M

y,

2""W‘""9‘9mu§éfl!w

is untrue;

 

u
u
.
a.
A
¢
«

y Vin-H.

-q“~~x~.p

~71.I-.-I’.~‘

.,....

 

 

 

LIBRARY

Michigan State
University

 

 

 

This is to certify that the

dissertation entitled

INTEGRATING HERBICIDE RESISTANT CORN (ZEA MAYS)
INTO WEED MANAGEMENT SYSTEMS FOR MICHIGAN

presented by

Brent Edward Tharp

has been accepted towards fulfillment
of the requirements for

Ph.D. degreein Crop and Soil Sciences

 

KW MO K550

\ ;/L'4/

/ Ma jop‘grofessor

Date {tin/Liar? ’4’; 2000

MSU i: an Affirmative Action/Equal Opportunity Institution 0-12771

 

 

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6/01 c:/ClRCIDateDue.p$5-p. 15

 

INTEGRATING HERBICIDE RESISTANT CORN (Zea mays)
INTO WEED MANAGEMENT SYSTEMS FOR MICHIGAN

By

Brent Edward Tharp

A DISSERTATION

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

DOCTOR OF PHILOSOPHY
Department of Crop and Soil Sciences

2000

ABSTRACT

INTEGRATING HERBICIDE RESISTANT CORN (Zea mays) INTO WEED
MANAGEMENT SYSTEMS FOR MICHIGAN

By

Brent Edward Tharp

The recent introductions of glufosinatc-resistant and glyphosate-resistant corn have
created new opportunities for weed management. Field trials were conducted from 1996 to
1999 to determine how these herbicide resistant corn hybrids could be integrated into weed
control systems for Michigan. Glufosinate and glyphosate have similar use characteristics,
but they are distinctly different compounds with unique modes of action. In dose-response
greenhouse trials, glufosinatc and glyphosate were equally effective on bamyardgrass
(Echinochloa crus-galli), common ragweed (Ambrosia artemisiifolia), fall panicum
(Panicum dichotomiflorum), giant foxtail (Setaria faberi), and large crabgrass (Digitaria
sanguinalis). Common lambsquarters (Chenopodium album) 4- to 8-cm in height was more
sensitive to glufosinatc than glyphosate. In contrast, 15- to 20-cm velvetleaf (Abutilon
theophrasti) was more sensitive to glyphosate than glufosinatc.

Glufosinate and glyphosate do not control weeds that emerge following application.
Late postemergence (LPOST) (13—cm average weed height) applications of these herbicides
adequately controlled many weed species, but reduced corn yields in a 1997 glyphosate-
resistant corn trial. Cultivation after early postemergence (EPOST) (5-cm average weed
height) applications of glufosinate or glyphosate applications increased weed control. Weed
control was often increased when glufosinatc or glyphosate were used with residual

herbicides. Consistency of weed control was reduced if the application rate of the residual

herbicide partner was reduced. A single EPOST application of glufosinate in corn planted
in reduced row spacings and at high populations did not provide adequate season-long weed
control. Corn yields were not affected by row spacing, but were increased when com
populations exceeded 72,900 plants/ha. Other weed management strategies need to be used
with glufosinatc or glyphosate to obtain season-long weed control without reducing corn
yields.

In no—tillage corn trials, glyphosate burndown timings can be delayed to enhance
weed control in the absence of a cover crop. Delayed burndown timings in an actively

growing wheat cover crop resulted in reduced corn populations and yields due to competition

from the cover crop.

ACKNOWLEDGMENTS

This dissertation would not have been possible without the guidance and support of
many individuals. Sincere appreciation is extended to Dr. Jim Kells who was instrumental
in guiding the direction of my graduate program, and who has provided me with many
rewarding experiences that have enhanced my professional growth. I am also grateful to Dr.
Donald Penner, Dr. Karen Renner, and Dr. Scott Swinton for their guidance and support
throughout my graduate career.

I thank Andy Chomas for leadership and assistance in field research, and who, along
with Gary Powell, always made summer field seasons fun and exciting. Special thanks goes
to fellow graduate students: Caleb Dalley, Jason Fausey, Nate Kemp, Chad Lee, Kelly
Nelson, Kyle Poling, Corey Ransom, Matt Rinella, Joe Simmons, Eric Spandl, Christy
Sprague, Sherry White, and Karen Zuver for great friendships, support, and assistance. The
assistance and friendships of Gabe Corey, Stephanie Eickholt, Kyle F iebi g, Jessica Leep, and
Pat O’Boyle are greatly appreciated.

I am extremely grateful and appreciative of the loving support my parents have
provided me throughout my graduate career. Finally, my personal relationship with Jesus

Christ has provided me with more than I will ever need, and all glory goes to Him.

iv

TABLE OF CONTENTS

LIST OF TABLES .................................................... vii
LIST OF FIGURES ..................................................... x
INTRODUCTION ...................................................... 1
CHAPTER 1
RESPONSE OF ANNUAL WEED SPECIES TO GLUFOSINATE AND
GLYPHOSATE ........................................................ 3
ABSTRACT ...................................................... 3
INTRODUCTION ................................................. 5
MATERIALS AND METHODS ...................................... 6
Single growth stage experiments ................................ 6
Multiple growth stage experiments .............................. 8
Statistical analysis ........................................... 9
RESULTS AND DISCUSSION ..................................... 11
Single growth stage experiments ............................... 11
Multiple growth stage experiments ............................. 12
LITERATURE CITED ............................................ 15
CHAPTER 2

INFLUENCE OF HERBICIDE APPLICATION RATE, TIMING, AND
INTERROW CULTIVATION ON WEED CONTROL, CORN (Zea mays) YIELD,
AND PROF ITABILITY IN GLUFOSINATE-RESISTANT AND GLYPHOSATE-

RESISTANT CORN ................................................... 23
ABSTRACT ..................................................... 23
INTRODUCTION ................................................ 25
MATERIALS AND METHODS ..................................... 26

Experimental Description .................................... 26
Statistical Analysis .......................................... 28
Profitability Analysis ........................................ 29
RESULTS AND DISCUSSION ..................................... 30
Effect of herbicide application timing on weed density ............. 30
Effect of herbicide rate and application timing on weed control ....... 30
Effect of cultivation and application timing on weed control ......... 34
LITERATURE CITED ............................................ 38

TABLE OF CONTENTS (cont.)

CHAPTER 3
WEED CONTROL AND CORN (Zea mays) YIELD FROM RESIDUAL
HERBICIDE COMBINATIONS WITH GLYPHOSATE AND GLUFOSINATE IN

CORN ............................................................... 48
ABSTRACT ..................................................... 48
INTRODUCTION ................................................ 50
MATERIALS AND METHODS ..................................... 51

Application timing of residual herbicide combinations .............. 52
Reduced application rates of residual herbicides ................... 53
Data collection and statistical analysis .......................... 54
RESULTS AND DISCUSSION ..................................... 55
Application timing of residual herbicide combinations .............. 55
Reduced application rates of residual herbicides ................... 58
LITERATURE CITED ............................................ 62
CHAPTER 4

EFFECT OF CORN (Zea mays) POPULATION AND ROW SPACING ON LIGHT
INTERCEPTION, WEED GROWTH, AND CORN YIELD IN GLUFOSINATE-

RESISTANT CORN ................................................... 73
ABSTRACT ..................................................... 73
INTRODUCTION ................................................ 85
MATERIALS AND METHODS ..................................... 77
RESULTS AND DISCUSSION ..................................... 81

Light interception ........................................... 81

Weed growth .............................................. 81

Corn yield ................................................. 83

LITERATURE CITED ............................................ 85
CHAPTER 5

DELAYED BURNDOWN TIMINGS IN N O-TILLAGE GLYPHOSATE-
RESISTANT CORN (Zea mays) PLANTED INTO SOYBEAN (Glycine max)

RESIDUE AND INTO A WHEAT (T riticum aestivum) COVER CROP ......... 94
ABSTRACT ..................................................... 94
INTRODUCTION ................................................ 96
MATERIALS AND METHODS ..................................... 98

No-tillage corn into soybean residue ............................ 98
No-tillage corn into a wheat cover crop ......................... 101
RESULTS AND DISCUSSION .................................... 104
No-tillage corn into soybean residue ........................... 104
No-tillage corn into a wheat cover crop ......................... 105
LITERATURE CITED ........................................... 108

vi

LIST OF TABLES

CHAPTER 1

RESPONSE OF ANNUAL WEED SPECIES TO GLUFOSINATE AND
GLYPHOSATE

Table 1. Statistical models fit to weed species and measures of quality of fit. . . 18

Table 2. GRSO values for weeds treated at a single growth stage. ........... 19
Table 3. GRSO values for weeds treated at multiple growth stages. .......... 20
CHAPTER 2

INFLUENCE OF HERBICIDE APPLICATION RATE, TIMING, AND
INTERROW CULTIVATION ON WEED CONTROL, CORN (Zea mays) YIELD,
AND PROFITABILITY IN GLUFOSINATE-RESISTANT AND GLYPHOSATE-
RESISTANT CORN

Table 1. Corn growth stages, weed heights, and weed densities at time of
herbicide application and cultivation. ................................. 41

Table 2. Assumption of costs used in profitability analysis. ................ 42
Table 3. Weed densities 18 d after glufosinatc and glyphosate applications. . . . 43

Table 4. Weed control, corn yield, and gross margins as affected by glufosinatc
application rate and timing, 1996 and 1997. ............................ 44

Table 5. Weed control, corn yield, and gross margins as affected by glyphosate
application rate and timing, 1996 and 1997. ............................ 45

Table 6. Weed control, com yield, and gross margins as affected by glufosinatc
application timing and cultivation, 1996 and 1997. ...................... 46

Table 7. Weed control, corn yield, and gross margins as affected by glyphosate
application timing and cultivation, 1996 and 1997. ...................... 47

vii

LIST OF TABLES (cont.)

CHAPTER 3
WEED CONTROL AND CORN (Zea mays) YIELD FROM RESIDUAL
HERBICIDE COMBINATIONS WITH GLYPHOSATE AND GLUFOSINATE IN

CORN

Table 1. Corn growth stage and weed density at POST, 1996-1999. ......... 64

Table 2. Effect of application timing of residual herbicide combinations with
glyphosate on weed control and corn yield. ............................ 65

Table 3. Effect of application timing of residual herbicide combinations with
glufosinatc on weed control and corn yield. ............................ 67

Table 4. Effect of reduced rates of residual herbicides in glyphosate tank mixtures
on weed control and corn yield. ..................................... 69

Table 5. Effect of reduced rates of residual herbicides in glufosinatc tank mixtures
on weed control and corn yield. ..................................... 71

CHAPTER 4

EFFECT OF CORN (Zea mays) POPULATION AND ROW SPACING ON LIGHT
INTERCEPTION, WEED GROWTH, AND CORN YIELD IN GLUFOSINATE-
RESISTANT CORN

Table 1. Actual corn populations for each year of the study. ............... 87

Table 2. Effect of row spacing and corn population on emergence of common
lambsquarters following an application of glufosinatc, 1998-1999. .......... 88

Table 3. Effect of row spacing and corn population on untreated common
lambsquarters biomass and seed production, 1998-1999. .................. 89

Table 4. Effect of row spacing, corn population, and weed control treatments on
corn grain yields, 1998-1999. ....................................... 90

CHAPTER 5

DELAYED BURNDOWN TIMINGS IN NO-TILLAGE GLYPHOSATE-
RESISTANT CORN (Zea mays) PLANTED INTO SOYBEAN (Glycine max)
RESIDUE AND INTO A WHEAT (T riticum aestivum) COVER CROP

Table 1. Weekly rainfall amounts, 1998 and 1999. ...................... 111

viii

LIST OF TABLES (cont.)

Table 2. Effect of delayed glyphosate bumdown applications on velvetleaf
control, velvetleaf biomass, and corn yield, 1998-1999. ........................ 112

Table 3. Effect of delayed glyphosate bumdown applications on wheat cover crop
and corn emergence, height, and yield, 1998-1999. ..................... 114

Table 4. Effect of banded applications of glyphosate on corn emergence, height,
and yield at delayed bumdown timings, 1998-1999 ..................... 116

ix

LIST OF FIGURES

CHAPTER 1

RESPONSE OF ANNUAL WEED SPECIES TO GLUFOSINATE AND
GLYPHOSATE

Figure 1. Example of log-logistic dose response curves for 5- to 6- cm velvetleaf
treated with glyphosate and glufosinatc. ............................... 21

Figure 2. Example of log-logistic dose response curves for 5- to 6- cm, 8- to 13-
cm, 15- to 20- cm velvetleaf treated with glyphosate. ..................... 22

CHAPTER 4

EFFECT OF CORN (Zea mays) POPULATION AND ROW SPACING ON LIGHT
INTERCEPTION, WEED GROWTH, AND CORN YIELD IN GLUFOSINATE-
RESISTANT CORN

Figure 1. Photosynthetic active radiation (PAR) intercepted by com canopies
among row spacings at weekly intervals in 1998 and 1999. ................ 91

Figure 2. Photosynthetic active radiation (PAR) intercepted by com canopies
among corn populations at weekly intervals in 1998 and 1999. ............. 92

Figure 3. Relationship of common lambsquarters seed production to shoot
biomass. ....................................................... 93

INTRODUCTION

The methods used to control weeds today are dramatically different from the methods
used fifty years ago. Prior to 1945, weeds were primarily controlled by tillage, crop rotation,
and manual labor. Since 1945, numerous synthetic herbicides have been developed and have
replaced dependency on other weed control methods. More effective weed management
practices need to be developed to meet future weed control needs.

Recent advances in science have allowed scientists to develop agricultural systems
which potentially provide effective, economical, and environmentally safe weed control.
Agronomists, plant breeders, and molecular biologists have been using biotechnology to
develop agronomic crops that are resistant to certain herbicides. Herbicide resistant crops are
designed to be insensitive to herbicide exposure, which would normally be lethal to the cr0p.
Herbicide resistant corn is a relatively new technology. Corn hybrids resistant to glufosinatc
and glyphosate have recently been introduced into the market. Glufosinate and glyphosate
are classified as non-selective, foliar applied herbicides. A non-selective herbicide is
generally toxic to all treated plants. Although these herbicides have similar use
characteristics, they are distinctly different compounds with unique modes of action. Very
little public information exists regarding the use of these herbicides in a production system
with corn resistant to these herbicides. Weed species differ in sensitivity to these herbicides
with some weeds more difficult to control than others. Research is needed to detemiine the

relative effectiveness of glufosinate and glyphosate on summer annual weed species, and

how these herbicides will perform in a row crop environment.

One of the major characteristics of glufosinatc and glyphosate is the lack of herbicidal
activity in the soil. These herbicides will not control weeds that emerge afier application.
Weed management systems that do not control weeds up to crop canopy closure will not
readily be adopted by farmers. Research is needed to identify weed management systems in
glufosinate- and glyphosate-resistant corn that consistently provide season-long weed
control.

Greenhouse trials were conducted to compare the relative effectiveness of glufosinatc
and glyphosate on several annual weed species common to Michigan. Field trials were
conducted to determine effective application timings and rates. Additional field trials were
conducted to identify weed management strategies that could be used with glufosinatc or
glyphosate to provide consistent weed control. The strategies that were investigated included:
i.) cultivation following an application of glufosinatc or glyphosate; ii.) residual herbicides
in combination with glufosinate or glyphosate; and iii.) planting high populations of corn in
reduced row spacings. In no-tillage glyphosate-resistant com, the effect of delayed bumdown

timings on weed control and corn yield was also investigated.

CHAPTER 1
RESPONSE OF ANNUAL WEED SPECIES TO GLUFOSINATE AND

GLYPHOSATE

Abstract. The recent introduction of glufosinatc-resistant and glyphosate-resistant crOps
provide growers with new options for weed management. Information is needed to compare
the effectiveness of glufosinate and glyphosate on annual weeds. Greenhouse trials were
conducted to determine the response of bamyardgrass, common lambsquarters, common
ragweed, fall panicum, giant foxtail, large crabgrass, and velvetleaf to glufosinatc and
glyphosate. The response of velvetleaf and common lambsquarters was investigated at
multiple stages of growth. Glufosinate and glyphosate were applied to each weed species at
logarithmically incremented rates. The glufosinatc and glyphosate rates which provided a 50
percent reduction in aboveground weed biomass, commonly referred to as GR50 values, were
compared using nonlinear regression techniques. Barnyardgrass, common ragweed, fall
panicum, giant foxtail, and large crabgrass responded similarly to glufosinatc and glyphosate.
Common lambsquarters 4 to 8 cm in height was more sensitive to glufosinate than
glyphosate. In contrast, 15- to 20-cm velvetleaf was more sensitive to glyphosate than
glufosinatc.

Nomenclature: Glufosinate, 2-amino-4-(hydroxymethylphosphinyl)butanoic acid;

glyphosate, N-(phosphonomethyl)glycine; bamyardgrass, Echinochloa crus-galli L. Beauv.

#‘ ECHCG; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed,
Ambrosia artemisiifolia L. # AMBEL; fall panicum, Panicum dichotomiflorum Michx. #
PANDI; giant foxtail, Setariafaberi Herrm. # SETFA; large crabgrass, Digitaria sanguinalis
L. Scop. # DIGSA; velvetleaf, Abutilon theophrasti Medicus. # ABUTH.

Additional index words: dose response, GRSO, Abutilon theophrasti, Ambrosia
artemisiifolia, Chenopodium album, Digitaria sanguinalis, Echinochloa crus-galli, Panicum
dichotomiflorum, Setaria faberi, ABUTH, AMBEL, CHEAL, DIGSA, ECHCG, PANDI,
SETFA.

Abbreviations: GRSO, rate causing 50% growth reduction.

 

‘ The letters following this symbol are a WSSA-approved computer code from

Composite List of Weeds, revised 1989. Available from WSSA.

4

INTRODUCTION

Glufosinate and glyphosate are nonselective, foliar-applied herbicides that have been
used for vegetative management in non-crop environments for several years (Wilson et al.
1985; Blackshaw 1989; Lanie et a1. 1994). In a fallow system of volunteer hard red winter
wheat (T riticum aestivum L. ‘Centurk 78’), Carlson and Burnside (1984) found that
glufosinatc toxicity was less than glyphosate at equal rates, and four times the application
rate of glufosinatc was required to achieve equivalent control of winter wheat as compared
to glyphosate. Before the advent of crops modified to resist glufosinate and glyphosate, the
herbicides could only be applied in cropping systems using controlled application systems
(Schweizer and Bridge 1982; Eberlein et al. 1993). The recent introduction of glufosinate-
and glyphosate-resistant crops has created new opportunities for use of these herbicides for
selective weed control in crop production. The performance of glufosinatc and glyphosate
has been evaluated on summer annual weeds in resistant crops (Lich et al. 1996; Tharp and
Kells 1997). Steckel et al. (1997a) reported that weed control using glufosinate was
influenced by application rate and weed growth stage. Similarly, Jordan et al. (1997)
concluded that application rate and timing were critical factors in the effectiveness of
glyphosate. Differential herbicide effectiveness is often associated with environmental
effects on herbicide absorption, translocation, and metabolism within a plant (Schultz and
Burnside 1980; Mersey et al. 1990; Steckel et al. 1997b). Although the effectiveness of
glufosinate or glyphosate on weeds has been explored, more information is needed for direct
comparison of the effectiveness of these herbicides on annual weeds.

Dose-response experiments are widely used to evaluate the effectiveness or activity of

herbicides (Heap and Morrison 1996; Sprague et a1. 1997). The results of these trials are
often analyzed and reported using dose-response curves (Streibig 1980; Seefeldt et a1. 1995).
Seefeldt et a1. (1995) referred to the comparison of two or more dose-response curves as
differential dose-response relationships. Values describing the growth response of a weed
to a herbicide are often estimated from the dose-response curves, and the values are used to
compare the effect being tested (Larke and Streibig 1995; Steckel et al. 1997b). One such
value is GRSO, the herbicide rate that decreases plant growth by 50%. Sandra] et al. (1997)
used log-logistic dose response curves to estimate and compare GR50 values of five herbicide
treatments applied to subterranean clover (T rifolium subterraneum L.). Carey et a1. (1997)
also compared GR50 estimates to determine the relative sensitivity of five plant species to two
herbicides.

The introduction of glufosinatc-resistant and glyphosate-resistant crops will place these
two herbicides in competition with each other in the herbicide market, and growers need
more information regarding their efficacy. Greenhouse trials were designed to compare the
relative effectiveness of glufosinatc and glyphosate on annual weed species. The statistical
analysis used in these trials enable direct comparisons of glufosinatc and glyphosate GR50

values on several annual weeds and on weeds growing at various stages of growth.

MATERIALS AND METHODS
Single growth stage experiments. Greenhouse trials were conducted to determine the
relative sensitivity of seven annual weed species to glufosinatc and glyphosate at one growth

stage. Bamyardgrass, common lambsquarters, common ragweed, fall panicum, giant foxtail,

large crabgrass, and velvetleaf were grown in a commercial potting mixture2 in 1L plastic
pots. Plants were grown in a 16 h photOperiod of natural lighting supplemented with sodium
halide lights providing a midday photosynthetic photon flux density of 1000 umol/mZ/s. Air
temperature was maintained at 27°C i 5 °C. Plants were watered and fertilized as needed to
insure maximum growth.

Commercial formulations of glufosinate and glyphosate were applied to weeds using a
continuous link belt sprayer equippexl with an 8001B flat fan nozz1e3 calibrated to deliver 234
L/ha at an operating pressure of 214 kPa. Grass weeds were thinned to five plants and
broadleaf weeds were thinned to one plant at least 7 days before treatment. The height of
bamyardgrass at application was 15 to 20 cm, common lambsquarters was 5 to 8 cm,
common ragweed was 8 to 10 cm, fall panicum was 5 to 10 cm, giant foxtail was 5 to 10 cm,
large crabgrass was 18 to 25 cm, and velvetleaf was 5 to 6 cm. Each weed species was
managed as an independent greenhouse trial. Each trial was a two-factor factorial designed
as a randomized complete block with four replications. Each trial was repeated in time. The
factors were herbicide and application rate. A commercial formulation of glufosinatc
ammonium or isopropylamine salt of glyphosate was applied to the seven weed species.
Herbicide application rates were logarithmically increased from a rate that caused no
herbicide injury to a rate that caused plant death in preliminary greenhouse screens for each

weed species (unpublished data). The application rates used on common ragweed were 0,

 

2Baccto professional planting mix, Michigan Peat Co., PO. Box 980129,
Houston, TX. 77098.

3Spraying Systems Co., PO. Box 7900, Wheaton IL 60189.

7

0.020, 0.040, 0.080, 0.16, and 0.32 kg ae/ha for glufosinatc and 0, 0.053, 0.11, 0.21, 0.42,
and 0.84 kg ae/ha for glyphosate. These rates in addition to 0.641 kg ae/ha glufosinate and
1.68 kg ae/ha glyphosate were applied to bamyardgrass, common lambsquarters, large
crabgrass, and velvetleaf. The lowest rate of 0.020 kg/ha glufosinatc was dropped from the
preceding rates and 1.28 kg/ha glufosinatc was added for the giant foxtail and fall panicum
trials. Spray grade ammonium sulfate was added to all herbicide mixtures at a rate of 2%
w/w. Above-ground biomass was harvested 14 d after treatment for all weed species except
common ragweed, which was harvested 21 d after treatment. The harvested plants were oven
dried at 110 C for 72 h and weighed. The response of each weed was expressed as dry weight
relative to the untreated plants and calculated by taking the dry weight of a treated plant and
dividing it by the mean dry weight of the untreated plants and multiplying this value by 100.
Multiple growth stage experiments. Common lambsquarters were grown in the greenhouse
to 4 to 8 cm, 10 to 15 cm, and 20 to 30 cm in height, and velvetleaf was grown to 5 to 6 cm,
8 to 13 cm, and 15 to 20 cm in height. The weeds were treated with glufosinatc or glyphosate
in order to determine the effect of weed size on the sensitivity of the weeds to the herbicides.
The environmental conditions used for growing and treating these weeds were identical to
the conditions in the single stage experiment.

Common lambsquarters and velvetleaf were managed as independent greenhouse trials
and were repeated in time. Each trial was a three-factor factorial designed as a randomized
complete block with four replications. The factors consisted of herbicide, herbicide
application rates, and weed grth stage. Glufosinate and glyphosate were applied to

common lambsquarters and velvetleaf using rates in logarithmic increments of 0, 0.0044,

0.017, 0.070, 0.28, 1.12, 4.48 kg/ha. Spray grade ammonium sulfate was included in the
herbicide mixture at a rate of 2% w/w. The above-ground biomass of the plants was
harvested 14 d after treatment and oven dried. The response of each weed was expressed as
dry weight reduction relative to the untreated plants.

Statistical analysis. Nonlinear dose-response curves were fitted, as described in
Schabenberger et al. (1999), to the data obtained on the various weed species (Figures 1 and
2). The log—logistic dose-response model properly represented the trends in grth reduction
for each weed species tested except bamyardgrass and common lambsquarters. As noted by
Brain and Cousens (1989), some species exhibit a hormetic growth increase at low rates of
applications. The term “hormesis” was coined by Southman and Ehrlich (1943) to describe
the stimulatory effect of an organism when exposed to subinhibitory concentrations of a
toxic substance. Ignoring this effect can bias the GR50 estimate as demonstrated by
Schabenberger et al. (1999). Horrnetic response was found in bamyardgrass. Therefore, the
log-logistic model:

a— 5
+ + e
1+ exp{,61n(x/GR50)}

 

[1]

where x is rate of application, 5 is the lower asymptote of relative growth, on is the upper

asymptote, and 0 relates to the rate of change near the inflection point, was modified to:

a— 6+ 7x
+ +e
1+ wexp{flln(x/GR50)}

 

y= 5 [2]

The variable, y, in equation [2] measures the hormetic effect and allows the dose-response

to grow beyond the upper asymptote and (I) = 1 + 2yGR50/(oc - 6). This version of the Brain-

Cousens model (Brain and Cousens 1998), which allows estimation of GRSO directly, is
derived in Schabenberger et al. (1999). Notice that for y= 0, equation [2] reduces to a log-
logistic model.

The sigmoidal shape of the log-logistic model was not supported by the common
lambsquarters data for which relative growth decreased with increasing rate of application
without inflection. Instead, a modified version of the Langmuir model (Seber and Wild 1989)

was chosen to depict dose-response in common lambsquarters. The model in terms of GR50
is:
l3
(x / GR50 )

= [3]
y a1+(x/GR50)fl +6

 

where again a is the upper asymptote.

Differences in GR50 values among herbicides or growth stages for a given species were
tested for significance with the sum of squares reduction test or “lack-of-fit F-test”. The
appropriate full model was fit to data combined across herbicides in the single growth stage
experiment or to data combined across herbicides and growth—stages in the multiple growth
stage experiment varying all parameters by treatments. Schabenberger et al. (1999) provides
a more detailed description of the method used analyze GRSO values and lists the SAS code
and assumptions of nonlinear model parameters used in the analyses. The models used for

each weed species and the resulting R2 and mean square error values are summarized in

Table 1.

10

RESULTS AND DISCUSSION

Single growth stage experiments. The GR50 values for glufosinatc and glyphosate were
similar for bamyardgrass, common ragweed, fall panicum, giant foxtail, and large crabgrass
(Table 2). Similarity in GR50 values indicates that the effectiveness of glufosinatc and
glyphosate are similar for these weed species. Contrary to our findings, Wilson et al. (1985)
reported greater common ragweed control from 1.1 kg ai/ha of glufosinate than from 1.1 kg
ai/ha of glyphosate. Common ragweed control was similar when they increased the
glyphosate rate to 1.7 kg/ha. The same study also reported that fall panicum control was
similar following early applications of glufosinatc and glyphosate; however, later
applications of 1.7 kg/ha glyphosate provided greater fall panicum control than the same rate
of glufosinate. In field trials, Lanie et al. (1994) showed similar control of bamyardgrass
after early applications of glufosinate and glyphosate, which was in agreement with the
results of our greenhouse trials (Table 2). However, later applications of 0.84 kg/ha
glyphosate provided greater bamyardgrass control than 0.84 kg/ha glufosinatc suggesting
bamyardgrass grth stage influences herbicide effectiveness (Lanie et al. 1994).

The GR50 values of glufosinatc and glyphosate differed for common lambsquarters and
velvetleaf (Table 2). More glyphosate was needed to reduce common lambsquarters biomass
by 50% when compared to glufosinatc, suggesting that common lambsquarters is more
susceptible to glufosinate than glyphosate. Our results agree with those of Wilson et al.
(1985) and Higgins et al. (1991), who reported that glufosinatc provided equal or greater
control of common lambsquarters than glyphosate applied at similar rates. The response of

velvetleaf was opposite to that of common lambsquarters. More glufosinatc was needed to

11

reduce velvetleaf biomass by 50% when compared to glyphosate, suggesting that velvetleaf
is more susceptible to glyphosate than glufosinatc (Table 2). However, at higher application
rates velvetleaf responded similarly to both herbicides (Figure 1).
Multiple growth stage experiments. Common Lambsquarters. Common lambsquarters was
more susceptible to glufosinatc than glyphosate in the single grth stage experiment.
Therefore, the influence of common lambsquarters growth stage on herbicide effectiveness
was examined. Glufosinate GR50 values were similar among the three growth stages of
common lambsquarters (Table 3). The GR50 values for glyphosate were also similar among
the growth stages. The response of common lambsquarters to glufosinatc or glyphosate is
not strongly influenced by the size of common lambsquarters in controlled environmental
conditions. However, the size of common lambsquarters has been shown to influence
herbicide efficacy in field experiments (Higgins et al. 1991; Krausz et al. 1996, Steckel et
al. 1997a). Glufosinate and glyphosate activity are also affected by environmental conditions
(Anderson et al. 1993a, 1993b; Devine et al. 1983; McWhorter and Azlin 1978). Plants
growing in typical greenhouse conditions of supplemented artificial light, constant
temperatures, and ample moisture tend to develop differently than plants growing in natural
environments. Differences in results from greenhouse and field experiments could be
attributed to differences in conditions of the two environments.

The GR50 value of glyphosate was greater than the glufosinatc GR50 value for 4- to 8-cm
tall common lambsquarters (Table 3). Glufosinate and glyphosate GR50 values were not
significantly different for 10- to 15-cm common lambsquarters. The glyphosate GR50 value

was more than twice as large as the glufosinate GR50 value for 20— to 30-cm tall common

12

lambsquarters, but this difference was not statistically significant (Table 3). This discrepancy
may be related to variability of the common lambsquarters data as expressed by a large mean
square error (Table l), which results in imprecise estimates of the GRSO parameter. The wide
range in common lambsquarters size may have contributed to the variability.

Velvetleaf Since velvetleaf was found to be more susceptible to glyphosate than glufosinatc
in the single growth stage experiments, the influence of the growth stage of velvetleaf on
herbicide effectiveness was examined. Figure 2 shows growth response curves for velvetleaf
at three growth stages to glyphosate. Although the shape of the curves are similar for each
growth stage, higher rates of glyphosate were required to induce a similar response on larger
velvetleaf (Figure 2). The GR50 values of glufosinatc and glyphosate increased at the later
velvetleaf growth stages (Table 3). The 15- to 20-cm velvetleaf required more glufosinate
and glyphosate for control than the 5- to 6-cm velvetleaf (Table 3). Smaller velvetleaf were
more sensitive to both herbicides than larger velvetleaf. Jordan et al. (1997) and Krausz et
a1. (1996) concluded that application timing was important for velvetleaf control with
glyphosate. Lich et a1. (1997) suggested that environmental conditions influence glyphosate
activity on velvetleaf. Similar conclusions were reported when glufosinatc was used for
velvetleaf control (Steckel et al. 1997a).

Glufosinate GR50 values were similar to glyphosate GR50 values for 5- to 6-cm and 8- to
13-cm velvetleaf, while glufosinatc GR50 values for 15- to 20-cm velvetleaf were greater than
glyphosate GR50 values (Table 3). The sensitivity of velvetleaf to glyphosate and glufosinatc
decreased as velvetleaf grew, and 15- to 20-cm velvetleaf was more sensitive to glyphosate.

The large overlapping leaves of velvetleaf makes entire coverage of the plant with spray

13

solution more difficult as the plant grows. Incomplete spray coverage would favor an easily
translocated herbicide such as glyphosate over a contact herbicide such as glufosinatc. The
difference in velvetleaf sensitivity to glufosinatc and glyphosate for the larger velvetleaf
could be related to the differences in the translocation patterns of these herbicides.

In conclusion, the response of various weed species to glufosinatc and glyphosate were
compared by fitting a nonlinear dose-response curve to each weed species. Appropriate
nonlinear models were chosen for statistical comparison of GR50 values. Based on our results
the relative effectiveness of glufosinatc and glyphosate were similar for five of the seven
weed species tested. However, common lambsquarters was more sensitive to glufosinatc than
glyphosate, and velvetleaf was more sensitive to glyphosate than glufosinatc under controlled
environmental conditions. With regard to the velvetleaf and common lambsquarters
experiments, glufosinatc and glyphosate activity was influenced by the growth stage of
velvetleaf but not common lambsquarters. The environmental conditions in which a weed
is growing may influence the degree by which herbicide efficacy is affected by growth stage.
Further research is needed to study the efficacy of glyphosate and glufosinatc under various
environmental conditions and to determine how these herbicides can be more effectively

used for annual weed control in herbicide resistant crops.

14

LITERATURE CITED

Anderson, D. M., C. J. Swanton, J. C. Hall, and B. G. Mersey. 1993a. The Influence of

temperature and relative humidity on the efficacy of glufosinatc-ammonium. Weed Res.
33:139--l47.

Anderson, D. M., C. J. Swanton, J. C. Hall, and B. G. Mersey. 1993b. The influence of soil
moisture, simulated rainfall and time of application on the efficacy of glufosinate-
ammonium. Weed Res. 33: 149--160.

Blackshaw, R. E. 1989. HOE-39866 Use in chemical fallow systems. Weed Technol. 3:420-
428.

Brain, P. and R. Cousens. 1989. An equation to describe dose responses where there is
stimulation of growth at low doses. Weed Res. 29: 93-96

Carey, J. B., D. Penner, and J. J. Kells. 1997. Physiological basis for nicosulfuron and
primisulfuron selectivity in five plant species. Weed Sci. 45:22—-30.

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

Devine, M. D., J. D. Bandeen, and B. D. McKersie. 1983. Temperature effects on glyphosate

absorption, translocation, and distribution in quackgrass (Agropyron repens). Weed Sci.
31 :461--464.

Eberlein, C. V., M. J. Guttieri, and F. N. Fletcher. 1993. Broadleaf weed control in potatoes
(Solanum tuberosum) with postemergence directed herbicides. Weed Technol. 7:298--303.

Heap, I. M. and I. N. Morrison. 1996. Resistance to aryloxyphenoxypropionate and
cyclohexanedione herbicides in green foxtail (Setaria viridis). Weed Sci. 44:25--30.

Higgins, J. M., T. Whitwell, and J. E. Toler. 1991. Common lambsquarters (Chenopodium
album) control with non-selective herbicides. Weed Technol. 5:884--886.

Jordan, D. L., A. C. York, J. L. Griffin, P. A. Clay, P. R. Vidrine, and D. B. Reynolds. 1997.
Influence of application variables on efficacy of glyphosate. Weed Technol. 11:354--362.

Krausz, R. F ., G. Kapusta, and J. L. Matthews. 1996. Control of annual weeds with
glyphosate. Weed Technol. 10:957--962.

Larke, P. E. and J. C. Streibig. 1995. Foliar absorption of some glyphosate formulations and
their efficacy on plants. Pestic. Sci. 44: 107--1 16.

15

Lanie, A. J ., J. L. Griffin, P. R. Vidrine, and D. B. Reynolds. 1994. Weed control with non-

selective herbicides in soybean (Glycine max) stale seedbed culture. Weed Technol. 8: 159--
164.

Lich, J. M., K. A. Renner, and D. Penner. 1997. Interaction of glyphosate with
postemergence soybean (Glycine max) herbicides. Weed Sci. 45: 12--21.

McWhorter, C. G. and W. R. Azlin. 1978. Effects of environment on the toxicity of

glyphosate to johnsongrass (Sorghum halepense) and soybeans (Glycine max). Weed Sci.
26:605--608.

Mersey, B. G., J. C. Hall, D. M. Anderson, and C. J. Swanton. 1990. Factors affecting the
herbicidal activity of glufosinatc-ammonium: absorption, translocation, and metabolism in
barley and green foxtail. Pestic. Biochem. Physiol. 37:90--98.

Sandra], G. A., B. S. Dear, J. E. Pratley, and B. R. Cullis. 1997. Herbicide dose rate response
curves in subterranean clover determined by a bioassay. Aust. J. of Exp. Agric. 37:67--74.

Schabenberger, O., B. E. Tharp, J. J. Kells, and D. Penner. 1999. Statistical tests for hormesis
and effective dosages in herbicide dose response. Agron J. 19:713-721.

Schultz, M. E. and O. C. Burnside. 1980. Absorption, translocation, and metabolism of 2,4-D
and glyphosate in hemp dogbane (Apocynum cannabinum). Weed Sci. 28:13--20.

Schweizer, E. E., and L. D. Bridge. 1982. Control of five broadleaf weeds in sugarbeets
(Beta vulgaris) with glyphosate. Weed Sci. 30:291--296.

Seber, G.A.F. and C. J. Wild. 1989. Nonlinear Regression. Wiley and Sons, New York.

Seefeldt, S. S., J. E. Jensen, and E. P. Fuerst. 1995. Log-logistic analysis of herbicide dose-
response relationships. Weed Technol. 9:218--227.

Sprague, C. L., E. W. Stoller, and L. W. Wax. 1997. Common cocklebur (Xanthium
strumarium) resistance to selected ALS-inhibiting herbicides. Weed Technol. 11:241--247.

Steckel, G. J ., L. M. Wax, F. W. Simmons, and W. H. Phillips 11. 1997a. Glufosinate efficacy
on annual weeds is influenced by rate and growth stage. Weed Technol. 11:484--488.

Steckel, G. J ., S. E. Hart, and L. M. Wax. 1997b. Absorption and translocation of glufosinatc
on four weed species. Weed Sci. 45:378--381.

Streibig, J. C. 1980. Models for curve-fitting herbicide dose response data. Acta Agriculturae
Scandinavica. 30:59--63.

l6

Tharp, B. E. and J. J. Kells. 1997. Weed management strategies in glufosinate resistant and
glyphosate resistant corn. North Central Weed Sci. Soc. Proc. 52:64.

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 Sci. 33:531-536.

17

Table 1. Statistical models fit to weed species and measures of quality of fit.

 

 

 

Single growth stage experiments Quality of Fit Measures
Model R2 Mean Square
Error
Barnyardgrass Brain Cousens 0.86 442
Common lambsquarters Langmuir 0.89 185
Common ragweed Log-logistic 0.89 172
Fall panicum Log-logistic 0.80 307
Giant foxtail Log-logistic 0.84 318
Large crabgrass Log-logistic 0.85 227
Velvetleaf Log-logistic 0.95 78

 

Multiple growth stage experiments

 

Common lambsquarters Langmuir 0.79 414
Velvetleaf Log-logistic 0.90 1 81

 

l8

Table 2. GR50 values for weeds treated at a single growth stage.

GR50 valuesa

 

 

Weed species Height Growth stage Glufosinate Glyphosate
cm true leaves
Barnyardgrass 15-20 2-3 0.15 a 0.16 a
Common lambsquarters 5-8 56 0.069 b 0.12 a
Common ragweed 8-10 6-7 0.063 a 0.064 a
Fall panicum 5-10 2-3 0.080 a 0.064 a
Giant foxtail 5-10 2-3 0.097 a 0.096 a
Large crabgrass 18-25 4-6 0.11 a 0.12 a
Velvetleaf 5-6 2-3 0.16 a 0.12 b

 

“GRSO values, within each weed and measurement, followed by the same lower case letter are
not significantly different at the 0.05 probability level.

19

Table 3. GRSO values for weeds treated at multiple growth stages.

GRSO valuesbc

 

Heighta Growth stagea Glufosinate Glyphosate

 

cm true leaves —— kg/ha
Common lambsquarters

4-8 4-6 0.14 B a 0.24 A a

10-15 8-10 0.20Aa 0.20Aa

20-30 10-12 0.16 A a 0.34 A a
Velvetleaf

5-6 2-3 0.037 A b 0.028 A 0

8-13 3-4 0.056 A ab 0.037 A b

15-20 5-6 0.12 A a 0.080 B a

 

aGrowth stage at application

bGR50 values, within each grth stage of each weed, followed by the same upper case letter
are not significantly different at the 0.05 probability level. (comparison within a row)

cGR50 values, within each herbicide for each weed, followed by the same lower case letter
are not significantly different at the 0.05 probability level. (comparison within a column)

20

Bmfimowiw Ea AIOIIV Eamonmbw 53» 8:8: «3:02?» E90 9 -m SM 8350 3:258 omow gamma—$2 mo oEmem .N 3:me

ATS— oa we:
make

_ fio 5.0
_ _ o

 

)

(lonuoa Jo %
IHDIEAA AUG

 

.I 02

 

 

 

we

21

.038:me 5:5 830.: .3263?»
Aifliv 80.8 2 -2 one A- :4: iv E92 8 -w AEOL 80-0 9 -m do.“ 8350 8:032 30v 3237?: .3 oEmem .N Emmi.

 

 

 

 

A72. 3 9:
mpg
S H mo 86 good
_ _ b o
mm
3
m. m
WM
3
a .u. m
n D
.or H
( l
02
m3
4

 

 

 

of

22

CHAPTER 2
INFLUENCE OF HERBICIDE APPLICATION RATE, TIMING, AND
INTERROW CULTIVATION ON WEED CONTROL, CORN (Zea mays) YIELD,
AND PROFITABILITY IN GLUFOSINATE-RESISTANT AND GLYPHOSATE-
RESISTANT CORN
Abstract: Field trials were conducted in 1996 and 1997 to determine the influence of
glufosinatc and glyphosate application rates, application timings, and interrow cultivation
on weed control, corn yield, and profitability. Glufosinate-ammonium rates ranged from 0.18
to 0.41 kg ai/ha, while rates for the isopropylamine salt of glyphosate ranged from 0.21 to
0.84 kg ae/ha. These herbicides were applied to weeds averaging 5-cm in height (EPOST)
and to weeds averaging 13-cm in height (LPOST). Increasing rates of glufosinatc and
glyphosate often increased weed control. Control of many of the weed species was enhanced
by delaying herbicide application timing. Weed control was most consistent from LPOST
applications of glufosinatc at 0.41 kg ai/ha or glyphosate at 0.84 kg ae/ha. Application rates
and timings did not influence gross margins for glufosinate or glyphosate treatments. Corn
yields were reduced due to incomplete weed control when the lowest rate of glufosinatc was
applied. Weed control from EPOST glufosinatc and glyphosate applications followed by
cultivation was similar to weed control from LPOST glufosinatc and glyphosate applications
without cultivation. Interrow cultivation following glufosinate or glyphosate application did
not affect corn yield, but did reduce gross margin over weed control costs with an

assumption of low corn price in the glyphosate trial.

23

Nomenclature: Glufosinate, 2-amino-4-(hydroxymethylphosphinyl)butanoic acid;
glyphosate, N-(phosphonomethyl)glycine; corn, Zea mays L. ‘DK 493GR’.

Additional index words: herbicide-tolerant crops, Abutilon theophrasti, Amaranthus
retroflexus, Chenopodium album, Setaria faberi, Zea mays, ABUTH, AMARE, CHEAL,
SETFA.

Abbreviations: EPOST, early postemergence; LPOST, late postemergence.

24

INTRODUCTION

The recent introduction of herbicide-resistant crops has created new Opportunities for
postemergence (POST) applications of herbicides traditionally considered non-selective,
such as glufosinatc and glyphosate. Glufosinate and glyphosate have been used for
vegetation management in the absence of a crop (Carlson and Burnside 1984; Wilson et al.
1985), but these herbicides are currently being used for weed control in herbicide resistant
corn hybrids and soybean varieties (Bertges et al. 1997; Probst et a1. 1997; Scott et al. 1998).

Performance of glufosinate and glyphosate is often influenced by environmental
factors (Anderson et al. 1993a, 1993b; McWhorter et al. 1980; Whitwell et al. 1980).
Herbicide performance can also be affected by timing of herbicide application as well as rate
of herbicide applied (Grichar 1997; O’Sullivan and Bouw 1997). Jordan et al. (1997)
evaluated efficacy of glyphosate applied at four rates and three timings in a fallow
environment. They found that glyphosate at earlier timings controlled weeds more
consistently than when applied at later timings, and increased glyphosate rates increased
control of large weeds. Krausz et a1. (1996) reported similar findings for glyphosate rates and
timings. Steckel et al. (1997) showed that rates and timing of application influenced the
performance of glufosinatc on several annual weed species.

In addition to managing herbicide rates and application timings, growers can
incorporate cultivation into management systems to enhance weed control (Buhler et al.
1992). Newsom and Shaw (1994) found that a timely cultivation following an herbicide
application in soybean (Glycine max) was beneficial for sicklepod (Cassia obtusifolia) and

pitted momingglory (Ipomoea lacunosa) control. One or two cultivations following herbicide

25

applications in no-tillage and chisel—plowed corn production systems has also been
demonstrated to control weeds adequately (Buhler et al. 1995).

Much of the previous research investigating glufosinatc and glyphosate activity has
been conducted in non-crop environments. With the development of herbicide resistant
crops, information is needed on application rates of glufosinatc and glyphosate, the time
these herbicides should be applied, and how other weed control methods can be integrated
with glufosinate and glyphosate for effective season long weed control. The objectives of our
research were to evaluate the influence of application rates, application timings, and interrow
cultivation on annual weed control, profitability, and corn yields in glufosinatc-resistant and

glyphosate-resistant corn.

MATERIALS AND METHODS
Experimental Description. Field trials were conducted in 1996 and 1997 at separate
locations on the Crop and Soil Sciences Research Farm at Michigan State University in East
Lansing, MI. The soil was a Capac loam (fine-loamy, mixed mesic Aerie Ochraqualf) with
a pH of 7.0 and 3.5% organic matter in 1996 and a Capac sandy loam with a pH of 6.4 and
2.6% organic matter in 1997. Both locations were moldboard plowed in the fall; secondary
tillage consisted of disking and field cultivation in the spring. Prior to field cultivation, a 46-
0-0 (N-P-K) fertilizer was broadcast at 305 kg/ha. A 6-24-24 fertilizer at 320 kg/ha in 1996
or 134 kg/ha in 1997 was banded 5 cm below and 5 cm to the side of the corn seed during
the planting operation. An experimental glufosinatc-resistant corn hybrid at 61,750 seed/ha

and an experimental glyphosate—resistant corn hybrid at 54,340 seed/ha were planted on May

26

17,1996. ‘Dekalb 493GR’ and an experimental glyphosate-resistant corn hybrid were planted
at 58,045 seed/ha and 50,635 seed/ha, respectively, on May 13, 1997. Plots consisted of four
rows spaced 76 cm apart with lengths of 10.7 m in 1996 and 9.1 m in 1997.
Glufosinate-resistant and glyphosate-resistant corn were planted as separate trials.
Treatments included glufosinatc at 0.18, 0.30, and 0.41 kg ai/ha or glyphosate 0.21, 0.42, and
0.84 kg ae/ha applied early postemergence (EPOST) or late postemergence (LPOST). Weeds
averaged 5-cm in height at the EPOST timing, while the weeds were 13-cm in height, 7 d
later, at the LPOST timing (Table 1). Additional treatments included glufosinatc at 0.30
kg/ha or glyphosate at 0.42 kg/ha applied EPOST or LPOST followed by cultivation.
Ammonium sulfate at 2% (w/w) was included with all glufosinatc and glyphosate
applications. Untreated and weed-free plots were included. Weed-free plots were treated with
a PRE application of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-
methylethyl)acetamide] at 2.24 kg/ha plus atrazine [6-chloro-N-ethyl-N’-(l-methylethyl)-
1,3,5-triazine-2,4-diamine] at 1.12 kg/ha followed by hand-weeding as needed. A POST
standard consisted of nicosulfuron {2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]
amino]sulfonyl]-N,N-dimethyl-3-pyridinecarboxamide} at 0.035 kg/ha plus dicarnba (3,6-
dichloro-2-methoxybenzoic acid) at 0.28 kg/ha plus non-ionic surfactant“ at 0.25% (v/v) plus
urea ammonium nitrate at 4.67 L/ha. Herbicides were applied with a tractor-mounted

compressed-air sprayer equipped with flat-fan nozzles5 and calibrated to deliver 187 L/ha at

 

4Activator-90, a mixture of alkyl polyoxyethylene ether and free fatty acids, from
Loveland Industries, Inc., PO. Box 1289, Greeley, CO 80632.

5TeeJet XR 8003. Spraying Systems Co., North Ave, Wheaton, IL 60188.

27

a pressure of 207 kPa. Cultivation was performed with a four-row C-shank cultivator
operated at a depth of 10 cm. Corn growth stages and weed densities were recorded in
untreated plots at each herbicide application in the study (Table 1). Weed densities at
cultivation were not recorded in untreated control plots because grasses were forming tillers
which made accurate densities difficult to determine. Visual estimates of weed control were
recorded 21 d after the LPOST herbicide application. Weed control was compared against
an untreated control using a scale of 0 (no control) to 100 (all plants dead).

Two 1900-cm2 quadrats per plot were established prior to EPOST and LPOST
applications of 0.30 kg/ha glufosinatc or 0.42 kg/ha glyphosate. Giant foxtail (Setariafaberz')
were thinned by hand to 10 plants/quadrat in 1996 and 20 plants/quadrat in 1997, because
high native populations of giant foxtail in these trials (Table 1) could interfere with broadleaf
weed populations. Within each quadrat, wooden toothpicks were placed in the soil adjacent
to the thinned grass and native broadleaf weeds. The toothpicks were used to distinguish
between weeds present at the time of application and weeds which emerged following
application. Weeds emerging after postemergence herbicide applications were recorded 18
d after herbicide application (Table 3).

Corn grain yield was determined by harvesting the center two rows of each plot with
a mechanical plot harvester. Corn grain yield was corrected to 15.5% moisture. Glyphosate-
resistant corn was not harvested in 1996 because it was destroyed prior to corn pollination
in compliance with federal regulatory restrictions.
Statistical Analysis. The glufosinatc-resistant corn trial and the glyphosate-resistant corn

trial were designed as randomized complete blocks with three replications. Visual estimates

28

of weed control were arcsine transformed prior to statistical analysis and subjected to
ANOVA procedures. Means of the transformed weed control data and corn yields were
separated using Fisher’s protected LSD at the P: 0.05 level of probability. Nontransformed
means are presented for clarity. Visual estimates of weed control and corn yields were
subjected to F -Max tests to test for homogeneity of variance among years (Kuehl 1994).
Data found to be homogenous were pooled over years.

Profitability analysis. The profitability analysis was based on gross margins over weed
control costs for each treatment. Weed control costs included herbicide treatment,
application, seed technology fee, and cultivation. These costs were added together as total
weed control costs. All other production costs were assumed to be fixed across treatments.
Gross margins over weed control costs were calculated by multiplying corn yield by com
price and subtracting total weed control costs. Gross margins were not calculated for weed
free treatments because weed control costs could not be estimated due to the difficulty of
assessing the value of handweeding. Gross margins over weed control costs for each
replication of each treatment were statistically analyzed using ANOVA, and means were
compared using Fisher’s protected LSD at the P: 0.05 level of probability.

A sensitivity analysis for corn prices of $60/Mg, $100/Mg, and $140/Mg were
included in the partial budget analyses. A technology fee is often assessed on herbicide
resistant seed. The technology fee for 80,000 seed of ‘DK 493RR’ was $18.00, while the
technology fee for 80,000 seed of ‘DK 493GR’ was $12.50. Prices for herbicides and custom

application fees were obtained from a custom pesticide applicator service". Doanes 1996

 

6 Dougherty Fertilizer Service. Franklin, IN 46131.

29

Agriculture Report7 was used to estimate cost for owning and operating a six row cultivator
on 76 cm row spacings and a trailer sprayer with 15 m booms. Operating costs included fuel,

maintenance, and labor. All costs are summarized in Table 2.

RESULTS AND DISCUSSION
Effect of herbicide application timing on weed density. Weed emergence typically occurs
during certain periods in a season (Ogg and Dawson 1984; Egley and Williams 1991;
Anderson and Nielsen 1996). Weed species that exhibit extended emergence patterns will
continue to emerge following an application of a non-residual POST herbicide such as
glufosinatc or glyphosate. More giant foxtail (p=0.028) and velvetleaf (A butilon theophrasti)
(p=0.124) emerged following an EPOST application of glufosinatc than the LPOST
application (Table 3). Similar trends were apparent for glyphosate. Giant foxtail (p=0.167),
common lambsquarters (Chenopodium album) (p=0.158), and velvetleaf (p=0.116)
emergence was greater following glyphosate EPOST compared to the LPOST timing (Table
3). The density of redroot pi gweed (Amaranthus retroflexus) were low and were not affected
by application timing. Glufosinate and glyphosate do not control weeds that emerge
following application. The lack of residual activity of glufosinatc and glyphosate makes

timing of herbicide application critical for optimum weed control.

 

7 1996. Estimated Machinery Operating Costs. Doanes Agriculture Report. 59:5-

30

Effect of herbicide rate and application timing on weed control. Glufosinate trial.
Control of giant foxtail, redroot pigweed, and velvetleaf was not affected by glufosinatc rates
at the EPOST timing (Table 4). However, control of all weeds except common lambsquarters
in 1996 was improved when increased rates of glufosinatc were applied at the LPOST
timing. Eberlien et al. (1993) and Steckel et al. (1997) similarly reported increased control
of giant foxtail and common lambsquarters as the rates of glufosinatc were increased. The
rate response could be attributed to weed size at application. Weeds were larger at the
LPOST timing compared to the EPOST timing (Table 1). The 0.18 kg/ha rate of glufosinatc
was too low to consistently control the larger weeds at the LPOST timing. Blackshaw (1989)
reported that grth stage influenced glufosinatc activity on two of six plant species. Under
greenhouse conditions, large velvetleaf required more glufosinatc to reduce plant biomass
by 50% than small velvetleaf (Tharp et al. 1999).

With the exception of common lambsquarters in 1996, LPOST applications of 0.30
kg/ha glufosinatc improved control of all weeds compared to EPOST timings (Table 4).
Control of common lambsquarters in 1997 and giant foxtail in both years from LPOST
applications of 0.41 kg/ha glufosinatc was greater than EPOST timings. Steckel et al. (1997)
showed that weed control from glufosinatc was most effective when applied to 10-cm tall
weeds compared to 5-cm and 15—cm tall weeds. They attributed erratic control of 15-cm tall
weeds to incomplete spray coverage. In our trials, reduced weed control from early
applications was likely due to emergence of weeds following application (Table 3).

Corn grain yields in plots treated with 0.18 kg/ha of glufosinate were reduced

compared to the yield of the weed-fi'ee treatment (Table 4). Reduction in corn yields from

31

lower herbicide rates is likely due to inadequate control of weeds that were present at
herbicide application. Corn yields were not affected by application timing, indicating that
competition from weeds which emerged following the EPOST timings was minimal. All
glufosinate treated plots yielded similar to the POST standard (Table 4).

Although differences in weed control were apparent among glufosinatc treatments,
gross margins among glufosinatc treatments were not statistically different (Table 4). The
trends that occurred for gross margins were similar to trends among corn yields,
demonstrating the importance of yields in determining profitability. Differences in gross
margins were not sensitive to corn price. Gross margins for the untreated plots were severely
lowered despite the lack of weed control costs. A weed control program including herbicides
is essential for high gross margins. Glufosinate-based weed control systems were as
profitable as the POST standard.
Glyphosate trials. Common lambsquarters control was improved when glyphosate rates were
increased fi'om 0.21 kg/ha to 0.84 kg/ha at the EPOST timing (Table 5). Control of all other
weeds at the EPOST timing were not significantly influenced by glyphosate rates. Increasing
the rate of glyphosate from 0.21 kg/ha to 0.84 kg/ha improved common lambsquarters and
redroot pigweed control at the LPOST timing. Giant foxtail control was not influenced by
glyphosate rates at either application timing. Increasing the rate of glyphosate has been
shown to improve control of broadleaf weeds, while not affecting giant foxtail control
(Krausz et al. 1996).

Common lambsquarters control in 1997 and giant foxtail control in both years was

greater for LPOST applications than EPOST applications at each rate of glyphosate (Table

32

5). Control of redroot pigweed and velvetleaf from LPOST applications of 0.84 kg/ha
glyphosate was improved compared to EPOST applications. Krausz et al. (1996) reported
that application timings did not influence giant foxtail and redroot pigweed control, but they
did not account for weeds that emerged following application in their visual ratings. As with
the glufosinatc trials, lower weed control from the EPOST timing could be attributed to
weeds which emerged following glyphosate application.

Corn yields were not significantly affected by glyphosate rates or application timings
compared with yields of weed-free plots (Table 5). However, yields in plots treated with an
EPOST application of 0.84 kg/ha glyphosate were significantly higher than yields of plots
treated with a LPOST application of glyphosate at the same rate. This yield reduction might
be attributed to early season competition from weeds that were not treated until the LPOST
timing. Carey and Kells (1995) and Tapia et al. (1997) reported that the duration of weed
interference with a corn crop is a critical factor in the determination of corn yield. As
herbicide timings are delayed the potential for yield reductions due to weed interference
increases. In these trials, weeds were always less than 10 cm tall at the EPOST timing, while
the highest density weed, giant foxtail, was 13 cm and 20 cm tall at the LPOST timing in
1996 and 1997, respectively (Table 1). Loux et a1. (1998) reported that com yield loss is
likely when weeds reach 15-cm in height before treatment.

Gross margins among glyphosate rates at each application timing were similar (Table
5). However, as with corn yields, gross margins were higher for 0.84 kg/ha of glyphosate
applied at the EPOST timing compared to the LPOST timing. Since the weed control costs

among these treatments are equal, the difference was due to lower corn yields from the

33

LPOST timing. The profitability of plots lacking weed control treatments was severely
reduced. Gross margins for many of the glyphosate based weed control programs exceeded
the gross margins for the POST standard at lower corn prices. However, the gross margins
of the POST standard were similar to the gross margins of the glyphosate treatments at
higher corn prices (Table 5). As the price of corn increased, the revenue for each treatment
increased while the weed control cost per treatment remained constant. Increased revenues
at constant costs resulted in increased gross margins across treatments, and a higher overall
mean square error when comparing differences in gross margins across treatments. A high
mean square error resulted in a large LSD value. Therefore, the gross margins of the POST
standard were not statistically different than the gross margins of the glyphosate treatments
as corn price increased.

Control of each weed species was enhanced when applications of glufosinatc and
glyphosate were delayed. LPOST applications of glufosinatc at 0.30 kg/ha or 0.41 kg/ha and
glyphosate at 0.42 kg/ha or 0.84 kg/ha provided at least 89% control of all weeds. Weed
control from glufosinatc never exceeded the EPOST standard of nicosulfuron plus dicarnba.
Giant foxtail and velvetleaf control was greater from LPOST applications of 0.42 kg/ha of
glyphosate compared to the POST standard. In the absence of weed control, corn yields were
severely reduced in both trials, indicating a high amount of weed competition.

Effect of cultivation and application timing on weed control. Glufosinate trial. Buhler
et al. (1995) demonstrated that annual grass control could be increased by following reduced
rates of preemergence applied herbicides with cultivation. Nearly all weeds which emerged

or recovered following an EPOST application of glufosinate were effectively removed by

34

cultivation (Table 6). Cultivation following EPOST glufosinate applications increased
control of redroot pigweed and velvetleaf compared to a LPOST glufosinate application
alone. Velvetleaf that emerged or recovered following a LPOST glufosinatc application was
effectively removed by cultivation. LPOST applications of glufosinate controlled all weed
species other than velvetleaf 292% and cultivation following the LPOST glufosinate
application only increased velvetleaf control. Cultivation alone did not adequately control
any of the weed species.

Mulder and Doll (1993) showed that cultivation increased weed control and did not
affect corn yields. Within an application timing, cultivation did not affect yield (Table 6).
Corn yields in weed free cultivated plots were higher than plots that were cultivated
following glufosinatc applications, indicating that weed control from the glufosinatc plus
cultivation was not complete. The remaining weeds competed with the corn enough to reduce
yields. At low corn prices, the cost of cultivation reduced gross margins over weed control
costs within the LPOST timings of glufosinatc (Table 6). However this difference was not
apparent at higher corn prices, because weed control costs remained constant as corn price
increased. Cultivation was more economically feasible as corn price increased. Using
cultivation as the only method of weed control resulted in severely reduced corn yields and
gross margins that were equivalent to untreated plots. In the absence of weeds, cultivation
did not affect yield.
Glyphosate trials. With the exception of velvetleaf, cultivation enhanced control of weeds
following an EPOST application of glyphosate (Table 7). Cultivation increased redroot

pigweed control following a LPOST application of glyphosate, but did not affect control of

35

other weed species at this timing. Cultivation following EPOST glyphosate applications
controlled all weeds 2 91%. Cultivation alone did not sufficiently control weeds.

Cultivation did not influence corn yields or gross margins in plots previously treated
with glyphosate (Table 7). Cultivation in the absence of weeds did not influence yield. Corn
yields in plots that received cultivation only were higher than the untreated plots but were
severely reduced compared with treatments that included a glyphosate application with or
without cultivation. At higher corn prices, cultivating without herbicide application increased
gross margins compared to untreated plots, but gross margins were much lower than plots
that were treated with glyphosate.

Weeds that emerge following application of glufosinate or glyphosate influence the
efficacy of these herbicides. Giant foxtail emergence following EPOST applications of
glufosinatc or glyphosate was often greater than giant foxtail emergence following LPOST
timings. Differences in weed emergence generally resulted in greater weed control from the
LPOST timings compared to the EPOST timings. Weed control was most consistent in plots
that were treated with LPOST applications of glufosinatc at 0.41 kg/ha or glyphosate at 0.84
kg/ha, the highest rates tested in these studies. Although weed control was often enhanced
from later timings of glufosinatc and glyphosate, the later timings can reduce corn yields by
extending the duration of weed interference with the corn. Since the risk of corn yield loss
increases as glufosinatc and glyphosate application timings are delayed, these herbicides
should be applied to weeds at the EPOST timing. Weed control from an EPOST timing is

often reduced from weed emergence following application. Cultivation following glufosinatc

36

and glyphosate applied at the EPOST timing is a cost effective weed management strategy
to increase weed control from early glufosinatc or glyphosate applications.

The EPOST and LPOST timings were only separated by an average of 7 d in which
time corn had developed 2 collars and weed heights doubled. The proper timing for POST
herbicide applications can occur within a short period. Therefore, a timely herbicide
application is a critical component of a successful POST weed control program. The
profitability of glufosinatc based weed control programs were similar to the POST standard,
and glyphosate based weed control programs often exceeded that of a POST standard. Corn
yield strongly influences the profitability of weed control programs. Further research is
needed to identify additional weed management strategies that will provide adequate weed
control from timely glufosinatc and glyphosate applications in corn without reducing yields

or profits.

37

LITERATURE CITED

Anderson, D. M., C. J. Swanton, J. C. Hall, and B. G. Mersey. 1993a. The influence of

temperature and relative humidity on the efficacy of glufosinatc-ammonium. Weed Res.
33:139-147.

Anderson, D. M., C. J. Swanton, J. C. Hall, and B. G. Mersey. 1993b. The influence of soil
moisture, simulated rainfall, and time of application on the efficacy of glufosinate-
ammonium. Weed Res. 33 : 149-1 60.

Anderson, R. L. and D. C. Nielsen. 1996. Emergence pattern of five weeds in the central
great plains. Weed Technol. 10:744-749.

Bertges, W. J ., M. D. Anderson, J. 1. Baldwin, G. G. Hora, J. C. Klauzer, D. J. Lamore, J. R.
Scoresby, and B. L. Woolley. 1997. Glufosinate plus atrazine coformulation for weed control
in field corn. Proc. North Central Weed Sci. Soc. 52:71.

Blackshaw, R. E. 1989. HOE-39866 use in chemical fallow systems. Weed Technol. 3:420-
428.

Buhler, D. D., J. F. Gunsolus, and D. F. Ralston. 1992. Integrated weed management
techniques to reduce herbicide inputs in soybean. Agron. J. 84:973-978.

Buhler, D. D., J. D. Doll, R. T. Proost, and M. R. Visocky. 1995. Integrating mechanical
weeding with reduced herbicide use in conservation tillage corn production systems. Agron.
J. 87:507-512.

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

Carey, J. B. and J. J. Kells. 1995. Timing of total postemergence herbicide applications to
maximize weed control and corn (Zea mays) yield. Weed Technol. 9:356-361.

Eberlein, C. V., M. J. Guttieri, and F. N. Fletcher. 1993. Broadleaf weed control in potatoes
(Solanum tuberosum) with postemergence directed herbicides. Weed Technol. 7:298-303.

Egley, G. H. and R. D. Williams. 1991. Emergence periodicity of six summer annual weed
species. Weed Sci. 39:595-600.

Grichar, W. J. 1997. Influence of herbicides and timing of application on broadleaf weed
control in peanut (Arachis hypogaea). Weed Technol. 11:708-713.

Jordan, D. L., A. C. York, J. L. Griffin, P. A. Clay, P. R. Vidrine, and D. B. Reynolds. 1997.
Influence of application variables on efficacy of glyphosate. Weed Technol. 11:354-362.

38

Kuehl, R. O. 1994. Statistical principles of research design and analysis. Belmont, CA:
Wadsworth , Inc. 686 p.

Krausz, R. F., G. Kapusta, and J. L. Matthews. 1996. Control of annual weeds with
glyphosate. Weed Technol. 10:957-962.

Loux, M. M., S. A. Grower, J. Cardina, P. Sprankle, N. J. Probst, M. W. Bugg, M. Spaur,
M. D. K. Owen, T. T. Bauman, S. E. Hart, R. S. Currie, D. L. Regehr, W. W. Witt, C. H.
Slack, J. J. Kells, W. G. Johnson, W. S. Curran, and R. G. Harvey. 1998. Determining the
critical period of weed management in glyphosate-tolerant corn: results of a multi-state
study. Proc. North Cent. Weed Sci. Soc. 53:66.

McWhorter, C. G., T. N. Jordan, and G. D. Wills. 1980. Translocation of l4C-glyphosate in
soybeans (Glycine max) and johnsongrass (Sorghum halepense). Weed Sci. 28:113-118.

Mulder, T. A. and J. D. Doll. 1993. Integrating reduced herbicide use with mechanical
weeding in corn (Zea mays). Weed Technol. 72382-389.

Newsom, L. and D. R. Shaw. 1994. Influence of cultivation timing on weed control in
soybean (Glycine max) with ACC 263,222. Weed Technol. 8:760-765.

Ogg, A. G. and J. H. Dawson. 1984. Time of emergence of eight weed species. Weed Sci.
32:327-335.

O’Sullivan, J. and W. J. Bouw. 1997. Effect of timing and adjuvants on the efficacy of
reduced herbicide rates for sweet corn (Zea mays). Weed Technol. 11:720-724.

Probst, N. J ., M. W. Bugg, and J. K. Soteres. 1997. Performance of glyphosate tolerant corn
in 1997. Proc. North Cent. Weed Sci. Soc. 52:71.

Scott R., D. R., Shaw, and W. L. Barrentine. 1998. Glyphosate tank mixtures with SAN 582

for bumdown or postemergence applications in glyphosate-tolerant soybean (Glycine max).
Weed Technol. 12:23-26.

Steckel, G. J ., L. M. Wax, F. W. Simmons, and W. H. Phillips 11. 1997. Glufosinate efficacy
on annual weeds is influenced by rate and growth stage. Weed Technol. 11:484-488.

Tapia, L. S., T. T. Bauman, R. G. Harvey, J. J. Kells, G. Kapusta, M. M. Loux, W. E.
Lueschen, M. D. K. Owen, L. H. Hageman, and S. D. Strachan. 1997. Postemergence

herbicide application timing effects on annual grass control and corn (Zea mays) grain yield.
Weed Sci. 45:138-143.

Tharp, B. E., O. Schabenberger, and J. J. Kells. 1999. Response of annual weeds to
glufosinatc and glyphosate. Weed Technol. 13:542-547.

39

Whitwell, T., P. Banks, E. Basler, and P. W. Santelmann. 1980. Glyphosate absorption and
translocation in berrnudagrass (Cynodon dactylon) and activity in horsenettle (Solanum
carolinense). Weed Sci. 28:93-96.

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 Sci. 33:531—536.

40

docm>23o 38.322 H.230 6030908883 82 u .5qu .oocowaofiemoa 380 H HmOmm a

 

 

 

 

 

3 mm mm mm _ 2 2 2 2 2 mm 03:023/

3 3 _ 3 mm mm _ 3 mm mm mm 8on5 888m

NC 32 mm mm 2. .3 2 _ mm $833382 308800

wmm mow Sn 2.: o: om: NE. 20 28380 2:30
”35:83 bfimcow 303

mm w 3 mm 2 m mm w .3 mm 2 m 03:63“;

3 a N 8 m 4 cm a N om w 4 Scene 888m

cm A m m 2 2 m cm A m m 2 2 m meutmgmnfifi 308380

3 m _ m m m cm 0 3s m 2 m m m cm 0 28x8 EEO
Eu 220: @063

mm mm 3 mm mm 2 mm mm 2 mm cm 2 A83 Ewsm

c v N m m m o v m m m M $828 6365

omfim 26% E00

 

3. 2 am 2 2 R 3“ 2 S R m... R 36% as can wee

 

.230 .5qu HmOmm .230 .5.qu HwOmm .230 .5qu HmOmm .230 .5qu HmOmm

 

32 eg— 32 032

 

223 882330 22.3 892825

“3323230 28 32382393 020252 00 635 8 $2320 383 28 £220: coo? .momfim £38m 3.50 .N 22$

41

Table 2. Assumption of costs used in profitability analysis.

 

Custom application costs
PRE $1 O/ha
POST $1 2/ha

Herbicide costs

ammonium sulfate $0.50/kg

dicarnba $303/L

glufosinatc $356/L

glyphosate $1 89/ L

nicosulfuron $ 1 /k g
Operating costs

Cultivator (6-row 76-cm row spacing) $8/ha

Sprayer (15-m boom) $0.60/ha
Technology fee

‘DK 493GR’b $12.50/unit“

‘DK 493RR’° $1 8/unit

 

a A unit of seed is estimated at 80,000 seed
b resistant to glufosinate
C resistant to glyphosate

42

Table 3. Weed densities 18 d after glufosinatc and glyphosate applications.

 

 

 

 

 

 

Weed density"1
SETFA CHEAL AMARE ABUTH

Glufosinateb plants/m2

EPOST 55.9 5.6 1.3 2.9

LPOST 20.2 3.9 1.3 0.0

p-value 0.028 0.707 1.00 0.124
Glyphosatec

EPOST 25.7 6.0 3.4 3.4

LPOST 9.8 2.2 3.4 0.9

p-value 0.167 0.158 0.993 0.116

 

“ SETFA, giant foxtail; CHEAL, common lambsquarters; AMARE, redroot pigweed;
ABUTH, velvetleaf. All data are pooled over 1996 and 1997.

b Application rate of 0.30 kg/ha. Ammonium sulfate was added at 2% (w/w).

c Application rate of 0.42 kg/ha. Ammonium sulfate was added at 2% (w/w).

43

.25 no.3 8 3323 3332033333 -833 33m 930 $26 8 22 833—332 a
.333 .x. N 8 0323 3333393333 83385 383383 833280310 9

598$ 2 222:3 325:3 53533 e 322:3 was: a 5353; 9 32333 3253 ..
.3350

3338 383 339 838333 33m 833 E8 ,3 22% E8 wax—3238 3 832328 383 333 333 32 382 083 883 3338 383 88 meSE 880 u
.38: 326 3283 333 ESQ 33m md<2< {bum 08823 .Ebmx‘. 60on5 38283 .52.)? ”3033333333— 8888 J<mm0 53380 38% {3.5m a

.Amodnmv 383 qu 3m 9 233388 388.23 338283 83 83 88— 0333. 05 >3 830:8 33328 a 3233 882 a

 

 

 

 

 

 

 

 

a :3 2: 2: 2: 2: 2: 83-303
3 8... 3 am a N2 2% 0 33 o o o o o 338:3
mg 3586
a S: a E a 33. 8.8 a... .3; as am a a a so a we a a .5033 + £8 + 853822
a 3.: a E a as. $3 a is a mm B a 3 8 .3 em a 3 59:
a 3: a :3 a 5. an? a one 23 a o 3 a Q a 8 3 4m 3833 :3
a 2 : a E a as. 8.9.. as 33 as 8 3 as B 8 pa 3 a a $0.:
a 3 : 3.. E a as. 3% .3 3; B E o 8 e 2 es 8 3 ow 3833 one
a :3: a g a :4 :3. 3 a; e K o a 8 am a 8 3 mm 50.:
a E: a E a 4:. :3. 3 83 33 mm o 3 o 8 3 3w 3 as $033 w 3 .o message
23 I 23 | new: a «is:
$32.; $38; .3253 £8 22: 3.593. 5232 32 32 <88 weep use 3533323
088 3338 383 396 meSE 380 3.338 383 3330 A<m30
a3338 38>?

 

«.32 333 002 $3333 333 033 30382333 83328.22w 3 3880.23 33 meBE 30% 333 .22» E8 .3338 885 .3 33$

44

.333 $3. 33 88.8 8338883 -88 38w 93v 383.0 :3 mHZ 3333333 3

.323 ..\o N 33 83.233 8383883 333338 3388383 88233303

33.8.8 9 353.338 82.2.; 5353.8 2 3329338 32.23; 53.8.; e 332238 32.333 .

.3338

3.338 383 338 888533 383 8:3 88 .3 3333 88 8323.538 .3 383—328 803 333 E30 333 88.3 89$ 88 3338 383 8.6 333.3338 880 3
2:5 82 :83 .333 :22 Eco .

88.3 8.6 3283 333 E393. 33m §< .<3Hmm 383032, .EDmaV ”38383 30838 dds)? 88333383— 30888 {3520 £883 383w .<3.Hmm 3
Amodufi: 383 Om; 3a 9 833388 33833333 33383.38me 83 83 88— 388 83 .3 386:8 383—8 3 33335 3382 .

 

 

 

 

 

 

 

 

 

33 8.: 2: 2: 2: 2: 2: 83-383
3 333 3 83. 3 8.3 8. : o 23.3 o o 3 3 3 338333
8.3 .3886
3 883 o 38 o 38 8.8 3.N 8o. : 03 8 3 8 m 8 U33 8 o 8 883 + 38.3 + 38.33822
3 8: Q3 38 03 3.33 8.33 3 2:: 3.N 8 3 .8 a 8 33 8 33 8 .583
3 38: a 2:3 3 3.3 8.33 a 8.: o 33 8 8 03 3w 3 2: 3 3w 883 3.3.3
33 8: 33.w 32: 3.. 38 $3. 3.w 33. : a 8 03 8 33 8 33 8 a 8 883
m 332 33 E: 33 38 $3. 3.N 8.: 33 3w 8 8 3 E B 8 3 mm 883 8.3
3 2:: 03 88 Q33 38 33.8 3 3:: 03 3.3 B 33 B :3 3 8 3 8 .883
3.N S: 33 $2 33 83 33.8 3.. 8.: o3 3. 3 mm 3 8 B3 8 B 8 $83 :3 33333330
33.3 I 33.3 | 33.32 2. 33.33
32.2.; 32.23; .3258 338 .232 3,533. 822.... 82 38: <88 328: 233 3833833
3338 3338 383 88 333338 3380 38338 38 3 880 immu
3888 38 3

 

.6333 33m 3333 $883 333 88 3038:3333 88033.33 .3 388.333 8 33888 338w 383 .3633 88 .3338 38 3 .3. 333.3

45

523$ 9 E2223 ”233% .552: e 222:3 £252” smsm. a 2 82983 368% u

.mumOo

3980 68>» :38 mccowbnzm US: 8th ES 3 22» 58 $3322: 3 cogs—:28 80>» 28 bao 33 Got 803 mumoa 75:8 v.33 :96 madman": 820 a

65:3 o\o~ “a 323 mm? 0323 8328825. .méwx end 83 BS sauna—Qua 825820 0

£3» :26 330m 8m 88 =< ““8302? .Ebmam 683w:— Hooao: .9723? 38:33383 :oEEou J<mmu £828 Eflw {Em—m n
.Amodnmv “we Om: as 9 wfivuooom “:80.“th bEmoanG .0: 225 8:2 088 05 3 326:9: 2828 a 23:3 332 a

 

 

 

 

 

 

 

 

a“ :3 2: 2: 2: 2: 02
a v? 2: 2: 2: 2: 9% 8:683
a 8m 9 am 0 N2 2% c 23 o o o o “.02
3 m? a 0% o 2: a: c on». o a. u S 0 mm 0 mm 8.» 8:26:
a 2 : m E a ME 3% 3 N3 0 8 a § 2 8 m 3 02
a 2: a 2: a M3. 8.3 0 wow u 3 a 3 a E a 8 a: 59:
a N: : a w: a MS 3% 2: :3 u E 0 2w 9 mm 9 2: 02
a o2: a g 2. EV 22$ 0 m; a 8 a Ma a 8 m 8 8> 28%
25 I 25 | 2&2
mzszm wzao; @253 980 222 :59: $25 imam «5% 8:325 02:8:
mega—mam
vflmoo 25:8 :83 :26 mamas: $80 25:8 :83 580 £888 :83 836830

 

«.32 98 com: £23355 98 wEEa 2:23:39: oEEmQBm 3 380%.? mm meHmE mme 98 .203 E8 .3980 :83 6 m3:&

46

.583 2 ”522:3 mgozm 588$ 2 622:3 3:8; 59%. _ m 2 222:3 misc?

.mumOo

3::8 :83 :32 35853 :28 8:: Eou 3 2:: Eco warms—2: 3 :83223 203 :5: 3:0 33 Soc 895 $80 6::8 :83 $>o 3&8: $80 0

2:5 3: so: 2% 222 88 2

AB}; o\o~ a: :o::: 33 Sufism EE:oEE< .méwv— NV: 33 88 :ozmozmam 889750 9

$3» :96 :28: 2m 8:: =< “3:033 .IHDmQ. ”:33wa 88:8 .mM<§< $852836:— :o:.::8 J<m$0 £828 EEw {Pfimm D
Amodumv 68 9mg :: 9 wEEoUom Babb: bESCEwG Ho: 80>» 3:2 2:3 2: 3 :uBozom 5:28 a 55:5 832 a

 

 

 

 

 

 

 

 

m 25.: 2: 2: 2: 2: 02
a S. : 2: 2: 2: 2: 8: 8:683
0 m8 0 N: a ma 3.: o 22 o o o o .02
a 08 a am a 2A 2:: a 2% 0 n u 2 w R 2 mm a: 322::
£02 a :2: SS 3.; m 3.: 2 a 2: 8 So «3 oz
2 2:: .2 mg a 3m 25. a 0:: m 3 a 3 a 8 m 3 a: :22:
3.2 m E: 38 3:. “2.: :2: 02: 3m 02 02
m a: a 22: a 2: 8.3 m 2. : 2 a 2 2m 3 a a a we: 50::
25 | 25 I 2::
WES; wzso; :25: 280 :22: :53 B232 2:5 flfimm 82325 was:
85:23::
”$80 3250 :33 :95 mama: 3:5 35:00 :83 £30 A:_ob:oo :83 Emmonnbmv

 

«.33 ::: coo: £28323 ::: wEE: :o:8=mm: Samonmbw 3 :28»? m: meBE View ::: .22» Eco ._o::oo :83 N £23m

47

CHAPTER 3
WEED CONTROL AND CORN (Zea mays) YIELD FROM RESIDUAL
HERBICIDE COMBINATIONS WITH GLYPHOSATE AND GLUFOSINATE IN
CORN

Abstract: Weed management strategies are needed to control weeds that emerge following
application of glyphosate or glufosinatc. Field trials were conducted from 1996 to 1999 to
determine if residual herbicides could be used with glyphosate or glufosinatc to provide
season-long weed control in glyphosate-resistant or glufosinatc-resistant com. Preemergence
(PRE) applications of the residual herbicides, acetochlor, atrazine, metolachlor, and
pendimethalin, followed by postemergence (POST) applications of glyphosate or glufosinate
were compared with POST tank mixtures of glyphosate or glufosinatc with the residual
herbicides. In general, weed control was similar for total POST compared to PRE followed
by POST systems. Residual herbicides used in combination with glyphosate or glufosinate
increased control of most weed species. Reducing the rate of residual herbicides reduced the
consistency of weed control. Tank mixtures with reduced rates of acetochlor plus atrazine
provided the most consistent weed control. Corn yields were not affected by application
timing, rates of the residual herbicides, or by whether residual herbicides were used. Using
reduced rates of residual herbicides with glyphosate or glufosinatc is a viable weed
management system in glyphosate-resistant and glufosinatc-resistant corn.

Nomenclature: acetochlor, 2-chloro-N-(ethoxymethy1)—N-(2-ethy1-6-

methylphenyl)acetamide plus MON 4660; atrazine, 6-chloro-N—ethyl-N’-(l-methylethyl)-

48

l,3,5-triazine-2,4-diamine; CGA-154281, 4-(dichloroacetyl)-3,4-dihydro-3-methy1-2H-1,4-
benzoxazine; flumetsulam, [N—(2,6-diflourophenyl)-5-(1,3,4,5,6,7-hexahydro-1 ,3-dioxo-2H—
isoindol-Z-y1)phenoxy] acetic acid; glufosinatc, 2—amino-4-
(hydroxymethylphosphiny1)butanoic acid; glyphosate, N—(phosphonomethyl)glycine;
metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1 -methylethyl)acetamide;
pendimethalin, N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine; com, Zea mays L.
‘DK 493GR’, ‘DK 493RR’.

Additional index words: acetochlor, atrazine, flumetsulam, glufosinatc-resistant corn,
glyphosate-resistant corn, herbicide tolerant crops, metolachlor, pendimethalin, soil-applied
herbicides.

Abbreviations: DAT, days after treatment; POST, postemergence; PRE, preemergence.

49

INTRODUCTION

Glyphosate and glufosinate have traditionally been used to control a wide spectrum
of weeds in non-crop environments. Recent advancements in biotechnology have resulted
in the development of crops that are resistant to POST applications of glufosinate or
glyphosate. These herbicides are currently used in resistant corn hybrids without crop injury
(Ateh and Harvey 1995; Culpepper and York 1999; Gower and Loux 1999).

Glufosinate is readily degraded by soil microorganisms (Smith 1989; Tebbe and
Reber 1991), and glyphosate is rapidly adsorbed to soil components (Sprankle et al. 1975a;
1975b). These characteristics typically eliminate herbicidal activity of glyphosate or
glufosinatc in the soil. Therefore, weeds that emerge following herbicide application are not
controlled. Late emerging weeds can be controlled by various weed management strategies.
Buhler et al. (1993) reported that interrow cultivation following a POST application of
herbicides at reduced rates increased control of common cocklebur (Xanthium strumarium).
Cultivation following a timely application of glufosinatc and glyphosate can also enhance
weed control (Tharp and Kells 1999). Watts et al. (1997) reported that PRE followed by
POST applications of herbicides increased control of sicklepod (Senna obtusifolia). Similar
sequential application methods were shown to consistently control annual grasses (Rabaey
and Harvey 1997). In glyphosate resistant soybeans, soil-applied herbicides are often applied
prior to POST glyphosate applications to provide residual weed control (Gonzini et al. 1999).
Van Wychen et a1. (1999) reported that in glufosinatc-resistant sweet corn, weed
management strategies that included residual herbicides or cultivation increased weed control

compared to glufosinatc applied alone. Weed control can be increased by delaying

50

glufosinatc or glyphosate applications (Tharp and Kells 1999). However, delayed
applications can result in soybean (Glycine max) and corn yield reductions from increased
weed competition (Carey and Kells 1995; Horak et a1. 1998).

As the use of glyphosate or glufosinate for weed control in corn increases, weed
management strategies that provide season-long weed control without reducing yields are
needed. In these trials, residual herbicides were used in combination with glufosinatc and
glyphosate. Weed control from PRE applications of residual herbicides followed by POST
glufosinatc or glyphosate applications were compared with POST tank mixtures of
glufosinatc or glyphosate with the residual herbicides. The effect of reduced rates of the

residual herbicide tank mix partners on weed control and corn yield was also investigated.

MATERIALS AND METHODS

Field trials were conducted in 1996, 1997, 1998, and 1999 at separate locations on
the Michigan State University Crop and Soil Sciences Research Farm in East Lansing. In
1996 and 1998, the soil was a Capac loam (fine-loamy, mixed mesic Aeric Ochraqualt) with
a pH of 7.0 and 3.5% organic matter, and in 1997 and 1999 the soil was a Capac sandy loam
with a pH of 6.5 and 2.6% organic matter. Research sites were moldboard plowed in the fall
of each year; secondary tillage consisted of disking and field cultivation in the spring of each
year. Prior to field cultivation, a 46-0-0 (N,P,K) fertilizer was broadcast and a 6-24-24
fertilizer was banded 5 cm below and 5 cm to the side of the corn seed during the planting

operation. Fertilizer rates were determined from results of soil analysis. All herbicides were

51

applied with a tractor-mounted compressed-air sprayer equipped with flat-fan nozzles8 and
calibrated to deliver 187 kg/ha at a pressure of 207 kPa. Experimental glyphosate-resistant
and glufosinatc-resistant hybrids were planted on May 17, 1996. An experimental
glyphosate-resistant hybrid and ‘Dekalb 493GR’ were planted on May 13, 1997. ‘Dekalb
493RR’ and ‘Dekalb 493GR’ were planted on May 11, 1998 and May 3, 1999. Plots
consisted of four rows spaced 76 cm apart with lengths of 10.7 in 1996 and 1998 and 9.1 m
in 1997 and 1999. Glufosinate-resistant and glyphosate-resistant corn were treated as
separate trials, and each trial was designed as a randomized complete block with three
replications. Com growth stages and weed densities were recorded in untreated plots at the
POST timing (Table 1). The average height of giant foxtail from 1996 to 1999 ranged from
5 to 8 cm and the average broadleaf weed height ranged from 3 to 6 cm at the POST timing.
Application timing of residual herbicide combinations. Glyphosate field trials. In 1996
to 1999, treatments in glyphosate-resistant corn consisted of POST tank mixtures of
glyphosate with residual herbicides and PRE applications of residual herbicides followed by
a sequential POST application of glyphosate. The residual herbicides included acetochlor at
1.8 kg/ha with MON 4660 (safener), atrazine at 1.1 kg/ha, metolachlor at 1.4 kg/ha with
CGA-154281 (safener), and pendimethalin at 1.7 kg/ha. Additional treatments included a
single POST application of glyphosate, untreated, and weed-free plots. Weed-free plots were

treated with a PRE application of a premix9 of metolachlor at 2.24 kg/ha plus atrazine at 1.12

 

8TeeJet XR 8003. Spraying Systems Co., North Ave., Wheaton, IL 60188.
9Bicep Lite 11, Novartis, Crop Protection Division, PO Box 18300, Greensboro,
NC 27419.

52

kg/ha followed by hand-weeding as needed. The glyphosate rate in 1996 and 1997 was 0.43
kg ae/ha, and in 1998 the rate was 0.63 kg/ha for the sequential and tank mixture treatments
and 0.43 kg/ha when glyphosate was applied alone. In 1999 all glyphosate rates were 0.63
kg/ha. Ammonium sulfate at 2% (w/w) was added to all treatments containing glyphosate.
Glufosinate field trials. In 1996 and 1997, POST tank mixtures of residual herbicides with
glufosinatc at 0.29 kg ai/ha and PRE applications of residual herbicides followed by
sequential POST applications of glufosinatc at 0.29 kg/ha were investigated. The residual
herbicides included acetochlor at 1.8 kg/ha with MON 4660 (safener), atrazine at 1.1 kg/ha,
metolachlor at 1.4 kg/ha with CGA—154281 (safener), and pendimethalin at 1.7 kg/ha.
Additional treatments included glufosinatc at 0.29 kg/ha applied alone, untreated, and weed-
free plots. All treatments containing glufosinatc included ammonium sulfate at 2% (w/w).
Reduced application rates of residual herbicides. Glyphosate field trials. Tank mixtures
of full and reduced rates of residual herbicides with glyphosate were tested in 1998 and 1999.
The full rates of the residual herbicides were acetochlor at 1.8 kg/ha with MON 4660
(safener), a pre-mixIO of acetochlor at 1.8 kg/ha with MON 4660 (safener) plus atrazine at
1.4 kg/ha, atrazine at 1.1 kg/ha, flumetsulam at 0.056 kg/ha, metolachlor at 1.4 kg/ha with
GOA-15428 (safener), and pendimethalin at 1.7 kg/ha. The reduced rates of the residual
herbicides were half the full rate. In 1998, the glyphosate rate was 0.63 kg/ha for the tank
mixture treatments and 0.43 kg/ha when glyphosate was applied alone. In 1999, all
glyphosate rates were 0.63 kg/ha. Ammonium sulfate at 2% (w/w) was added to all

treatments containing glyphosate.

 

loHarness Xtra 5.6L, Monsanto, St. Louis, MO 63167

53

Glufosinate field trials. In 1998 and 1999, glufosinatc at 0.29 kg/ha was applied in POST
tank mixtures with full and reduced rates of the residual herbicides. The full rates of the
residual herbicides were acetochlor at 1.8 kg/ha with MON 4660 (safener), a pre-mix5 of
acetochlor at 1.8 kg/ha with MON 4660 (safener) plus atrazine at 1.4 kg/ha, atrazine at 1.1
kg/ha, flumetsulam at 0.056 kg/ha, metolachlor at 1.4 kg/ha with CGA-15428 (safener), and
pendimethalin at 1.7 kg/ha. The reduced rates of the residual herbicides were half the full
rate. All treatments containing glufosinatc included ammonium sulfate at 2% (w/w).

Data collection and statistical analysis. Visual estimates of weed control were recorded 28
d after the POST application in all trials. Weed control was compared against an untreated
control using a scale of (0 no control) to 100 (all plants dead). Corn grain yield was
determined by harvesting the center two rows of each plot with a mechanical plot harvester.
Corn grain yield was corrected to 15.5% moisture. Glyphosate-resistant corn was destroyed
prior to corn pollination in 1996 to comply with federal regulatory restrictions.

Visual estimates of weed control were arcsine transformed prior to statistical analysis
and subjected to AN OVA procedures. Means of the transformed weed control data and corn
yields were separated using Fishers protected LSD at the P: 0.05 level of probability.
Nontransformed means are presented for clarity. Visual estimates of weed control and corn
yield were subjected to F-Max tests to test for homogeneity of variance among years (Kuehl
1994). Data found to be homogenous were pooled over years. Differences in herbicide

performance among years are likely due to differences in environmental conditions.

54

RESULTS AND DISCUSSION

Application timing of residual herbicide combinations. Glyphosate field trials. Control
of each weed species in the trial except velvetleaf in 1996 and 1999 generally increased when
a residual herbicide was used in combination with glyphosate (Table 2). The effect of
atrazine application timing on velvetleaf control varied each year. In 1997 and 1998, POST
tank mixtures of glyphosate and atrazine provided more complete velvetleaf control than
sequential applications. However, in 1996 and 1999, timing of atrazine application had no
effect on velvetleaf control. Control of velvetleaf was 90% or greater every year from
combinations of glyphosate and pendimethalin, and control was always less than 90% from
combinations of glyphosate and metolachlor and acetochlor. Control of redroot pigweed and
giant foxtail was not affected by the timing of residual herbicide applications. Common
lambsquarters and redroot pigweed were controlled 290% regardless of the timing of
residual herbicide application. Likewise, common lambsquarters control was similar from
PRE applications of residual herbicides followed by glyphosate compared to POST
glyphosate tank mixtures in soybean trials conducted by Gonzini et al. (1999). When
metolachlor and pendimethalin were tank mixed with glyphosate, common ragweed control
was increased compared to sequential applications in 1997, but was not affected by
application timing in other years. POST tank mixtures increased weed control compared to
sequential applications in five instances. In contrast, using sequential applications increased
weed control over tank mixtures in two instances.

Despite some differences in weed control between application timings, differences

in corn yields were not apparent (Table 2). Corn yields in plots treated with residual

55

herbicide combinations were similar to weed-free plots. Plots treated with glyphosate alone
yielded similar to plots treated with residual herbicides and weed-free plots, suggesting that
weeds that emerged after the glyphosate application did not compete with corn enough to
reduce yields. Dalley et al. (In Press) showed that weeds that emerged following POST
glyphosate applications did not reduce corn yields. Bosnic and Swanton (1997) reported that
the time of weed emergence relative to corn emergence is critical in determining the
competitive effects of weeds to corn. They showed that bamyardgrass seedlings that emerged
after the 4-leaf corn stage reduced corn yields less than 6% (Bosnic and Swanton 1997).
Weeds that emerge afier crop emergence are less likely to cause yield reductions than weeds
that emerge with the crop (Knake and Slife 1965; Murphy et a1. 1996). Corn yields in
untreated plots were severely reduced from intense weed competition.

Glufosinate field trials. The addition of residual herbicides increased control of each weed
species in the trial except common lambsquarters in 1996 (Table 3). Velvetleaf control was
not affected by application method for any of the residual herbicide combinations. Van
Wychen et al. (1999) similarly showed that the application timing of atrazine combinations
with glufosinatc did not influence common lambsquarters, common ragweed, or velvetleaf
control. Control of redroot pigweed and common ragweed exceeded 90% for all residual
herbicide combinations (Table 3). Similar levels of common ragweed control were reported
by Culpepper and York (1999). Sequential applications of acetochlor or metolachlor
followed by glufosinatc controlled common lambsquarters in 1997 greater than the POST
tank mix combinations of these herbicides (Table 3). This difference in control could be

attributed to poor common lambsquarters control from glufosinate. Although glufosinatc

56

applied alone in 1996 controlled common lambsquarters 93%, glufosinatc applied alone in
1997 controlled common lambsquarters only 76%. In 1997, the density of common
lambsquarters at application was more than five time higher than in 1996 (Table 1). The high
density of common lambsquarters plants in 1997 might have resulted in incomplete herbicide
exposure which would decrease the effectiveness of glufosinate. Steckel et a1. (1997)
attributed reduced glufosinatc activity on giant foxtail to inadequate herbicide coverage. The
residual tank mix partners likely prevented new emergence of common lambsquarters.
Unlike the other residual herbicide partners, atrazine has foliar activity on common
lambsquarters. Therefore, common lambsquarters control was complete and not affected by
the application timing of atrazine. Giant foxtail control was greater when glufosinatc was
applied as a POST tank mixture with atrazine compared to a sequential application (Table
3). Giant foxtail control was increased when a PRE application of pendimethalin was
followed by a POST application of glufosinatc compared to a POST tank mixture of these
herbicides. A PRE application of a herbicide with residual grass activity followed by a POST
application of glufosinatc has been shown to increase the consistency of annual grass control
(Van Wychen et al. 1999; Culpepper and York 1999). Glufosinate tank mixtures with
residual herbicides provided more complete control of weeds than sequential applications
in two instances. Weed control was greater from sequential applications compared to tank
mixtures in three instances. In most cases, the timing of the residual herbicide combinations
did not affect weed control (Table 3). In cases where weed control was increased by

application timing, corn yields were not affected.

57

Corn yields in plots treated with residual herbicide combinations with glufosinatc
were similar to the yields of the weed-free plots (Table 3). Corn in plots treated with
glufosinatc applied alone also yielded similar to weed-free plots. As with the glyphosate field
trial, weeds emerging after herbicide application did not reduce corn yield. Corn yields in
untreated plots were severely reduced as a result of intense weed competition.

Reduced application rates of residual herbicides. Glyphosate field trials. Velvetleaf
control was not increased when acetochlor, metolachlor, or the reduced rate of atrazine were
tank mixed with glyphosate (Table 4). Reduced rates of acetochlor plus atrazine and atrazine
resulted in reduced control of velvetleaf compared to the full rates. Control of redroot
pi gweed was increased by tank mixing the residual herbicides with glyphosate, and reduced
rates of all residual herbicide tank mix partners controlled redroot pigweed similar to full
rates. Tank mixing glufosinate with each of the residual herbicides, except metolachlor or
pendimethalin, increased common ragweed control in 1998. Common ragweed control was
lower when application rates of acetochlor and atrazine were reduced in 1998, and when
rates of flumetsulam and metolachlor were reduced in 1999. Common lambsquarters control
was increased when each of the residual herbicides were included with glyphosate compared
to glyphosate applied alone. Weed control is often increased when residual herbicides are
applied in combination with glyphosate compared to glyphosate applied alone (Gonzini et
al.1999; Wilson et a1. 1985). Reducing the application rate of acetochlor and flumetsulam
decreased common lambsquarters control. In 1998, giant foxtail control was increased when
glyphosate was tank mixed with each of the residual herbicides compared to glyphosate

applied alone, except for the reduced rate of atrazine (Table 4). In 1999, glyphosate tank

58

mixtures with acetochlor, acetochlor + atrazine, and metolachlor increased control of giant
foxtail compared to glyphosate applied alone. Control of giant foxtail in 1998 decreased
when the rate of atrazine was reduced. Otherwise, giant foxtail control was not affected by
application rate of residual herbicide combinations. O’Sullivan and Bouw (1993) reported
consistent grass control from reduced rates of metolachlor plus cyanazine, but grass control
was lower from reduced rates of cyanazine plus atrazine. Previous research showed potential
antagonism when glyphosate is tank mixed with other herbicides (Lich et a1. 1997; Selleck
and Baird 1981). Although glyphosate tank mixtures with atrazine delayed the development
of injury symptoms (data not shown), tank mixing residual herbicides with glyphosate did
not reduce the overall control of any of the weed species in these trials. Weed control was
often increased when a residual herbicide was included with glyphosate, and was less
consistent when rates of the residual herbicides were reduced. Weed control was reduced in
nine of 60 instances when residual herbicide application rates were reduced. Corn yields
were similar among full and reduced rates of all residual herbicide combinations and were
comparable to yields in the weed-free plots (Table 4). Corn yields in the untreated plots were
significantly reduced indicating high weed competition.

Glufosinate field trials. Velvetleaf control was increased by tank mixing glufosinatc with
acetochlor plus atrazine, flumetsulam, pendimethalin, and the fill] rate of atrazine (Table 5).
Previous research has also shown increased weed control when glufosinatc is applied in
combination with residual herbicides (Culpepper and York 1999; Wilson et al. 1985).
Reduced rates of acetochlor plus atrazine, flumetsulam, and pendimethalin reduced

velvetleaf control. Common ragWeed control was increased when acetochlor, acetochlor plus

59

atrazine, atrazine, or full rates of flumetsulam and metolachlor was added to glufosinate.
Control of common ragweed decreased when application rates of acetochlor and flumetsulam
were reduced. Reduced application rates of residual herbicide combinations did not affect
control of redroot pigweed compared to full rates. When the application rate of acetochlor
was reduced, common lambsquarters control decreased. All residual herbicide combinations,
except the reduced rate of metolachlor, increased common lambsquarters control in 1998 and
redroot pigweed control compared in both years to glufosinate applied alone. Herbicide
combinations which included atrazine provided greater than 96% control of common
lambsquarters and redroot pigweed. Culpepper and York (1999) similarly reported increased
common lambsquarters and smooth pigweed (A maranthus hybridus) control when
glufosinatc was tank mixed with atrazine. In 1998, giant foxtail control was increased when
each of the residual herbicide combinations were used, and control was lower when
application rates of metolachlor and pendimethalin were reduced (Table 5). In 1999, giant
foxtail control was increased when each of the residual herbicides except atrazine were used
with glufosinate, but was not influenced by application rate of the residual herbicides. Weed
control was often increased when residual herbicides were used with glufosinate. Reducing
the rates of residual herbicides reduced weed control in eight of 60 instances. As with
glyphosate, none of the residual herbicides tank mixed with glufosinate reduced control of
any weed species, and corn yield was not affected.

Large differences in weed control were not apparent when residual herbicides were
applied PRE and followed by a POST timing of glyphosate or glufosinatc compared to POST

tank mixtures of the residual herbicides with the glyphosate or glufosinate. Of the residual

60

tank mix partners evaluated in this research, the acetochlor plus atrazine tank mixture with
glyphosate or glufosinate provided the most consistent control of all weed species, except
velvetleaf. Reducing the application rates of the residual herbicides often decreases the
consistency of weed control.

Many factors need to be considered when deciding whether or not to use residual
herbicides with glyphosate or glufosinatc. Weeds that emerge following glyphosate or
glufosinatc applied alone will not likely reduce corn yields. However, weed control from
single POST applications of these herbicides alone is often less than 90%. Residual herbicide
combinations with glyphosate or glufosinatc increase season-long weed control and should
minimize weed seed production, but corn yields will not likely be increased. Growers will
need to assess the value of maintaining a high level of weed control throughout the season
with no apparent corn yield increases when deciding if a residual herbicide is needed with

glyphosate or glufosinate.

61

LITERATURE CITED

Ateh, C. M. and R. G. Harvey. 1999. Annual weed control by glyphosate in glyphosate-
resistant soybean (Glycine max). Weed Technol. 13:394-398.

Bosnic, A. C. and C. J. Swanton. 1997. Influence of bamyardgrass (Echinochloa crus-galli)
time of emergence and density on corn (Zea mays). Weed Sci. 45:276-282.

Buhler, D. D., J. L. Gunsolus, and D. F. Ralston. 1993. Common cocklebur (Xanthium
strumarium) control in soybean (Glycine max) with reduced bentazon rates and cultivation.
Weed Sci. 41:447-453.

Carey, J. B. and J. J. Kells. 1995. Timing of total postemergence herbicide applications to
maximize weed control and corn (Zea mays) yield. Weed Technol. 92356-361.

Culpepper, A. S. and A. C. York. 1999. Weed management in glufosinate-resistant corn (Zea
mays). Weed Technol. 13:324-333.

Dalley, C. D., J. J. Kells, and K. A. Renner. In Press. Weed interference in glyphosate
resistant corn and soybeans as influenced by treatment timing and row spacing. Proc. North
Central Weed Science Society. In Press.

Gonzini, L. C., S. E. Hart, and L. M. Wax. 1999. Herbicide combinations for weed
management in glyphosate-resistant soybean (Glycine max). Weed Technol. 13:354-360.

Gower, S. A., M. M. Loux, J. Cardina, P. L. Sprankle, and N. J. Probst. 1999. Determining
the critical period of weed interference in glyphosate-tolerant corn: results of a multi-state
study. Weed Sci. Soc. ofAm. Abstr. 39:55.

Horak, M. J ., P. F. Reese Jr., J. L. Flint, T. Roebke, N. Gubbiga, T. Bauman, W. Johnson,
D. Null, W. Curran, J. Getting, T. Hoverstad, S. Hart, G. Harvey, G. Kapusta, M. Loux, M.
Owen, K. Renner, C. Slack, M. VanGessel. 1998. Early season weed control in roundup
ready soybean: effect on yield. Proc. North Central Weed Sci. Soc. 53:130.

Knake, E. L. and F. W. Slife. 1965. Giant foxtail seeded at various times in corn and
soybeans. Weeds 13:331-334.

Menbere, H. and R. L. Ritter. 1999. Weed management systems utilizing glufosinate and
glufosinatc-resistant crops. Weed Sci. Soc. of America Abstr. 39:68.

Murphy, S. D., Y. Yakubu, S. F. Weise, and C. J. Swanton. 1996. Effect of planting patterns

and inter-row cultivation on competition between corn (Zea mays) and late emerging weeds.
Weed Sci. 44:856-870.

62

Rabaey, T. L. and R. G. Harvey. 1997. Sequential applications control woolly cupgrass
(Eriochloa villosa) and wild-proso millet (Panicum miliaceum) in corn (Zea mays). Weed
Technol. 11:537-554.

Smith, A. E. 1989. Transformation of the herbicide [14C ] glufosinatc in soils. J. Agric. Food
Chem. 37:267-271.

Sprankle, P., W. F. Meggitt, and D. Penner. 1975b. Adsorption, mobility, and microbial
degradation of glyphosate in the soil. Weed Sci. 23:229-234.

Sprankle, P., W. F. Meggitt, and D. Penner. 19753. Rapid inactivation of glyphosate in the
soil. Weed Sci. 23:224-228.

Steckel, G. J ., L. M. Wax, F. W. Simmons, and W. H. Phillips 11. 1997. Glufosinate efficacy
on annual weeds is influenced by rate and grth stage. Weed Technol. 11:484-488.

Tebbe, C. C. and H. H. Reber. 1991. Degradation of (CM) phosphinothricin (glufosinatc) in
soil under laboratory conditions: effects of concentration and soil amendments on l4C02
production. Biol. and Fertility of Soils 11:62-67.

Tharp, B. E. and J. J. Kells. 1999. Influence of application rate, timing, and interrow
cultivation on weed control and corn yield in glufosinatc-resistant and glyphosate-resistant
corn. Weed Technol. 13:807-813.

Van Wychen, L. R., R. G. Harvey, M. J. VanGessel, T. L. Rabaey, and D. J. Bach. 1999.
Efficacy and crop response to glufosinatc-based weed management in PAT-transformed
sweet corn (Zea mays). Weed Technol. 13:104-111.

Watts, J. R., E. C. Murdock, G. S. Stapleton, and J. E. Toler. 1997. Sickelpod (Senna

obtusifolia) control in soybean (Glycine max) with single and sequential herbicide
applications. Weed Technol. 1 1:157-163.

63

 

 

 

 

 

NN 2 9. N “N N a, NN 3:02;

o a 2 N N N o oN Beams 8858

N a a : N N N : N 83% 683m

«N N e: N e N 8 N messages 8868

oNN NZ N NE so 9% NNN 33 NE Essa Ego
NEESNE 36:00 80>)

Na ON M: 2 2 ON 2 2 Es “swam

N N N N N 4 N N m5:8 @553
owflm 53on 800

aom_ Noon Nom_ oma_ mama Nam_ Name omo_
23.5 20¢ magma—:0 ENE 22% QHMmOSQNA—mv

 

32.82 .32 a base 303 new swam 53% 58 .N a3

64

.00NmonN5w msg#00300 3008300 =N 00 0003 NB €33
ficm N 05:30 8:80:52. NEE mod 203 0.080 00Nm0:§_w =N 003 E .0:0_N 00:03 mNB 00Nmonabw c003 Nfiwx mvd EN $000.08:
028 033 0% 30000000 05 00.0 Néwx mod mNB 008 05 mag E 98 .N£0N wx mud NB 3% EN 003 E 030 00Nmonmbw 05. N

.Amodumv “m0“ qu 0N 00 $5088 000000.20 30.30%sz 00: 0003 00:2 0:80 0:0 3 0030:00 50300 N 0323 £802 a

 

 

 

 

 

 

N _N.N_ cm: on: 02 03 03 03 2: cm: 03 03 03 .000.“ 0003
n mus o o o o o o o o o o o 008N050:
N No.2 0 0w 0 R 0 0w 0 mm 000 N0 a mu 0 on N E 0 we a Q. 0N m0 000? 00Nmosmbw
N 0%: 0 3 N 00 2N 3 0 0m 2N 00 n S a mo N mm N 02 N 00 0N m0 028 x08
N mm. 2 a 00 N mm on 5 0 0m 0 0w 0 mm 0 mm N 00 00 N0 N 3 0N 3 2330003 535056000
N 3.: N 00 0 00 0N m0 0 no N ma 0 mm a no N mm 0 we a ow 0 mm 58 x08
N 3.: N 00 0 S 00 mm 0 mm 00 S a mm 0 mm N mm 0 E a mm 00 um 3:00:08 002030008
Nmo.: 0% N2: N00 NON: N02 Nwo Noe Nww 03 N m0 N3 58 x03
N 00.: 0 3 N 03 0N 3 N N0 N 03 N mo DN 00 N m0 00 5 0 vw 0N 8 3:00:08 0ENN0N
N 3.: N 00 0 00 N mm N 3 N 03 0N 0w 0N 00 N mm 0 mu 0 mm 00 mm 0:8 VENH
N 3.: N mm N 00 0N 00 N mm 00N 3 0N a LN 00 N 0w 00 mm 0 ow 0 K 3000008 00200008
I N532 I 60280 .x.
<mHMm immu 003 mag .33 003 mm<2< 003 mag 33 003 swEE: ”9003.0:sz £3,
“20% gaging 005.80 020303
E00 4mg EHDm<

 

100000 0003

 

«20% 500 “EN 30:00 0003 :0 00Nm0nnbw 5:» 82305800 020300: 330600 m0 was“: nouNozmmN .00 80mm .N 0303

65

.00:NtN> 0: 0>N0 m02N> 000N000 <>OZ< E 000208 00: 0003 303 00000003 05 000N088. 000 NHN0 38000 000 .3 N
.002 - 33 8N0» 00>0 02000 00—03 800 o

.003 - 002 $30» 00>0 00—000 1?me 08N {3330 .9332 0808““ 8&w {hr—{mm ”88.800383 008800 {ammo

M0003mN0 808800 {Emu/2 M0003w8 008000 .25 ”(800023 .05sz 800828? HmOm 08 00% 0 mm 00080000 $808 30050
008.80 006500: £3 00300ng

00 08008 8N0 HmOm .008 8N0 ”BNmonmbw 00 8008285 HmOm 3 0030200 008.80 006500: 05 .80 80085:? mam 4N0=0000m u

66

23:5 fem “a 08.23 83.8828 8:205 8.860me wEEwEoo 85:58: =< 4“me mmd 803 238 826820 A.
Amodumv “mo: qu cm 9 @6808 680mb: 358$:me 8: 203 3:2 08% 2t 3 330:8 5:28 a 55:? £802 a

 

 

 

 

 

 

0% 3d 2: 2: 2: 2: 2: 2: .02: 303
u SN 0 o o o o o 228%:
0% 2; v S c 3 a .8 0 2w 5. g o a 2% 2258i

3 m3 0 8 B a a 3 a.“ Ma 2“ mm a 8 :8 is
o Em a a n 3 a mo 3 3 a 8 a: No 3:538 £25323

3 N3 % 3 o E a 2: a 2: n 3 o 3 :8 veg
0% 2% m a o 8 a mo 3 8 pm mm o M: 3:538 5202906

m :3 08 m2: m2: «2: 22: a8 is :5
an 2 .o c Q a 2: a 2: a 8 a 2: pm No 3538 8:23

o a; pm Ma w 2 a mg a 8 an mm 0 2w 56 is
o 2.x nu mm a 2: a Ma 2.. Ma pm mo 0 mm 3:538 szoeog

l £32 | 6980 X
“222 EB fiemm 32 82 Ems? 52:2 52m? :58: 8:30:23 macaques» .23
8:th 0205.82
émmo
28:80 @003

 

«.22» Eco 98 35:8 to?» so 89:8me 53, 2835:5800 02058: 3323.: Mo wEE: 5:32QO .«0 Beam .m. flag

67

808:? o: 8:: 82? 8:88: <>OZ< E :o::_o:_ 8: 883 $2: 8::83 :8 8885:: 8.: :8: 35:8 :83 _
.32 - com: .88.» 8>o :28: 883 :80 u

.32 - 82 .282 $3 838 <88 as 8:32 .22... .58... :88: an: .588 mm.o:~=cm:§_ 8888 :28:

6838: 88:80 .AmmSZ 683$: 82:8 .9232 @8838.» .mhbmxx .882??? .50: 2: 8:: : mm 8:802 8:8: 835 v
.858: 8:858: 5:: Samongfi

:0 882:: 2:8 HmOm .58 2:8 88.8:me mo 8:82:28 .50: .3 830:8 858: 0:858: 05 mo 8:823: mam ._m::o:8m u

68

Amodnmv :8: Om:— 8 8 $688: 888:: 3:82:38 8: 883 8:8. 888 2: .3 830:8 5:28 a 55:5 382 :

 

 

 

69

 

 

 

 

: 8.: 2: 2: 2: 2: 2: 2: 2: :2: :25.
o 8.8 o c o o o o o 8:225
.: 8.: 8 8 o 8 o 8 8 2: o 8 o 8 8 8 25: 8:822:
.: 2.: B 8 2: 8 m 2: 2: 8 u 8 a 8 2: 8 82:2
: 8.: B 8 o 8 a 2: u 8 o 8 .: 8 .: 8 =2 5:506:22:
.: 8.: 2: 8 2: 8 8 8 c 8 o 8 .: 8 u E 82:2
.: 8. : .: 8 .: 8 8 2w 2: 8 8 8 : 8 8 8 :3 528222:
a 8.: B 8 8 8 U .8 2: 8 8 2w : 8 2: 8 32:2
.: 8.: :2: 8 8 2: n. 8 a 2: 2: 8 2: 8 2: 8 :3 2:328:
.: :2 B 8 o 8 : 8 2: 8 B 8 a 8 8 8 B262
.: 8.: 8 8 c 5 a 2: .: 8 a 2: a 2: o 8 =2 8:8:
.: 8.: : 8 a 2: a 2: a 2: m 2: : 2: 2: 8 82:2
.: 8.: m 8 m 2: a 2: a 2: n: 8 m 2: a 8 :2 02:58:83:
.: 8.: 2: 8 .: 8 8 .2: .: 8 B 8 .: 8 8 8 B262
.: 8.: a 8 m 2: .: 8 2: 8 : 8 m 2: 8 8 :3 528::
II 2&2 ll 75:8 o\o
22> EB 88: 28: :88 88: 88: 52:2 :53 .22 55:8 :8 as:
<n:.mm AME)? 8:82:98.
20:80 :83

 

«2:» :80 :8 35:8 :83 :o 8252:: :8: 882383 5 8:838: 83:88: mo 8:: 88:8: :0 88:5 .2: 83:8

.8529, 0: «>2 829» 8:83 <>OZ< E vows—2: go: 203 32a moaéooa EB @8395 H8 8% 3589 303 v

.232?wa wfifificoa Baogmub =« 9 new? $3

AEBV o\om Hm Sufism EacofiE< .méwx $6 895 8:8 8&8:an =« 32 E .28? 3:an mm? mammogngw 5:3 «Emu. $3 28 352585 0:53: x83 2:
H8 «£3 mod 33 BS Saga—w 05 wofl 5 .88 =3 2: has 895 $2033: :26me 05 mo mutt 3262 2; .mme 54 8 Eifiogvcom ES .mfiwx v; 3
820282: .afiwv— $06 3 82388:: dakwx _._ “a 05523 6me v." 8 campus 33 max w." 3 83868 653 mg «a 8208on 203 8:2 :3 2F 0
.32 - wag £3» B>o @203 41530 98 dag/x .533 4838.“ 82m infirm—m mmuutascmn§_

:oEEoU J<mmu 683me 88:50 rims; 6833a 8068 dds)? emu—82m; .EDm< donmozamm HmOm 05 Ban u mm 3982 $523 33; A.

70

.306an 88 qu S“ 9 wSEOoom Eobfifi bEmanwB Ho: 803 .833 ~88. 2: 3 326:8 5:38 a £53» £822 a

 

71

 

 

 

 

 

 

a" 8.: 8: 8: 2: OS 8: 8: 8: ”Ba 8%,
u m? o o o o o o o «.8322;
a 8.: u a 0 mm m G 8 8 a 8 8 8 8 8 2.2m @3882»
e... 2.: 8 8 0 mm a 8 EU 8 a 8 on 8 n 8 8262
as 2 .2 B 8 p 8 an 8 E S m 8 B 8 a 8 =3 £23828
fi 8.: Ba 8 p 8 a S 8.... S a 8: .3 mm 8 8 8262
e... 3.: Ba 8 a 8 t 8 EU a a 8: 8 .8 8 a. =e 820222:
on 8.: 8 8 v we 8 8 z a a 8: 8 mm B S 8262
a 3.: u a u we 0 8 Ba 8 a 8: 8 mm a 8 :3 528358
8 8.: 8 mm :8 8 an 8 B 8 a 8 a 2: .3 8 8262
a... 8.: 8 8 B R an 8 pm 8 a 8: a 2: DB 8 :3 EEE
a“ 8.: an 8 a 8 m 2: m o: a 2: a 2: o 8 @882
an 8.: m 8 a 8 a o: a o: m 2: a 2: a 8 E 0553:2698“
a a: an 8 a 8 8 8 Ba 8 a 8: B 8 E :w 8262
an 8.: pm 8 a o: o a a 8 a 2: fl 8 t 8 :3 52833
..| «£32 I 35:8 o\o
22> 68 8: 8: 48:5 gum—>2 88: M8: :53. use 558 88 85
£85 mm<2< 8:883
n—ObcoU U003

 

«22» 88 98 35:8 too? so 85:28 x53 0858.33 5 82058: 3:38: .«o 8:2 :3st mo Seam .m 033.

.8553 ca 96: 32a.» 3:83 <>OZ< E 82:05 8: 203 «.83 029683 was BEBE: H8 8% 35:8 303 v

ABE; fem 3 03:3. 8:80:58 39.35 336823 wfifificoa 3:288:

=< .méwx omd 3 3:9? 33 0358.35 .28 :3 05 has 203 82035: 3662 2: .3 3:2 30sz ugh .mev— 5; 3 5352533 28 £53 v; 3
820282: 6me 036 an 82:325.: 653 _._ a ofimmbm 6me v; “a 333m 33 «£3 w; “a 82868 $53 m4 8 8208on 825 85 :3 2:1 0
.82 - M32 .28» as 333 flag as gums? .Ebm< age :5» 25mm 3553382

8888 J<mmo 633me 29:88 Jmm2< 68>)me 8062 .md<§< «Suez?» .EHDm< .cocmozgaa HmOm 05 Eta u mm @0388 mwcre :25; n

72

CHAPTER 4
EFFECT OF CORN (Zea mays) POPULATION AND ROW SPACING ON LIGHT
INTERCEPTION, WEED GROWTH, AND CORN YIELD IN GLUFOSINATE-
RESISTANT CORN
Abstract: Management of com row spacings and populations have been used for many years
to increase corn (Zea mays) productivity. In 1998 and 1999, corn was grown in 38-, 56-, and
76-cm row spacings at populations averaging 59,300, 72,900, and 83,900 plants/ha.
Glufosinate at 0.29 kg/ha was applied to common lambsquarters (Chenopodium album)
averaging 5 cm in height in each plot. Weed emergence following herbicide application was
monitored and no differences were found among row spacings or corn populations. Common
lambsquarters biomass and seed production were reduced when grown under canopies of
corn planted in populations exceeding 72,900 plants/ha. Row spacing did not influence the
growth or seed production of common lambsquarters. Early season interception of
photosynthetic active radiation (PAR) by com canopies increased as row spacings decreased,
but differences were not apparent later in the season. Interception of PAR among corn
populations were similar throughout the season. Corn yields were not affected by row
spacing, but were increased when com populations exceeded 72,900 plants/ha. When

averaged over row spacings and populations, com yields were reduced by weeds not

73

controlled by glufosinate. Planting corn in reduced row spacings at high populations did not
significantly increase weed control following a POST application of a non-residual herbicide.
Nomenclature: glufosinate, 2-amino-4-(hydroxymethylphosphinyl)butanoic acid; common
lambsquarters, Chenopodium album L.#” CHEAL; corn, Zea mays L. ‘DK 493GR’.
Additional index words: crop density; herbicide tolerant crops; narrow row corn.
Abbreviations: DAP, days after planting; DAT, days afier treatment; PAR, photosynthetic

active radiation; POST, postemergence; PRE, preemergence.

 

“ Letters following this symbol are a WSSA-approved computer code from
Composite List of Weeds, Revised 1989. Available from WSSA, 810 East 10m Street,

Lawrence, KS 66044-8897.

74

INTRODUCTION

Corn yields have dramatically increased since the beginning of the 20th century
(Warren et al. 1998). Cardwell (1982) reported how changes in production practices
contributed to corn yield increases from 1930 to 1980 in Minnesota. He estimated that the
adoption of hybrid corn contributed to 58% of the yield increase, and 23% of the yield
increase was attributed to increasements in weed control. The adoption of new technologies
has also contributed to corn yield increases. For instance, the advancement of power driven
equipment over horse-drawn equipment has allowed farmers to reduce com row spacings and
increase plant densities. Cardwell (1982) attributed a 25% yield increase to increased plant
densities and reduced row spacings. Other factors such as soil erosion and occurrences of
new insect pests have had negative effects on corn yields.

The effects of row spacing on corn grain yield is quite variable throughout the
literature. Some researchers have reported yield increases when row spacings were reduced
(Lutz et al. 1971; Murphy et al. 1996; Ottman and Welch 1989; Porter et al. 1997) while
others have reported no effect on yields (Alessi and Power 1974; Johnson et al. 1998; Nunez
and Kamprath 1969; Westgate et al. 1997). Increased corn populations have consistently
resulted in yield increases (Cox 1997; Nunez and Kamprath 1969; Westgate et al. 1997).
Fulton (1970) reported yield increases from corn planted in a 50 cm row spacing at 54,000
plants/ha when soil moisture levels were high. Polito and Voss (1991) cautioned that
adoption of techniques such as reduced row spacing and higher populations for higher yields
pose greater risks to producers who cannot control water availability.

75

Managing row spacings and plant populations alter the spacing of plants between and
within rows. These changes influence corn growth and development. Seedlings grown in
close proximity to each other express phytochrome mediated responses by developing
narrow leaves, long stems, and less massive roots (Kasperbauer and Karlen 1994). Planting
corn in a pattern that equalizes the spacing of plants within and between rows can increase
plant biomass and leaf area index (Bullock et al. 1988). These types of spatial changes affect
the architecture and light dynamics of a corn canopy. Reduced row spacings increased the
total interception of photosynthetic active radiation (PAR) by the corn canopy and
redistributed the radiation towards the top of the canopy (Ottman and Welch 1989). Tetio-
Kagho and Gardner (1988) reported a similar trend of light distribution as plant density was
increased. Photosynthetic efficiency, corn growth, and ultimately grain yield is influenced
by the distribution and interception of PAR within a corn canopy.

Reduced row spacings and increased corn populations are thought to increase weed
control by increasing the competitiveness of a crop with the weeds and by reducing the light
transmittance to the soil surface. Teasdale (1995) showed that reduced row spacing and
increased populations decreased weed growth in the absence of herbicides and shortened the
time to canopy closure by one week. Many research trials have been conducted to investigate
the potential for reducing herbicide use by reducing the row spacing or increasing the
population of corn. F orcella et al. (1992) and Teasdale (1995) found that weed control from
reduced herbicide rates was increased in narrow row compared to wide row environments.
In contrast, Johnson et al. (1998) showed little benefit to reduced row spacings as a method

76

for reducing herbicide inputs. Other trials have been conducted to determine the influence
of row spacing and corn populations on weeds that emerge later in the season. Weed biomass
was reduced when com populations were increased (Tollenaar et al. 1994) and row spacings
were reduced (Murphy et al. 1996).

The emergence of weeds following application of non-residual herbicides, such as
glufosinate, is a potential weakness of weed management systems that include these
herbicides. Glufosinate controls early-season weeds in corn, but additional weed
management strategies are needed for season-long weed control. The objectives of this
research was to determine if glufosinatc-resistant corn planted in reduced row spacings and
at increased populations would increase weed control following a timely application of
glufosinatc. Data evaluated in this trial include PAR interception, weed emergence, weed

growth, and corn yield.

MATERIALS AND METHODS

Field trials were conducted in 1998 and 1999 on the Crop and Soil Sciences Research
Farm at Michigan State University in East Lansing, MI. The soil was a Capac sandy loam
(fine-loamy, mixed mesic Aeric Ochraqualt) with a pH of 7.1 and 3.5% organic matter in
1998, and a pH of 5.9 and 1.4% organic matter in 1999. A 46-0-0 (N-P-K) fertilizer was
broadcast prior to corn planting each year. Fertilizer rates were determined from the results
of soil analysis. A custom built toolbar equipped with John Deere planter units was used to
plant ‘DK 493GR’ corn in row spacings of 38 cm, 56 cm, and 76 cm on May 27, 1998 and

77

 

May 14, 1999. The corn was planted at low, medium, and high population regimes for each
row spacing. The number of emerged corn plants in the center rows of each plot were
counted to determine actual populations (Table 1).

Two weed control treatments were applied within each row spacing and corn
population. Glufosinate at 0.29 kg/ha plus ammonium sulfate at 2% (w/w) was applied when
weeds averaged 5 cm in height 36 days after planting (DAP) in 1998 and 28 DAP in 1999.
Weed—free plots were included at all row spacing and corn population combinations. Weed-
free plots were treated with a PRE application of a premix12 of S—metolachlor at 2.24 kg/ha
plus atrazine at 1.12 kg/ha followed by hand-weeding as needed. Herbicides were applied
with a tractor-mounted compressed-air sprayer equipped with flat-fan nozzles13 and
calibrated to deliver 187 L/ha at a pressure of 207 kPa. Plots were 3 m wide and 10.7 m long
in 1998 and 9.1 m long in 1999.

The amount of photosynthetic active radiation (PAR) transmitted through the corn
canopy to the soil surface was measured in 1998 and 1999 on a weekly basis beginning when
com had developed 5 to 6 collars and ending at corn silking, at which time it was assumed

that cano closure was complete. A SunScan” canop anal sis system was used for these
Py Y y

 

12Bicep Lite 11 Magnum, Novartis, Crop Protection Division, PO Box 18300,
Greensboro, NC 27419.

l3TeeJet XR 8003VS. Spraying Systems Co., North Ave., Wheaton, IL 60188.

‘4 Delta-T Devices Ltd. 128 Low Road, Burwell, Cambridge CBS OEJ, England.

78

measurements, and was comprised of a 1 m long probe used to detect the amount of PAR
transmitted to the soil surface below the corn canopy, and a beam fraction sensor used to
detect direct and diffuse PAR outside the canopy. The probe was aligned down the center of
adjacent corn rows in each weed-free plot. The beam fraction sensor was situated on a tripod
and placed outside of the corn canopy. The two devices were used to record simultaneous
PAR measurements. Three measurements were recorded in each plot near solar noon at each
weekly interval. The percent PAR intercepted by the canopy was determined by dividing the
transmitted PAR by the incident PAR and multiplying by 100. In 1998, the 63 DAP light
measurement date was discarded because corn in all plots were showing symptoms of
moisture stress (rolled leaves) which skewed PAR readings.

Common lambsquarters was the dominant weed species present each year. Three
1060 cm2 quadrats were randomly established between the center two rows of each plot
previously treated with glufosinatc. Ten days following glufosinate application common
lambsquarters that showed symptoms of potential recovery from herbicide application were
marked with toothpicks within each quadrat. The marked weeds enabled us to distinguish
between common lambsquarters that emerged following herbicide application from those
that recovered from the glufosinatc application 21 days after treatment (DAT).

In 1998 two common lambsquarters plants in the center of the plots were covered
with an inverted 10 cm clay pot prior to application of glufosinate to protect the plants from
exposure to the herbicide. The clay pots were removed within an hour following herbicide
application and the common lambsquarters remained undisturbed until seed drop. In 1999,

79

the number of common lambsquarters plants were increased to three per plot to reduce
variability. The aboveground shoots of these plants were harvested at the initiation of seed
drop on September 29, 1998 and September 28, 1999. The harvested shoots were dried in a
100°C oven for at least 5 d and dry weights were determined. Achenes containing seed,
referred to as seed from this point forward, were separated from the harvested shoots and
weighed. The number of seed per plant was estimated by multiplying the weight of seed per
plant by the average weight of ten random samples of 100 seed.

Corn grain yield was determined by harvesting the center five rows of the 38-cm row
spacing plots, the center three rows of the 56-cm row spacing plots, and the center two rows
of the 76-cm row spacing plots with a mechanical plot harvester. Corn grain yield was
corrected to 15.5% moisture.

Each trial was designed as a split-split-plot with four replications. The whole-plot
was row spacing, the sub-plot was com population, and the sub-sub-plot was weed control
treatments. All data were subjected to ANOVA procedures. Data were pooled over main
effects when the p-values of the interactions were greater than 0.05. PAR and yield data were
separated using Fisher’s protected LSD at the P: 0.05 level of significance. Common
lambsquarters emergence, biomass, and seed production data were variable. Therefore,
means were separated using Fisher’s protected LSD at the P= 0.10 level of significance. All
data were subjected to F-Max tests to test for homogeneity of variance among years (Kuehl

1994). Data found to be homogenous were pooled over years.

80

RESULTS AND DISCUSSIONS

Light interception. The amount of light intercepted by a corn canopy typically increases as
row spacing is decreased (Bullock et al. 1988; Murphy et al. 1996; Ottman and Welch 1989).
Averaged over populations, corn grown in 38—cm row spacings intercepted more PAR than
76-cm spacings up to 50 DAP in 1998 and 42 DAP in 1999 (Figure l). The difference in
PAR interception among row spacings never exceeded 20%. The maximum level of
intercepted PAR never exceeded 95%. Differences in the levels of PAR interception among
row spacings were not apparent later in the season (Figure 1). Westgate et al. (1997) also
reported no differences in maximum PAR levels as row spacings were decreased.

Averaged over row spacings, corn planted at low populations intercepted similar
amounts of PAR than corn planted at higher populations (Figure 2). In contrast, previous
researchers have reported an increase in light interception when com populations were
increased (Murphy et al. 1996; Tollenaar 1994; Westgate et al. 1997). This contradiction
could be due to differences in the range of populations investigated. The average difference
between the low and high populations in our trials was 24,600 plants/ha, which was less than
the range tested by others.
Weed growth. Smith (1988) reported that glufosinate is rapidly degraded by soil
microorganisms. Therefore, the herbicidal activity of glufosinatc in most soils is severely
diminished, and weeds that emerge following application are not controlled. Corn grown in
reduced row spacings and at increased populations did not affect weed emergence following
application of glufosinate (Table 2). Johnson et al. (1998) similarly reported that row spacing

81

has little impact on mid-season weed densities. Peak time and periodicity of weed emergence
was similarly not affected by row spacing in soybeans (Mulugeta and Boerboom 1999).
Tollenaar et al. (1994) reported that in both high (144 plants/m2) and medium (79 plants/m2)
weed pressures the number of weeds at corn silking did not differ among corn populations.

Biomass of untreated common lambsquarters was reduced in 38-cm and 56-cm row
spacings compared to 76-cm row spacing (Table 3). Common lambsquarters biomass was
also reduced as corn populations increased. McLachlan et al. (1993) showed that as light
interception by a corn canopy increased, the total dry matter accumulation of redroot
pigweed (Amaranthus retraflexus) decreased. Tollenaar et al. (1994) and Murphy et al.
(1996) also reported a reduction in weed biomass from increased corn densities or reduced
row spacings, while Johnson et al. (1998) reported no effect of row spacing on weed
biomass. McLachlan et al. (1993) and Murphy et al. (1996) showed that increased light
interception by com canopies decreased weed growth. In our trials, differences in common
lambsquarters biomass and seed production were most pronounced among corn populations.
However, light interception among corn populations were similar throughout the season,
suggesting that factors other than light interception were contributing to the differences in
weed growth. In addition to light, corn and weeds compete for water and nutrients. Tollenaar
et al. (1994) suggested that increased corn populations enhanced the competitive effects of
corn with weeds. Such a shift in competitive balance would result in decreased weed growth.

Since high soil fertility levels were maintained in both years and large differences in light

82

interception were not apparent, competition for moisture would have most likely contributed
to decreased weed growth.

The number of seed produced by each common lambsquarters plant was also reduced

as corn populations increased, but was not affected by row spacings (Table 3). McLachlan
et al. (1993) similarly found that redroot pigweed inflorescence was reduced as corn
populations increased. There was a strong correlation (r2=0.93) between the number of seed
produced by each common lambsquarters plant and shoot biomass (Figure 3). Crook and
Renner (1990) also reported a correlation between common lambsquarters seed production
and shoot biomass.
Corn yield. Johnson et al. (1998) and Teasdale (1995) showed that, regardless of the level
of weed control, yields were similar for corn planted in 76-cm row spacings to corn planted
in 38-cm row spacings. In our trials, corn yields were also similar among row spacings
(Table 4). In contrast, Murphy et al. (1996) and Porter et al. (1997) reported yield advantages
as row spacings were reduced. Hybrid selection and environmental conditions could attribute
to the differences in yield results. In our trials, corn yields were highest at 73,000 plants/ha
and 84,000 plants/ha (Table 4). Porter et al. (1997) reported that com yields were higher at
populations of 79,000 and 86,000 plants/ha in some years, but were unaffected by
populations in other years. Cox (1997) reported optimum corn yields from populations
averaging 75,300 plants/ha in dry environmental conditions and populations exceeding
88,900 plants/ha in favorable environmental conditions, indicating a strong environmental
influence to yield determination.

83

Corn yields were reduced in plots that were treated with glufosinatc compared to
weed-free plots (Table 4). This difference was may be due to weed competition from weeds
that were not controlled by glufosinate.

Murphy et al. (1996) stressed the importance of early-season weed control, and
suggested that cultivation and corn planted in narrow rows might be a potential integrated
weed management system that could be used to control late-season weeds. A timely
application of glufosinate will provide adequate early-season weed control, but other weed
control measures are needed for weeds that emerge later in the season. Weed emergence
following an application of glufosinatc was not affected by planting corn in narrow row
spacings and at high populations. Weed growth was reduced under a canopy of corn planted
in 38-cm and 56-cm row spacings and at populations exceeding 73,000 plants/ha. Corn
yields were not increased by narrow row spacing but were increased when populations were
increased. The results of this research suggest that increasing corn populations to 73,000
plants/ha may reduce weed growth and increase corn yields under the environment where
these studies were conducted. However, weed emergence following an application of
glufosinate was not affected by increased corn populations. Our research does not support
the hypothesis that com planted in narrow rows at high populations reduces weed emergence.
Further research is needed to identify other weed management strategies that can be used to

increase weed control following application of a non-residual herbicide, such as glufosinatc.

84

LITERATURE CITED

Alessi, J. and J. F. Power. 1974. Effects of plant population, row spacing, and relative
maturity on dryland corn in the northern plains. 1. Corn forage and grain yield. Agron. J.
66:316-319.

Bullock, D. G., R. L. Nielsen, and W. E. Nyquist. 1988. A growth analysis comparison of
corn grown in conventional and equidistant plant spacing. Crop Sci. 28:254-258.

Cardwell, V. B. 1982. Fifty Years of Minnesota Corn Production: Sources of Yield Increase.
Agronomy Journal. 74:984-990.

Cox, W. J. 1997. Corn silage and grain yield responses to plant densities. J. Prod. Agric.
10:405-410.

Crook, T. M. and K. A. Renner. 1990. Common lambsquarters (Chenopodium album)
competition and time of removal in soybeans (Glycine max). Weed Sci. 38:358-364.

Forcella, F ., M. E. Westgate, and D. D. Wames. 1992. Effect of row width on herbicide and
cultivation requirements in row crops. Am. J. of Altem. Agric. 7:161-167.

Fulton, J. M. 1970. Relationships among soil moisture stress, plant populations, row spacing
and yield of corn. Can. J. of Plant Sci. 50:31-38.

Johnson, G. A., T. R. Hoverstad, and R. E. Greenwald. 1998. Integrated weed management
using narrow corn row spacing, herbicides, and cultivation. Agron. J. 90:40-46.

Kasperbaueer, M. J. and D. L. Karlen. 1994. Plant spacing and reflected far-red light effects
on phytochrome-regulated photsynthate allocation in corn seedlings. Crop Sci. 34:1564-
1569.

Kuehl, R. O. 1994. Statistical Principles of Research Design and Analysis. Belmont, CA:
Wadsworth , Inc. 686 p.

Lutz, J. A., H. M. Camper, and G. D. Jones. 1971. Row spacing and population effects on
corn yields. Agron. J. 63:12-14.

McLachlan, S. M., M. Tollenaar, C. J. Swanton, and S. F. Weise. 1993. Effect of com-

induced shading on dry matter accumulation, distribution, and architecture of redroot
pigweed (Amaranthus retroflexus). Weed Sci. 41:568-573.

85

Murphy, S. D., Y. Yakubu, S. F. Weise, and C. J. Swanton. 1996. Effect of planting patterns
and inter-row cultivation on competition between corn (Zea mays) and late emerging weeds.
Weed Sci. 44:856-870.

Mulugeta, D. and C. M. Boerboom. 1999. Seasonal abundance and spatial pattern of Setaria

faberi, Chenopodium album, and Abutilon theophrasti in reduced-tillage soybeans. Weed
Sci. 47:95-106.

Nunez, R., and E. Kamprath. 1969. Relationships between N response, plant population, and
row width on growth and yield of corn. Agron. J. 61:279-282.

Ottman, M. J. and L. F. Welch. 1989. Planting patterns and radiation interception, plant
nutrient concentration, and yield in corn. Agron J. 81:167-174.

Polito, T. A. and R. D. Voss. 1991. Corn yield response to varied producer controlled factors
and weather in high yield environments. J. Prod. Agric. 4:51-57.

Porter, P. M., D. R. Hicks, W. E. Lueschen, J. H. Ford, D. D. Wames, and T. R. Hoverstad.
1997. Corn response to row width and plant population in the northern corn belt. J. Prod.
Agric. 10:293-300.

Smith, A. E. 1988. Persistence and transformation of the herbicide (14C)glufosinate-

ammonium in prairie soils under laboratory conditions. J. of Agric. and Food Chem. 36:393-
397.

Teasdale, J. R. 1995. Influence of narrow row/high population vorn (Zea mays) on weed
control and light transmittance. Weed Technol. 9:113-118.

Teito-Kagho, F. and F. P. Gardner. 1988. Responses of maize to plant population density.
1. Canopy development, light relationships, and vegetative growth. Agron. J. 80:930-935.

Tollenaar, M., A. A. Dibo, A. Aguilera, S. F. Weise, and C. J. Swanton. 1994. Effect of crop
density on weed interference in maize. Agron J. 86:591-595.

Warren, G. F. 1998. Spectacular increases in crop yields in the United States in the twentieth
century. Weed Technol. 12:752-760.

Westgate, M. E., F. Forcella, D. C. Reicosky, and J. Somsen. 1997. Rapid canopy closure
for maize production in the northern US corn belt: Radiation-use efficiency and grain yield.
Field Crops Res. 49:249-258.

86

Table 1. Actual corn populations for each year of the study.

 

Actual populations“

 

 

 

 

Population regime 1998 1999 Average
plants/ha

High 84,800 83,000 83,900

Medium 72,000 73,800 72,900

Low 59,000 59,700 59,300

 

3 populations averaged over row spacings

87

Table 2. Effect of row spacing and corn population on emergence of cormnon lambsquarters
following an application of glufosinatc, 1998 -1999.

 

Common lambsquarters

 

emergence
-— plants/m2 —

Row spacing“

38 cm 21.5 a

56 cm 17.4 a

76 cm 21.5 a
Average corn populationb

59,300 plants/ha 20.6 a

72,900 plants/ha 17.1 a

83,900 plants/ha 21.2 a

 

a Means are pooled over corn populations and represent a main effect. Means for row spacing
followed by the same letter within a column are not significantly different according to an
LSD test (P=0.10).
b Means are pooled over row spacings and represent a main effect. Means for corn population
followed by the same letter within a column are not significantly different according to an
LSD test (P=0.10).

88

Table 3. Effect of row spacing and corn population on untreated common lambsquarters
biomass and seed production, 1998-1999.“

 

 

Biomass Seed production
— grams — — no. of seed/plant —

Row spacingb

38 cm 28.0 b 26,770 a

56 cm 29.4 b 26,990 a

76 cm 38.4 a 34,860 a
Average corn populationc

59,300 plants/ha 48.4 a 47,740 a

72,900 plants/ha 27.2 b 24,090 b

83,900 plants/ha 20.2 b 16,910 b

 

“ Common lambsquarters plants were not exposed to glufosinatc application.

b Means are pooled over corn populations and represent a main effect. Means for row spacing
followed by the same letter within a column are not significantly different according to an
LSD test (P=0.10).

c Means are pooled over row spacings and represent a main effect. Means for corn population

followed by the same letter within a column are not significantly different according to an
LSD test (P=0.10).

89

Table 4. Effect of row spacing, corn population, and weed control treatments on corn grain
yields, 1998-1999.

 

 

Corn yield
_ Mg/ha __

Row spacinga

38 cm 10.61 a

56 cm 10.64 a

76 cm 10.56 a
Average corn populationb

59,300 plants/ha 10.22 b

72,900 plants/ha 10.75 a

83,900 plants/ha 10.86 a
Weed controlc

Glufosinate treated plots 10.42 b

Weed-free plots 10.80 a

 

“ Means are pooled over corn populations and weed control treatments and represent a main
effect. Means for row spacing followed by the same letter within a column are not
significantly different according to an LSD test (P=0.05).
b Means are pooled over row spacings and weed control treatments and represent a main
effect. Means for corn population followed by the same letter within a column are not
significantly different according to an LSD test (P=0.05).
° Means are pooled over corn populations and row spacings and represent a main effect.
Means for weed control treatments followed by the same letter within a column are not
significantly different according to an LSD test (P=0.05).

90

[338 cm .56 cm I76 cm

 

100

1998 a a a

PAR intercepted by com canopy (%)

 

 

 

 

 

 

100

 

 

1999 a a aaa aaa

a a
90 abb

a
80-4
70-

60-

PAR intercepted by com canopy (%)
U!
0

 

 

 

 

 

 

 

35 42 49 56 63 7o 77
Days after corn planting

Figure 1. Photosynthetic active radiation (PAR) intercepted by com canopies among
row spacings at weekly intervals in 1998 and 1999. Data is averaged over corn
populations and represents whole-plot effects. Bars that are followed by the same letter
within weekly light measurement dates were not significantly different according to

an LSD test (P=0.05). 91

 

El Low I Medium I High

100

 

1993 baba a a a
90-

80-

60- a a a
50- a
40-

30-

20- a

PAR intercepted by com canopy (%)

 

 

 

 

 

 

100

 

1999

90. aaa “‘3
80-
7o-
60-
50-

   

40- a
30-
20- 3'

PAR intercepted by com canopy (%)

10-

 

 

 

 

 

 

 

35 42 49 56 63 70 77

Days after corn planting

Figure 2. Photosynthetic active radiation (PAR) intercepted by com canopies among
corn populations at weekly intervals in 1998 and 1999. Data is averaged over row
spacings and represents sub-plot effects. Bars that are followed by the same letter
within weekly light measurement dates were not significantly different according to
an LSD test (P=0.05). 92

.93 u me 8: - 5.8 u > a 8: $.88

65 .6 22:33. .mmmEoE. 80am 8 :26:on comm mcotmsgnEfl 8888 we mEmcocfioM .m. Miami

 

 

 

AEEEmEEmV
86:65 895
03 on F oo F om om ow om o

oooow
oooov

, oooom
oooow
ooooor
coco?

 

 

(wad/peas IO 'ou)
uogronpord peas Sieuenbsquie| UOUJUJOQ

93

CHAPTER 5
DELAYED BURNDOWN TIMINGS IN NO-TILLAGE GLYPHOSATE-
RESISTANT CORN (Zea mays) PLANTED INTO SOYBEAN (Glycine max)
RESIDUE AND INTO A WHEAT (T riticum aestivum) COVER CROP
Abstract: Field trials were conducted in 1998 and 1999 to determine the effect of delayed
bumdown timings on weed control and yield of no-tillage glyphosate-resistant corn planted
into soybean residue or into a wheat cover crop. Bumdown treatments containing glyphosate
were applied to both trials when the corn was planted (PRE), when the corn began to emerge
(SPIKE), or when the corn had developed three leaves (3-LEAF). When corn was planted
into soybean residue, glyphosate applied at SPIKE or 3-LEAF followed by a sequential
glyphosate application controlled velvetleaf 291% and corn yields were similar to the weed-
free plots. Glyphosate tank mixtures with residual herbicides provided less than 60%
velvetleaf control. Corn yields among the bumdown treatments were directly related to
velvetleaf control. In the wheat cover crop trial, wheat treated at the PRE timing was
completely controlled and corn yields were similar to the weed-free plots. As bumdown
timings were delayed, corn emergence and yields were severely reduced. Applying
glyphosate to 25-cm wide strips of wheat over the corn row at corn planting and following

with delayed bumdown timings increased corn emergence and yield.

94

Nomenclature: glyphosate, N-(phosphonomethyl)glycine; velvetleaf, Abutilon theophrasti
Medicus it“5 ABUTH; corn, Zea mays L. ‘DK 493RR’; winter wheat, T riticum aestivum L.
Additional index words: herbicide tolerant crops; acetochlor, atrazine, Abutilan theaphrasti,
ABUTH.

Abbreviations: POST, postemergence; PRE, preemergence; SPIKE, at corn emergence; 3-

LEAF, corn with three developed leaves.

 

‘5 Letters following this symbol are a WSSA-approved computer code from
Composite List of Weeds, Revised 1989. Available from WSSA, 810 East 10th Street,
Lawrence, KS 66044-8897.

95

INTRODUCTION

Tillage has been an integral part of corn production since corn was first produced.
However, advancements in farm machinery, adoption of herbicides, and concerns about soil
erosion have reduced the reliance of tillage. Many factors associated with corn production
are affected when tillage is reduced. Fortin (1993) reported that com development was
delayed and soil temperatures were reduced within the seed zone in no-tillage systems
compared to conventional tillage systems. In Indiana, reduced tillage systems are adequately
adapted to well-drained soils, but are not adapted to poorly drained, fine-textured soils
(Griffith et al. 1973). No-tillage systems also affect weed control. Some weeds are more
difficult to control in reduced tillage systems (Buhler 1992). Larger densities of green foxtail
and redroot pi gweed were reported in no-tillage compared to conventional tillage systems,
and horseweed appeared only in no-tillage systems (Buhler 1992; Wrucke and Arnold 1985).
The mean depth of weed emergence was found to be more shallow in no-tillage systems than
conventional (Buhler and Mester 1991). Mulugeta and Stoltenberg (1997) found that 74%
of the total viable weed seed in no- tillage systems were in the top 10-cm of the soil profile
compared to 43% in conventional tillage. Differences in weed control have caused farmers
to develop weed management strategies that are unique to no-tillage systems.

Cover cropping systems can be easily integrated into no-tillage corn production.
Proper management of the cover crop is needed to obtain the beneficial aspects of a cover
cropping system. When managed appropriately, cover crops can reduce water and wind
erosion (Frye et a1. 1983; Smith et al. 1987), sequester excess nitrates (Jackson et al. 1993;

Shipley et al. 1992), provide nitrogen to succeeding crops (Mitchell and Teel 1977; Wagger

96

1989a), increase soil properties (Benoit et al. 1962), provide a favorable environment for
predatory insects (Bugg 1991; Clark et al. 1993; Kaakeh and Dutcher 1993), and suppress
weeds (Lal et al. 1991; Weston 1996).

Cover crops are often desiccated prior to corn planting to provide non-living surface
residues, but can be managed to provide living surface residues, as in living mulch systems
(Echtenkarnp and Moomaw 1989; Eberlien et al. 1992). Delaying the timing of cover crop
dessication will increase the biomass of the cover crop and ultimately result in higher surface
residues and accumulated nitrogen (Smith et al. 1992; Clark et al. 1997a). Wagger (1998b)
cautioned that managing a cover cr0p for additional growth and nitrogen accumulation
should not delay corn planting.

Reports on the influence of cover crop dessication timing on corn growth and yield
are mixed. Munawar et al. (1990) and Raimbault et al. (1991) reported yield reductions from
cover crops desiccated later in the season, while others reported yield reductions from early
timings of dessication (Moschler 1967; Clark et al. 1995; Clark et al. 1997b). These
differences were attributed to differences in soil moisture and potential allelopathic effects.
Munawar et al. (1990) stated that early season cover crop growth depleted soil moisture, but
that during years of high spring rainfall the removal of soil moisture might be advantageous.
Clark et al. (1997b) concluded that soil moisture conservation by cover crop residues was
more important than spring water depletion in determining corn yield. Rainfall patterns are
important factors to consider in cover crop management. Weston (1996) reported allelopathic

effects of many plant species used as cover crops on other plant species. Tollenaar et al.

97

(1993) and Raimbault et al. (1991) suggested that reductions in corn development and yield
could be caused by allelopathic interactions.

In no-tillage corn production, non-selective herbicides with no soil residual activity,
such as glyphosate or paraquat, are often applied before planting to remove existing
vegetation that would have been removed by tillage in conventional systems. These
applications are commonly referred to as bumdown applications. Herbicides with residual
herbicidal activity in the soil are often combined with the non-selective herbicides to provide
season long weed control (Wilson et al. 1985; Blackshaw 1989). The availability of
glyphosate-resistant corn hybrids provides the opportunity to effectively remove vegetation
after corn emergence. The objective of this research was to determine if glyphosate applied
alone and in tank mixtures with residual herbicides could be delayed to increase weed control
and crop yield in no-tillage corn planted into soybean residue. Delayed glyphosate bumdown
applications were also examined in no-tillage corn planted into a wheat cover crop to

determine the effect of the cover crop on corn yield.

MATERIALS AND METHODS
No-tillage corn into soybean residue. Field trials were conducted in 1998 and 1999 on the
Crop and Soil Sciences Research Farm at Michigan State University in East Lansing, MI.
The soil was a Capac sandy loam (fine-loamy, mixed mesic Aeric Ochraqualf) with a pH of
6.2 and 3.1% organic matter in 1998 and a Capac sandy clay loam with a pH of 6.9 and 2.9%
organic matter in 1999. A 34-0-0 (N —P-K) fertilizer was broadcast prior to corn planting and

a 6-24-24 fertilizer was banded 5 cm below and 5 cm to the side of the corn seed during the

98

planting operation. Fertilizer rates were determined fi'om the results of a soil analysis. ‘DK
493RR’ hybrid com was no-till planted into soybean residue on May 14, 1998 at a
population of 53,100 seeds/ha and on May 10, 1999 at a population of 71 ,600 seeds/ha. Plots
consisted of four rows spaced 76 cm apart with lengths of 10.7 m. The trials were not
irrigated. Weekly rainfall amounts are listed in Table 1.

Four bumdown herbicide treatments were applied at three timings. The bumdown
herbicide treatments included glyphosate at 0.84 kg ae/ha applied alone, glyphosate at 0.84
kg/ha tank mixed with atrazine [6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-
diamine] at 1.12 kg ai/ha, glyphosate at 0.84 kg/ha tank mixed with a premix"’ of acetochlor
at 0.88 kg ai/ha plus atrazine at 0.7 kg/ha, and an initial application of 0.84 kg/ha glyphosate
applied alone followed by a sequential application of glyphosate at 0.84 kg/ha. Each of the
bumdown herbicide treatments were applied before corn emergence (PRE), when com in the
weed free plots was beginning to emerge (SPIKE), and when com in the weed free plots had
three visible leaves (3-LEAF). The average height of velvetleaf was 1.3 cm at the PRE
timing in both years, 3.8 cm in 1998 and 2.5 cm in 1999 at the SPIKE timing, and 3.8 cm at
the 3-LEAF timing in both years. Velvetleaf in plots receiving sequential treatments were
2.5 to 5 cm tall. Sequential treatments were applied 25 and 24 days following the PRE
application, 28 and 27 days following the SPIKE application, and 27 and 19 days following
the 3-LEAF application in 1998 and 1999, respectively. Ammonium sulfate at 2% (w/w) was

included with all glyphosate applications. Weed-free plots were treated with a PRE

 

16Premix of acetochlor [2-chloro-N-(ethoxymethyl)-N—(2-ethyl-6-methylphenyl)
acetamide] plus atrazine plus MON 4660 (safener)

99

application of a premixl7 of metolachlor at 2.24 kg/ha plus atrazine at 1.12 kg/ha followed
by hand-weeding as needed. Herbicides were applied with a tractor-mounted compressed-air
sprayer equipped with flat-fan nozzles18 and calibrated to deliver 187 L/ha at a pressure of
207 kPa.

Velvetleaf was the dominant weed species present in both years. Visual estimates of
velvetleaf control were compared against untreated velvetleaf using a scale of 0 (no control)
to 100 (all plants dead) 28 d after the final POST sequential application. Aboveground shoots
of velvetleaf within a 930-cm2 quadrat in 1998 and a 1900-cm2 quadrat in 1999 were
harvested from each plot 80 days after corn planting. The harvested shoots were oven dried
for at least 5 d and dry weights were determined. Corn grain yield was determined by
harvesting the center two rows of each plot with a mechanical plot harvester. Corn grain
yield was corrected to 15.5% moisture.

The trial was a two-factor factorial designed as a randomized complete block with
three replications. The factors were bumdown herbicide treatments and bumdown
application timings. Visual estimates of weed control were subjected to AN OVA procedures.
Weed free plots were not included in the statistical analysis of the weed control data since
the values contained no variance. Means of the weed control data and corn yield were

separated using Fisher’s protected LSD at the P= 0.05 level of probability. Visual estimates

 

l7Premix of S-metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-
1-methylethyl)acetamide, S—enantiomer] plus atrazine plus CGA-154281 [4-
(dichloroacetyl)-3 ,4-dihydro-3-methyl-2H-l ,4—benzoxazine]

18TeeJet XR 8003VS. Spraying Systems Co., North Ave., Wheaton, IL 60188.

100

 

of weed control and corn yields were subjected to F-Max tests to test for homogeneity of
variance among years (Kuehl 1994). Data found to be homogenous were pooled over years.
No-tillage corn into a wheat cover crop. Field trials were conducted in 1998 and 1999 on
the Crop and Soil Sciences Research Farm at Michigan State University in East Lansing, MI.
The soil was a Capac sandy loam (fine-loamy, mixed mesic Aerie Ochraqualf) with a pH of
6.2 and 3.1% organic matter in 1998 and a pH of 6.1 and 1.0% organic matter in 1999. A 34-
0-0 (N-P-K) fertilizer was broadcast prior to corn planting and a 6-24-24 fertilizer was
banded 5 cm below and 5 cm to the side of the corn seed during the planting operation.
Fertilizer rates were determined from the results of a soil analysis. ‘DK 493RR’ was no-till
planted into an actively growing winter wheat cover crop on May 13, 1998 at a population
of 70,400 seeds/ha and on May 10, 1999 at a population of 71,600 seeds/ha. Plots consisted
of four rows spaced 76 cm apart with lengths of 10.7 m. The wheat cover crop was drilled
the previous fall of each year. The wheat was 30 to 40-cm tall when com was planted in 1998
and 25 to 48 cm tall in 1999. Perrnethrin at 0.11 kg ai/ha was broadcast over the entire study
on June 27 , 1998 and June 15, 1999 to minimize corn damage from arrnywonns (Pseudaletia
unipuncta).

Four bumdown herbicide treatments were applied at three timings. The bumdown
herbicide treatments included glyphosate at 0.84 kg ae/ha applied alone, glyphosate at 0.84
kg/ha tank mixed with atrazine at 1.12 kg ai/ha, glyphosate at 0.84 kg/ha tank mixed with
a premix of acetochlor at 0.88 kg ai/ha plus atrazine at 0.7 kg/ha, and an initial application
of 0.84 kg/ha glyphosate applied alone followed by a sequential application of glyphosate

at 0.84 kg/ha. Each of the bumdown herbicide treatments were applied before com

101

emergence (PRE), when com in the weed free plots was beginning to emerge (SPIKE), and
when com in the weed free plots had three visible leaves (3-LEAF). The height of the wheat
averaged 38 cm and 33 cm at the PRE timing, 53 cm and 36 cm at the SPIKE timing, and
61 cm and 46 cm at the 3-LEAF timing in 1998 and 1999, respectively. The sequential
glyphosate applications were applied 26 and 24 days following the initial application of
glyphosate at the PRE timing, 32 and 34 days following the initial SPIKE glyphosate
application, and 27 and 19 days following the initial 3-LEAF glyphosate application in 1998
and 1999, respectively. Weed-free plots were treated with a PRE application of glyphosate
at 0.84 kg/ha plus a formulated premix of metolachlor at 2.24 kg/ha plus atrazine at 1.12
kg/ha followed by hand-weeding as needed. An additional treatment was glyphosate at 0.84
kg/ha applied in 25-cm wide bands directly over the corn row. The banded glyphosate was
applied immediately following corn planting with a tractor-mounted compressed-air sprayer
equipped with even flat-fan nozzles'9 and calibrated to deliver 187 L/ha at a pressure of 207
kPa. Plots previously treated with the PRE banded glyphosate applications were treated with
the four bumdown herbicide treatments broadcast at the SPIKE timing and at the 3-LEAF
timing.

Weed populations were negligible both years. Visual estimates of wheat control was
compared against untreated wheat plants using a scale of 0 (no control) to 100 (all plants
dead) 28 d after the 3-LEAF bumdown timing. The emergence and height of com plants

were recorded 45 days after planting from the center two rows of each plot in both years.

 

l9TeeJet 80015EVS. Spraying Systems Co., North Ave., Wheaton, IL 60188.

102

Corn grain yield was determined by harvesting the center two rows of each plot with a
mechanical plot harvester. Com grain yield was corrected to 15.5% moisture.

The trial was a two-factor factorial designed as a randomized complete block with
four replications. The factors were bumdown herbicide treatments and bumdown application
timings. In addition, a factorial arrangement of bumdown herbicide treatments at the SPIKE
and 3-LEAF bumdown timings were applied where glyphosate had been applied PRE in
bands. Since most of the wheat control data was between 80 and 100%, the data were arcsine
transformed prior to statistical analysis and subjected to ANOVA procedures. Means of the
transformed wheat control data, corn emergence, height, and yield data were separated using
Fishers protected LSD at the P= 0.05 level of probability. Nontransformed means are
presented for clarity. All data were subjected to F -Max tests to test for homogeneity of

variance among years (Kuehl 1994). Data found to be homogenous were pooled over years.

103

RESULTS AND DISCUSSION

No-tillage corn into soybean residue. Glyphosate has negligible herbicidal activity in most
soils. Therefore, weeds that emerge after a glyphosate application will not be controlled
unless a residual herbicide is also applied. Glyphosate applied once did not provide season-
long control of velvetleaf (Table 2). However, sequential applications of glyphosate provided
the greatest control of velvetleaf, and dramatically increased velvetleaf control at each timing
compared to a single application of glyphosate. Gonzini et al. (1999) also reported increased
weed control in soybeans from sequential applications of glyphosate compared to glyphosate
applied once. When the initial applications of glyphosate were delayed the sequential
applications were subsequently delayed resulting in increased velvetleaf control.

A tank mixed residual herbicide or a sequential application of glyphosate generally
increased velvetleaf control compared to glyphosate applied once. Velvetleaf control was
increased and velvetleaf biomass was reduced as the bumdown application timings were
delayed (Table 2). The 3-LEAF timing resulted in greater control of velvetleaf by glyphosate
than the PRE timing because less velvetleaf emerged following the 3—LEAF application
compared to the PRE timing. Glyphosate plus atrazine applied at the 3-LEAF timing
controlled velvetleaf greater than when applied at the PRE timing. In addition, velvetleaf
control increased and velvetleaf biomass decreased as applications of glyphosate plus
acetochlor/atrazine were delayed. Control of broadleaf weed species is often increased when
herbicides are tank mixed with atrazine (Culpepper and York 1999; Wilson et al. 1985).

Corn yield trends were similar to the trends of velvetleaf control. Carey and Kells

(1995) showed that weeds emerging with corn can potentially reduce yield if the weeds are

104

not removed before they reach 10 cm in height. In our trials, velvetleaf height never
exceeded 10 cm when bumdown applications were applied, and yields were increased as
bumdown timings were delayed (Table 2). Therefore, yield reductions in our trials were
likely caused by competition from weeds that emerged after application. Sequential
applications of glyphosate and delayed applications of bumdown treatments that included
residual herbicides resulted in corn yields that were similar to the weed-free plots (Table 2).
Corn yields in plots treated with glyphosate applied once at all bumdown timings were lower
than yields of weed-free plots. Sequential glyphosate applications and residual herbicide
treatments controlled velvetleaf long enough to prevent corn yield reductions from velvetleaf
competition.
No-tillage corn into a wheat cover crop. All of the bumdown herbicide treatments applied
at the PRE timing completely controlled the wheat cover crop (Table 3). Control was
reduced as the application timings were delayed. As herbicide timings are delayed, plants
grow larger and become increasingly difficult to control. Tank mixtures of glyphosate with
atrazine reduced cover crop control, compared to glyphosate applied alone, when
applications were delayed. Antagonism of the herbicidal activity of glyphosate plus atrazine
tank mixtures has been reported on other plant species (Selleck and Baird 1981). Sequential
applications of glyphosate did not increase control of the wheat cover crop compared to a
single application of glyphosate.

Residual herbicides or sequential applications of glyphosate did not affect the
emergence and height of corn compared to glyphosate applied alone (Table 3). The

emergence and height of corn were significantly reduced as bumdown timings were delayed.

105

Within each herbicide treatment, corn emergence and height at the PRE timing were similar
to corn in the weed-free plots. However, corn emergence and height were reduced in
treatments where herbicide application was delayed beyond the PRE timing. Reductions in
corn emergence are likely attributed to soil moisture depletion by the competing wheat cover
crop. Less than 3 cm of rainfall occurred within 4 weeks of corn planting in 1998, and less
than 4 cm of rain fell between 3 and 6 weeks after planting in 1999 (Table 1). In addition to
moisture stress, differences in corn height could be due to competition for available nitrogen
between the corn and the wheat cover crop.

Corn yields were similar among burndown herbicide treatments within each
bumdown timing (Table 3). Within each bumdown herbicide treatment, delayed applications
resulted in reduced yields compared to the PRE timing and compared to the yields of the
weed-free plots. The reductions in yield are likely due to reduced corn emergence and
moisture stress from the competing wheat cover crop. In plots where the cover crop was
controlled at planting, com yields were similar to the yields in weed-free plots.

Plots previously treated with banded applications of glyphosate contained a larger
number of corn plants that grew taller and yielded more than corn in plots that were not
treated with banded applications of glyphosate (Table 4). Eberlein et al. (1992) reported
reduced soil moisture availability and reduced yields of corn planted into a partially
suppressed alfalfa sod. Corn yields were not reduced by the presence of a living crimson
clover mulch (T rifolium incarnatum), as long as the clover was dessicated in strips greater
than 60% of the total area (Kumwenda et al. 1993). When bumdown treatments were applied

at the SPIKE and 3-LEAF timing, the glyphosate treated strips of wheat provided an

106

environment that was conducive to corn germination, but the plants that emerged in the 3-
LEAF timing plots were stunted from competition with wheat that remained in the interrow.
Corn yields were also reduced when bumdown treatments were delayed to 3-LEAF
compared to SPIKE timings.

Results of this research suggest that bumdown herbicide application can be delayed
to increase weed control in glyphosate-resistant no-tillage corn in the absence of a cover
crop. The addition of residual herbicides with glyphosate increased velvetleaf control but did
not provide adequate control with a single treatment weed management system. Sequential
applications of glyphosate provided the greatest weed control and corn yields. Corn yields
increased when velvetleaf control increased. In our trials, very few winter annuals were
present at corn planting. Corn yields would likely be reduced if large populations of weeds
were present at corn planting.

The presence of an actively growing cover crop at the time of corn planting will
strongly influence the success of delayed bumdown timings. Delayed bumdown timings in
an actively growing wheat cover crop resulted in reduced corn populations and yields. The
wheat competed with the corn for moisture and nutrients. This data suggests that bumdown
of a wheat cover crop should occur no later than the time non-irrigated corn is planted to

avoid yield loss from competition.

107

LITERATURE CITED

Benoit, R. E., N. A. Willits, and W. J. Hanna. 1962. Effect of rye winter cover crop on soil
structure. Agron. J. 54:419-420.

Blackshaw, R. E. 1989. HOE-39866 use in chemical fallow systems. Weed Technol. 3:420-
428.

Bugg, R. L. 1991. Cover crops and control of arthropod pests of agriculture. In W. L.
Hargrove (ed). Cover cr0ps for clean water. Ankeney, IA p. 157-163.

Buhler, D. D. 1992. Population dynamics and control of annual weeds in corn (Zea mays)
as influenced by tillage systems. Weed Sci. 40:241-248.

Buhler, D. D. and T. C. Mester. 1991. Effect of tillage systems on the emergence depth of
giant foxtail (Setarz'afaberi) and green foxtail (Setaria viridis). Weed Sci. 39:200-203.

Carey, J. B., and J. J. Kells. 1995. Timing of total postemergence herbicide applications to
maximize weed control and corn (Zea mays) yield. Weed Technol. 9:356-361.

Clark, M. S., J. M. Luna, N. D. Stone, and R. R. Youngman. 1993. Habitat preferences of
generalist predators in reduced-tillage corn. J. Entomol. Sci. 28:404-416.

Clark, A. J ., A. M. Decker, J. L. Meisinger, F. R. Mulford, and M. S. McIntosh. 1995. Hairy
vetch kill date effects on soil water and corn productivity. Agron. J. 87:579-585.

Clark, A. J ., A. M. Decker, J. J. Meisinger, and M. S. McIntosh. 1997a. Kill date of vetch,
rye, and a vetch-rye mixture. 1. Cover crop and corn nitrogen. Agron. J. 89:427-434.

Clark, A. J ., A. M. Decker, J. J. Meisinger, and M. S. McIntosh.1997b. Kill date of vetch,
rye, and a vetch-rye mixture. 11. Soil moisture and corn yield. Agron. J. 89:427-434.

Culpepper, A. S. and A. C. York. 1999. Weed management in glufosinate-resistant corn (Zea
mays). Weed Technol. 13:324-333.

Eberlein, C. V., C. C. Sheaffer, and V. F. Oliveira. 1992. Corn grth and yield in an alfalfa
living mulch system. J. Prod. Agric. 5:332-339.

Echtenkamp, G. W., and R. S. Moomaw. 1989. No-till corn production in a living mulch
system. Weed Technol. 32261-266.

Fortin, M. C. 1993. Soil temperature, soil water, and no-till corn development following in-
row residue removal. Agron. J. 85:571-576.

108

trey-en“— -r

Frye, W. W., J. H. Herbek, and R. L. Blevins. 1983. Legume cover crops in production of
no-tillage corn. In W. Lockeretz. (ed.) environmentally sound agriculture. Praeger
Publishers, NY. pp. 179-191.

Gonzini, L. C., S. E. Hart, and L. M. Wax. 1999. Herbicide combinations for weed
management in glyphosate-resistant soybean (Glycine max). Weed Technol. 13:354-360.

Griffith, D. R., J. V. Mannering, H. M. Galloway, S. D. Parsons, and C. B. Richey. 1973.
Effect of eight tillage-planting systems on soil temperature, percent stand, plant growth, and
yield of corn on five Indiana soils. Agron. J. 65:321-326.

Jackson, L. E., L. J. Wugland and L. J. Strivers. 1993. Winter cover crops to minimize
nitrate losses in intensive lettuce production. J. of Agric. Sci. 121:55-62.

Kaakeh, W. and J. D. Dutcher. 1993. Rates of increase and probing behavior of

Acythosiphonpisum (Homoptera: Aphidiae) on preferred and nonpreferred host cover crops.
Environ. Entomol. 22: 1016-1021.

Kuehl, R. O. 1994. Statistical Principles of Research Design and Analysis. Belmont, CA:
Wadsworth , Inc. 686 p.

Kumwenda, J. D. T., D. E. Radcliffe, W. L. Hargrove, and D. C. Bridges. 1993. Reeseeding
of crimson clover and corn grain yield in a living mulch system. Soil Sci. Soc. Am. J.
57:517-523.

Lal, R., E. Regnier, D. S. Eckert, W. M. Edwards, and R. Hammond. 1991. Expectations of
cover crops for sustainable agriculture. In W. L. Hargrove (ed.) Cover crops for clean water.

Soil and Water Conserv. Soc., Ankeny, IA. pp. 1-11.

Mitchell, W. H., and M. R. Teel. 1977. Winter-annual cover crops for no-tillage corn
production. Agron. J. 69: 569-572.

Munawar, A., R. L. Blevins, W. W. Frye, and M. R. Saul. 1990. Tillage and cover crop
management for soil water conservation. Agron. J. 82:773-777.

Mulugeta, D. and D. E. Stoltenberg. 1997. Weed and seedbank management with integrated
methods as influenced by tillage. Weed Sci. 45:706-715.

Raimbault, B. A., T. J. Vyn, and M. Tollenaar. 1991. Com response to rye cover crop, tillage
methods, and planter options. Agron J. 83: 287-290.

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

109

Shipley, P. R., J. J. Meisinger, and A. M. Decker. 1992. Conserving residual corn fertilizer
nitrogen with winter cover crops. Agron J. 84:869-876.

Smith, M. S., W. W. Frye, and J. J. Varco. 1987. Legume winter cover crops. Advances
in Soil Sci. 7:96-139.

Smith, M. A., P. R. Carter, and A. A. Imholte. 1992. Conventional vs. no-till corn following
alfalfa/grass: timing of vegetation kill. Agron. J. 84:780-786.

Tollenaar, M., M. Mihajlovic,and T. J. Vyn. 1993. Corn growth following cover crops:
influence of cereal cultivar, cereal removal, and nitrogen rate. Agron J. 85:251-255.

Wagger, M. G. 1989a. Time of dessication effects on plant composition and subsequent
nitrogen release from several winter annual cover crops. Agron. J. 81:236-241.

Wagger, M. G. 1989b. Time of dessication effects on plant composition and subsequent
nitrogen release from several winter annual cover crops. Agron. J. 81:236-241.

Weston, L. A. 1996. Utilization of allelopathy for weed management in agroecosystems.
Agron. J. 88:860-866.

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 Sci. 33:531-536.

Wrucke, M. A. and W. E. Arnold. 1983. Weed species distribution as influenced by tillage
and herbicides. Weed Sci. 33:853-856.

110

Table 1. Weekly rainfall amounts, 1998 and 1999.

Weeks after 1998 1999
corn planting

 

 

 

 

cm

1 0 1.1
2 0.4 2.6
3 1.5 0.9
4 0.9 1.9
5 2.3 0.1
6 0 0.4
7 3.6 1.8
8 5.0 6.0
9 0 2.0
10 1.8 0

11 0 3.6
12 0.4 1.4

 

lll

 

 

 

 

a v2: 8 cm a a: a 8 "Eu:

2.2: a." om :2 a a. 55m

3% 38 3; 0;. my:
ocoobo £95 wEEc Esowfizm

m wnA: o n 6 me a on 35538 889330

8 Se as K a 3m n 3 058582838 .83 aaofiaa

a 5.2 B a. o E 9 am usage 33 28230

0 v: a N: a 2 w o E 253 23230
ecouto 595 8:62:85 “66.58: gonfism

I new: I .x.
22» EB 32 82 832:5
iobcoa
ummmEoE «3:31; mm6=6>_o>

 

«.003 98 M32 .2?» ES 28 £8805 $83622, .3950 98:02? no 326233 E5863 oawmonmbm Rim—ow mo Subm— N chk

112

Amodnnc “m2 qu 5 2 wEEouua Bonfirv

bEmocEm_m Ho: 8m 5:2 2:8 2: 3 830:8 Ba 35 382 .35ng 02053: 85363 was $55: 85883 we coonBE 2: E0838 382 b
.Godun: :2 a3 5 9 3288

880%6 388$:me 8: 2a 5:3 083, we 3 326:8 8a 35 882 acute :88 a 28838 98 62585 02038: 55853 ~96 “go—com 2a 882 u
.23an :2 QB 5

8 $3808 Bahama bagmawa 8: 2m 8:2 083. 05 3 325:8 2m 35 882 Quote :38 m 382%: 98 $583 E3955 ~26 @208 En .882 a

.9553 E8 Btu want ow 33,033 33 .«mo_3>_o> u

douse—qua =35:ch HmOm 38m 05 Ban c wm 3982 $.83 13% > a

65:5 gm 8 0828 538955 wows—05 Savannah =4:N

 

a 2:: o o 2: 8a 803
a 5.: 3 n: moo ”2qu

35.: E u 2 m a Him
an 3.2 w 3 B m: 83 3 BE 3:538 33230
23:: 3: 3a 3% “Em:
e“ 3.2 8 cm 8 m: 2U mm 55m

a mg em a: a E mm S BE angfisfieam 33 23230
a 8.2 B S c S 3 cm ”EBA

m 6.2 8 mm 8 %_ 03 8 595

8 N3 8 S 8 8m E R BE 2:5: 83 23230
3 £3 89 mm o o3 x mm ”2qu

3; B“ o: Sam 8 2 895

o of. a N2 m om: w o BE 8% 33239

hEocoahzac wEEc :3853m x 325me 3655: Esowfism

 

3:53 N 2ch

113

 

ovafi 0 mm o fim own "$544.
n 3; a Q 9 9m 9 ca gmm
m 36 m ow m WV m 2: mam

”30qu Ease mEE: gocgm

a va a Q a 5m m a 3:533 882735
9 mm.» a Q a 9m n S oENmbmtoEooEom 33 882330
an m: a Q a 3 a ; ESE“ 33 28330
em a; a w? a Z a a 8% 23230

38% SEE EuEEub 02058; gownsm

 

 

 

 

| SEE | I Eu I - 38 83:31 - c\o
22> uEmEI uuocuwbEm iobcoo uEmta>
9:0
500 830 3055

 

«.33 EB
moi .203 can .Ewfi: .oocowHoEo 2.50 van .8280 Q80 H260 30:3 no 323239“ E5853 Emmonmbw woxflow mo 80am .M. m3:&

114

.Godumv .8. am; 5 9 3288 55:6

bEmucEmG .0: Eu 5:2 25% 2: 3 830:8 8m 35 ESE Savannah @2058: EsovEsp Es meEc sac—V53 mo 8.50885 2: 682%: £822 .
Amodnmv $8 qu an 8 @6508

Subaru baggy—Ev, 8: 8m 8:3 2:3 05 an 330:8 8m 35 £802 Sofie :88 a 582%“ 28 288305 02958; 558.55 B>o “go—com 8a 832 u
.333: :2 a3 5

9 $6825 Subway bEmucEwB 8: Rm 3:2 08mm 2t 3 330:8 2m 35 332 Joyce :88 a 3889: was mwfifiu 5982.59 H26 “go—con Ba 2502 a

.3983 58 Sam 9% me 3288 $582582 u

.335 85853 m<mq-m 05 Can my mm 3382 mwcufi :26; a
.CEBV AXVN :w 0323 EamoEEa c2535 mucogmob :< a

 

a 3m 9 w? an 3 a 2: 0% 803
2“ 8.0 c m? we 3 0 mm ”25;.
B S .w o 9? Ba 3 a 3 595
a 3.2 a 2% a 3‘ m 2: BE 3:533 33230
M c; a 3m 0 M: c 3 "Em?
08 Nos 0 a? 8 3 0 mm 595
a No.0 Du Odo m WV N 2: gm oENmbwio—noguom + owmmong_mv
E :.o a 3m 08 2 n mm “2qu
B m? 0 Q 3 3 o a 8:55
a 9% a new a 3 a 2: BE 2:53 + 382259
t 3m v mom 08 3 0 mm ”23a
08 m3 0 a? B E a 8 55m
m 9% a 5% a 3 a 2: BE 3% 23230

.30508323 wag: gowEsm x “5832“ 02056: :Bovfism

 

3.283 m mEarm

115

Table 4. Effect of banded applications of glyphosate on corn emergence, height, and yield
at delayed bumdown timings, 1998 and 1999.

 

 

 

Com
Variable Banded glyphosatea Emergenceb Heightb Yield
plants/m row — cm — — Mg/ha —
Banded glyphosate (main effect)c
Yes 4.0 a 50 a 8.27 a
No 3.3 b 39 b 6.69 b
Bumdown timing >< Banded glyphosate (interaction)d
SPIKE Yes 4.0 a 51 a 8.70 a
No 3.6 b 43 c 7.44 b
3-LEAF Yes 4.0 a 48 b 7.83 b
No 3.1 c 35 d 5.94 c

 

“ Glyphosate plus ammonium sulfate applied in 25-cm bands directly over corn row at

planting.

b Measurements recorded 45 days after corn planting.

‘ Means are pooled over bumdown timings and bumdown herbicide treatments and represent
a main effect. Means that are followed by the same letter are not significantly
different according to an LSD test (P=0.05).

d Means are pooled over bumdown herbicide treatments and represent the interaction of
bumdown timing and banded glyphosate. Means that are followed by the same letter
are not significantly different according to an LSD test (P=0.05). Abbreviations:
SPIKE, bumdown treatments broadcast at corn emergence; 3-LEAF, bumdown
treatments broadcast when com had developed three leaves.

116