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THESIS
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This is to certify that the

dissertation entitled

EFFECT OF ROW SPACING AND GLYPHOSATE TREATMENT TIMING
0N CORN (Zea mazs L.) AND SOYBEAN (Eyeine max (L.) Merr.)
YIELD, SUBSEQUENT WEED GROWTH AND SOIL MOISTURE

 

presented by

caleb D. Dalley

has been accepted towards fulﬁllment
of the requirements for

 

 

Ph.D. degreein Crop and Soil Sciences

.JZ)‘

M or professor

Date Twig, I! 90602,

 

MS U is an Afﬁrmative Action/Equal Opportunity Institution O~ 12771

 

 

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PLACE IN RETURN BOX to remove this checkout from your record.
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6/01 c:/ClRC/DateDue.p65-p.15

.- _ - . . nun—HA —_—--A- 0—4....___.._._ ;--— _-—_.-..-—.— .—

EFFECT OF ROW SPACING AND GLYPHOSATE TREATMENT TIMING ON
CORN (Zea mays L.) AND SOYBEAN (Glycine max (L.) Merr.) YIELD,
SUBSEQUENT WEED GROWTH AND SOIL MOISTURE
By

Caleb D. Dalley

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

2002

ABSTRACT
EFFECT OF ROW SPACING AND GLYPHOSATE APPLICATION TIMING ON
CORN (Zea mays L.) AND SOYBEAN (Glycine max (L.) Merr.) YIELD,
LIGHT INTERCEPTION, SUBSEQUENT WEED GROWTH,
AND SOIL MOISTURE IN CORN
By
Caleb D. Dalley

With the introduction of glyphosate resistant corn (Zea mays L.) and soybean (Glycine
max (L.) Merr.), appropriate glyphosate application timings need to be determined to
avoid yield loss from weed interference. From 1998 to 2001 research was conducted at
two locations in Michigan (East Lansing and Clarksville) to study the effects of
glyphosate application timing and row spacing on glyphosate resistant corn and soybean
yield, light interception by com and soybean, weed growth following applications, and
soil moisture in corn. Glyphosate was applied at 0.84 kg ae/ha when weed canopy height
reached 5, 10, 15, 23, and 30 cm. In 1999, under high weed density (1270 weeds/m2) and
lower than normal rainfall, soybean yield losses occurred when weeds reached 15 and 23
cm in height (V2 and V3 growth stage, respectively) when planted in 19 and 38 cm rows,
respectively. Under these same conditions, corn yield was reduced by weed interference
when weeds reached 10 and 15 cm in height (V4 and V5 growth stage, respectively)
when com was planted in 38 and 76 cm rows, respectively. Under less competitive
growing conditions (180 weeds/m2 and above normal rainfall), corn and soybean yield
losses occurred only after weeds exceeded 30 cm in height (>V9 and >V5 growth stage,
respectively). In each year of this study, allowing weeds to reach 30 cm in height (>V5

growth stage) did not reduce yield of soybean planted in 76 cm rows. Narrow row

soybean (19 and 38 cm rows) and com (38 cm rows) yielded greater than wide row

soybean (76 cm rows) and com (76 cm rows) in three of four years. Planting soybean in
narrow rows resulted in higher levels of light interception than soybean in wide rows
throughout the growing season. Narrow row corn intercepted more light than wide row
corn early in the season, but light interception was similar for both row spacings at
tasselling. Planting soybean in narrow rows reduced growth of late emerging weeds.
This was not generally true for narrow row corn. Sequential glyphosate applications in
corn reduced weed biomass in com at the earliest application timing but did not increase
corn yield. Delaying glyphosate application until weeds reached 30 cm in height did not
signiﬁcantly reduce soil moisture in corn compared to the weed free control. Soil
moisture was reduced in weedy corn compared to weed free corn. Delaying glyphosate
applications until weeds reached 23 or 30 cm in height did not result in reduced soil
moisture in corn. At lower depths, soil moisture was greater in corn when glyphosate
was applied to 23 and 30 cm weeds compared to the weed free control. This likely
occurred due to reduced root development due to early season weed competition. When
glyphosate applications were made to 5 cm tall weeds at Clarksville, soil moisture was
reduced compared to the weed free control. This was likely due to competition for soil
moisture from weeds emerging following glyphosate applications. Corn planted in wide
row spacings had greater soil moisture than narrow row corn at the 0-18 cm depth at East

Lansing and at the 18-36 cm depth at Clarksville.

 

ACKNOWLEDGMENTS

I would like to thank my advisor, James J. Kells, for the support, guidance, and
encouragement that he has given me. Additionally, I would like give a sincere thanks to
my graduate committee members; Christina D. DiFonzo, Donald Penner, and Karen A.
Renner for their work in helping me complete my doctoral degree program.

I would like to thank the Corn Marketing Program of Michigan, and the Michigan
Soybean Promotion Committee for ﬁnancially supporting this research.

I would also like to thank Andy Chomas, Gary Powell, and Keith Dysinger for
their work in helping me establish and complete my ﬁeld experiments. I would also like
to thank Brian Long and Cal Bricker for their help with soil moisture measurements. I
would also like to thank my fellow graduate students, Mark Bernards, Trevor Dale, Jason
Fausey, Aaron Franssen, Corey Guza, Heather Johnson, Nate Kemp, Chad Lee, Eric
Nelson, Kyle Poling, David Pratt, Adrienne Rich, Marulak Simarmata, Joseph Simmons,
Christy Sprague, Brent Tharp, and Sharon White for their work in the ﬁeld, their help in
coursework, and their friendship. I would also like to thank the undergraduate students,
Stephanie Eickholt, Patrick O’Boyle, and Jarrod Thelen for their hard work.

I would especially like to thank my wife, Mindy, for her encouragement and
patience with me while completing my education along with my son Benjamin. I would
also like to thank my parents, Dale Z and Valene Dalley, for their sacrifices in raising me
and for their support in helping me reach my educational goals. Ultimately, I thank God,
my Heavenly Father, whom has blessed me immeasurably and has given me the strength

and wisdom to complete this work.

iv

TABLE OF CONTENTS

LIST OF TABLES ................................................................................... vi
LIST OF FIGURES ................................................................................. vii
CHAPTER 1
Abstract ....................................................................................... 1
Introduction .................................................................................... 2
Materials and Methods ...................................................................... 6
Results and Discussion ..................................................................... 13
Literature Cited ............................................................................. 33
CHAPTER 2
Abstract ...................................................................................... 55
Introduction .................................................................................. 56
Materials and Methods ..................................................................... 59
Results and Discussion .................................................................... 63
Literature Cited ............................................................................. 72

LIST OF TABLES

Table 1.1. Treatment dates and crop and weed heights at East Lansing, MI ............... 37
Table 1.2. Treatment dates and crop and weed heights at Clarksville, MI .................. 38

Table 1.3. Weed densities, and calculated competitive load values at East Lansing and
Clarksville in 1998, 1999, 2000, and 2001 ....................................................... 39

Table 1.4. Monthly precipitation recorded at the Michigan State University Department
of Horticulture Teaching and Research Center, East Lansing, MI, and at the Clarksville
Horticulture Experiment Station in Clarksville, MI ............................................ 40

Table 1.5. Adjusted determination coefﬁcient (R2) values for corn and soybean yield and
application timing parameters ..................................................................... 41

Table 1.6. Corn yield following a single glyphosate application compared to sequential
glyphosate applications ............................................................................. 42

Table 1.7. Weed biomass in corn that received a single glyphosate application compared
to sequential glyphosate applications .............................................................. 43

Table 1.8. Biomass of weeds emerging after glyphosate application and in untreated corn
and soybean in 1998 and 1999 and in corn at Clarksville in 2001 ........................... 44

Table 2.1. Treatment dates, crop height and growth stage and average weed canopy
height at each treatment timing at East Lansing and Clarksville, MI ........................ 75

Table 2.2. Weed densities and calculated competitive load at East Lansing, and
Clarksville in 2001... ................................................................................ 76

vi

LIST OF FIGURES

Figure 1.1. Effect of glyphosate application timing on corn yield at East Lansing in
1998, 1999, and 2000 and combined from Clarksville and East Lansing in 2001. Yields
from 2001 were averaged over row spacing. WF = weed free, UNT = untreated. Yield
means with different lower case letters are signiﬁcantly different (p < 0.05) comparing
treatment timings within row spacing. Yield means with different upper case letters are
signiﬁcantly different (p < 0.05) comparing row spacing within treatment timings ....... 41

Figure 1.2. Effect of timing of weed control on corn yield in at Clarksville in 1998 and
1999, averaged over row spacings. WF = weed free, UNT = untreated. Yield means with
different lower case letters are signiﬁcantly different (p < 0.05) ............................. 42

Figure 1.3. Effect of glyphosate application timing on soybean yield at East Lansing.
WF = weed free, UNT = untreated. Data from 2001 were averaged over row spacing.
Yield means with different lower case letters are signiﬁcantly different (p < 0.05)
comparing treatment timings within row spacing. Yield means with different upper case
letters are signiﬁcantly different (p < 0.10) comparing row spacing within each treatment
timing ................................................................................................. 43

Figure 1.4. Effect of timing of weed control on soybean yield in at Clarksville. WF =
weed free, UNT = untreated. Yield means with different lower case letters are
signiﬁcantly different (p < 0.05) comparing treatment timings within row spacing. Yield
means with different upper case letters are signiﬁcantly different (p < 0.10) comparing
row spacing within each treatment timing ........................................................ 44

Figure 1.5. Effect of glyphosate application timing on corn and soybean yield at East
Lansing. WF = weed free, UNT = untreated. In 1998, glyphosate was not applied to 5
and 30 cm tall weeds in soybean. Yield means with different lower case letters are
signiﬁcantly different (p < 0.10) comparing treatment timings within crops. Yield means
with different upper case letters are signiﬁcantly different (p < 0.10) comparing corn and
soybean within treatment timings .................................................................. 45

Figure 1.6. Effect of glyphosate application timing on corn and soybean yield at
Clarksville. WF = weed free, UNT = untreated. Yield means with different lower case
letters are signiﬁcantly different (p < 0.05) comparing treatment timings within each row
spacing. Yield means with different upper case letters are signiﬁcantly different (p <
0.10) comparing corn and soybean within treatment timings 46

Figure 1.7. Linear regression of yield and timing of glyphosate application based on the
product of weed density and weed height of corn planted in 38 and 76 cm rows and

soybean planted in 19, 38, and 76 cm rows at East Lansing (1998, 1999, 2000, 2001) and
Clarksville (1998, 1999, and 2001) ............................................................... 47

vii

Figure 1.8. Effect of row spacing on light interception by com and soybean at East
Lansing. Means with different lower case letters are signiﬁcantly different (p < 0.05)
comparing row spacings within each measurement trmrng48

Figure 1.9. Effect of row spacing on light interception by corn and soybean in 1998 and
1999 at Clarksville. Means with different lower case letters are signiﬁcantly different (p
< 0.05) comparing row spacings within each measurement timing ........................... 49

Figure 1.10. Biomass of weeds emerging following glyphosate applications in soybean
and corn at East Lansing. Means with different lower case letters are signiﬁcantly
different (p < 0.05) comparing treatment timing within each row spacing. Means with
different upper case letters are signiﬁcantly different (p < 0.10) comparing row spacing
within each treatment timing ........................................................................ 50

Figure 2.1. Predicted soil moisture retention curves for soil textures at East Lansing and
Clarksville using a mathematical equations for estimating soil water potential from soil
texture (Saxton, et a1 1986) ......................................................................... 73

Figure 2. 2. Biomass of weeds emerging following glyphosate applications at Clarksville
and East Lansing. Biomass means with different lower case letters are signiﬁcantly
different (p > 0.05) for comparisons of application timing within each row spacing.
Biomass means with different upper case letters are signiﬁcantly different (p > 0.05) for
comparisons of row spacing within each treatment timing .................................... 74

Figure 2.3. Corn yield averaged across row spacing at Clarksville and East Lansing.
Yield means with different lower case letters are signiﬁcantly different. Fisher’s
Protected LSD (or = 0.10) procedures were used for means separation ...................... 75

Figure 2.4. Soil moisture in weed free and weedy corn averaged across row spacing at
Clarksville at 0-18, 18-36, 36-54, 54-72, and 72-90 cm depths and rainfall during the
same time period. Asterisks denote a signiﬁcant difference in soil moisture at p <

0.10 ..................................................................................................... 76

Figure 2.5. Soil moisture in weed free and weedy corn averaged across row spacing at
East Lansing at 0-18, 18-36, 36—54, 54-72, and 72-90 cm depths and rainfall during the
same time period. Asterisks denote a signiﬁcant difference in soil moisture at p <

0.10 ...................................................................................................... 77

Figure 2.6. Soil moisture at Clarksville averaged across row spacing in weed free corn
and when weeds were controlled using glyphosate when 5, 23, and 30 cm in height.
Fishers Protected LSD (or = 0.10) was used for means separation ........................... 78

Figure 2.7. Soil moisture at East Lansing averaged across row spacing in weed free corn
and when weeds were controlled using glyphosate when 5, 23, and 30 cm in height.
Fishers Protected LSD (or = 0.10) was used for means separation ........................... 68

viii

Figure 2.8. Soil moisture in weed free corn in 38 and 76 cm row spacings at Clarksville
at 0-18, 18-36, 36-54, 54-72, and 72-90 cm depths and rainfall during the same time
period. Asterisks denote a signiﬁcant difference in soil moisture at p < 0.10 ............... 69

Figure 2.9. Soil moisture in weed free corn in 38 and 76 cm row spacings at East Lansing

at 0-18, 18-36, 36-54, 54-72, and 72-90 cm depths and rainfall during the same time
period. Asterisks denote a signiﬁcant difference in soil moisture at p < 0.10 ............... 70

ix

CHAPTER 1

EFFECT OF GLYPHOSATE APPLICATION TIMING AND ROW SPACING
ON CORN AND SOYBEAN YIELD, LIGHT INTERCEPTION,
AND WEED GROWTH
ABSTRACT
With the introduction of glyphosate resistant crops, determination of appropriate
herbicide application timings is needed. Corn and soybean were planted in narrow and
wide row spacings to determine the effect of glyphosate application timing and row
spacing on crop yield and weed control in these two crops. Glyphosate was applied at
0.84 kg ae/ha when native weeds populations reached 5, 10, 15, 23, and 30 cm in height.
Under highly competitive growing conditions (below normal rainfall; 53% of normal, and
high weed density; 1200 weeds/m2), corn yield was ﬁrst reduced when weeds reached 10
and 15 cm in height (V4 and V5, respectively) when com was planted in 38 and 76 cm
rows, respectively. Under similar conditions, soybean yield was ﬁrst reduced when
weeds reached 15 and 23 cm (V2 and V3, respectively) when planted in 19 and 38 cm
rows, respectively. Yield losses occurred only in the untreated control when soybean was
planted in 76 cm rows (>V5 growth stage). Corn and soybean yielded higher but were
more susceptible to early season weed interference when planted in narrow than in wide
rows in three out of four years. Regression analysis of com yield and application timing
parameters (weed height, crop growth stage, days after crop emergence) along with
combinations of application timing parameters and weed density and competitive load
was performed. Using the product of weed height and density as the independent

variable resulted in the best ﬁt for both corn and soybean. High weed densities increase

if:
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the risk of yield loss and must be considered when determining the appropriate timing for
total POST herbicide applications such as glyphosate. Inter-row light interception was
greater when com and soybeans were planted in narrow rows. Biomass of weeds
emerging after glyphosate applications was greater when soybeans were planted in 76
than in 19 or 38 cm rows, but was generally similar for both row spacings of corn.
Sequential glyphosate applications in corn reduced weed biomass in corn at the earliest
application timing but did not increase corn yield.

Nomenclature: Glyphosate, N-(phosphonomethyl) glycine; com ‘DK 493RR’, Zea mays
L.; soybean ‘Pioneer 92871’, Glycine max (L.) Merr.

Key words: Narrow row corn, narrow row soybean, light intensity, shading, weed

interference, weed competition.

INTRODUCTION
Weeds compete with crops for soil moisture, sunlight, and nutrients, and when any of
these resources become limited, crop and weed growth can be constrained. The period
of time in which weed control efforts affect crop yield is known as the critical period
for weed control and is deﬁned by two terms, the weedy period (how long weeds can
remain within a crop before yield losses begin to occur), and the weed free period (how
long weed control efforts must be maintained to prevent yield loss) (Weaver et a1.
1992). Hall et al. (1992) reported that the beginning of the critical period for weed
control in corn varied according to weed densities and environmental conditions more

than the end of the critical period of weed control. The introduction of herbicide

llr

resistant crops has not only greatly enabled the study of the critical period for weed
control but has also created a necessity for its study.

The ﬁrst genetically transformed herbicide resistant crop, glyphosate resistant
soybean (Glycine max L.), (Padgette et a1. 1995) was ﬁrst grown commercially in
1996. Today, glyphosate resistant corn hybrids (Zea mays L.), cotton (Gossypium
hirsutum L.), and canola (Brassica napus L.) cultivars are also commercially available
and research is being conducted on other crops not yet available commercially. With
glyphosate resistant crops, most annual weeds and some perennial weeds can be
controlled with a postemergence (POST) glyphosate (N -phosphonomethyl glycine)
application. Weed control is achieved with increased ﬂexibility when using glyphosate
resistant crops due to glyphosate’s effectiveness on controlling weeds regardless of size
(Krausz et a1. 1996; Ateh and Harvey 1999; Tharp and Kells 1999; and Tharp et al.
1999), although higher application rates may be needed to control larger weeds (Krausz
et al. 1996). The most important factor in correctly timing glyphosate applications is
avoidance of yield loss due to early season weed interference (Krausz et al. 2001).
Greater understanding of the critical period of weed control can help growers make
better decisions regarding timing of herbicide application, especially when planting
herbicide resistant crops.

The proper glyphosate application timing needed to achieve satisfactory weed
control without yield loss due to competition is not well understood. Many studies have
been conducted to examine the effect of weed removal timing on crop yield, however,
in these studies weed removal timing has been based on a variety of factors including

weed height (Knake and Slife 1969; Krauz et a1. 2001), crop growth stage (Hall et al.

1992; VanGessel et al. 2001), and days or weeks after crop emergence (Horn and
Burnside 1985). Tharp and Kells (1999) stated that com yield reductions, when
glyphosate was applied late-POST, may be due to early season weed interference.
Carey and Kells (1995) concluded that delaying POST herbicide applications increased
the probability of yield reductions in corn. Horn and Burnside (1985) found that
soybean yield loss was not reduced when weeds were removed four weeks after
emergence, but that when weeds remained in the crop for six weeks, yields were
reduced in ﬁve of eight experiments. VanGessel et al. (2000) stated that weed control
and yield were greatest when glyphosate was applied to soybean at the one- to three-
trifoliate stage (18 to 28 days after planting (DAP)). Krausz et al. (2001) found that
soybean could tolerate weed interference until weeds reached 30 cm in height (V5 leaf
stage; 6 to 7 weeks after planting) without sacriﬁcing yield.

Using a single factor as a basis for application timing is commonly used when
conducting research on the effects of herbicide application timing on weed interference.
However, other factors such as environmental conditions and weed density and species
composition may also play an important role in determining when yield losses ﬁrst
occur. Also it is not clear which parameter, weed height, crop growth stage, days after
crop emergence DAE, etc. , should be used as a basis for timing herbicide applications.

Glyphosate has no residual herbicidal activity in most soils, becoming tightly
bound to soil particles where it is readily degraded by soil microorganisms (Sprankle et
al. 1975). Therefore, a single application of glyphosate can result in unsatisfactory
weed control due to emergence and growth of weeds following glyphosate application.

Interest has grown in cultural practices that, when combined with glyphosate

application, would achieve satisfactory weed control. One cultural practice is planting
in narrow rows. Interest in narrow rows is associated with quicker canopy closure
(Peters et al. 1965; Wax and Pendleton 1968; Mickelson and Renner 1997). It is
thought that with quicker canopy closure, weed emergence and growth would
subsequently be reduced. Mickelson and Renner (1997) reported that soybean planted
in narrow rows had 30% less weed biomass than soybean planted in wide rows.
Johnson et al. (1998) reported no weed control improvements when row spacing was
reduced in corn, and Teasdale (1995) found limited weed control improvements when
com was planted in narrow rows suggesting that using narrow rows and increasing corn
populations would decrease the critical period for weed competition by one week.
Interest in narrow rows also stems from yield advantages associated with
planting in narrow rows which are generally attributed to more efﬁcient use of sunlight,
especially early in the growing season. Narrow row soybeans have an increased
number of pods per plant, branching, plant survival and harvest index (Ethridge 1989;
Ikeda 1992; Board and Harville 1993, Egli 1994; Board and Harville 1996; Bullock et
al. 1998). Nelson and Renner (1999) reported an $18 to $26/acre increase in gross
margin for narrow row compared to wide row soybean. For corn, the advantages of
narrow row widths are more variable. Bullock et al. (1988) found that com planted at
equidistant spacing in 38 cm rows consistently yielded greater than corn planted in 76
cm rows at the same population, while Westgate et al. (1997) reported no yield gains
for corn planted in narrow rows. Widdicombe (2000) reported that the advantages of
using narrow row spacings for corn were greater at central and northern locations than

at southern locations in Michigan. This is consistent with the ﬁndings of Paszkiewicz

(1997) who suggested that the greatest yield response to narrow rows occurred north of
US. Interstate 90.

Herbicide resistant crops allow for studying weed control in different crops
using identical weed control practices. A greater understanding of the yield beneﬁts of
narrow row spacings and the effects of row spacing on weed control and weed
interference is needed to reﬁne weed control recommendations for herbicide resistant
cr0ps. The objectives of this study were: 1) to determine the effect of row spacing and
glyphosate application timing on corn and soybean yield, 2) to determine the effect of
row spacing on light interception and weed growth 3) to determine the effect of
glyphosate application timing on weed growth 4) to compare the relative tolerance of

soybean and corn to early season weed interference.

MATERIALS AND METHODS

To study the effects of weed interference in corn and soybean, ﬁeld experiments
were conducted from 1998 to 2001 at two locations: the Crop and Soil Sciences Research
Farm at Michigan State University in East Lansing, Michigan, and the Clarksville
Horticulture Experiment Station in Clarksville, Michigan.

At East Lansing, experiments were conducted on the same ﬁelds in 1998 and
2000, with an adjacent ﬁeld used in 1999 and 2001. The soil type in 1998 and 2000 was
Capac sandy loam (ﬁne-loamy, mixed mesic Aeric Ochraqualfs), 55.8% sand, 30.2% silt,
and 14.0% clay, with pH 6.6 and 2.9% organic matter. In 1999 and 2001 the soil was a
Capac sandy clay loam (ﬁne-loarny, mixed mesic Aeric Ochraqualfs), 51.9% sand, 28.3%

silt, and 19.8% clay, with pH 6.5 and 3.0% organic matter.

At Clarksville, experiments were conducted in 1998, 1999, and 2001 on three
different ﬁelds. The soil type in 1998 was a mix of Dryden sandy loam (coarse-loamy,
mixed, mesic Oxyaquic Hapludalfs) and Lapeer sandy loam (coarse-loamy, mixed, mesic
Typic Hapludalfs), 74.6% sand, 17.1% silt, and 8.4% clay, with pH 6.7 and 1.8% organic
matter, and in 1999 and 2001 the soil type was Lapeer loam (coarse-loamy, mixed, mesic
Typic Hapludalfs), 47.1% sand, 36.2% silt, and 16.7% clay, with pH 6.8 and 2.0%
organic matter.

Fields were fall chisel plowed followed by a ﬁeld cultivator in the spring for
seedbed preparation. Prior to planting corn plots were fertilized with granular urea (46-0-
0) at 300 kg/ha using a broadcast applicator and incorporated using a ﬁeld cultivator.
Nitrogen fertilizer was not applied to soybean plots because of the symbiotic relations of
soybean with nitrogen ﬁxing Rhizobiumjaponicum bacteria. Soil tests showed that
adequate levels of phosphorous and potassium were present and therefore no additional
fertilizers were applied.

A glyphosate resistant full season com hybrid (DK 493RR') and an indeterminate
Group II soybean cultivar (928712) were planted at 77,000 and 422,000 seeds/ha,
respectively. Corn was planted in two row spacing (38 and 76 cm row widths) and
soybean was planted in three different row spacings (19, 38, and 76 cm row widths).
Plant populations remained constant across the different row spacings of corn and
soybean. At East Lansing, corn and soybean were planted on May 19, 1998, May 14,

1999, May 25, 2000, and May 14, 2001. At Clarksville, corn and soybean were planted

 

' DeKalb Genetics Corp, Monsanto Co., 800 N. Lindbergh Blvd, St. Louis, MO 63167

2 Pioneer Hi-Bred International, 400 Locust Street, Suite 800, Des Moines, IA 50306

on May 19, 1998, and May 17, 1999, and corn was planted on May 4, 2001. Corn and
soybean were planted using a customized toolbar with John Deere planter units designed
to plant in either 38 or 76 cm row widths. The 19 cm row width for soybean was
accomplished by planting double rows; using the 38 cm spacing, two passes were made
with the planter with the second pass planted directly between the rows from the ﬁrst
pass with the planter. Plot size was 3 m wide by 10.7 m long for each experiment.
Weeds were controlled using glyphosate3 at 0.84 kg ae/ha plus 2% ammonium
sulfate at different timings based on weed height. Weeds were controlled when the
average weed canopy height reached 5, 10, 15, 23, and 30 cm in height, although not all
treatment timings were included in all experiments (Tables 1.1 and 1.2). Weed free and
untreated controls were included for each crop species and each row spacing. Weed
control in weed free plots was accomplished through preemergence application of a
commercial premix4 of S-metolachlor at 0.57 kg/ha plus atrazine at 0.45 kg/ha in corn,
and by using a commercial formulation of alachlor5 at 0.91 kg/ha in soybean followed by
POST glyphosate applications or hand weeding as needed in both crops. All herbicides
were applied using a tractor-mounted compressed-air sprayer calibrated to deliver 187

L/ha at 207 kPa using ﬂat fan nozzles“.

 

3 Roundup Ultra, Monsanto Co., 800 N. Lindbergh Blvd, St. Louis, MO 63167
4 Bicep Lite 11 Magnum, Syngenta Crop Protection, Inc., PO. Box 18300, Greensboro,

NC 27419
5 Lasso, Monsanto Co., 800 N. Lindbergh Blvd, St. Louis, MO 63167

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

Experimental Design: At East Lansing, the experimental design was a split-split-plot
each year. The whole plot was crop species (corn or soybean), the sub-plot was row
spacing (19, 38, and 76 cm widths), and the sub-sub-plot was time of glyphosate
application (based on weed height). In 1998, glyphosate was applied in soybean plots
when weeds were 10, 15, and 23 cm in height, and when weeds were 5, 10, 15, 23, and
30 cm in height in corn plots. Glyphosate was applied when weeds were 5, 10, 15, 23,
and 30 cm in height in corn and soybean in 1999, 2000, and 2001. Additionally, at East
Lansing, sequential glyphosate applications were made in corn in each year of this study.
Glyphosate (0.84 kg ae/ha) was applied when weed canopy height reached 5, 15, and 15
cm which was followed by a second glyphosate application (0.42 kg ae/ha) when weeds
in the untreated control reached 30 cm. These treatment timings were compared to weed
free and untreated controls. Table 1.1 summarizes treatment dates, crop heights, and
growth stages for experiments conducted at East Lansing. A 3-m border of corn and a 3-
m border of soybean separated each whole plot of corn and soybean to minimize the risk
of border effects.

At Clarksville, smaller experiments were established in support of the larger
experiments conducted at East Lansing. In 1998 and 1999, corn and soybean were
planted in a split-split plot design. Crop species (corn or soybean) was the main plot, row
spacing (38 and 76 cm row widths for corn, and 19, 38, and 76 cm row widths for
soybean) was the sub-plot, and timing of herbicide application was the sub-sub-plot.
Glyphosate, at 0.84 kg ae/ha, was applied when weeds were 15 cm in height, which was
compared to weed free and untreated controls. As at East Lansing, a 3 m border of corn

and a 3 m border of soybean separated each whole plot of corn and soybean. In 2001,

corn was planted in a split-plot design. The whole plot was row spacing (38 and 76 cm
row widths), and the sub-plot was time of herbicide application. Glyphosate, at 0.84 kg
ae/ha, was applied when weeds reached an average canopy height of 5, 10, 15, 23, and 30
cm. Table 1.2 summarizes treatment timing dates, crop heights, and growth stages for

experiments conducted at Clarksville.

Weed Density: Weed density was measured to quantify the presence and density of the
native populations of weeds used in experiment. Weed densities, by species, were
measured in the untreated plots using three 14 by 75 cm quadrats randomly placed
parallel to the middle of the crop rows. Quadrats were marked with ﬂags so that
density measurements would come from the same locations within each plot each time
they were measured. Densities were measured when weeds were 5, 10, and 15 cm in
height. Weeds present in these studies include velvetleaf (Abutilon theophroasti Medic.
#7 ABUTH), redroot pigweed (Amaranthus retroﬂexus L. # AMARE), common
ragweed (Ambrosia artemisiifolia L. # AMBEL), common lambsquarters
(Chenopodium album L. # CHEAL), jimsonweed (Datura stamonium L. # DATST),
bamyardgrass (Echinachloa crus-galli Beauv. # ECHCG), fall panicum (Panicum
dichotomilﬂorum Michx. # PANDI), giant foxtail (Setaria faberi Herrm. # SETFA),
yellow foxtail (Setaria glauca Beauv. # SETLU), green foxtail (Setaria viridis Beauv. #

SETVI), and Eastern black nightshade (Solanum ptycanthum Dun. # SOLPT). Table

 

7 Letters following this symbol are a WSSA-approved computer code from Composite
List of Weeds, Revised 1989. Available only on computer disk from WSSA, 810 East 10th

Street, Lawrence, KS 66044-8897.

10

1.3 summarizes weed densities measured when weeds were 15 cm in height each year
at each location. Using competitive indices for each weed species, competitive load
was calculated by summing the product of weed densities and their respective indices.
Competitive load is used by the 8WeedSOFT Advisor weed management software to
predict yield losses due to weed competition.

Crop Yields: Corn and soybean were harvested at maturity. Corn was harvested using a
plot combine with a 5-row header designed to harvest corn planted in either 38 or 76 cm
rows. The center two rows were harvested from corn in 76 cm row widths (an area of 16
m2), and the 5 middle rows were harvested from corn in 38 cm row widths (an area of 20
m2). Soybean were harvested using a plot combine with a 1.5-meter header so that the
center 2 rows were harvested from soybean in 76-cm rows, 4 rows were harvested from
soybean in 38 cm rows, and 8 rows were harvested from soybean in 19 cm rows (an area
of 16 m2 for each). Yield and moisture were measured and recorded with a datalogger on
the plot combine. Corn yield was adjusted to 15.5% moisture, and soybean yield was

adjusted to 13% moisture. Yields were calculated according to area harvested.

Light Interception: The amount of light transmitted through the crop canopy was
measured in weed free plots using a SunScan Canopy Analysis Systemg. The Sunscan
system consists of three components; 1) a wand one meter long and 13 mm wide with
sensors spaced every 15.6 mm along the length of the wand with a spectral response of

400-700 nm which was used to measure light beneath the crop canopy, 2) a tripod

 

8 WeedSOFT, 362 Plant Sciences Bldg, University of Nebraska, Lincoln, NE 68583-

0915

" Dynamax, Inc., 10808 Fallstone #350, Houston, Tx. 77099

11

mounted sensor which was used to measure both incident and diffuse light above the crop
canopy, and 3) a handheld Psion Workabout'o data logger that combined simultaneous
measurements of light above and beneath the crop canopy. Light transmission, as a
percent of incident, was automatically calculated as each measurement was taken, and
was recorded by the datalogger. Light measurements were taken from the center of the
crop row aligning the wand parallel to the crop row. Crops were planted in a north-south
orientation so the amount of light intercepted by the crop canopy at solar noon was
closely related to canopy closure. Three measurements were taken from weed free plots
of corn and soybean. Light measurements were taken weekly at or near solar noon at each
site beginning after the 5 or 6 collar stages in corn and continuing through pollination
when vegetative growth of corn ceased and canopy closure was assumed to have reached

its maximum.

Weed Biomass: Weed biomass measurements were taken in all treatments between weed
ﬂowering and seed set. All weeds within 15 by 150-cm quadrats (0.225 m2) randomly
placed perpendicular lengthwise to the crop row were harvested at the soil surface. Two
quadrats were harvested from each plot. Weed densities, by species, were also recorded,
and harvested weeds were placed in paper bags and dried in a forced air oven. When dry,

weeds were weighed and biomass calculated as g/mz.

Statistical Analysis: All data were analyzed using SAS 6.12 and 8.00 softwarel 1. The
PROC GLM procedure in SAS was used for analysis of variance and appropriate F-tests

were run to determine signiﬁcance. Regression analysis of yield and the application

 

'0 Psion Digital, 1810 Airport Exchange Blvd., Suite 500, Erlanger, KY 41018
” SAS Institute Inc., 100 SAS Campus Drive, Cary, NC 27513

12

timing parameters weed height, crop height, crop growth stage, DAE and growing degree
days after emergence (GDDAE) and the products of these parameters with weed density
and competitive load was performed separately for each row spacing of corn and
soybean. This analysis was performed to determine which application timing parameter

best predicted yield loss due to weed interference.

RESULTS AND DISCUSSION
Growing conditions: The different environmental conditions and weed densities that
occurred during the years this research was conducted enabled for studying the effects of
weed interference on corn and soybean under different growing conditions. These
conditions resulted in varying levels of stress on corn and soybean, which resulted in
differing effects of weeds on corn and soybean during this four year study. Weed
densities and rainfall amounts are presented in Table 1.3 Table 1.4, respectively. At East
Lansing, weed densities were greatest in 1999 (1270 weeds/m2), and lowest in 2000 (180
weeds/m2), with intermediate densities in 1998 (563 weeds/m2) and 2001 (319 weeds/m2)
(Table 1.3). At Clarksville, weed densities were lowest in 1998 (89 weeds/m2) and
greatest in 2001 (881 weeds/m2). In 1998 and 1999 rainfall was below normal in May
and June. In 2000, rainfall was above normal (especially in May). Rainfall in the 2001
growing season was variable. May rainfall was two times greater than normal, which
was followed by drought conditions in July and August. Therefore, weed interference
was greatest in 1998 and 1999 at East Lansing due to high weed densities and lower than
normal early season rainfall, and was lowest in 2000 at East Lansing due to low weed

density (180 weeds/m2), and above normal early season rainfall. At Clarksville weed

l3

interference was relatively low in 1998 due to low weed density and higher in 2001 due
to high weed density.

Corn yield: The time at which corn yield was reduced due to weed interference varied
according to the environmental conditions under which it was grown. In 1998 and 1999,
under highly competitive growing conditions (low precipitation, high weed density),
yield losses occurred earlier than in 2000 and 2001 when growing conditions were less
competitive (normal or above normal precipitation and lower weed densities). At East
Lansing in both 1998 and 1999 there was a signiﬁcant interaction between time of
glyphosate application and row spacing for corn yield. Corn planted in narrow rows
yielded more but was more susceptible to early season weed interference than corn in
wide rows (Figure 1.1).

In 1998, when com was planted in 38 cm rows, yield was greatest when weeds
were controlled when 5 cm in height and was reduced by weed interference when weeds
reached 15 cm or more (V4 growth stage) (Figure 1.1). Corn yield was also reduced in
the weed free control compared to yield when weeds were controlled at 5 cm. Yield loss
in the weed free control was most likely related to disturbance cause by light interception
measurements taken in these plots. In future years separate weed free plots were
included for taking these measurements. When corn was planted in 76 cm rows, yield
was reduced when weeds reached 30 cm or more (V7 growth stage) compared to when
weeds were controlled at 10 cm in height. Yield was similar in all other treatment
timings. Corn planted in 38 cm rows yielded more than corn planted in 76 cm rows only
at the earliest glyphosate application timing (5 cm weed height), and in the untreated

control (Figure 1.1).

14

In 1999, corn yield was reduced by weed interference when weeds reached 10 cm
or more (V4 growth stage) when com was planted in 38 cm rows, and when weeds
reached 15 cm or more (V5 growth stage) when com was planted in 76 cm rows. Corn
yield was greater when com was planted in 38 cm row than in 76 cm rows if weeds were
controlled before reaching 10 cm in height (Figure 1.1).

Under lower levels of weed interference (2000), there was no signiﬁcant
interaction between row spacing and time of glyphosate application; however differences
between time of glyphosate application and differences between row spacing were both
signiﬁcant. In 2000, corn yield was reduced by weed interference when weeds were
allowed to reach 30 cm or more (V9 growth stage) (Figure 1.1). Corn planted in narrow
rows yielded 6% greater than com planted in wide rows averaged across all treatment
timings (data not shown).

Corn responded differently in 2001 than in any of the previous years due to the
environmental extremes under which it was grown. May was more than twice as wet as
normal (at East Lansing 145 mm of rainfall in May 2001; normal rainfall is 69 m)
(Table 1.4). This was followed by a drought period lasting from the end of June through
the ﬁrst half of August. From June 23 through August 15 (53 days) only 29 mm of rain
fell at East Lansing. Normal rainfall during this time period is 141 mm. High rainfall,
combined with moderate weed densities, resulted in less competitive growing conditions
early in the season. The drought conditions later in the season, during pollination, when
com is most susceptible to yield loss from moisture stress (Classen and Shaw 1970),
resulted in yield losses not entirely associated with weed interference. For these reasons,

results differed from previous years. First, there was no signiﬁcant yield increase

15

associated with planting corn in narrow rows (data not shown). Porter et a1 (1997) also
found that climatic conditions that limited corn yield also limited the advantage gained by
planting corn in narrow rows. Because of the drought conditions, the photosynthetic
advantage of planting in narrow rows was negated by the lack of soil moisture, which
was evident due to observed leaf wilting, which limited crop production. Because there
was no signiﬁcant interaction between time of glyphosate application and row spacing,
and no signiﬁcant difference between row spacings, corn yield was averaged across row
spacing. Corn yield was reduced by weed interference if weeds were allowed to reach 23
cm in height or more (V5 growth stage) (Figure 1.1). In the untreated control, 96% yield
loss occurred.

Yield losses also occurred when glyphosate was applied to weeds 5 cm in height
(V3 grth stage) (Figure 1.1). Yield losses at the 5 cm timing may be attributed to
interference from weeds emerging alter the glyphosate application. This was the only
time during this four year study that weeds emerging after glyphosate applications could
be implicated in reducing corn yield. This may be in part due to the drought conditions
that occurred during the summer months of 2001. Under these conditions, late emerging
weed competition for soil moisture may have caused reduced corn yield when glyphosate
was applied to weeds 5 cm in height. In the ﬁrst three years of this study, weeds that
emerged after glyphosate applications did not negatively affect corn yield possibly
because soil moisture was more plentiful due to higher rainfall in these years.

Sequential glyphosate applications in corn had no affect on yield. In all cases,
corn yield following one glyphosate application was similar to corn yield following

sequential glyphosate applications (Table 1.6). This is consistent with the ﬁndings of

16

Murphy et al. (1996) who found that late emerging weeds were less likely to reduce corn
yield than weeds emerging with corn.

At Clarksville in 1998 and 1999, com yield was not reduced by weed interference
when weeds were allowed to reach 15 cm in height (V6-V7 grth stage) (Figure 1.2).
Yield losses occurred only in the untreated control and there was no signiﬁcant yield
increase when com was planted in narrow rows.

Delaying POST herbicide applications in corn increases the risk of yield loss due
to weed competition (Knake and Slife 1968; Carey and Kells 1995; Tharp and Kells
1999). Results from this study showed that as glyphosate applications were delayed,
yield losses occurred in all four years of this study. Planting corn in narrow rows
increased yield in three of four years. However when corn was grown under high levels
of weed interference, the yield advantage diminished as glyphosate applications were
delayed. This research supports the ﬁndings of others that planting in more equidistant
row spacings does increase yield (Stickler 1964; Lutz et al. 1971; Nielsen et al. 1988;
Murphy et al. 1996; Porter et al. 1997; Widdicombe 2000).

Soybean yield: In 1998 and 1999, when soybeans were grown under high levels of weed
interference (low rainfall, high weed density), there was a signiﬁcant interaction between
time of glyphosate application and row spacing for soybean yield at East Lansing. As in
corn, yield advantages existed for soybean planted in narrow rows, but they were more
susceptible to early season weed interference than soybean planted in wide row spacings
(Figure 1.3). In 1998, weed interference resulted in yield losses when weeds were
allowed to reach 23 cm or more (V5 growth stage) when soybean were planted in 19 cm

rows. Yield losses occurred only in the untreated controls when soybean were planted in

17

38 or 76 cm rows (>V5 growth stage). In 1999, weed interference reduced soybean yield
when glyphosate applications were delayed until weeds reached 15 cm or more (V2
growth stage) when soybean were planted in 19 or 38 cm rows, while yield reductions
occurred only in the untreated control when soybean were planted in 76 cm rows (>V5
growth stage). In both 1998 and 1999 there was a yield advantage for narrow row
soybean. Soybean planted in both 19 and 38 cm rows yielded more than soybean in 76
cm rows, but only when weeds were controlled before reaching 15 cm in height.
Soybean planted in 19 and 38 cm rows yielded similarly in nearly all treatment timings.

In 2000, when soybeans were grown under less stressful conditions, soybean yield
was reduced by weed interference only in the untreated control (>V5 growth stage) in all
row spacings (Figure 1.3). However, there were signiﬁcant differences between row
spacings for soybean yield. When averaged across all treatment timings, soybean planted
in 19 cm rows yielded 5% more than soybean planted in 38 cm rows, and yielded 9%
more than soybean planted in 76 cm rows. Soybean planted in 38 cm rows yielded 4%
more than soybean planted in 76 cm rows (data not shown). However, when separated
into individual treatment timings, soybean planted in 19 cm rows yielded more than
soybean planted in 76 cm rows in the weed free and untreated controls and when
glyphosate was applied to weeds 10 and 15 cm in height. Soybean planted in 38 cm rows
generally yielded similar to soybean planted in both 19 and 76 cm rows (Figure 1.3).

The drought conditions in 2001 affected soybean yield similarly to that of corn
yield. Drought conditions limited yield of soybean planted in narrow rows which were
similar to yields of soybean planted in wide rows. Because there was no signiﬁcant

difference between row spacing and no interaction between row spacing and time of

18

glyphosate application yields were averaged across row spacings (Figure 1.3). Soybean
yield losses due to weed interference occurred only in the untreated control (>V4 growth
stage) where yield losses of 54% occurred.

At Clarksville, soybean responded similarly to weed interference in both 1998 and
1999. In 1998, there was a signiﬁcant interaction between time of glyphosate application
and row spacing for soybean yield. In the weed free control, soybean planted in 19 and
38 cm rows yielded more than soybean planted in 76 cm rows (Figure 1.4). There were
no signiﬁcant differences in yield due to row spacing at any other treatment timing.
When soybeans were planted in 19 cm rows, yield was reduced when weeds were
allowed to reach 15 cm. When soybeans were planted in 38 and 76 cm rows, yield losses
occurred only in the untreated control. In 1999, yield was signiﬁcantly reduced only in
the untreated control and there were no signiﬁcant differences in yield due to row spacing
(Figure 1.4). When averaged over years, weed free soybean planted in 19 cm rows
yielded 7% and 10% more than soybean planted in 38 and 76 cm rows, respectively, and
soybean planted in 38 cm rows yielded 2% more than in 76 cm rows (data not shown).

Delaying glyphosate application in soybean resulted in yield losses in the ﬁrst two
years of this study at both locations, but not during the ﬁnal two years of this research.
Differences in the results between these years were likely related to differences in degree
of weed interference related to the environmental conditions and weed density in which
the soybean were grown. VanGessel et a1. (2000) also reported that under severe weed
interference, yield reductions occurred that did not occur under lower levels of weed
interference. 'Krausz et al. (2001) reported that delaying glyphosate applications in

soybean until weeds reach 30 cm in height did not reduce grain yield when soil moisture

l9

was adequate. Our research results showed that soybean yield was ﬁrst reduced by early
season weed competition at smaller weed heights when soybeans were planted in narrow
rows (19 and 38 cm) than in wide rows (76 cm). Soybean planted in 19 and 38 cm rows
yielded similarly in most cases, however, soybean in both of these narrow row spacings
yielded more than soybean planted in 76 row spacings in three out of the four years this
study was conducted. However, in 1998 and 1999, there was no yield advantage for
narrow rows when glyphosate applications were delayed until weeds reached 15 cm or
more, possibly due to high levels of weed interference. Many researchers have also
found that there is a yield advantage for planting soybean in narrow row spacings
(Wiggins 1939; Ethredge et al. 1989; Ikeda 1992; Egli 1994; Board and Harville 1996;
Bullock et al. 1998) and the results of this research support these ﬁndings.

Corn and soybean yield: To compare the effects of weed interference on corn and
soybean, yields were converted to percent of the weed free control except in 1998 when
yields were adjusted to a percent of when glyphosate was applied to weeds 10 cm in
height because of yield losses in the weed free control. In 1998, weed interference
reduced corn yield more than soybean yield when glyphosate was applied to weeds 15
and 23 cm in height and in the untreated control, while yields were similar at all other
treatment timings (Figure 1.5). In 1999, weed interference reduced corn yield more than
soybean yield when weeds were allowed to reach 23 cm or more. In 2000, there were no
differences in yield comparing corn and soybean at any of the glyphosate application
timings, but soybean yield was reduced more than corn yield in the untreated control. In
2001, corn yield was reduced more by weed interference than was soybean yield when

glyphosate was applied when weeds were 5 or 23 cm in height. Corn yield was also

20

reduced more than soybean yield in the untreated controls (Figure 1.5). At Clarksville,
there were no signiﬁcant difference comparing corn and soybean yield at any treatment
timing in 1998 or 1999 (Figure 1.6).

Corn yield was affected more by early season weed competition than soybean
yield. Delaying herbicide applications reduced corn yield to a greater extent than
soybean yield, especially under highly competitive growing conditions such as high weed
density and low rainfall. In addition, under season-long weed interference, corn yield
was reduced more than soybean yield in three of four years at East Lansing (Figure 1.5).
Knake and Slife (1969) also found that early season weed competition had greater effects
on corn than on soybean. They stated that the greatest competitive effect of weeds on
soybean appears to occur after the start of the reproductive stage while early weed
competition delayed corn tassel emergence.

Corn growth and development is quite different than that of soybean (especially
that of indeterminate soybean cultivar types grown in Michigan and used in this study).
Corn tassel and ear shoots initiation is complete at about the V5 grth stage (Ritchie
and Hanway 1992). Although corn is most susceptible to environmental stresses during
silking (Classen and Shaw 1970), stress to corn during ear shoot initiation may also result
in reduced yield potential. Unlike corn, soybeans are most susceptible to environmental
stresses during the pod ﬁll stage (Doss et a1. 1974). However, stresses occurring during
vegetative growth may reduce branching and therefore yield potential. Early season
weed competition may also remove nutrients from the soil making them unavailable later
in the season. In 2001, season-long weed interference with corn and soybean growing

under drought conditions resulted in 94% and 56% yield loss in corn and soybean,

21

respectively. Yield losses in the untreated controls were greater for corn than soybean
likely due to the timing of drought conditions (from June 23 to August 15, 2001) which
occurred during corn pollination while drought conditions subsided during soybean pod
ﬁll.

Our results indicate that under highly competitive growing conditions (high weed
density and less than normal rainfall) weeds need to be controlled as early as S cm in
height (15 to 23 DAE, at the V3 corn leaf stage) to prevent yield loss in narrow row corn
and before weeds exceed 15 cm (21 to 30 DAE, at the V2 to V3 soybean leaf stage) to
prevent yield loss in narrow row soybean. Weeds in corn planted in wide rows needed to
be controlled before they reached 10 cm in height (19 to 26 DAE, at the V4 corn leaf
stage). Weeds reaching 30 cm in height (31 to 38 DAE, at the V4 to V5 soybean leaf
stage) in wide row soybean did not reduce soybean yield in any year in which this study
was conducted.

Results showed that early season weed control is more important when crops are
planted in narrow rows. Under competitive growing conditions, a yield advantage
existed for narrow rows only when weeds were controlled before weeds reached 10 cm in
corn and 15 cm in soybean. However, under less stressful growing conditions
(moderately low weed density and above normal rainfall), delaying glyphosate
applications until weeds reached 30 cm did not greatly affect corn or soybean yields, and
narrow rows yielded higher regardless of when weeds were controlled.

Planting in narrow row spacings increases soybean yield (Wiggans 1939; Ethridge
1989; Ikeda 1992; Board and Harville 1993, Egli 1994; Board and Harville 1996; Bullock

et al. 1998) and corn yield (Hunter et al. 1970; Nielsen 1988; Porter et al. 1997; Bullock

22

et al. 1988; Widdicombe 2000). This research supports those ﬁndings. In three of four
years, corn and soybean yielded higher when planted in narrow rows than in wide rows.
In 2001, when there was no yield advantage to narrow rows, drought conditions limited
crop growth, and was likely responsible for eliminating the advantage of narrow rows.
Similarly, Taylor (1980) reported that decreasing row spacing increased soybean yield
only when seasonal water supply was plentiful and Porter et al. (1997) found that narrow
rows increased corn yield only under favorable growing conditions.

Predicting occurrence of yield loss: Herbicide application timings are frequently based
on a variety of parameters such as days or weeks after crop emergence (Horn and
Burnside 1985), crop height or growth stage (Hall et al. 1992; VanGessel et al. 2000), or
weed height (Knake and Slife 1969; Krausz et al. 2001). In this study timing of
glyphosate application was based on weed height. However, crop growth stage, crop
height, DAE and GDDAE were also recorded (Tables 1.1 and 1.2). Competitive load
(WeedSoft) was also calculated using weed densities and competitive indices for weeds
in corn and soybean (Table 1.3). Simple linear regression analysis of crop yield response
to these application timing parameters resulted in fairly low coefﬁcients of determination
(R2). Of the four application timing parameters (weed height, crop height, crop growth
stage, DAE and GDD, the highest R2 value was obtained using weed height as the
independent variable (Table 1.5). Using competitive load values as a regression
parameter did not improve the linear ﬁt compared to that of weed density. In theory
competitive load provides the ability to sum the density and competitiveness of the
individual weed species that accounts for the differences in competitive ability of

different weed species. Furthermore, the competitive indices used in calculating the

23

competitive load are derived from results of studies comparing the effects of season-long
weed interference and may not be appropriate for calculating yield losses due to weed
competition early in the growing season. Competitive indices must continue to be reﬁned
to more accurately predict the effects of weed interference.

The weed height at which yield losses ﬁrst occurred varied considerably during
the different growing seasons. Other factors such as weather, crop growth stage, weed
density or weed composition may have signiﬁcantly inﬂuenced yield results. Yield
losses have been shown to increase with increased weed density. When the product of
weed density and the application timing parameters were used, R2 values improved
considerably (Table 1.5). The best ﬁt was obtained by using the product of weed height
(cm) and weed density (weeds/m2) as the independent variable for regression analysis of
corn and soybean yield (Figure 1.7). These regressions had R2 values of 0.31, 0.35, and
0.05 for soybean planted in 19, 38, and 76 cm rows, respectively, and 0.39, and 0.45 for
corn planted in 38 and 76 cm rows, respectively. The R2 values were not improved by
using competitive load values in place of weed density (Table 1.5).

Weed density appears to be one of the most important factors in determining
when yield losses begin to occur due to weed interference. Combining weed density with
weed height, crop height, crop growth stage, DAE, or GDDAE greatly improved the ﬁt of
linear regression. In this study, the combination of weed height and weed density was the
best predictor of yield loss for both corn and soybean. This suggests that using a single
variable such as weed height or crop growth stage as a basis for timing glyphosate
applications without considering weed density is an incorrect practice. High weed

densities increase the risk of yield loss and must be considered when determining the

24

appropriate timing for total POST herbicide applications such as glyphosate. Further
research needs to be conducted on the effects of weed density on early season weed
competition.

Corn and soybean planted in narrow rows were affected differently than corn and
soybean planted in wide rows. As row spacing narrowed in both crOps the inﬂuence of
early season weed interference increased. This showed that com and soybean planted in
narrow rows were more susceptible to yield loss from weed interference than corn and
soybean planted in wide rows. The R2 values for soybean planted in 76 cm rows were
very low in all regression analyses because signiﬁcant yield losses did not occur at any of
the treatment timings resulting in a slope approaching zero. However yield losses did
begin to occur at some point following that ﬁnal glyphosate application to 30 cm weeds
(V5 growth stage) as yields in the untreated controls were reduced considerably.

Effect of row spacing on canopy closure: Planting corn and soybean in narrow rows
increased light interception early in the growing season at both Clarksville and East
Lansing in each year of this study. However there were signiﬁcant differences between
years and locations and therefore data is presented separately for each year and location.
At East Lansing in 1998, light interception by soybean planted in 19 and 38 cm rows
reached nearly 100% at 75 DAE while light interception by soybean in 76 cm rows
reached only 81% (Figure 1.7). Light intercepted by com was similar for both row
spacings in all measurements taken. In 1999, light interception reached 98% in soybean
planted in narrow rows (19 and 38 cm) at 62 DAE, while light interception by soybean
planted in 76 cm rows was only 79%. Corn planted in narrow rows intercepted more

light than corn in wide rows until 62 DAE. Light interception reached 92% and 89% for

25

corn planted in 38 and 76 cm rows, respectively. In 2000, light interception reached 98%
or more at 64 and 72 DAE in soybean planted in 19 and 38 cm rows, respectively, while
light interception reached only 85% in soybean planted in 76 cm rows. Light interception
in narrow row com was greater than in wide row corn up to 64 DAE when light
interception reached 92% and 88% in corn planted in 38 and 76 cm rows, respectively.

In 2001, drought conditions limited crop growth, under these conditions light interception
was reduced in all row spacings. At 72 DAE, light interception was 92%, 67% and 59%
in soybean planted in 19, 38, and 76 cm rows. Light interception was greater when com
was planted in narrow rows until 59 DAE. At 59 DAE there was a drop in light
interception in corn that is likely due to soil moisture stress resulting in leaf wilting and
rolling. Due to drought conditions, light interception reached only 83% and 77% in corn
planted in 38 and 76 cm rows, respectively (Figure 1.7).

Light interception results were similar for Clarksville. Light interception was
always higher in soybean planted in narrow rows (19 and 38 cm) than soybean planted in
76 cm rows (Figure 1.8). Light interception was greater in corn planted in 38 cm rows
than in 76 cm rows at 52 DAE in 1998 and up to 49 DAE in 1999. Soybean planted in
narrow rows reached 92% in 1998 and 99% in 1999, while corn planted in narrow rows
reached only 88% in 1998 and 91% in 1999. Soybean planted in wide row spacings
reached only 68% in 1998 and 79% in 1999, while corn in wide rows reached 86% in
1998 and 93% in 1999 (Figure 1.8).

It is apparent that the advantage of reducing row spacing to achieve quicker
canopy closure is greater for soybean than for corn. It is also apparent that reducing row

spacing in soybean allows for a more complete canopy closure as light interception was

26

always higher when soybeans were planted in narrow rows compared to soybean planted
in wide rows. This advantage was not so evident for corn planted in narrow rows. Light

interception was greater in com planted in narrow than in wide rows early in the season,

but light interception was similar for both row spacings of corn when maximum canopy

closure was reached.

Planting in narrow rows hastened canopy closure, but more importantly, planting
soybean in narrow rows resulted in more complete canopy closure than wide row
soybean. At its maximum canopy closure, light interception was also greater for soybean
planted in narrow rows than for corn planted in either narrow or wide rows. Because
narrow row soybean intercepted 98% or more of the sunlight, very little was available for
weed growth under the crop canopy. Canopy closure occurred later for wide row
soybean, and levels of light intercepted never reached that of soybean in narrow rows,
which made them less competitive with late emerging weeds.

Light interception was greater in corn early in the growing season, and canopy
closure occurred at nearly the same time as soybean in narrow rows. However, the levels
of light interception never reached that of narrow row soybean. Because of this, when a
single application of glyphosate is the only method of weed control, corn would likely be
more susceptible to reduced weed control than narrow row soybean due to growth of late
emerging weeds.

Effect of crop row spacing and glyphosate application timing on weed biomass:
Row spacing affected weed growth following glyphosate applications. The biomass of
weeds emerging in corn and soybean following glyphosate application was different each

year and are presented separately. In 1998, biomass of weeds harvested from corn at East

27

Lansing was greater when glyphosate was applied to 5 cm weeds compared to all other
treatment timings (Figure 1.9). All other treatment timings were similar and there were
no differences due to row spacing. Following glyphosate applications, very little weed
growth occurred in soybean regardless of row spacing. There were no signiﬁcant
differences in weed biomass in soybean related to treatment timing or row spacing.

In 1999, weed biomass in corn was greatest when glyphosate was applied when
weeds were 5 cm in height (Figure 1.9). There were no signiﬁcant differences in weed
biomass due to row spacing. Weed growth in corn was greater than in soybean averaged
across all treatment timings, and also in the untreated control. In soybean, time of
glyphosate application did not affect weed growth in any of the row spacings, however,
weed growth was greatest when soybean were planted in 76 cm rows compared to either
19 or 38 cm rows when averaged across all treatment timings (Figure 1.9).

In 2000, neither row spacing nor time of weed control affected weed growth
following glyphosate applications in corn (Figure 1.9). However, time of weed control
and row spacing did affect weed growth following glyphosate applications in soybean.
Weed grth was greatest when soybean were planted in 76 cm rows, and weed growth
was reduced as glyphosate applications were delayed (Figure 1.9).

In 2001, there was a signiﬁcant interaction between time of glyphosate
application and row spacing for both corn and soybean at East Lansing. Biomass of
weeds emerging following glyphosate application was greatest in corn when weeds were
controlled at 5 cm in height for both row spacings (Figure 1.9). Weed biomass was also
greater in corn planted in 76 than in 38 cm row when weeds were controlled at 5 cm in

height, but was similar between row spacings at all other treatment timings. Biomass of

28

weeds emerging in soybean was greatest when weeds were controlled at 5 cm for
soybean planted in 76 cm rows. There were no signiﬁcant differences in weed growth
following glyphosate applications for soybean planted in 19 or 38 cm rows. Weed
biomass was greater when soybean were planted in 76 cm rows compared to soybean in
19 or 38 cm rows when weeds were controlled at 5 or 10 cm in height. At later treatment
timings there were no signiﬁcant differences in weed biomass due to row spacing
(Figure 1.9).

Sequential glyphosate applications in corn reduced weed biomass when the
sequential application followed the initial application to weeds 5 cm in height in each
year of this study (Table 1.7). Weed biomass was also reduced when the sequential
application followed the application the 10 cm weeds in 2000. In all other cases, weed
biomass was similar comparing corn treated with a single or sequential glyphosate
applications. While weed control was improved in some cases with sequential
glyphosate applications, yield was not increased (Table 1.6).

At Clarksville in 1998, weed biomass in corn and soybean treated with glyphosate
when weeds were 15 cm in height was very low (Table 1.8). There was no signiﬁcant
difference in weed biomass due to row spacing in either crop. In 1999, weed biomass
was greater when soybean were planted in wide versus narrow rows when glyphosate
was applied when weed reached 15 cm in height. Weed biomass was not measurable in
any other treatment. In 2001, the biomass of weeds decreased in corn as glyphosate
applications were delayed (Table 1.8). There were no signiﬁcant differences in weed

biomass due to row spacing.

29

These results showed that planting in narrow rows reduced weed growth in
soybean, but planting corn in narrow rows generally did not have this effect. Planting
corn in narrow rows did not decrease weed growth following glyphosate applications,
except at the 5 cm weed height treatment timing in 2001 at East Lansing. Johnson et al.
(1998) also reported that planting corn in narrow rows did not reduce weed biomass
following herbicide applications. Murphy et al. (1996) reported that com planted in
narrow rows reduced weed growth when com was planted at high densities. Tharp and
Kells (2001) found that reducing row spacings and increasing corn population reduced
growth of common lambsquarters. Our study did not examine the interaction between
different crop populations and row spacings.

Delaying glyphosate applications may reduce the growth of weeds following
glyphosate applications. Tharp and Kells (1999) also found that weed control increased
when glyphosate and glufosinate applications were delayed to a late POST timing
compared to an early POST timing because weed emergence and growth was less
following the late POST herbicide application. At East Lansing, delaying glyphosate
applications reduced weed biomass in corn and soybean in two of four years. However,
delaying glyphosate applications beyond 15 cm weed heights in both row spacings of
corn and 23 cm weed heights in wide row soybean did not further reduce biomass. When
soybeans were planted in narrow rows, there was no advantage to delaying glyphosate
applications. Sequential glyphosate applications reduced weed biomass in corn at the
earliest glyphosate application all four years but did not increase corn yield.

Proper timing of glyphosate applications in glyphosate resistant crops is necessary

to avoid yield losses due to early season weed interference. Our research showed that

30

corn and soybean are more susceptible to early season weed interference when grown
under stressful environmental conditions such as high weed densities. Under high levels
of weed interference and narrow rows, corn yield was reduced when weeds reached 10
cm in height. Under similar conditions soybean yield was reduced when weeds reached
15 cm in height or more when soybean was planted in 19 cm rows. Our results showed
that com and soybean planted in narrow rows growing with high levels of weed
interference are more susceptible to yield losses compared to wide row corn and soybean.
However, when weeds were controlled early, corn and soybean planted in narrow rows
yielded more than wide rows in 3 of 4 years. Under less stressful growing conditions,
corn tolerated weeds until reaching 30 cm, while soybean yields were not reduced until
after weeds exceeded 30 cm in height. Under drought conditions there was no yield
advantage to narrow rows in either corn or soybean.

Weed densities varied considerably in the four years this research was conducted.
When high weed densities occurred, yield losses were observed at earlier application
timings (smaller weed heights) than when low weed densities occurred. Combining weed
height and weed density to create a 3-dimensional parameter appeared to be the best
predictor of yield loss due to weed interference for both corn and soybean.

Soybean planted in narrow rows (either 19 or 38 cm) suppressed weed growth
following glyphosate applications. This was likely due to the earlier canopy closure and
the higher levels of light interception by soybean in narrow rows. Planting corn in
narrow rows did not greatly affect weed grth following glyphosate applications. This
may be due to the less complete canopy closure in corn compared to soybean, which

allowed weeds to survive and grow under the corn canopy. Although delaying

31

glyphosate applications reduces biomass of weeds emerging following treatment, it
increases the risk of yield losses due to weed competition. Additionally, when soybeans
were planted in narrow rows (19 or 38 cm) there was no advantage in weed control
gained by delaying glyphosate applications. Also, weeds emerging following glyphosate
applications did not reduce yield in either corn or soybean except under drought
conditions that occurred in 2001. Therefore, delaying glyphosate applications to achieve

better weed control will not generally be beneﬁcial to crop yield.

32

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Board J. E., and B. G. Harville. 1996. Growth dynamics during the vegetative period
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Bullock, D., S. Khan, and A. Rayburn. 1998. Soybean yield response to narrow rows is
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Carey, J. B., and J. J. Kells. 1995. Timing of total postemergence herbicide applications
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Classen, M.M., and RH. Shaw. 1970. Water deﬁcit effects on com. 11. Grain
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Ethredge, W. J. Jr, D. A. Ashley, and J. M. Woodruff. 1989. Row spacing and plant
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33

Jackson, L. A., G. Kapusta, and D. J. S. Mason. 1985. Effect of duration and type of
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glyphosate. Weed Technol. 10:957-962.

Krausz, R. F., B. G. Young, G. Kapusta, and J. L. Matthews. 2001. Inﬂuence of weed
competition and herbicides on glyphosate-resistant soybean (Glycine max). Weed
Technol. 15:530-534.

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

Mickelson, J. A., and K. A. Renner. 1997. Weed control using reduced rates of
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43 1 -437.

Murphy, S. D., Y. Yakubu, S. F. Weise, and C. J. Swanton. 1996. Effect of planting
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emerging weeds. Weed Sci. 44:856-870.

Nelson, K. A., and K. A. Renner. 1999. Weed management in wide— and narrow-row
glyphosate resistant soybean. J. Prod. Agric. 12:460-465.

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Padgette, S. R., K. H. Kolacz, X. Delannay, D. B. Re, D. J. LaVallee, C, N, Tinius, W. K.
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Paszkiewicz, S. R. 1997. Narrow row width inﬂuence on corn yield. In Proc. 51St Annu.
Corn and Sorghum Res. Conf., Chicago, IL. Am. Seed Trade Assoc., Washington
D. C.

Peters, E. J ., M. R. Gebhardt, and J. F. Stritzke. 1965. Interrelations of row spacing,
cultivation, and herbicides for weed control in soybean. Weeds. 13:285-289.

34

Porter, P. M., D. R. Hicks, W. E. Lueschen, J. H. Ford, D. D. Wames, and T. R.
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35

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36

Table 1.1. Treatment dates and crop and weed heights at East Lansing, MI.

 

---------- Treatmenta Timing [Weed Height (cm)]------

 

 

 

 

Year PREb 5 10 15 23 30
Date treated 1998 5/20 6/15 6/18 6/22 6/27 6/30
Days after emergenceC 1998 -3 23 26 30 35 38
GDDAE (base 42) 1998 0 409 491 618 786 877
Corn heightd 1998 0 20 25 30 58 64
Corn leaf stage 1998 0 V3 V4 V4 V6 V7
Soybean height 1998 0 10 13 15 23 ”--
Soybean leaf stage 1998 0 V2 V3 V3 V5 --
Date treated 1999 5/15 6/5 6/9 6/1 1 6/15 6/21
Days aﬁer emergence 1999 -6 15 19 21 25 31
GDDAE (base 42) 1999 0 313 458 538 640 764
Corn height 1999 0 13 18 28 33 53
Corn leaf stage 1999 0 V3 V4 V5 V6 V7
Soybean height 1999 0 8 10 13 18 23
Soybean leaf stage 1999 0 V1 V2 V2 V3 V5
Date treated 2000 5/25 6/22 6/23 6/26 6/29 7/4
Days after emergence 2000 -7 22 23 26 29 34
GDDAE (base 42) 2000 0 529 553 642 705 833
Corn height 2000 0 35 36 46 56 71
Corn leaf stage 2000 0 V6 V6 V7 V8 V9
Soybean height 2000 0 13 13 15 18 25
Soybean leaf stage 2000 0 V2 V2 V3 V4 V5
Date treated 2001 5/14 6/13 6/17 6/19 6/22 6/25
Days after emergence 2001 -9 21 25 27 30 33
GDDAE (base 42) 2001 0 366 495 561 628 703
Corn height 2001 0 10 18 23 25 36
Corn leaf stage 2001 0 V3 V4 V5 V5 V6
Soybean height 2001 0 8 10 15 18 20
Soybean leaf stage 2001 0 V2 V2 V3 V4 V4

 

 

a Glyphosate was applied at 0.84 kg ae/ha.

b Abbreviations: PRE = preemergence, GDDAE = growing degree days after crop
emergence.

c Height is reported in cm.

d Corn and soybean were planted May 19, 1998, May 14, 1999, May 25, 2000, and May
14, 2001.

e ‘--‘ indicates no treatment or measurements taken.

37

Table 1.2. Treatment dates and crop and weed heights at Clarksville, MI.

 

----------- Treatmenta Timing [Weed Height (cm)]--------

 

 

 

Year PRE” 5 10 15 23 30
Date treated 1998 5/ 19 --° -- 6/25 -- -_
Days after emergencec 1998 -4 -- -— 33 -- _-
GDDAE (base 42) 1998 0 -- -- 796 -- --
Corn heightd 1998 0 -- -- 23 -- --
Corn leaf stage 1998 0 -- -- V7 -- --
Soybean height 1998 0 -- -- 18 -- -_
Soybean leaf stage 1998 0 -- -- V5 -- --
Date treated 1999 5/ 19 -- -- 6/15 -- __
Days after emergence 1999 -5 -- -- 22 -- -—
GDDAE (base 42) 1999 0 -- -- 587 -- --
Corn height 1999 0 -- -- 13 -- --
Corn leaf stage 1999 0 -- -- V6 -- -_
Soybean height 1999 0 -- -- 15 -- --
Soybean leaf stage 1999 0 -- -- V3 -- --
Date treated 2001 5/4 6/4 6/1 1 6/14 6/19 6/22
Days after emergence 2001 -7 24 31 34 39 42
GDDAE (base 42) 2001 0 360 503 604 749 813
Corn height 2001 0 10 23 33 43 53
Corn leaf stage 2001 0 V2 V4 V5 V6 V7

 

 

a Glyphosate was applied at 0.84 kg ae/ha.

b Abbreviations: PRE = preemergence, GDDAE = growing degree days after crop
emergence.

° Corn and soybean were planted May 19, 1998, and May 17, 1999. Corn was planted
May 4, 2001.

d Height is reported in cm.

6 ‘--‘ indicates no treatment or measurements taken.

38

Table 1.3. Weed densities, and calculated competitive load values at East Lansing and
Clarksville in 1998, 1999, 2000, and 2001.3

 

 

 

 

 

 

 

ABUTHa ANGR AMBEL AMARE CHEAL SOLPT DATST Total Com Soy
Year weeds/m2 at East Lansing CLb

1998 92 18 O 29 424 0 0 563 4674 2263
1999 22 1011 0 97 140 0 0 1270 3094 1559
2000 11 10 0 39 120 0 0 180 1414 694

2001 5 194 0 25 35 55 5 319 834 491
weeds/m2 at Clarksville Corn Soy

1998 0 0 0 28 61 0 0 89 722 356
1999 0 0 1 16 391 0 0 407 3984 1638
2001 0 80 0 41 760 0 0 881 7860 3244

 

 

a Reported weed densities were measured when weed canopy height was 15 cm.

b ABUTH = velvetleaf, ANGR = annual grass (including giant foxtail, green foxtail,
yellow foxtail, bamyardgrass, and fall panicum), AMBEL = common ragweed, AMARE
= redroot pigweed, CHEAL = common lambsquarters, SOLPT = Eastern black
nightshade, and DATST = jimsonweed.

c CL = Competitive Load which is derived by summing the products of weed densities
and competitive indices (CI) for each weed. CIs for corn are ABUTH = 4.2, ANGR
(giant foxtail) = 1.2, AMBEL = 10, AMARE = 4.0, CHEAL = 10. SOLPT = 2.0 and
DATST = 4.0. CIs for soybean are ABUTH = 4.8, ANGR (giant foxtail) = 0.5, AMBEL
= 10, AMARE = 4.0, CHEAL = 4.0. SOLPT = 2.0 and DATST = 4.0.

39

Table 1.4. Monthly precipitation recorded at the Michigan State University Department
of Horticulture Teaching and Research Center, East Lansing, MI, and at the Clarksville
Horticulture Experiment Station in Clarksville, MI.

 

 

 

 

 

 

 

 

East Lansing ------------- Clarksville -------------
1998 1999 2000 2001 30-year 1998 1999 2001 30-year

Month precipitation (mm)

May 49 42 135 145 69 39 67 136 74
June 55 42 80 85 89 34 102 67 97
July 57 103 93 24 76 41 100 23 60
August 83 44 86 41 79 55 79 103 92
Total 244 231 394 254 313 168 348 329 322

 

 

40

Table 1.5. Adjusted determination coefﬁcient (R2) values for corn and soybean yield and

application timing parameters.

 

 

 

 

 

 

 

Soybean 1 Corn
Row spacing (cm)
19 38 76 38 76
DAEI 0.1437 0.0330 0.0064 0.0170 0.1661
V-stage 0.1489 0.0428 -0.0071 0.0395 0.1220
GDD (base 42) 0.1684 0.0493 -0.0060 0.0525 0.1628
Crop ht 0.1846 0.0737 -0.0098 0.0814 0.1426
Weed ht 0.2042 0.1340 -0.0131 0.1582 0.2579
CL 0.0773 0.0030 -0.0125 -0.0077 0.1288
Weed density 0.2278 0.2406 0.0445 0.1855 0.1990
DAE x CL 0.0734 -0.0023 -0.0130 -0.0076 0.1567
V-stage x CL 0.0949 0.0047 -0.0126 0.0093 0.1966
GDD (base 42) x CL 0.1091 0.0098 -0.0123 0.0046 0.1826
Crop ht x CL 0.1195 0.0191 -0.0117 0.0234 0.1822
Weed height x CL 0.1854 0.0725 -0.0061 0.0417 0.2354
DAE x weed density 0.2682 0.2566 0.0264 0.1767 0.3679
V-stage x weed density 0.2731 0.2598 0.0279 0.3147 0.3834
GDD (base 42) x weed density 0.2820 0.2817 0.0380 0.2878 0.3878
Crop ht x weed density 0.2918 0.2989 0.0323 0.3220 0.4029
Weed height x weed density 0.3012 0.3437 0.0352 0.3829 0.4430

 

1 Abbreviations: CL = Michigan competitive load, GDD = growing degree days after crop

emergence, DAE = days after crop emergence.

41

Table 1.6. Corn yield following a single glyphosate application compared to sequential

glyphosate applications.a

 

 

 

 

 

 

 

 

 

 

1998 1999 2000 2001
Weed row spacing (cm)
height 38 76 38 76 38 76 38 76
(cm)b Se C corn yield (100 kg/ha)
5 No 113ad 101a 114a 105a 101a 95a 54a 60a
5 Yes 107a 1023 113a 112a 102a 1013 63a 72a
10 No 106a 104a 103a 101a 101a 95a 78a 77a
10 Yes 103a 103a 97a 96a 105a 99a 63a 72a
15 No 96a 98a 99a 93a 109a 93a 72a 69a
15 Yes 100a 96a 98a 82a 103a 98a 74a 64a

 

aGlyphosate application rates: single application; 0.84 kg ae/ha, sequential applications;
0.84 kg ae/ha followed by 0.42 kg ae/ha.

bWeed height at initial glyphosate application timing.
cSeq = Sequential glyphosate application made when weeds in the untreated control

reached 30 cm in height.

dMeans within columns followed by different lower case letters are signiﬁcantly different
at p < 0.10 comparing corn yield within each application timing (initial weed height).

42

Table 1.7. Weed biomass in corn that received a single glyphosate application compared
to sequential glyphosate applicationsa

 

 

 

 

 

 

 

 

 

 

1998 1999 2000 2001
Weed row spacing (cm)
height 38 76 . 38 76 38 76 38 76
(cm)b Seqc corn yield (kg/ha)
5 No 52ad 81a 120a 101a 17a 13a 100a 228a
5 Yes 0b 0b 8b 8a 1b 1b 0b 45b
10 No 17a 53a 28a 5b 17a 20a 52a 59a
10 Yes 0a 22a 108a 98a 0b 3b 14a 0a
15 No 10a 3a 42a 119a 4a 1 1a 24a 26a
15 Yes 0a 15a 35a 155a 0a 1a 0a 1a

 

aGlyphosate application rates: single application; 0.84 kg ae/ha, sequential applications;
0.84 kg ae/ha followed by 0.42 kg ae/ha.

bWeed height at initial glyphosate application timing.

CSeq = Sequential glyphosate application made when weeds in the untreated control
reached 30 cm in height.

dMeans within columns followed by different lower case letters are signiﬁcantly different
at p < 0.10 comparing weed biomass within each application timing (initial weed height).

43

Table 1.8. Biomass of weeds emerging after glyphosate application and in untreated corn
and soybean in 1998 and 1999 and in corn at Clarksville in 2001.

 

 

 

 

 

 

 

 

 

 

 

 

 

--------- Time of glyphosate application [weed height (cm)]---------
5 10 15 23 30 UNT
Row Width weed biomass in corn (kg ha'l) in 1998
38 cm -- 0 bbAc -- -- 1250 aA
76 cm -- -- 9 bA -- -- 1500 aA
weed biomass in soybean (kg ha") in 1998
19 cm -- -- 0 bA -- -- 983 aA
38 cm -- -- 0 bA -- -- 1350 aA
76 cm -- -- 0.5 bA -- -- 1130aA
weed biomass in corn (kg ha") in 1999
38 cm -- -- 0 bA -- -- 3010 aA
76 cm -- -- 0 bA -- -- 3230 aA
weed biomass in soybean (kg ha") in 1999
19 cm -- -- 0 bB -- -- 3345 aA
38 cm -- -- 0 b8 -- -- 3164 aA
76 cm -- _ -- 44 bA -- -- 3929 aA
weed biomass in corn (kg ha") in 2001
38 cm 707 bA 525 bA 99 cA 9 cA 15 cA 8040 aA
76 cm 962 bA 453 cA 224 dA 30 dA 59 dA 7880 aA

 

act

indicates no treatment at this timings.
b Means within rows followed by different lower case letters are signiﬁcantly different at
p < 0.05.

c Means within columns followed by different upper case letters are signiﬁcantly different
at p < 0.05.

44

Row Spacing I 38 cm 76 cm

 

     
   

14000

A A A B A A

be ab c b c ab C A B
8400 - z '

Corn yield (kg/ha)

 

Corn yield (kg/ha)

 

Corn yield (kg/ha)

 

Corn yield (kg/ha)

 

 

 

WF 5 10 15 23 30 UNT
Timing of glyphos ate application [Weed height (cm)]

Figure 1.1. Effect of glyphosate application timing on corn yield at East Lansing in 1998,
1999, and 2000 and combined from Clarksville and East Lansing in 2001. Yields from
2001 were averaged over row spacing. WF = weed free, UNT = untreated. Yield means
with different lower case letters are signiﬁcantly different (p < 0.05) comparing treatment
timings within row spacing. Yield means with different upper case letters are
signiﬁcantly different (p < 0.05) comparing row spacing within treatment timings.

45

 

1.99,?

Corn yield (kg/ha)

 

Corn yield (kg/ha)

 

 

 

WF 15 UNT
Timing of glyphosate application [Weed height (cm)]
Figure 1.2. Effect of timing of weed control on corn yield in at Clarksville in 1998
and 1999, averaged over row spacings. WF = weed free, UNT = untreated. Yield
means with different lower case letters are signiﬁcantly different (p < 0.05).

46

Row Spacing I 19 cm I 38 cm El 76 cm

 

A6500

N

S AAB AABB AAA. AAA ,..__,..1998
5200- , ~ - ~
é aba
E

0

">1.

H

N

o

.9

a

CD

 

Soybean yield (kg/ha)

 

 

Soybean yield (kg/ha)

 

Soybean yield (kg/ha)

 

 

WF

5 1 15 23 30 UNT
Timing of glyphos ate application [Weed height (cm)]
Figure 1.3. Effect of glyphosate application timing on soybean yield at East Lansing.
WF = weed ﬁee, UNT = untreated. Data from 2001 were averaged over row spacing.
Yield means with different lower case letters are signiﬁcantly different (p < 0.05)
comparing treatment timings within row spacing. Yield means with different upper
case letters are signiﬁcantly different (p < 0.10) comparing row spacing within each
treatment timing.

47

Row Spacing I 19 cm I 38 cm 76 cm

A A B 1998
4000- A ,A A -_ , ,

5000

 

    

3000 r

2000 .. . E .... . -.. ....... . ¥ ......

Soybean yield (kg/ha)

1000 - ‘ixr _____

 

 

 

6000

4800 -

3600 -

2400 -

Soybean yield (kg/ha)

 

 

 

WF 15 UNT
Timing of glyphos ate application [Weed height (cm)]

Figure 1.4. Effect of timing of weed control on soybean yield in at Clarksville. WF =
weed free, UNT = untreated. Yield means with different lower case letters are
signiﬁcantly different (p < 0.05) comparing treatment timings within row spacing.
Yield means with different upper case letters are signiﬁcantly different (p < 0.10)
comparing row spacing within each treatment timing.

48

I Corn Soybean

1998

 

120

Yield (% of weed free)

 

Yield (% of weed free)

 

A

O

2

h-

'u

0

0

B

‘8 ‘ iii
a"!

s a.

V 1133.;
13 a.“

'2 ti

-“-’ at:

>- - 2%

 

 

 

Yield (% of weed free)

 

WF 5 10 15 23 30
Timing of glyphos ate application [Weed height (cm)]

Figure 1.5. Effect of glyphosate application timing on corn and soybean yield at East
Lansing. WF = weed free, UNT = untreated. In 1998, glyphosate was not applied to 5 and
30 cm tall weeds in soybean. Yield means with different lower case letters are
signiﬁcantly different (p < 0.10) comparing treatment timings within crops. Yield means
with different upper case letters are signiﬁcantly different (p < 0.10) comparing corn and
soybean within treatment timings.

49

I Corn Soybean
A A A A 1999

 

120
100 r
80 -

60-

Yield (% of weed free)

 

 

120
100 -
80 r
60 t
40 -

Yield (% of weed free)

 

 

 

WF 15 UNT
Timing of glyphos ate application [Weed height (cm)]

Figure 1.6. Effect of glyphosate application timing on corn and soybean yield at
Clarksville. WF = weed free, UNT = untreated. Yield means with different lower case
letters are signiﬁcantly different (p < 0.05) comparing treatment timings within each
row spacing. Yield means with different upper case letters are signiﬁcantly different (p
< 0.10) comparing corn and soybean within treatment timings.

50

—-e-— 19 cm + 38 cm --A-- 76 cm

 

 

 

 

 

 

 

 

 

 

“It
'5
8
3
‘0-
6
§
:9.
.2
>' 38 cm: y = -0.8138x + 101.12; 1?.2 = 0.3829
20 -
76 cm: y = -0.687x + 100.22; 1?.2 = 0.4430
0 I I 1 I
120 a I
,s. A“ a g 2 a Soybean
, 8 A
i I z
100 -...;,- g :F‘,‘ -. 5.1-... -3... .2. .-_ .,_-.__:_,_,_., ._ _ ,--;_' ,__ ; ... ,. t .- .-.;.; ..... . ._
éaei'tr- ‘
’3" to '- 8 ° 9 “.Ta“-~..
3380- o a o , -~--...._.-
'6 o 8 0 0 ° — I
‘2
«5 60 -
§ 19 cm; y = -0.6613x + 99.45; 12.2 = 0.3012
'6
E 40 -
T 38 cm: y =-0.5753x + 101.23; 1?.2 = 0.3437
20 -
76 cm: y = -0.l926x + 102.31; 1?.2 = 0.0352
0 I l I I
0 9 18 27 36

Weed density (1000 weeds/m2) x Weed ht (cm)

Figure 1.7. Linear regression of yield and timing of glyphosate application based on the
product of weed density and weed height of corn planted in 38 and 76 cm rows and

soybean planted in 19, 38, and 76 cm rows at East Lansing (1998, 1999, 2000, 2001)
and Clarksville (1998, 1999, and 2001).

51

Row Spacing I 19 cm I 38 cm 76 cm

 

 

 

 

as; as... 8. a... an:

 

 

A

 

{av noun—8.3... Em:

.. db. rxuauu ritmahﬁwammsutﬁ

2000
aa aa aa

grdxé gvgﬁmwawm

., ... a m... 1.18.8183.“
agcaannggn
a a. .l...;:.

a .. Eﬂgg

 

 

 

Soybean

0000000
208642
1]

Aﬁb ﬂex—09.83 E»:

37 41 49 55 64 72 77 85

37 41 49 55 64 72 77 85

 

 

 

 

_ _
0 0
2

_ _ _
0 0 0
8 6 4

as; E23235 E»:

59 66 72 82

36 43 51

Days after crop emergence

59 66 72 82

36 43 51

Figure 1.8. Effect of row spacing on light interception by com and soybean at East

Lansing. Means with different lower case letters are signiﬁcantly different (p < 0.05)

comparing row spacings within each measurement timing.

52

Row Spacing I 19 cm I 38 cm I 76 cm

120 JSoybean Com 1m
100 ‘ - ,.

 

   
 

Light interception (%)

 

 

 

 

100 -

Light interception (%)

 

 

 

35 42 49 64 70

Days after crop emergence

Figure 1.9. Effect of row spacing on light interception by com and soybean in 1998
and 1999 at Clarksville. Means with different lower case letters are signiﬁcantly
different (p < 0.05) comparing row spacings within each measurement timing.

53

Row Spacing I 19 cm I 38 cm 76 cm

 

     

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

    

%; 1:: _ Soybean Corn 1998
'55 120 _ Untreated = 1623 kg/ha Untreated = 1945 kg/ha
g A A
E 90 - a AA
.91 ab
3 601 AAA AAA AAA :3
8 30 "‘ a a
B 0 1 a gr] I am I a as}, I I I
10 15 23 5 10 15 23 30

g 1:: _ Soybean Com 1999
a 120 Untreated = 3479 kg/ha A A
m _
N
E 90 ‘ B B: B B A B B A
:5 60 ‘ .3 A A A
w: .-
0 30 ‘ a
g 0 'i I a a I a a I I

5 10 15 23 30
g 1:: _ Soybean Corn 2000
'5 120 Untreated = 3263 kg/ha Untreated = 3805 kg/ha
E 90 ‘ ‘B B 3‘ B B A
.2 60 _ ab
"3 bc AAA AA AA AA AA
3 30- a c AAA aa aa 3 aa AA
é’ 0 - aﬁ » a a a a 83 33 d —1—-—1—-—1—-l—1—-—r—1

5 10 15 23 30 5 10 15 23 30
é; 60" Soybean Corn 2001

(Untreated = 7725 kg/ha

 

 

 

Timing of glyphos ate application [Weed height (cm)]

Figure 1.10. Biomass of weeds emerging following glyphosate applications in soybean
and corn at East Lansing. Means with different lower case letters are signiﬁcantly
different (p < 0.05) comparing treatment timing within each row spacing. Means with
different upper case letters are signiﬁcantly different (p < 0.10) comparing row spacing
within each treatment timing.

54

CHAPTER 2

EFFECT OF GLYPHOSATE APPLICATION TIMING AND ROW SPACING
ON SOIL MOISTURE AND CORN YIELD
ABSTRACT

Weed interference reduces crop yield in part through competition for soil moisture.
Glyphosate resistant corn was grown in 38 and 76 cm row spacings to examine the
effects of weed competition on soil moisture and corn yield. Soil moisture was measured
at 5 depths in 18 cm increments. Weeds were controlled with glyphosate at 0.84 kg/ha
when weeds reached average canopy heights of 5, 10, 15, 23, and 30 cm. Biomass of
weeds emerging after glyphosate application decreased as applications were delayed to
taller weed heights. At East Lansing, planting com in narrow rows reduced weed
biomass emerging after glyphosate applications only at the earliest application timing (5
cm weeds). Yield losses occurred when glyphosate was applied at the 5 cm weed height,
likely due to competition from these late emerging weeds. Corn yield was also reduced
when weeds reached 23 cm in height or more at both locations prior to herbicide
application. Although delaying glyphosate applications to 23 cm weed heights reduced
corn yield, soil moisture was not reduced at any depth in this treatment compared to the
weed free control at either location. Season-long weed interference reduced soil moisture
compared to the weed free control at both locations, although the depths at which
differences occurred differed by location. At Clarksville, there were differences in
percent soil moisture from late June through early August in the shallowest measured soil

depths, and from mid-July through the end of the season in the deepest measured depths.

55

At East Lansing, season-long weed interference reduced soil moisture only at the three
shallowest measured depths. Row spacing did not greatly affect soil moisture in corn.
Nomenclature: Glyphosate; corn ‘DK 493RR’, Zea mays L.

Key words: Narrow row corn, glyphosate resistant, weed interference, weed competition.

INTRODUCTION
Weeds interfere with crops by competing for soil moisture, sunlight, and mineral
nutrients and may also produce allelochemicals. Weeds that are allowed to remain
growing within a crop reduce yield. These yield losses increase as the period of weed
duration within that crop increases (Hill and Santelmann 1969; Knake and Slife 1969;
Hall et a1. 1992; Weaver et al. 1992). The point in time where the continued presence of
weeds causes yield losses is known as the beginning of the critical period of weed
control. The time at which continued weed control efforts do not affect crop yield is the
end of this critical period (Weaver et al. 1992). This period is different for most crops
and is inﬂuenced by weed density and environmental conditions, as well as cultural
practices such as tillage and row spacing (Banks et a1. 1985; Hall et al. 1992). Knake and
Slife (1969) reported that giant foxtail interference had a greater effect on corn than on
soybean. Van Acker et al. (1993) reported that the beginning of the critical weed
removal period was earlier under high weed densities. This makes deﬁning a critical
period for weed control for even one crop a very difﬁcult task. A greater understanding
of how weeds interfere with crops is needed to enhance understanding of the critical

period of weed control.

Hall et a1. (1992) found that the beginning of the critical period of weed control

56

was highly variable and was greatly inﬂuenced by weed density and environmental
factors while the end of the critical period for weed control inﬂuenced less by
environmental factors. They stated that knowing when weeds ﬁrst begin to impact crop
yield would not likely alter weed management practices because timing of herbicide
application and cultivation is generally based on weed susceptibility or crop growth
stage. This was prior to the introduction of herbicide resistant crops. Herbicide resistant
crops generally allow for weed control through use of nonselective postemergence
herbicides. Glyphosate resistant soybean, the ﬁrst herbicide resistant crop developed
using recombinant DNA technologies, was commercially introduced in 1996 (Padgette et
al. 1995). The introduction of herbicide resistant crops has increased the importance of
knowing how long weeds that emerge with the crop can remain growing with the crop
before yield losses occur. Glyphosate has been shown to effectively control weeds 20 to
30 cm in height or more, although higher application rates are generally needed to control
larger weeds (Krauz et al. 1996; Ateh and Harvey 1999). Because glyphosate can
effectively control larger weeds, glyphosate applications in glyphosate resistant crops do
not need to be based solely upon weed susceptibility or crop growth stage. However,
glyphosate applications must be made early enough to avoid yield losses due to early
season weed interference.

To understand the critical period of weed control in corn, more information on
factors that make weeds competitive with corn is needed. One of the factors that most
limits corn production is soil moisture (Classen and Shaw 1970; Karlen and Camp 1985).
Weeds compete with crops for soil moisture, yet research reporting the inﬂuence of weed

competition on soil moisture and corn yield is limited. Soil moisture from 0 to 150 cm in

57

depth was reduced in dryland cotton growing with silverleaf nightshade compared to
cotton growing alone and yield losses were 20% greater in dryland cotton compared to
irrigated cotton competing with silverleaf nightshade at similar weed densities (Green et
al. 1988). Nightshade competing with tomato reduced soil water content in the topsoil (O
to 60 cm) but not in the subsoil (60 to 150 cm) (McGiffen et al. 1992). Soil moisture at O
to 54 cm depths was reduced in Spanish peanuts growing in the presence of weeds
compared to weed free peanuts (Hill and Santelmann 1969). Peanut yields were reduced
as the length of time that weeds were allowed to persist increased. Soil water content and
soybean yield were reduced in conventionally tilled and in no-till soybean as sicklepod
density increased from 50,000 to 200,000 plants/ha (Banks et al. 1985). Feltner et a1.
(1969) found that tall waterhemp competition for soil moisture in grain sorghum was
greatest early in the growing season at soil depths below 50 cm. Yet, Knake and Slife
(1969) found no signiﬁcant differences in soil moisture at the 0 to 60 cm depth measured
mid-season in corn and soybean when giant foxtail was removed at 8, 15, 23, and 30 cm
in height compared to weed free and untreated corn and soybean. More information is
needed on competition for soil moisture between corn and weed infestations to gain a
greater understanding of the role of soil moisture in early season weed competition. In
each of these studies row spacing remained constant and was not compared. Increasing
interest in narrow row culture adds importance to understanding what effects its adoption
will have on crop growth parameters such as soil moisture.

Narrow row spacings for corn have been shown to increase yield (Hunter et al.
1970; Nielsen 1988; Bullock 1988; Porter et al. 1997; Widdicombe 2000). Narrow rows

may also affect weed control (Teasdale 1995). It is not well known what effect narrow

58

row spacing of corn may have on soil moisture. However, narrow row spacings may
inﬂuence soil moisture because of the more equidistant distribution of crop plants.
However, if planting density is increased with the adoption of narrow rows, use of soil
moisture may increase due to a higher number of plants competing for soil moisture.
The objectives of this study were 1) to determine the effect of glyphosate
application timing on corn yield and soil moisture, 2) to quantify the effect of season-
long weed growth on soil moisture in corn, and 3) to study the effect of row spacing on

soil moisture in corn.

MATERIALS AND METHODS

To study the effects of weeds on soil moisture and corn yield, ﬁeld research trials
were conducted at two locations in 2001. These studies were conducted at the Crop and
Soil Sciences Research Farm in East Lansing, Michigan, and at the Clarksville
Horticulture Experiment Station in Clarksville, Michigan. The soil type at East Lansing
was a Capac sandy clay loam (ﬁne-loamy, mixed mesic Aerie Ochraqualfs) (47% sand,
28% silt, and 25% clay) with pH 6.5 and 3.0% organic matter. The Capac soil series is
somewhat poorly drained with moderately slow permeability (National Cooperative Soil
Survey). The soil type at Clarksville was a Lapeer sandy loam (coarse-loamy, mixed,
mesic Typic Hapludalfs) (47% sand, 28% silt, and 17% clay) with pH 6.8 and 2.0%
organic matter. The Lapeer soil series is well drained with moderate permeability
(National Cooperative Soil Survey). Fields were prepared conventionally at both sites,
which included chisel plowing in the fall followed by a ﬁeld cultivator in the spring for

seedbed preparation. Plots were fertilized with granular urea (46-0-0) at 300 kg/ha prior

59

to planting and incorporated using a ﬁeld cultivator. Fertilizer application rate was based
on soil test recommendation for a yield goal of 10,000 kg/ha.

A full season glyphosate resistant corn hybrid ('DK 493RR) was planted in 38
and 76 cm rows on May 4, 2001 at Clarksville, and on May 14, 2001 at East Lansing.
Both row spacings were planted at 77,000 seed/ha to allow a direct comparison of the
effects of row spacing on yield and soil moisture.

Glyphosate2 at 0.84 kg ae/ha plus 2% v/v ammonium sulfate was applied when
the average weed canopy height reached 5, 10, 15, 23, and 30 cm at each location. Weed
free and untreated controls were also included at each location. Corn height and growth
stage and days after crop emergence for each treatment timing are shown in Table 2.1.
Weeds were controlled in weed free treatments with a preemergence (PRE) application of
a commercial premix3 of S-metolachlor at 0.57 kg/ha plus atrazine at 0.45 kg/ha followed
by glyphosate at 0.84 kg/ha to control weeds not controlled by the PRE herbicide
application. Weed densities were determined at the 5, 10 and 15 cm weed height
treatment timings. Table 2.2 presents weed densities by species and competitive load4

when weeds were 15 cm in height.

 

l Dekalb Gentics Corp., Monsanto Corp., 800 N. Lindbergh Blvd., St. Louis, MO 63167
2 Roundup Ultra, Monsanto Co., 800 N. Lindbergh Blvd., St. Louis, MO 63167

3 Bicep Lite 11 Magnum, Syngenta Crop Protection, Inc., PO. Box 18300, Greensboro,

NC 27419

4 WeedSOF T, 362 Plant Sciences Bldg, University of Nebraska, Lincoln, NE 68583-

0915

60

Experimental Design: Experiments were arranged in a split-plot design with four
replications. Row spacing (38 and 76 cm row widths) was the whole plot. Time of
herbicide application, based on weed height, was the sub-plot. Plot size was 3 m wide by
10.7 m long. Plot width allowed for four rows of corn when planted in 76 cm rows, and
seven rows of corn when 38 cm row widths were used.

Weed biomass: Weeds that emerged following glyphosate applications, and weeds in the
untreated control, were harvested from corn plots near the end of August. All weeds
within a randomly placed 15 by ISO-cm quadrat (0.225 m2) were harvested at the soil
surface. Quadrats were placed with the longwise perpendicular to the crop row. Two
quadrats were harvested from each plot. Weed densities, by species, were also recorded,
and harvested weeds were placed in paper bags and dried in a forced air oven. Upon
drying, weeds were weighed and biomass calculated as g/mz.

Corn Yield: Corn was harvested using a plot combine with a 5-row header designed to
harvest corn planted in either 38 or 76 cm rows. The center two rows were harvested
from corn in 76 cm row widths (an area of 16 m2), and the 5 middle rows were harvested
from corn in 38 cm row widths (an area of 20 m2). Grain yield and moisture were
measured and recorded with a datalogger on the plot combine. Corn yield was adjusted to
15.5% moisture and calculated according to area harvested.

Soil moisture: Soil moisture measurements were taken using a TRIME-T35 tube access
probe. The tube access probe consisted of a cylindrical body with four spring-mounted,
curved aluminum plates located on opposite sides of the probe body that act as TDR-

wave guides, similar to a rod-probe. The tube access probe was used to measure soil

 

5 IMKO Micromultechik, GmbH Im Stock 2 D-76275, Ettlingen, Germany.

61

moisture from the inside of the one meter plastic access tubes in length used in this study.
Tubes were inserted into the soil by removing soil with augers with a diameter slightly
less than that of the access tubes to allow for good soil contact with the access tubes.
Each access tube is equipped with a metal cutting ring to ease insertion into the soil. To
insert the tubes into the soil, a steel tube designed to ﬁt inside the access tube and rest
against the cutting ring was inserted into the tubes. A metal cap was then placed on the
steel tube. Tubes were then hammered into the augured holes using a rubber mallet.
Tubes were inserted following crop emergence and were placed within one of the center
corn rows spaced evenly between two corn plants where competition for soil moisture
between corn plants would be greatest. One access tube was placed in each plot. Care
was taken to cause as little soil disturbance as possible during the insertion process.

Soil moisture measurements were taken at 5 depths (0-18, 18-36, 36-54, 54-72,
and 72-90 cm) within the corn row on a weekly basis starting when weeds reached 5 cm
in height and continuing until corn reached physiological maturity (black layer).
Measurements were recorded as percent volumetric soil moisture.

Field capacity (-33 kPa) and permanent wilting point (~1500 kPa) were estimated
(Figure 2.1) using a mathematical equation that estimates soil water potential from soil
texture (Saxton et a1. 1986). Field capacity was 26.5% and 24.2% soil moisture (v/v) at
East Lansing and Clarksville, respectively. The permanent wilting point was 14.7% and
11.4% soil moisture (v/v) at East Lansing and Clarksville, respectively.

Statistical Analysis: All data were analyzed using SAS" 8.00 software. The PROC

GLM procedure in SAS was used for analysis of variance and appropriate F-tests were

 

6 SAS Institute Inc., Cary, NC 27513

62

run to determine signiﬁcance. Soil moisture for each depth and measurement timing
were analyzed separately. Combining all depths within measurement timings often

resulted in interactions between soil depth and weed control timing.

RESULT AND DISCUSSION
Weed biomass: Biomass of weeds emerging following glyphosate applications was
much greater at Clarksville than at East Lansing. Biomass of weeds emerging in corn
following glyphosate application was greatest at the earliest treatment timings in both
row spacings at both locations and decreased as glyphosate applications were delayed
(Figure 2.2). At both locations, weed biomass from weeds emerging after glyphosate
application at 15, 23 and 30 cm weed heights were similar.

At Clarksville, there was no signiﬁcant difference in weed biomass due to row
spacing (Figure 2.2). However, biomass of weeds emerging following glyphosate
applications was greatest at the earliest application timings and decreased at later
application timings. However, weed biomass was similar when glyphosate applications
were made to weeds 15 cm in height or more. Teasdale (1998) also found that com row
spacing did not greatly inﬂuence velvetleaf biomass and Johnson et al. (1998) found that
row spacing had little impact on giant foxtail and common ragweed control.

At East Lansing, when glyphosate was applied to 5 cm weeds, late emerging
weed biomass was greater when com was planted in wide row compared to narrow rows
(Figure 2.2). However, there were no differences in weed biomass due to row spacing at
any other glyphosate application timing. These ﬁndings were similar to those of Murphy

et al. (1996) who found that narrow row spacings reduced the biomass of late emerging

63

weeds and those of Teasdale (1995) who found that weed control using reduced herbicide
rates was more consistent when using narrow rows. Tharp and Kells (2001) also found
greater weed control using late-postemergence applications compared to early-
postemergence applications of glyphosate or glufosinate in herbicide resistant corn.

Our results showed that weeds will emerge following glyphosate application to 5
cm weeds in corn. Weed growth following glyphosate applications may be reduced in
narrow compared to wide row corn at this early application timing. If glyphosate
application is delayed until weeds reach 23 or 30 cm fewer weeds will emerge and weed
growth will be reduced but this delay in herbicide application increases the risk of yield
loss due to weed interference. However, delaying glyphosate applications until weeds
reached 15 cm in height (6 to 10 days after the 5 cm weed height) also reduced weed
growth but did not negatively affect crop yield.

Corn yield: Corn yield was reduced at both locations when weeds were allowed to reach
23 cm or more in height (Figure 2.3). Conventional corn yield was reduced if weeds
were allowed to reach 23 cm or more in one of three years in Illinois (Knake and Slife
1969). Carey and Kells (1995) and Tharp and Kells (2001) found that delaying herbicide
applications to late-postemergence timings increased the risk of yield loss due to weed
interference in Michigan.

Yield loss also occurred when glyphosate was applied to weeds 5 cm in height
(Figure 2.3). This yield loss may be attributed to interference from weeds emerging after
glyphosate was applied (Figure 2.2).

No signiﬁcant differences in yield occurred due to row spacing (data not shown).

Narrow rows have been shown to increase corn yield (Bullock et al. 1988; Nielsen 1988;

64

Murphy et al. 1996), or to have no effect on yield (Johnson et al. 1998; Teasdale 1998).
In this study, drought conditions limited corn production at both locations. Drought
conditions occurred due to a dry period beginning in late June and lasting through early
August. Only 23 mm of rain fell over a 48 day period (June 22 through August 8) at
Clarksville and 29 mm of rain fell at East Lansing over a 54 day period (June 23 through
August 15), which also coincided with corn pollination in this study. Corn has been
shown to be most susceptible to drought stress during pollination; in some cases yield
losses of 40-50% can occur (Classen and Shaw 1970). Porter et al. (1997) also found that
yield advantages gained by planting in narrow rows occurred only under optimal growing
conditions. Yield gains for narrow rows are generally attributed to increased light
interception through increased leaf area index allowing for less intraspeciﬁc competition
for sunlight (Bullock et al. 198 8). Furthermore, the spacing between corn plants affects
photosynthate allocation due to differences in far-red light reﬂection and absorption
(Kasperbauer and Karlen 1994). They found that plants receiving a higher ratio of far-
red to red light due to closer spacing developed longer leaves and stems and a less
massive root system. In this study soil moisture was likely more limiting than
photosynthetic efﬁciency. Therefore, the stresses caused by drought may have
eliminated any photosynthetic advantage normally gained by adopting more equidistant
row spacings.

Soil moisture: Due to differences in soil type, and rainfall, soil moisture is presented
separately for each location. At Clarksville, weed competition reduced soil moisture in
weedy compared to weed free corn at the upper soil depths early in the season and at the

lower soil depths later in the season. Soil moisture was reduced by weeds at the 0 to 18

65

cm depth from 52 to 94 DAP (Figure 2.4). At 97 DAP (August 9); 37 mm of rain fell at
Clarksville increasing soil moisture. After this point, soil moisture at the 0-18 cm depth
was similar in the weed free and weedy corn for the remainder of the growing season due
to more frequent rainfall. At the 18-36 cm depth, soil moisture was signiﬁcantly less in
weedy corn than in weed free corn only at 59 DAP and there were no differences in soil
moisture at any point in the growing season at the 36-54 cm depth. At the lowest two
measured depths, soil moisture was signiﬁcantly reduced by weeds beginning at 87 DAP
at 54-72 cm and at 77 DAP at 72-90 cm. Soil moisture continued to be less in weedy
than in weed free corn at these soil depths throughout most of the remainder of the
growing season (Figure 2.4). Corn growth was greatly impaired in untreated plots, in
many cases failing to produce a tassel or ear. Reduced soil moisture at the lower soil
depths was likely due to increased soil moisture depletion due to the presence of weeds in
these plots.

In contrast, at East Lansing, soil moisture was reduced in weedy corn, compared
to weed free corn, at many sampling dates throughout the growing season at the 0-18, 18-
36, and 36-54 cm soil depth (Figure 2.5). However, weeds did not reduce soil moisture at
the 54-72 or 72-90 cm depth (with the exception of one sampling date at the 54-72 cm
depth).

The results at Clarksville were similar to those of Green et al. (1988) who found
that differences in soil moisture in dryland cotton growing with and without silverleaf
nightshade in a ﬁne dandy loam soil occurred ﬁrst in the upper part of the soil proﬁle. At
this time little or no change in soil moisture occurred at lower depths. Soil moisture was

also reduced at lower soil depths later in the season and continued to be less throughout

66

the remainder of the growing season. F eltner et al. (1969) found the greatest differences
in soil moisture due to tall waterhemp interference in grain sorghum in a silt loam soil
was at the 36-80 cm depths, and differences in soil moisture were greatest from early
June through mid-August.

At East Lansing, competition for soil moisture was greatest at the upper soil
depths, and was not observed deep in the soil proﬁle. Similarly, black nightshade and
Eastern black nightshade growing with tomatoes in a silt loam soil reduced soil moisture
in the top 60 cm of soil while reductions in soil moisture were not observed at 60-150 cm
depths (McGiffen et al. 1992). The differences in results between these two locations
may be due to differences in weed composition and density or drainage differences
related to subsoil characteristics. The most prevalent weed at Clarksville was common
lambsquarters, while the most prevalent weed at East Lansing was giant foxtail, and weed
density was nearly three times greater at Clarksville than at East Lansing (Table 2.2).
The soil type at Clarksville was a Lapeer sandy loam, which is well drained due to the
sandy loam subsoil that generally extends down to at least 1.5 meters (National
Cooperative Soil Survey). This differs from the Capac loam soil at East Lansing which is
somewhat poorly drained due to the loam or clay loam subsoil which generally has a
seasonal high water table (15-45 cm below the soil surface) occurring sometime between
October and May (National Cooperative Soil Survey). Additionally, the East Lansing
ﬁeld site contains drainage tile. These differences in soil type and weeds likely
contributed to the differences in soil moisture response to weed competition at these two

locations.

67

Effect of glyphosate application timing on soil moisture: Corn yield was reduced when
weeds were allowed to reach 23 cm or more (Figure 2.2). Weeds emerging after
glyphosate applications to 5 cm weeds also reduced corn yield. One of the main
objectives of this study was to determine the effects of early season weed interference on
soil moisture in corn. At Clarksville, no differences in soil moisture were found at 35
DAP (when weeds were between 5 and 10 cm in height) for any treatment timing at any
soil depth (data not shown). Signiﬁcant differences in soil moisture did not occur until
52 DAP (6 days after glyphosate was applied to 23 cm tall weeds and 3 days after
glyphosate was applied to weeds 30 cm in height). At this time soil moisture was 2.4%
less in weedy than in weed free corn at the 0-18 cm depth, but soil moisture was not
signiﬁcantly reduced in any other treatment compared to the weed free control (Figure
2.6). While soil moisture was not signiﬁcantly reduced prior to the 23 or 30 cm weed
height glyphosate application timings, weeds were apparently competing for soil
moisture prior to glyphosate application to these larger weeds as soil moisture was
becoming depleted in the upper 18 cm of soil. However, soil moisture continued to be
plentiful due to frequent rainfalls (48 mm of rain fell during the period when weeds grew
from 5 cm to 30 cm; 31-49 DAP).

Following the glyphosate application to 30 cm tall weeds (49 DAP) at Clarksville,
rainfall was less frequent. Over the following 46 days only 24 mm of rain fell at
Clarksville resulting in very dry soil conditions. Soil moisture in the upper 36 cm of soil
was reduced below the estimated permanent wilting point (11.4%) from 77 to 94 DAP in
weed free corn plots with soil moisture being reduced even more in the untreated control

(Figure 2.4). However, delaying glyphosate applications until weeds reached 23 or 30

68

cm in height did not result in reduced soil moisture in corn compared to the weed free
control (Figure 2.6). Soil moisture was reduced compared to the weed free control when
glyphosate was applied to 5 cm tall weeds at the 0-18 cm depth at 77 and 94 DAP, and at
the 54-72 and 72-90 cm soil depths at corn physiological maturity (136 DAP). This
depletion in soil moisture was likely due to weed growth following glyphosate
application (Figure 2.2). Soil moisture was greater in corn when glyphosate was applied
at the 23 and 30 cm weed heights than in the weed free control at the 72-90 cm depth at
77 DAP and at the 54-72 cm depth at 94 DAP (Figure 2.6). Pavlychenko (1937) found
that crops competing with weeds had smaller root systems. Greater soil moisture in corn
when weeds were allowed to reach 23 or 30 cm may be due to reduced corn rooting depth
due to weed interference prior to glyphosate applications.

At East Lansing, the effect of glyphosate treatment timing on soil moisture
differed from what occurred at Clarksville. Soil moisture was generally similar for all
glyphosate treatment timings, which were generally similar to soil moisture in the weed
free control (Figure 2.6).

Difference in the way that soil moisture responded to timing of glyphosate
applications at these two locations may be due to differences in weed growth following
applications. Biomass of weeds emerging following glyphosate applications at the 5 cm
weed height timings was much greater at Clarksville than at East Lansing (Figure 2.2),
and consequently the effect of these late emerging weeds on soil moisture was also
greater at Clarksville than at East Lansing.

Delaying glyphosate applications until weeds reached 23 or 30 cm in height did

not cause soil moisture to become depleted compared to the weed free control at either

69

location. Soil moisture was not reduced in weedy corn compared to weed free corn until
weeds had exceeded 30 cm in height. However, when glyphosate was applied to 5 cm
tall weeds, soil moisture was reduced at Clarksville compared to the weed free control,
likely due to late season weed emergence.

Effect of row spacing on soil moisture: There was greater soil moisture in wide rows at
Clarksville from 77 to 143 DAP only at the 18-36 cm depth (Figure 2.8). In contrast,
there was no difference in soil moisture due to row spacing at East Lansing at this depth
(Figure 2.9). However, at the 0-18 cm depth at East Lansing soil moisture was greater in
76 cm rows from 91-137 DAP. Soil moisture measurements were taken within the crop
row. Corn planted in wide rows would be expected to have increased competition for soil
moisture between corn plants due to higher density within the row which would reduce
soil moisture more than corn planted narrow rows that are spaced further apart within the
row. However, corn planted in narrow rows may also be competing more strongly for
soil moisture with corn plants in adjacent rows than corn planted in wide rows. Corn
roots have been shown to extend horizontally 90-120 cm, and vertically to depths of 1.8-
2.1 m (Miller 1916; Weaver et al. 1922). This exceeds the distance between both row
spacings of corn in this study indicating that com roots likely were interlapping both in
the narrow and wide rows. However, corn planted in narrow rows likely had increased
interlapping of corn roots from adjacent rows due to their proximity that was likely
responsible for reducing soil moisture to a greater extent compared to corn in wide row
spacings. Even greater differences in soil moisture may have been found if measurements
had been taken half-way between rows instead of from within the crop row. This would

have increased the average distance of the nearest corn plant ﬁom the TDR-access tube

70

from 83 to 380 mm for corn planted in 76 cm rows, whereas the average distance would
have only increased from 166 to 190 mm in corn planted in 38 cm rows. However, soil
moisture was measured within the row in this study because this corresponds with the
location of the highest density of corn roots and subsequently would be the best indicator
of soil moisture availability as corn roots absorb water more rapidly near the plant than at
a distance (Davis 1940).

In this study, differences in soil moisture were not detected early in the season.
This would suggest that weed competition for soil moisture was not the primary factor
responsible for observed yield losses due to early season weed interference (Figure 2.2).
Yield losses that occurred when glyphosate was applied to 5 cm tall weeds are likely due
to interference from late emerging weeds. While weed competition did reduce soil
moisture, yield losses in corn with season-long weed interference cannot be attributed
entirely to reduced soil moisture. Soil moisture in corn where weeds were controlled at 5
cm in height was similar to that in the weedy control for most of the growing season (data
not shown), yet yield losses in the weedy control were nearly 95% while yield loss when
glyphosate was applied to weeds 5 cm in height was only 9%.

While differences in soil moisture were not apparent early in the season in this
study, higher than normal rainfall occurred during this period. In a season with lower
than normal rainfall, differences in soil moisture would likely occur earlier. Further
investigation into components responsible for yield loss due to early season weed
interference in corn is needed. Greater knowledge in this area would help in predicting
yield losses due to weed interference and in formulating recommendations for weed

control in herbicide resistant crops to avoid these yield losses.

71

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glyphosate-resistant soybean (Glycine max). Weed Tech. 13: 394-398.

Banks, P. A., T. N. Tripp, J. W. Wells, and J. E. Hammel. 1985. Effects of tillage on
sicklepod (Cassia obtusifolia) interference with soybeans (Glycine max) and soil
water use. Weed Sci. 34:143-149.

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. 25:254-
258.

Carey, J. B., and J. J. Kells. 1995. Timing of total postemergence herbicide applications
to maximize weed control and corn yield. Weed Technol. 91356-361.

Classen, M. M. and R. H. Shaw. 1970. Water deﬁcit effects on com 11. Grain
component. Agron. J. 622652

Davis, C. H. 1940. Absorption of soil moisture by maize roots. Botan. Gaz. 101:791-
805.

F eltner, K. C., H. R. Hurst, and L. B. Anderson. 1969. Tall waterhemp competition in
grain sorghum. Weed Sci. 34:214-216.

Green, J. D., D. S. Murray, and J. F. Stone. 1988. Soil water relations of silverleaf
nightshade (Solanum elaeagnifolium) with cotton (Gossypium hirsutum). Weed
Sci. 36:740-746.

Hall, M. R., C. J. Swanton, and G. W. Anderson. 1992. The critical period of weed
control in grain corn (Zea mays). Weed Sci. 40: 441-447.

Hill, L. V., and P. W. Santelmann. 1969. Competitive effects of annual weeds on
Spanish peanuts. Weed Sci. 17:1-2.

Hunter, R. B., L. W. Kannenberg, and E. E. Gamble. 1970. Performance of ﬁve maize
hybrids in varying plant populations and row widths. Agron. J. 62:255-256.

Johnson, G. A. , T. R. Hoverstad, and R. E. Greenwald. 1998. Integrated weed

management using narrow row corn spacing, herbicides, and cultivation. Agron.
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Karlen, D. L., and C. R. Camp. 1985. Row Spacing, Plant population, and water
management effects on corn in the Atlantic coastal plain. Agron. J. 77:393-398.

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Kasperbauer, M. J ., and D. L. Karlen. 1994. Plant spacing and reﬂected far-light effects

on phytochrome-regulated photosynthate allocation in corn seedlings. Crop Sci.
34:1564-1569.

Knake, E. L., and F. W. Slife. 1969. Effect of time of giant foxtail removal from corn
and soybeans. Weed Sci. 17:281-283.

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

McGiffen, M. E., Jr., J. B. Masiunas, and M. G. Huck. 1992. Tomato nightshade
(Solanum nigrum L. and S. ptycanthum Dun.) effects on soil water content. J.
Amer. Soc. Hort. Sci. 117:730-735.

Miller, E. C. 1916. Comparative study of the root system and leaf areas of and the
sorghums. J. Ag. Res. 9:311-332.

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

National Cooperative Soil Survey. Soil Survey Division, Natural Resources
Conservation Service, United States Department of Agriculture. Ofﬁcial Soil
Series Descriptions [Online WWW]. Available URL:
"http://www.statlab.iastate.edu/soils/osd/" [Accessed 3 June 2002].

Nielsen, R. L. 1988. Inﬂuence of hybrids and plant density on grain yield and stalk
breakage in corn grown in 15-inch row spacing. J. Prod. Agric. 1: 190-195.

Padgette, S. R., K. H. Kolacz, X. Delannay, D. B. Re, D. J. LaVallee, C, N, Tinius, W. K.
Rhodes, Y. I. Otero, G. F. Barry, D. a. Eichholtz, V. M. Peschke, D. L. Nida, N.
B. Taylor, and G. M. Kishore. 1995. Development, identiﬁcation, and
characterization of a glyphosate-tolerant soybean line. Crop Sci. 35: 1451-1461.

Pavlychenko, T. K. 1937. Quantitative study of the entire root systems of weed and crop
plant under ﬁeld conditions. Ecology. 18:62-79.

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.

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73

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

Teasdale, J. R. 1998. Inﬂuence of corn (Zea mays) population and row spacing on corn
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Weaver, S. E., M. J. Kropff, and R. M. W. Groeneveld. 1992. Use of ecophysiological
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planting date on corn (Zea mays L.) grain and silage production in Michigan.
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74

Table 2.1. Treatment dates, crop height and grth stage and average weed canopy
height at each treatment timing at East Lansing and Clarksville, MI.

-------- Treatment Timing (Weed Height (cm))------

 

 

Location PRE 5 10 15 23 30
Date treated CHES 5/4 6/4 6/1 1 6/14 6/19 6/22
Days after planting CHES 0 31 ' 38 41 46 49
Corn height CHES 0 10 23 33 43 53
Corn leaf stage CHES 0 V2 V4 V5 V6 V7
Date treateda ELANb 5/ 14 6/ 13 6/17 6/19 6/22 6/25
Days after planting ELAN 0 30 34 36 39 42
Corn heightC ELAN 0 10 18 23 25 36
Corn leaf stage ELAN 0 V3 V4 V5 V5 V6

 

3 Corn was planted May 14, 2001 at East Lansing, and May 4, 2001 at Clarksville.
b ELAN = East Lansing, CHES = Clarksville.
c Height is reported in cm.

75

Table 2. 2. Weed densities and calculated competitive load at East Lansing, and
Clarksville in 2001.21

 

 

 

 

ANGRT’ CHEA AMAR ABUT SOVLPT DATST TOTAL CLC
Location , weeds/m“
ELAN 194 35 25 5 55 5 319 834
CHES 80 760 41 O 0 0 881 7860

 

 

a Reported weed densities were measured when weed canopy height was 15 cm.

b ANGR = annual grass (including giant foxtail, green foxtail, yellow foxtail,
bamyardgrass, and fall panicum), CHEAL = common lambsquarters, AMARE = redroot
pi gweed, SOLPT = Eastern black nightshade, and DATST = jimsonweed.

c CL = competitive load which is derived by summing the products of weed densities and
competitive indices (CI) for each weed. CIs used for corn in Michigan are ABUTH =
4.2, ANGR (giant foxtail) = 1.2, AMBEL = 10, AMARE = 4.0, CHEAL = 10. SOLPT =
2.0 and DATST = 4.0.

d ELAN = East Lansing, CHES = Clarksville.

76

 

Clarksville

 

 

Field Capacity = 24.2%

 

Permanent Wilting Point = 11.4%

1

 

/

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

-2100 -1800 -1500 -1200 -900 -600 -300 0
East Lansing
Field Capacity = 26.5%
Permanent Wilting Point = 14.7%
/
g t
-2100 -1800 -1500 -1200 -900 -600 -300 0

Soil water potential (kPa)

45
40
35
30
25
20
15
10

45
40
35
30
25
20
15
10

Soil moisture (% v/v)

Soil moisture (% v/v)

Figure 2.1. Predicted soil moisture retention curves for soil textures at East Lansing and

Clarksville using a mathematical equations for estimating soil water potential from soil

texture (Saxton, et a1 1986).

77

Row Spacing I 38 cm @ 76 cm
1200

 

Clarksville

Untreated = 7960 kglha

    

1050 -

900 -

750 r

600 -

450 ~

Weed biomass (kg/ha)

300 -

 

 

280

 

  

East Lansing
245 -
Untreated = 7725 kglha
210 -
175 .
140 -

105 ~

Weed biomass (kg/ha)

70-

 

 

 

Glyphos ate application timing [Weed height (cm)]

Figure 2. 2. Biomass of weeds emerging following glyphosate applications at

C larksville and East Lansing. Biomass means with different lower case letters are
signiﬁcantly different (p > 0.05) for comparisons of application timing within each row
spacing. Biomass means with different upper case letters are signiﬁcantly different (p >
0.05) for comparisons of row spacing within each treatment timing.

78

 

 

12000
a a Clarksville

10000 a

8000 -

6000 -

4000 -

Weed biomass (kg/ha)

 

 

East Lansing

6000 -

4000 *

Weed biomass (kg/ha)

2000 -

 

 

 

WF 5 10 15 23 30 UNT
Glyphos ate application timing [Weed height (cm)]

Figure 2.3. Corn yield averaged across row spacing, Clarksville and East Lansing. Yield
means with different lower case letters are signiﬁcantly different. Fisher’s Protected LSD
(or = 0.10) procedures were used for means separation.

79

- A "Weed Free +Weedy

Volumetric Soil Moisture (% v/v)

54-72 cm

 

 

 

 

 

 

10

5 72-90 cm
A 0
5 4
g 2 I I
C.-
-§ 0 .lllll . .ul L-“l1.l I.L_.__I.I_L
m I ﬁ I I If I

31 45 59 73 87 101 115 129 143

Days after Planting

Figure 2.4. Soil moisture in weed free and weedy corn averaged across row spacing at
Clarksville at 0-18, 18-36, 36-54, 54-72, and 72-90 cm depths and rainfall during the
same time period. Asterisks denote a signiﬁcant difference in soil moisture at p < 0.10.

80

- A -Weed Free +Weedy

Volumetric Soil Moisture (% v/v)

 

 

 

 

 

 

28 42 56 70 84 98

Rainfall (cm)

4

2 7 n

0-LI_I_L|I .. - III I I L [4%] - TJI - lll|_l_l‘_r
112 126 140

Days after Planting

Figure 2.5. Soil moisture in weed free and weedy corn averaged across row spacing at
East Lansing at 0-18, 18-36, 36-54, 54-72, and 72-90 cm depths and rainfall during the
same time period. Asterisks denote a signiﬁcant difference in soil moisture at p < 0.10.

81

Time of glyphosate application [Weed height (cm)]
I5cm I23cm I30cm EiWeedFree 136DAP

 

LSD=NS LSD=NS LSD=NS LSD=2.3 LSD=2.8

 

 

 

94 DAP
LSD=1 .8 LSD=2.3 LSD=NS LSD=2.8 LSD=2.9

 

 

 

 

 

 

77 DAP
LSD=1 .5 LSD=NS LSD=NS LSD=NS LSD=2.8

 

 

 

 

Volumetric Soil Moisture (% v/v)

 

 

 

35 52 DAP
LSD=NS LSD=NS LSD=NS LSD=NS LSD=NS

 

      

 

18-36 36-54 54-72
Soil depth (cm)

Figure 2. 6. Soil moisture at Clarksville averaged across row spacing in weed free corn
and when weeds were controlled using glyphosate when 5, 23, and 30 cm in height.
Fishers Protected LSD (or = 0.10) was used for means separation.

82

Time of glyphosate application [Weed height (cm)]
I5cm I23 cm B30cm IWeedIh'ee

137 DAP

 

LSD=NS LSD=NS LSD=2.2 LSD=NS

LSD=NS

 

40

 

98 DAP

 

LSD=NS LSD=NS LSD=NS LSD=NS

LSD=NS

 

 

 

71 DAP

 

LSD=NS LSD=NS LSD=NS LSD=NS

LSD=NS

 

 

H N (A)
O\ A N C c 00 a A N
1 l 1

HNWA

 

Volumetric Soil Moisture (% v/v)

 

 

 

 

     
    

LSD=1.4 LSD=NS

 

 

 

24
16 -
8 _
0 _
0-18 18-36 36—54 54-72
Soil depth (cm)

  

SD=NS

try-:2-
. . .1
L .

Figure 2. 7. Soil moisture at East Lansing averaged across row spacing in weed free
corn and when weeds were controlled using glyphosate when 5, 23, and 30 cm in

height. Fishers Protected LSD (a = 0.10) was used for means separation.

83

Row Spacing - -A- - 38 cm +76 cm

18-36 cm

Volumetric Soil Moisture ("/o v/v)

 

 

 

 

 

 

 

- -A
12
A 3 72-90 cm
a 4
g 2 I l
G...
’g 0 A llllll In '14.. I Irljll1- I‘l'i Ill
1::
31 45 59 73 87 101 115 129 143

Days after Planting

Figure 2. 8. Soil moisture in weed free corn in 38 and 76 cm row spacings at Clarksville
at 0-18, 18-36, 36-54, 54-72, and 72-90 cm depths and rainfall during the same time
period. Asterisks denote a signiﬁcant difference in soil moisture at p < 0.10.

84

Row Spacing - -A- - 38 cm +76 cm

Volumetric Soil Moisture (% v/v)

 

 

 

 

 

 

Rainfall (cm)

4

2 - I

0_L|I|I|I - r- all] 1.‘ lg] A 'J.'L,
112 126 14

28 42 56 70 84 98 0

Days after Planting

Figure 2. 9. Soil moisture in weed free corn in 38 and 76 cm row spacings at East
Lansing at 0-18, 18-36, 36-54, 54-72, and 72-90 cm depths and rainfall during the same
time period. Asterisks denote a signiﬁcant difference in soil moisture at p < 0.10.

85