96%) and
soybean yield than the standard PRE alone (Table 3). The addition of ammonium sulfate

47

48

Inhlej. Total and average monthly rainfall in Cass County, Michigan for 1986, 1987 and

 

 

 

 

 

1988.
Total monthly rainfall
Year April May June July
(cm)
1986 5.8 12.6 14.8 12.6
1987 7.2 8.8 5.8 7.1
1988 9.3 4.6 1.3 5.4

30 year ave 9.5 7.9 10.1 9.4

 

49

 

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51

(20.4 g/L spray volume) to glyphosate did not improve horseweed control. O’Sullivan et
al. (1981) and Suwunnamek and Parker (1975) observed increased glyphosate activity with
the addition of ammonium sulfate. Glyphosate when applied either in 98 or 206 L/ ha
carrier volumes, obtained similar horseweed control. Downs (1981) reported similar
findings, however Jordan (1981), O’Sullivan et al. (1981), Buhler and Burnside (1983) and
Carlson and Burnside (1984) observed increased glyphosate phytotoxicity with reduced
carrier volumes.

The lowest rate of 2,4-D ester (0.56 kg/ ha) applied EPP provided greater than 95%
horseweed control. When applied two weeks prior to soybean planting, no soybean injury
or crop yield reduction was observed with 2,4-D at rates up to 1.1 kg/ ha. The addition of
2,4-D ester (0.56 kg/ha) to glyphosate (0.2 kg/ha) provided horseweed control equivalent
to 2,4-D ester applied alone.

The experimental herbicides HOE-39866 and BAS-514 provided excellent control of
horseweed when applied EPP. HOE-39866 obtained greater than 95% control of horseweed
at the lowest rate tested (0.56 kg/ ha). HOE-39866 applied EPP significantly improved
soybean yield as compared to the standard PRE herbicide program. Application of BAS-
514 (0.07 kg/ha) provided greater than 98% horseweed control in 1987 and 1988. Soybean
injury was observed with BAS-514 at rates greater than 0.14 kg/ha in 1987.' During the
dry 1988 growing season, crop injury was significant at the 0.25 kg/ha herbicide rate.
Soybean yields, however were not significantly reduced by this injury (Table 3).

In 1988, paraquat was applied early preplant at 0.56 kg/ ha followed by the standard
PRE. This treatment provided excellent (97%) horseweed control. The horseweed density
in this treatment consisted of 13 severely stunted plants/m2. This herbicide program did
not significantly improve soybean yield as compared to the standard PRE alone. Paraquat

applied in this manner may suppress horseweed, however with normal rainfall these plants

52

may recover.
. The herbicides glyphosate, 2,4-D ester, HOE-39866 or BAS-514 provided excellent
control of horseweed when applied EPP. The use of EPP herbicide applications
demonstrated outstanding weed control consistency in the environmentally diverse three

year study. Fawcett et al. (1983) reported similar consistency among EPP applications.

Preemergence applications. The PRE application of paraquat (0.56 kg/ ha) plus linuron
(0.84 kg/ ha) plus metolachlor (2.2 kg/ ha) plus non-ionic surfactant (0.25% v/v) is commonly
used by Michigan no-tillage soybean producers. The paraquat treatment desiccates
horseweed plants, however the plants recover by generating new growth from axillary
meristems within two to four weeks after treatment. The addition of BAS-514 to this
standard PRE combination significantly improved horseweed control over the standard PRE
combination alone (Table 4). In 1987, BAS-514 (0.07 kg/ ha) added to the standard PRE
provided 94% horseweed control. Soybean injury at this rate was slight, however as BAS-
514 rates increased, soybean injury increased significantly. In 1988, BAS-514 caused
significant crop injury at all rates and provided only fair to good horseweed control. These
results may be due to inadequate rainfall for prOper herbicide placement in the soil profile.

Greater horseweed control was obtained when glyphosate was substituted for paraquat
in the standard PRE herbicide program. At the 0.84 kg/ha rate, glyphosate provided
between 78% and 89% horseweed control in the three year study. The 0.84 kg/ha rate of
glyphosate resulted in significantly greater soybean yield than paraquat in the 1986 and 1988
growing seasons. Glyphosate provided more consistent horseweed control than paraquat
or HOE-39866 in the three environmentally different growing seasons.

A glyphosate spray solution was applied PRE to horseweed foliage with and without

linuron. When applied without linuron, glyphosate was applied with surfactant (0.5% v/v)

53

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54
to the horseweed foliage PRE. After the glyphosate had dried on the leaf surface, linuron
was applied late PRE to the horseweed. Antagonism was not apparent since horseweed
control did not significantly differ in these different application methods. Selleck and Baird
(1981) observed similar results with glyphosate and linuron on annual weedy species.

The nonselective herbicide HOE-39866 was also substituted for paraquat in the standard
PRE treatment in 1987 and 1988. When applied at 0.84 kg/ha, HOE-39866 provided 91%
and 73% (Table 4) control of horseweed in 1987 and 1988, respectively. The application
of HOE-39866 (0.84 kg/ ha or greater) provided significantly greater horseweed control than
paraquat in 1987 and 1988. Soybean yields were significantly greater in the HOE-39866
treatments than in plots receiving paraquat. Similar observations were reported by Wilson
et al. (1985).

The addition of chlorimuron to linuron (1:16) in the standard PRE program consistently
improved horseweed control (Table 5). Horseweed control and soybean yield were
significantly improved without crop injury at total herbicide rates of 0.84 and 0.62 kg/ha in
1987 and 1988, respectively. Good horseweed control was obtained at all rates tested in
1987, however less control was obtained in 1988 probably due to the drought conditions.

The substitution of metribuzin (0.42 kg/ ha) for linuron in the standard PRE provided
greater horseweed control in all years (Table 5). This concurs with observations made by
Kapusta (1979). Adding chlorimuron to metribuzin (1:10) improved horseweed control
compared to metribuzin applied alone at the same total herbicide rate. The use of
metribuzin with or without chlorimuron provided significantly greater horseweed control in
all years and resulted in significantly greater soybean yields in 1986 and 1988 than with
linuron in the standard PRE herbicide program.

The herbicides imazaquin and imazethapyr did not provide adequate control of

horseweed when substituted for linuron in the standard PRE herbicide program. Slack et

55

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56
al. (1988) observed similar results when imazaquin or imazethapyr was applied with
paraquat. Since these herbicides consistently provided inadequate horseweed control in
1986 and 1987 they were not examined in 1988.

There appear to be several options available for improving horseweed control with PRE
application of herbicide. The use of the nonselective postemergence herbicides glyphosate
or HOE-39866 provide significantly greater horseweed control than paraquat. The addition
of BAS-514 to paraquat improves horseweed control, however soybean injury was evident.
The soil active herbicides metribuzin and metribuzin plus chlorimuron (10: 1) provided
significantly greater horseweed control than linuron in all years tested. The addition of
chlorimuron to linuron (1:16) significantly improved horseweed control in three of four
observations.

PRE applications of soil active herbicides in 1988 provided less horseweed control than
the previous two years. This was possibly due to the inadequate rainfall for proper
incorporation of these herbicides. According to Fawcett et al. (1983), this is one of the main
disadvantages of PRE herbicide applications in no-tillage. Sequential applications of
herbicides reduce the risk of herbicide failure under these environmental conditions.

EPP applications of nonselective herbicides followed by the standard PRE applied
herbicide provided more effective and consistent horseweed control than PRE applications
of nonselective herbicides. This is explained in part by (1) the EPP program received a
second herbicide application containing paraquat PRE, and (2) due to taller horseweed

plants at the PRE application time.

Sequential Applications. Preliminary research was conducted in 1987 to examine sequential
applications of soil active herbicides for control of horseweed. The sequential treatments

were designed such that 2/3 of the total herbicide rate was applied EPP with a non-ionic

57
surfactant. The remaining 1/3 of the herbicide was applied PRE with paraquat (0.56 kg/ ha)
plus metolachlor (2.2 kg/ha) plus non-ionic surfactant (0.25% v/v). The herbicide linuron
(0.84 kg/ha) and metribuzin (0.42 kg/ha) were applied PRE and sequentially in 1987.
Sequential applications of these herbicides provided equal or greater horseweed control and
soybean yield than a single PRE application of the same total amount of herbicide (Table
6).

The research program was expanded in 1988 to include the herbicides linuron plus
chlorimuron (16:1) and metribuzin plus chlorimuron (10:1). The herbicides linuron and
linuron plus chlorimuron provided significantly greater horseweed control when applied
sequentially. Sequential applications of linuron provided significantly greater soybean yields
than a single PRE application. Metribuzin applied sequentially with and without
chlorimuron also provided greater horseweed control than the single PRE application.
Ritter and Harris (1982) and Fawcett et al. (1983) observed similar results with sequential
applications.

The large differences in horseweed control between sequential and PRE applications
in 1988 were probably due to inadequate rainfall for incorporation of the herbicides
following the PRE application. The EPP portion of the sequential application received
adequate rainfall for herbicide incorporation, however little rainfall occurred after the PRE
application resulting in poor weed control. This demonstrates the potential advantages of

an EPP or sequential herbicide program.

58

flfable 6. Comparison of single preemergence and sequential application of soil active
herbicides for control of horseweed.

 

 

 

 

 

 

 

1987
Soybean Horseweed
Rate injury __§&nm;l___ Density Soybean
Herbicide EPP' PRE" 4 WAP 7 WAP 12 WAP 7 WAP yi_e_ld_
(kg/ h=11) (%) (%) (PltS/mz) (kg/ha)
Linuron 0.84 0 60 55 15 1766
Linuron 0.56 0.28 o ' 68 58 7 1763
Metribuzin 0.42 0 87 82 3 2130
Metribuzin 0.28 0.14 2 100 100 0 2464
Untreated 0 0 0 212 79
LSDwm)‘ n.s. 17 22 27 570
1988
Linuron 0.84 0 15 18 27 261
Linuron 0.56 0.28 8 75 82 6 1083
Linuron +
chlorimuron 0.63 0 28 33 68 801
Linuron +
chlorimuron 0.35 0.28 0 92 90 2 865
Metribuzin 0.42 0 70 75 1 1384
Metribuzin 0.28 0.14 0 88 91 2 1187
Metribuzin +
chlorimuron 0.42 0 60 60 5 961
Metribuzin +
chlorimuron 0.28 0.14 0 97 95 1 1352
LSDMS)‘ 6 20 20 32 554

 

'Herbicides applied EPP include surfactant (0.25% v/v).

I’PRE herbicide applications include paraquat (0.56 kg/ ha) plus metolachlor (2.2 kg/ ha) plus
surfactant (0.25% v/v).

‘Comparisons valid within columns of a given year.

BIBLIOGRAPHY

Brown, S. M. and T. Whitwell. 1988. Influence of tillage on horseweed, Coma canadensis.
Weed Tech. 2:269-270.

Buhler, D. D. and O. C. Burnside. 1983. Effect of spray components on glyphosate
toxicity to annual grasses. Weed Sci. 31:124-130.

Carlson, K. L. and O. C. Burnside. 1984. Comparative phytotoxicity of glyphosate, SC-
0224, SC-0545 and HOE-00661. Weed Sci. 32:841-844.

Downs, J. P. 1981. Evaluations of glyphosate rates and carrier volumes for no-till. Proc.
North Cent. Weed Control Conf. 36:92.

Elmore, C. D. and L. G. Heatherly. 1983. Preplant tillage effects on the weed flora in
soybeans. Abst. Weed Sci. Soc. Am. 23:66-67.

Fawcett, R. S., M. D. K. Owen and P. C. Kassel. 1983. Early preplant treatments for weed
control in no-till corn and soybeans. Proc. North Central Weed Control Conf. 38:112-
117.

Jordan, T. N. 1981. Effects of diluent volumes and surfactant on the phytotoxicity of
glyphosate to bermudagrass (cynodon dactylon). Weed Sci. 29:79-83.

Kapusta, G. 1979. Seedbed tillage and herbicide influence on soybean (Glycine max) weed
control and yield. Weed Sci. 27:520-526.

McCutchen, T. C. and R. M. Hayes. 1983. Control of horseweed, cocklebur, and
smartweed in no-till soybeans. Proc. So.- Weed Sci. Soc. 37:20-26.

O’Sullivan, P. A., J. T. O’Donovan and W. M. Hamman. 1981. Influence of non-ionic
surfactants, ammonium sulphate, water quality and spray volume on the phytotoxicity
of glyphosate. Can. J. Plant Sci. 61:391-400.

Ritter, R. L. and T. C. Harris. 1982. Weed control in no-tillage corn and full season no-
tillage soybeans on the eastern shore of Maryland. Proc. Northeastern Weed Sci. Soc.
36:18.

Selleck, G. W. and D. D. Baird. 1981. Antagonism with glyphosate and residual herbicide
combinations. Weed Sci. 29:185-190.

Slack, L. H., R. B. Wells and W. W. Witt. 1988. Chlorimuron, imazaquin, and imazethapyr
performance in no-tillage, full season soybeans. Abst. Weed Sci. Soc. Am. 28:82-83.

59

60

Stougaard, R. N., G. Kapusta and G. Roskamp. 1984. Early preplant herbicide applications
for no-till soybeans weed control. Weed Sci. 32:293-298.

Suwunnamek, U. and C. Parker. 1975. Control of Cyperus rotunda: with glyphosate: the
influence of ammonium sulphate and other additives. Weed Res. 15:13-19.

Wilson, H. P., T. E. Hines, R. R. Bellinder and J. A. Grande. 1985. Comparisons of HOE-
39866, Sc-0224, paraquat, and glyphosate in no-till corn (Zea mays). Weed Science
33:531-536.

Wilson, J. S. and A. D. Worsham. 1988. Combinations of nonselective herbicides for
diffith to control weeds in no-till corn (Zea mays), and soybeans (Glycine max).
Weed Sci. 36:648-652.

CHAPTER4

HORSEWEED [Carma madam} (L.) Cronq.] CONTROL
WITH FOLIAR APPLIED HERBICIDES

ABSTRACT

The nonselective foliar applied herbicides glyphosate, HOE-39866, paraquat and 2,4-
D ester were applied to horseweed at 5, 10, and 20-cm average horseweed heights.
Glyphosate (0.42 kg/ha) effectively controlled S-cm horseweed, however 0.84 kg/ ha were
needed to control 10 and 20-cm tall horseweed. HOE-39866 (0.84 kg/ ha) and paraquat
(0.56 kg/ha) did not provide adequate control of 5 and 10-cm horseweed but obtained
greater than 93% control of 20-cm horseweed in 1987. HOE-39866 provided excellent
horseweed control at all timings in 1988. Significantly less control was obtained with
paraquat at horseweed heights of 10 and 20 cm in 1988. Applications of 2,4-D ester (1.12
kg/ha) provided less than 83% horseweed control in both years.

Applications of the selective postemergence soybean herbicides bentazon and
chlorimuron did not provide consistent effective control of 5, 10 and 20-cm tall horseweed.
These herbicides provided some control of 5-cm tall horseweed, however due to horseweed
height variability, control was not complete. _

Ropewick applications of glyphosate decreased horseweed flower production, however

horseweed control was inadequate. Horseweed control was greater with two applications

61

62

from opposite directions than from single applications. Complete horseweed control by this
method was difficult to obtain due to variability in horseweed height and apparent acropetal
translocation of glyphosate. Plant regrowth from axillary meristems below the point of
herbicide application was observed with this system of application. Nomenclature:
glyphosate, N-(phosphonomethyl)glycine; HOE-39866, ammonium-(3-amino-3-carboxy
propyl)-methyl-phosphinate; paraquat, 1-1’-dimethyl-4-4’-bipyridinium ion; 2,4-D, (2,4-
dichlorophenoxy) acetic acid; bentazon, 3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-
one 2,2-dioxide; chlorimuron, 2-[[[[[4-chloro-6-methoxy-2-pyrimidinyl]amino]carbonyl]
amino]sulfonyl]benzoic acid; horseweed, Cornea canadensir (L.) Cronq. #1 ERICA; soybean,
Glycine max (L.) Merr. # GLXMA.

 

1Letters following this symbol are WSSA-approved computer code from Composite List of
Weeds, Weed Sci. 32, Suppl. 2. Available from WSSA, 309 W. Clark St., Champaign, IL 61820.

INTRODUCTION

Effective horseweed control in no-tillage soybean production requires complete control
of existing vegetation prior to crop emergence (Anonymous, 1983) as well as full season
weed control. Nonselective foliar herbicides. are usually applied in combination with soil
active herbicides to control existing vegetation and later germinating weed species (Fawcett
et al., 1983). When soil active herbicides fail to control germinating weeds, mechanical row
cultivation often provides ineffective control due to the firm soil conditions (Richey et al.,
1977 ). No-tillage producers must rely upon selective postemergence herbicides (Fawcett,
1983; Kapusta, 1979) or ropewick application of nonselective herbicides for postemergence
weed control.

Ropewick herbicide application is a method by which a nonselective systemic herbicide,
such as glyphosate, is applied to weeds extending above the crop canOpy. The ropewick
applicator uses loosely woven nylon wick to convey herbicide, by capillary movement, to
weedy plants contacting the wick. The applicator height is adjustable to allow for varying
crop height. This method of glyphosate application provides selective control of tall weeds
without crop injury (Dale, 1981).

Researchers have examined the use of nonselective herbicides in combination with soil
active herbicides for control of horseweed in no-tillage soybeans. Only limited research has
examined the effect of horseweed height on the degree of control provided by nonselective
herbicide applications. In Texas, Henniger et al. (1989) observed effective control of

horseweed rosettes with 2,4-D, glyphosate and HOE-39866. The herbicide 2,4-D applied

63

64
at high rates and HOE-39866 effectively controlled 10-cm tall horseweed. HOE-39866 was
the only herbicide to effectively control 30-cm tall horseweed. Paraquat did not provide
adequate horseweed control according to Henniger et al. (1989).

Limited research has examined horseweed control with selective postemergence
herbicides. Hagood and Davis (1986) reported unsatisfactory control of horseweed with
either acifluorfen [5-[2-chloro-4-(trifluoro methyl)phenoxy]-2-nitrobenzoic acid] or bentazon.
Currently, no research has examined ropewick application of glyphosate for control of
horseweed.

Studies were conducted in 1987 and 1988 to identify nonselective and selective
herbicides which provide effective and consistent horseweed control. Studies examined the
effect of herbicide rate and horseweed height on control. A study was also conducted to

examine horseweed control from ropewick application of glyphosate.

MATERIALS AND METHODS

Nonselective herbicide applications. Research was conducted in Shiawassee and Ingham
Counties in 1987 and 1988, respectively. The soil type in 1987 was a Macomb loam (Fine-
loamy, mixed, mesic Aquallic Hapludalfs) with 1.4% organic matter and a 6.5 pH. The soil
at the 1988 site consisted of Riddles-Hillsdale sandy loam complex (Riddles, F inc-loamy,
mixed, mesic Typic Hapludalfs; Hillsdale, Coarse-loamy, mixed, mesic Typic Hapludalfs)
with 1.5% organic matter and a pH of 5.8. The studies were arranged as a randomized
complete block with four and three replications in 1987 and 1988, respectively. Plot areas
were at least 3 m wide by 9 m in length.

Herbicide applications were made with a tractor mounted compressed air sprayer

calibrated to deliver 98 L/ ha or 206 L/ ha at 207 kPa. Glyphosate treatments were applied

65

at 98 L/ ha carrier volume. Herbicides were applied at three timings with average
horseweed heights of 5, 10 and 20 cm. The environmental conditions for each timing are
shown in Table 1. Horseweed densities averaged 267 and 124 plants/m2 in 1987 and 1988,
respectively.

Horseweed control was visually evaluated two, four and six weeks after treatment.
Evaluations were based on a scale where zero was no visible horseweed injury and 100
represented dead plants. Data were subjected to analysis of variance with means separated

by least significant difference at the 0.05 level of significance.

Selective herbicide applications. Field experiments were conducted in Shiawassee County
in 1987 and 1988. The soil at the 1987 site was a Macomb loam with 1.4% organic matter
and a 7.1 pH. The 1988 location had a Boyer loamy sand (Coarse-loamy, mixed, mesic
Typic Hapludalfs) with 1.2% organic matter and a pH of 5.0. Treatments were arranged
as a randomized complete block with three replieations in both years. Plots at all locations
were at least 3 m wide and 9 m in length.

The soybean varieties ‘Hodgson 78’, ‘BSR 101’ were no-tillage planted in 18 and 71-
cm row spacings in 1987 and 1988, respectively. Soybeans were planted on May 23, 1987
and May 26, 1988. Prior to soybean emergence, a preemergence application of paraquat
(0.56 kg/ha) plus linuron (0.84 kg/ha) plus metolachlor (2.2 kg/ha) plus non-ionic
surfactant2 (0.25% v/v) was made to the 1987 experiment site. Imazaquin (0.14 kg/ha) was
substituted for linuron in the above treatment at the 1988 location.

Herbicide applications were made with a tractor mounted compressed air sprayer
calibrated to deliver 281 L/ ha at 345 kPa. Selective postemergence herbicides were applied

to average horseweed regrth of 5, 10 and 20 cm in height. Table 2 shows the

 

2x-77. Valent U.s.A. Corp., 1333 N. California Blvd., Walnut Creek, CA 94596.

Table 1. Environmental conditions for nonselective foliar herbicide applications in 1987

and 1988.

66

 

Date

Time

Cloud cover (%)

Air temperature (°F)

Relative humidity (%)

Leaf surface moisture

(1 =wet, 5 = dry)

Horseweed height (cm)

ave (range)

Date

Time

Cloud cover (%)

Air temperature (°F)

Relative humidity (%)

Leaf surface moisture
(1=wet, 5 = dry)

Horseweed height (cm)
ave (range)

 

 

1987
5/ 13 6/2 6 / 13
4 pm 8 am 8 pm
10 20 Clear
70 73 69
40 80 72
4 1.5 4
5 (0.6-7.5) 10 (0.6-29) 20 (3-51)
1988
6/3 6/17 6/27
7 am 7 am 7 pm
10 10 10
48 55 82
68 68 20
4 3.5 5
5 (2-15) 10 (3-18) 20 (4-48)

 

132112. Environmental conditions for selective foliar herbicide applications in 1987 and

67

 

1988.
1987

Date 6/10 6/24 7/4
Time 10 am 6 pm 9 am
Cloud cover (%) Clear Clear 10
Air temperature (°F) 60 93 64
Relative humidity (%) 50 48 56
Leaf surface moisture

(1=wet, 5=dry) 3.5 4 3
Horseweed height (cm)

ave (range) 5 (3-30) 10 (4-48) 20 (4-74)
1988

Date 6 / 17 6/23 7/ 1
Time 10 am 8 am 8 pm
Cloud cover (%) Clear 25 Clear
Air temperature (°F) 77 75 65
Relative humidity (%) 45 40 44
Leaf surface moisture

(1 =wet, 5 = dry) 5 5 5
Horseweed height (cm)

ave (range) 5 (1-10) 10 (3-23) 20 (3-35)

 

 

 

68
environmental conditions for each application timing. Horseweed densities averaged 152
and 105 plants/m2 at the 1987 and 1988 research sites, respectively.
Visual evaluation of horseweed control and soybean injury was taken at 2, 4 and 6
weeks after treatment. Data were subjected to analysis of variance with means separated

by least significant difference at the 0.05 level of significance.

Ropewick application of glyphosate. Research was conducted in 1987 at the Michigan State
University Soil Science farm to examine ropewick applications of glyphosate for control of
horseweed. The soil type was a Riddles-Hillsdale sandy loam with 1.5% organic matter and
5.8 pH. Treatments were designed as a randomized complete block with three replications.
Plot dimensions were 3 m wide and 11 m in length.

Treatments were applied with a tractor mounted ropewick, 2.5 m length, travelling 5.6
km/hr. A solution containing 120 g glyphosate/L was applied to horseweed plants on July
14, 1987. Treatments consisted of one and two ropewick applications with opposite
directions of travel with application heights of 38, 64 or 89-cm. Horseweed plants averaged
84 cm in height with a range of 15-140 cm. The 38 cm applicator height would not be
practical since soybean height would exceed the applicator height at the time of application.
Horseweed density averaged 20 plants/m2 at. this site.

Evaluation for horseweed control was taken visually. Control ratings were based on
0-100 scale with 0 representing no horseweed injury and 100 being horseweed death. The
percentage of the total horseweed population having flowers was determined 57 days after
treatment. Treatment percentages were calculated from stand counts taken within three
randomly chosen 1 m2 areas for each plot. These data were subjected to analysis of

variance with means separated by least significant difference to the 0.05 level.

RESULTS AND DISCUSSION

Nonselective herbicide application. Foliar application of glyphosate at 0.43 and 0.84 kg/ ha
provided greater than 87% control of 5 cm tall horseweed in 1987 and 1988 (Table 3).
Glyphosate applied at 0.42 kg/ ha provided sigiificantly less control of horseweed heights
geater than 5 cm. Horseweed control was not significantly affected by plant height in
either year to glyphosate rates of 0.84 kg/ha. When horseweed height exceeded 5 cm, the
0.84 kg/ha rate of glyphosate provided sigiificantly geater horseweed control than the 0.42
kg/ha rate. During the 1987 gowing season, glyphosate (0.84 kg/ha) applied to 5 and 10-
cm tall plants obtained horseweed control superior to the herbicides HOE-39866, paraquat
or 2,4-D ester.

When applied to 5 and 10-cm plant heights, the herbicides HOE-39866 and paraquat
did not provide adequate horseweed control in 1987. Horseweed plants in these plots were
desiccated, however new gowth was initiated from apical or axillary meristems within three
weeks after treatment. Greater then 93% control of horseweed was obtained when either
HOE-39866 (0.84 kg/ha) or paraquat (0.56 kg/ha) were applied to 20-cm tall horseweed in
1987. Wilson et al. (1985) observed geater horseweed control when HOE-39866 was
applied to taller horseweed plants. The large increase in horseweed control provided by
paraquat may be due in part to the 8 pm. time of application (Table l). Putnam and Ries
(1968) observed geater weed control from evening paraquat applications than mid-day
applications. Evening applications allowed paraquat to be transported from the leaves in

a non-toxic state. This allowed geater paraquat distribution in the plant prior to light

69

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71
conversion into a toxic state.

HOE-39866 (0.84 kg/ha) applied to 5, 10 and 20-cm tall horseweed provided geater
then 93% control in 1988. Paraquat (0.56 kg/ ha) applications in 1988 provided 91% control
of 5 cm tall plants and less control of 10 and 20 cm horseweed heights. Application of 2,4-
D ester provided less than 83% horseweed control in 1987 and 1988.

The contact herbicides HOE-39866 and paraquat provided better horseweed control
in 1988 than in 1987. Weather conditions for each year were responsible in part for the
difference in horseweed control. As shown in Table 4, the 1987 research site received
adequate rainfall, however in 1988, rainfall was below normal during the months of May and
June. Since there was adequate rainfall in 1987, horseweed plants gew actively and were
able to overcome the temporary stress caused by the herbicide. Plants gowing in 1988 were
moisture stressed before and after herbicide applications. The combination of moisture and

herbicide induced stress reduced the chances for plant recovery and regowth.

Selective herbicide applications. Horseweed control obtained with selective postemergence
herbicides varied a geat deal from 1987 to 1988. This variability is due in part to the
differing weather patterns. The 1987 gowing season received adequate rainfall, however
both 1988 locations received below normal rainfall in the months of May and June (Table
5). Due to the dry soil conditions, soybean seed planted at the Shiawassee 1988 location
did not germinate uniformly.

Postemergence application of bentazon (0.84 kg/ ha) provided geater than 91% control
of 5-cm tall horseweed in 1987 (Table 6). Liquid nitrogen fertilizer (28% nitrogen) with
bentazon was sigiificantly more effective than crop oil concentrate (COC) at the 10-cm
horseweed height application. The application of bentazon (1.12 kg/ha) plus COC (2.3

L/ha) to 5-cm tall horseweed was the only 1988 bentazon treatment that provided geater

72

131219.40 Total and average monthly rainfall for the nonselective herbicide research
conducted in 1987 and 1988.

 

 

 

 

 

 

 

Total monthly rainfall
Shiawassee countL Ingham county
Month 1987 Avegge 1988 Averag
(cm)
April 5.8 12.6 14.8 12.6
May 7.2 8 8 5 8 7 1
June 9.3 4.6 1.3 5.4

July 9.5 7.9 10.1 9.4

 

73

Table 5. Total and average monthly rainfall for selective foliar herbicide research conducted in
1987 and 1988, Shiawassee County.

 

 

 

 

 

Total monthly rainfall
Year April May June July
(an)
1987 4.6 6.4 4.9 7.0
1988 9.7 1.1 1.2 8.9

30 year ave 7.2 6.5 8.4 6.9

 

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76
than 80% horseweed control. Horseweed control with bentazon applied with either 28%
nitrogen or crop oil concentrate in 1988 was similar. Horseweed control with bentazon
generally decreased as horseweed height increased. Taller horseweed plants were
suppressed by bentazon, however regrowth from apical meristems or axillary meristems was
observed approximately two weeks after treatment.

Chlorimuron applied at 9 and 13 g/ ha with non-ionic surfactant provided less than
84% horseweed control in 1987. Chlorimuron did not provide adequate horseweed control
at the 1988 location. Chlorimuron effectively destroyed horseweed apical meristems and
suppressed plant regrowth in 1987. This herbicide did not appear to cause plant death,
instead plant growth was suppressed temporarily. Approximately four to six weeks after
herbicide treatment, new horseweed shoots were observed at axillary meristems.

The selective postemergence herbicides acifluorfen, lactofen [(i)-2-ethoxy-1-methyl-2-
oxoethyl 5-[2-chloro-4»(trifluoromethyl)phenoxy]-2-nitrobenzoate], fomosafen [5-[2-chloro-
4-(trifluoromethyl)phenoxyl]-N-(methylsulfonyl)-2-nitrobenzamide] and chloramben [3-
amino-2,5-dichloro benzoic acid] were also evaluated. Application of these herbicides
provided less than 35% horseweed control regardless of horseweed height. In most cases,
the plots appeared similar to the untreated control plots.

Currently available selective postemergence herbicide options for use in soybeans do
not provide consistent, effective horseweed control. Bentazon will provide control of 5—cm
horseweed under optimal growing conditions, however control is often inconsistent.
Chlorimuron appears to temporarily suppress the growth of small horseweed for up to four
to six weeks after treatment.

Consistent and complete horseweed control with selective postemergence herbicides
is currently not feasible for several reasons. Currently available herbicides do not effectively

control horseweed greater than 5 cm in height. This is due in part to the ability of

77
horseweed plants to initiate regrth from axillary meristems making it difficult to provide
complete horseweed control. Horseweed height within a field varies greatly at the time of
herbicide application (Table 2). Due to this height variability and the inability of herbicides
to effectively control large horseweed, complete horseweed control is very difficult with

single applications of currently available selective postemergence herbicides.

Ropewick application of glyphosate. Ropewick applied glyphosate generally did not provide
adequate horseweed control (Table 7). The only treatment providing adequate horseweed
control required two applications in opposite directions 38 cm above the soil surface. This
treatment, however is not practical since crop height at the time of application would exceed
the applicator height causing substantial herbicide injury to the crop.

Horseweed control and number of flowering plants were influenced by the amount of
plant tissue contacting the herbicide during application. Two ropewick applications of
glyphosate made in opposite directions provided significantly greater horseweed control and
significantly fewer flowering plants than a single ropewick application at 38 and 89 cm
applicator heights. Plants receiving two applications were completely dessicated on both
sides of the plant. The single applications caused plant desiccation only to the side of the
plant contacted by the herbicide. The opposite side of the plant remained green and
continued to grow and flower at axillary meristems. Horseweed control increased and
flowering decreased significantly as the ropewick applicator height decreased. Horseweed
plants encountered very little herbicide induced desiccation below the point of herbicide
contact. This suggests that glyphosate did not translocate downward in the plant treated at
this stage of physiological development.

Complete control of horseweed or reduction of flowering is not feasible with ropewick

applications of glyphosate. Without basipetal translocation, glyphosate cannot effectively

78

Table 7. Ropewick application of glyphosate for controlling horseweed and reducing flower
production in 1987.

 

 

 

 

 

 

 

Horseweed
Number of Applicator Control Flower production
apphgg’ tigns hgight 45 DAT 57 DAT
(CHI) (%)
0 -- 0 100
1 38 58 25
64 72 28
89 45 57
2 38 95 1 1
64 65 30
89 70 38

 

“Comparisons valid within columns.

79
destroy horseweed axillary meristems below the point of herbicide contact. Destruction of
these axillary meristems is essential to prohibit further growth and flower production by
horseweed plants.

In summary, broadcast application of glyphosate is the only currently registered
nonselective herbicide program which provides consistent and effective control of
horseweed. When applied at 0.84 kg/ ha, horseweed control was not significantly reduced
by horseweed heights up to 20 cm. The selective foliar applied herbicides bentazon and
chlorimuron provided inconsistent control and suppression of 5-cm tall horseweed. The
most probable cause for horseweed control inconsistency was the variability of horseweed
height at the time of herbicide application. Ropewick application of glyphosate provided

inadequate control of horseweed and flower production.

BIBLIOGRAPHY

Anonymous. 1983. Control vegetation for successful no-till corn. Conservation Tillage
Guide. Successful Farming, Des Moines, IA p.14.

Dale, J. E. 1981. The rope-wick applicator - A new method of applying glyphosate. Proc.
Southern Weed Sci. Soc. 31:332.

Fawcett, R. S. 1983. Weed control in conservation tillage systems. Proc. North Central
Weed Control Conf. 38:67.

Fawcett, R. S., M. D. K. Owen and P. C. Kassel. 1983. Early preplant treatments for weed
control in no-till corn and soybeans. Proc. North Central Weed Control Conf. 38:112-
117.

Hagood, E. 8. Jr. and P. H. Davis. 1986. Horseweed control in no-till corn and soybeans.
Proc. Northeast Weed Sci. Soc. 40:26.

Henniger, C. G., J. W. Keeling and J. R. Abernathy. 1989. Horseweed Conyza canadensis
(L.) Cronq. Control in Conservation Tillage Systems. Weed Sci. Soc. Am. Abst. 19:12.

Kapusta, G. 1979. Seedbed tillage and herbicide influence on soybean (Glycine max).
Weed Control and Yield. Weed Sci. 27 :520-526.

Putnam, A. R. and S. K. Ries. 1968. Factors influencing the phytotoxicity and movement
of paraquat in quackgrass. Weed Sci. 16:336-339.

Richey, C. B., D. R. Griffith and S. D. Parsons. 1976. Yields and cultural energy
requirements for corn and soybeans with various tillage-planting systems. Adv. Agron.
29: 141-182.

Wilson, H. P., T. E. Hines, R. R. Bellinder and J. A. Grande. 1985. Comparisons of HOE-

39886, 800224, paraquat, and glyphosate in no-till corn (Zea mays). Weed Sci. 33:531-
536.

80