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(936170

This is to certify that the
dissertation entitled

TWO SOYBEAN CROPPING SYSTEM CHALLENGES —
MANGANESE FERTILIZER ANTAGONISM OF
GLYPHOSATE, AND SCHEDULING IRRIGATION TO
INCREASE SOYBEAN YIELD

presented by

MARK L. BERNARDS

has been accepted towards fulﬁllment
of the requirements for the

Ph. D. degree In CROP AND SOIL SCIENCE

 

 

Kim.

Major Professor's Signature

57/2/12?

Date

MSU is an Afﬁrmative Action/Equal Opportunity Institution

 

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PLACE IN RETURN Box to remove this checkout from your record.
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TWO SOYBEAN CROPPING SYSTEM CHALLENGES -
MANGANESE FERTILIZER ANTAGONISM OF GLYPHOSATE,
AND SCHEDULING IRRIGATION TO INCREASE SOYBEAN YIELD
By

Mark L. Bernards

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

2004

ABSTRACT
TWO SOYBEAN CROPPING SYSTEM CHALLENGES -
MANGANESE FERTILIZER ANTAGONISM OF GLYPHOSATE,
AND SCHEDULING IRRIGATION TO INCREASE SOYBEAN YIELD
By

Mark L. Bemards

Michigan soybean (Glycine max) producers have observed antagonism of glyphosate
efﬁcacy in tank-mixtures with foliar manganese (Mn) fertilizers. The effects of four Mn
fertilizer formulations on glyphosate efﬁcacy, absorption, and translocation were
evaluated. The fertilizers, Mn-lignin sulfonate (Mn-LS) and manganese sulfate (MnSO4),
antagonized glyphosate efﬁcacy by reducing glyphosate absorption and translocation.
Mn-ethylaminoacetate (Mn-BAA) antagonized glyphosate efficacy by reducing
translocation. Mn-EDTA did not antagonize glyphosate efﬁcacy nor affect absorption
and translocation. Adding ammonium sulfate (AMS) increased the efﬁcacy, absorption,
and translocation of glyphosate for each Mn fertilizer tank-mixture, but did not
consistently overcome the antagonism. Citric acid and EDTA also increased glyphosate
efﬁcacy, but were no more effective than AMS. Mn-EAA, applied less than 3 d before
glyphosate, reduced glyphosate efficacy on velvetleaf (Abutilon theophrasti) but not giant
foxtail (Setariafaberi) or common larnbsquarters (Chenopodium album). Glyphosate
efficacy declined as the amount of Mn in the tank-mixture was increased from Mn-EAA,
Mn-LS, and W804. Electron Paramagnetic Resonance (EPR) spectra showed that
glyphosate coordinates with Mn2+ in aqueous solution. Producers should be cautious
when tank-mixing glyphosate and Mn fertilizers, especially when environmental

conditions favor poor weed control. A fully chelated Mn-EDTA formulation (1 :1 molar

ratio) is the least likely to reduce glyphosate efﬁcacy. AMS at 2% (w/w) is needed for
maximum glyphosate efﬁcacy.

Soybean producers with center-pivot irrigation systems in the Great Lakes region
have reported inconsistent yield response to irrigation. Five irrigation schedules, based
on soybean growth stage and the soil moisture, were evaluated for their effect on soybean

grain yield. Deﬁcit and full season treatments were irrigated to maintain volumetric soil

. .fil'n

moisture (V SM) at 25% and 50% of capacity, respectively, for the entire growing season.
The grth stage treatments of ﬂowering (R1-R2), pod development (RB-R4), and seed

ﬁll (RS-R6), were irrigated to maintain VSM at 25% of capacity before reaching the

 

designated growth stage, and at 50% of capacity thereafter. Soil moisture was measured
weekly to a depth of 0.9 m using a TDR technology. Yields of soybean in the full
season, ﬂowering, and pod development treatments were statistically equal each year.
The yield of soybeans irrigated beginning at seed ﬁll was less than that of the full season
treatment, but greater than that of the deﬁcit irrigation treatment. Soybean yield
increases were related to increased numbers of seeds and pods per plant, and increased
seed size. In 2003, yields of eight cultivars responded to the ﬁve irrigation schedules
similarly. The irrigation schedule did not affect full season weed control when
glyphosate was applied twice, but when glyphosate was applied only once, weed control
was reduced for treatments that were not irrigated until after ﬂowering. Soybean
producers in the Great Lakes Region may effectively increase yield of irrigated soybean
by planting high-yielding varieties, and irrigating to maintain soil moisture levels above

50% of capacity, beginning at ﬂowering or pod development.

Copyright © Mark L. Bemards

All Rights Reserved

A......- -n‘ .._.R..

 

Dedicated to my wife, Alysson,

and to my parents, Dennis and Patience

" ‘. .1".

 

ACKNOWLEDGEMENTS

I have been richly blessed by the association of many outstanding mentors and
colleagues. I would especially like to thank Dr. Kurt Thelen for inviting me to be his
student, providing ﬁnancial support, encouraging me to gain new skills and share my
research, trusting me to get the job done, and giving me the freedom to engage in many
service and leadership activities.

A special thank you to my committee: Dr. Donald Penner - for sharing knowledge,
research ideas, stories, and laughs; Dr. Christine DiFonzo — for letting your creative spirit
shine in the classroom and expressing conﬁdence in my abilities; Dr. Douglas Buhler —
for having a minute when I needed advice, and for asking thought-provoking questions;
and Dr. Ted London -—— for providing needed insight on how to conduct irrigation research.

I am grateful to many for their willingness to help with my research, namely, Bill
Widdicombe, Keith Dysinger, Brad Fronning, John Boyse, Amanda Borel, Pingping
Jiang, Jan Michael, Brian Long, Mike Boring, Rachel McGuire, Matt Lund, Erin Bosch,
Travis Canﬁeld, Heather Holdaway, Teresa Koppin, Chad Lee, Karen Renner, Gary
Powell, Anatoliy Kravchenko, and Dave Francis and the staff of SWMREC.

And a heartfelt thank you — To Rene Scoresby, for introducing me to MSU. — To Jim
Kells, for a great sales pitch. — To my fellow graduate students, for your friendship and
interest. — To the faculty and staff of the Department of Crop and Soil Science. — To good
roommates and friends. — To my family, for being supportive and excited about the
opportunities I was given. — To my dear wife, Alysson, for always believing that I would
succeed. — And to my Heavenly Father, for sustaining me throughout my program and for

preserving my life when I was on a roll.

vi

TABLE OF CONTENTS

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

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

CHAPTER 1

GLYPHOSATE EFFICACY IS ANTAGONIZED BY MANGANESE .......................... 1
Abstract ................................................................................................................... 1
Introduction ............................................................................................................. 2
Materials and Methods ............................................................................................ 4
Results and Discussion ............................................................................................ 8
Literature Cited ...................................................................................................... 15

CHAPTER 2

GLYPHOSATE COORDINATION WITH MANGANESE IN SOLUTION AND ITS

EFFECT ON GLYPHOSATE ABSORPTION AND TRANSLOCATION ..................... 29
Abstract .................................................................................................................. 29
Introduction ............................................................................................................ 30
Materials and Methods .......................................................................................... 34
Results and Discussion .......................................................................................... 38
Literature Cited ...................................................................................................... 46

CHAPTER 3

IRRIGATION SCHEDULING TO INCREASE SOYBEAN YIELD IN THE GREAT

LAKES REGION ............................................................................................................... 58
Abstract .................................................................................................................. 58
Introduction ............................................................................................................ 59
Materials and Methods ........................................................................................... 63
Results and Discussion .......................................................................................... 68
Literature Cited ...................................................................................................... 78

vii

LIST OF TABLES

CHAPTER 1
Table 1. The effect of Mn formulation, glyphosate salt, and ammonium sulfate on control
of giant foxtail and velvetleaf 14 d after treatment in a greenhouse bioassay ....... 17
Table 2. The effect of mixing order in preparing glyphosate-Mn tank-mixtures on control
of giant foxtail and velvetleaf 14 d after treatment in a greenhouse bioassay ....... 18
Table 3. The effect of adjuvants and Mn formulation on velvetleaf and giant foxtail
control 14 d after glyphosate application in a greenhouse bioassay ...................... 19
Table 4. The effect of glyphosate rate, Mn rate, and ammonium sulfate on control of
velvetleaf 14 d after treatment in greenhouse bioassays ........................................ 20
CHAPTER 2
Table 1. The interaction of Mn fertilizer formulation and ammonium sulfate on
velvetleaf control in glyphosate tank-mixtures ...................................................... 49
Table 2. The effect of Mn formulation, ammonium sulfate, and time on absorption of
l4C-glyphosate by velvetleaf ................................................................................. 50
Table 3. The effect of Mn formulation and time on translocation of l4C-glyphosate from
tank-mixtures prepared without ammonium sulfate .............................................. 51
Table 4. The effect of Mn formulation and time on translocation of 14C-glyphosate from
tank-mixtures prepared with ammonium sulfate .................................................... 52

Table 5. Analysis of Mn fertilizers for cations antagonistic to glyphosate efﬁcacy ......... 53

CHAPTER 3
Table 1. Weather conditions during the 2001, 2002, and 2003 growing seasons ............ 81
Table 2. Date and amount of irrigation events during the 2001, 2002, and 2003 growing
seasons ................................................................................................................... 82
Table 3. Soybean yield as affected by year, irrigation treatment, and variety ................. 83
Table 4. Water use efﬁciency and irrigation water use efﬁciency ................................... 84
Table 5. Soybean yield components as affected by irrigation treatment .......................... 85
Table 6. Soybean yield components as affected by variety .............................................. 86
Table 7. Plant height as affected by irrigation and variety ............................................... 87
Table 8. Lodging as affected by irrigation and variety ..................................................... 88
Table 9. Weed control and soybean yield as affected by irrigation treatment and weed
control treatment with glyphosate ......................................................................... 89
Table 10. Yield components as affected by weed control treatment ................................ 90
Table 11. Soybean aphid count averages based on the MSU scale in 2003 ..................... 90
Table 12. Estimates of soybean aphid population differences between irrigation
treatments .............................................................................................................. 91

viii

 

LIST OF FIGURES

CHAPTER 1
Figure 1. Giant foxtail and velvetleaf control 14 d after treatment with glyphosate-Mn
fertilizer tank-mixtures in greenhouse bioassays ................................................... 22
Figure 2. Common lambsquarters control 28 d after treatment with glyphosate-Mn
fertilizer tank-mixtures in a soybean ﬁeld in East Lansing, MI ............................ 24
Figure 3. The effect of application timing on glyphosate efﬁcacy when glyphosate and
Mn were applied separately ................................................................................... 26
Figure 4. The effect of separate applications of Mn and glyphosate on velvetleaf control
14 d aﬂer treatment ................................................................................................ 28
CHAPTER 2
Figure 1. A depiction of glyphosate acid (N-phosphonomethylglycine) .......................... 54
Figure 2. Continuous wave EPR spectra of 1.125 mM glyphosate + 0.75 Mm Mn
solution at room temperature ................................................................................. 55
Figure 3. Continuous wave EPR spectra of 1.125 mM glyphosate + 0.75 Mm Mn
solution frozen to the temperature of liquid helium ............................................... 56
Figure 4. The ratio of four 3-pulsed ESEEM EPR spectra ............................................... 57
CHAPTER 3
Figure 1. Weekly soil moisture levels of ﬁve irrigation treatments in 2001 .................... 92
Figure 2. Weekly soil moisture levles of ﬁve irrigation treatments in 2002 .................... 93
Figure 3. Weekly soil moisture levels of ﬁve irrigation treatments in 2003 .................... 94

ix

CHAPTER 1

GLYPHOSATE EFFICACY IS ANTAGONIZED BY MANGANESE

Abstract: Michigan soybean producers have observed that glyphosate efficacy is
sometimes reduced in tank-mixtures with foliar manganese (Mn) fertilizers. The

objectives of this study were to evaluate the effects of Mn formulation, Mn application

 

timing, tank-mixture adjuvants, and Mn rate on glyphosate efﬁcacy. Three Mn 1“
formulations, Mn-ethylaminoacetate (Mn-BAA), Mn-lignin sulfonate (Mn-LS), and i
manganese sulfate monohydrate (MnSO4) reduced glyphosate efficacy in greenhouse and

ﬁeld bioassays, but Mn-ethylenediaminetetraacetate (Mn-EDTA) did not. Mn-EAA E

applied less than 3 d before glyphosate reduced glyphosate efﬁcacy on velvetleaf but not
giant foxtail or common lambsquarters. The antagonism increased as the interval
between treatment applications was shortened, but did not appear when Mn was applied 1
d or more after glyphosate to velvetleaf. Including the adjuvants ammonium sulfate
(AMS), EDTA, or citric acid in the glyphosate-Mn tank-mixture increased control of
giant foxtail and velvetleaf, but only matched the efﬁcacy of the glyphosate plus AMS
control in three combinations: AMS with Mn-LS on velvetleaf, citric acid with MnSO4
on giant foxtail, and EDTA with Mn-EAA on giant foxtail. AMS increased the
glyphosate efﬁcacy in Mn tank-mixtures as much as, or more than, citric acid and EDTA,
with two exceptions: EDTA with Mn-EAA on giant foxtail, and citric acid with MnSO4
on velvetleaf. Control of velvetleaf declined as the amount of Mn from Mn-EAA, Mn-

LS, and W804 in the tank-mixture increased.

Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. #1
CHEAL; giant foxtail, Setariafaberi Herrm. # SETFA; velvetleaf, Abutilon theophrasti
Medicus. # ABUTH; soybean, Glycine max L.

Additional Index Words: fertilizer, hard-water antagonism, herbicide interaction,
micronutrient, split application.

Abbreviations: AMS, ammonium sulfate; EDTA, ethylenediaminetetraacetate;
glyphosate-IPA, isopropylamine salt of glyphosate; glyphosate-K, potassium salt of
glyphosate; Mn-EAA, manganese sulfate with ethylaminoacetate chelate; Mn-EDTA,
manganese chelated by ethylenediaminetetraacetate; Mn-LS, manganese sulfate with

lignin sulfonate chelate; MnSO4, manganese sulfate monohydrate; IPA, isopropylamine.

INTRODUCTION

Manganese (Mn) is the most common soybean micronutrient deﬁciency in Michigan.
It occurs on mildly acidic or alkaline sands, high organic matter soils, and alluvial soils
derived from calcareous materials where Mn availability is restricted by low Mn
concentrations and/or high soil pH (Tisdale et al. 1993). Mn deﬁciency appears as
interveinal chlorosis in newly emerging tissue (Mengel and Kirkby 1987). Both soil- and
foliar-applied Mn can effectively alleviate deﬁciency symptoms and enhance seed yield
(Gettier et a1. 1984, 1985). Much less Mn is required when it is foliar applied than when
it is soil banded or broadcast, although multiple foliar applications are necessary during

some growing seasons (Gettier et a1. 1985). Typical foliar Mn fertilizers include MnSO4

 

1 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.

and several Mn chelates, including citric acid, EDTA, glucoheptonate, and lignin
sulfonate (Bailey et al. 2002; Carnberato 2001; Heckman 2000).

The appearance of Mn deﬁciency symptoms frequently coincides with time of post
emergence herbicide applications, and producers have tank-mixed herbicides and
fertilizers to reduce application trips and costs. Heckman et al. (1999) reported that
MnSO4 did not affect the weed control efﬁcacy of four soybean herbicides, aciﬂourfen,
chlorimuron-ethyl, imazethapyr, and bentazon, and the herbicides did not interfere with
soybean utilization of Mn. However, with the widespread adoption of glyphosate-
resistant soybean lines, reports have indicated that glyphosate efficacy is reduced when
tank-mixed with some Mn fertilizers (Bailey et al. 2002).

Glyphosate, like many other aminopolyacids, acts as a chelating agent and forms
stable complexes with di- and trivalent metal cations (Lundager Madsen et al. 1978;
Glass 1984). Hard-water cations such as Ca“, Mg”, and Fe3+ reduce glyphosate efﬁcacy
by complexing with glyphosate to form salts that are not readily absorbed by plants
(Thelen et al. 1995a). The antagonism caused by hard-water cations may be overcome by
increasing the glyphosate concentration relative to the cation content, by adding a chelate
such as EDTA or citric acid, or by adding an excess of monovalent cations such as NH4+,
K+, or Na+ to the spray mixture (Buhler and Burnside 1983a, 1983b; Shea and Tupy
1984; Thelen et al. 1995a, 1995b). Glyphosate efficacy is also antagonized when plants
secrete cations, e.g., Ca2+, to the leaf surface, where it presumably forms insoluble
complexes with glyphosate before glyphosate penetrates the plant cuticle (Hall et al.

2000)

The objectives of this research were to evaluate the effect of Mn fertilizer
formulations, the timing of Mn fertilizer application, and adjuvants on glyphosate

efﬁcacy, and to describe the effect of varying the relative rates of Mn and glyphosate.

MATERIALS AND METHODS

Greenhouse Bioassays. Bioassays were conducted between October 2000 and June 2003
in greenhouses located on the Michigan State University campus in East Lansing, MI.
Natural light was supplemented by high-pressure sodium lights that produced a
photosynthetic photon ﬂux density of 200 uE/mZ/s. The photoperiod was 16/8 light/dark,
and the temperature was 23 d: 3 °C. Velvetleaf and giant foxtail seed were planted in
potting mix2 in 945-ml plastic pots. Plants were thinned to one velvetleaf or three giant
foxtail plants per pot prior to receiving treatment. Pots were randomly assigned to
treatments. Each experiment consisted of four replications and was conducted twice.
When treated, velvetleaf had six leaves (approximately 14 cm tall) and giant foxtail had
ﬁve (approximately 20 cm tall). Treatments were applied using a single tip track sprayer
using a TP8001 ﬂat fan nozzle3 at a pressure of 170 kPa. Unless noted, treatment
mixtures were prepared with tap water. Plants were rated visually on a scale of 0
(untreated and healthy) to 10 (dead plant) 7 and 14 d after treatment. Ratings were
converted to percentages for statistical analysis.

Low glyphosate and high Mn rates were selected to maximize the likelihood of

observing Mn effect on glyphosate. The glyphosate rates of 0.28 or 0.45 kg ae/ha used

 

2 BacctoO High Porososity Professional Potting Mix, Michigan Peat Company, PO. Box 980129,
Houston, TX 77098

3 TeeJet", Spraying Systems Co., PO. Box 7900, Wheaten, IL 60189

were signiﬁcantly lower than the labeled rate of 0.84 kg/ha listed for most annual weeds.
The rate of Mn fertilizer applied was 05-25 kg Mn/ha, greater than the minimum rate of
0.1-0.2 kg/ha recommended for foliar application, although rates of 1-2 kg/ha are
frequently used (Carnberato 2001). The treatment rate used for the three liquid fertilizer
formulations, Mn-EAA4, Mn-EDTAS, and Mn-LSG, on a volumetric basis, was 9.4 L
fertilizer/ha. However, the Mn analysis of the formulations varied, consequently the

mass of Mn applied at this volumetric rate differed between fertilizer formulations. The

x. ”swanky
.

recommended rates listed on the label of each liquid fertilizer for foliar application to

 

ﬁeld crops were: Mn-EAA, 2.3 to 4.7 L/ha; Mn-EDTA, 2.3 to 23 L/ha; Mn-LS, 4.7 to

twuuni '1

9.4 L/ha. For the balance of the paper, fertilizer rates will be listed on a kg Mn/ha basis.
The dry Mn fertilizer, MnSOr7, was applied at 2.5 kg Mn/ha.

Tank-mixture antagonism. Four commercial Mn fertilizers were tank-mixed with
glyphosate to determine how different Mn formulations affected glyphosate efficacy.
The fertilizers Mn-EAA at 0.54 kg/ha, Mn-EDTA at 0.74 kg/ha, Mn-LS at 0.59 kg/ha,
and MnSO4 at 2.5 kg/ha were tank-mixed with two formulations of the isopropylamine

salt of glyphosate8 (glyphosate-IPA). In a second study, a glyphosate-IPA formulation

 

‘ POST-MAN Liquid Mn (5% Mn), Traylor Chemical & Supply Co., Inc., 1911 Traylor Boulevard,
Orlando, FL 32804

5 TRACO 6% Mn, Traylor Chemical & Supply Co., Inc., 1911 Traylor Boulevard, Orlando, FL 32804

6 METAGRO Liquid Mn (5% Mn), Traylor Chemical & Supply Co., Inc., 1911 Traylor Boulevard,
Orlando, FL 32804

7 Tecmangam (32% Mn), Tetra Micronutrients, PO. Box 73087, Houston, TX 77273

' Roundup Ultra”, Roundup UltraMAXW, Monsanto Company, 800 North Lindbergh Boulevard, St.

Louis, MO 63167

and a potassium salt of glyphosate9 (glyphosate-K) were tank-mixed with the four Mn
fertilizers to compare how different glyphosate salt formulations responded to Mn tank-
mixtures. The glyphosate rate for both experiments was 0.28 kg/ha, and the AMS rate
was 20 g/L in a spray volume of 190 L/ha. Treatments were applied to both giant foxtail
and velvetleaf plants.

To determine how the order of adding ingredients to the spray mixture affected
glyphosate efﬁcacy, mixing order treatments using only MnSO4 were prepared as
follows: (1) AMS (if used) followed by glyphosate followed by MnSO4 or (2) MnSO4
followed by glyphosate followed by AMS (if used).

Timing of separate applications. Mn-EAA was chosen as the Mn source because it
consistently antagonized glyphosate in tank-mixtures with and without AMS. Mn-EAA
was applied to giant foxtail and velvetleaf 6, 4, and 2 d before glyphosate, in tank-
mixtures with glyphosate, and 2 d aﬁer glyphosate to determine how glyphosate efficacy
was affected when Mn fertilizer was applied either before or aﬁer glyphosate. In a
second set of experiments, Mn-EAA was applied to velvetleaf 3 d, 2 d, 1 d, and l h
before glyphosate, in tank-mixture with glyphosate, and 1 h, l d, and 2 d after glyphosate.
Two formulations of glyphosate-IPA were used. Treatment rates were glyphosate at 0.28
kg/ha, Mn-EAA at 0.54 kg/ha, and AMS at 20 g/L in a spray volume of 190 L/ha.
Tank-mixture adjuvants. Glyphosate-Mn tank-mixtures were prepared with different
adjuvants to determine if the Mn reduction of glyphosate efﬁcacy could be overcome.

The fertilizers Mn-EAA at 0.54 kg/ha, Mn-EDTA at 0.74 kg/ha, Mn-LS at 0.59 kg/ha,

 

9 Roundup WeatherMAXT“, Monsanto Company, 800 North Lindbergh Boulevard, St. Louis, MO

63167

 

and W804 at 2.5 kg/ha were tank-mixed with two formulations of glyphosate-IPA at
0.45 kg/ha in a spray volume of 190 um. Three adjuvants were tested: AMS at 20 g/L,
citric acid at 21 g/L, and the tetrasodium salt of EDTA at 38 g/L.

Variable rates of glyphosate and manganese fertilizer. Velvetleaf was treated with
varying rates of Mn fertilizer and one glyphosate-IPA formulation to determine if
reducing the rate of Mn would reduce its negative effect. Tank-mixtures were prepared
using distilled water at a spray volume of 190 L/ha. Six rates of glyphosate-IPA, 0.0, 0.2,
0.4, 0.8, 1.7, and 3.3 kg/ha, were tank-mixed with six rates of each Mn formulation, 0.0,
0.1, 0.2, 0.5, 0.9, 1.8 kg/ha, and two rates of AMS, 0 and 20 g/L.

Field Trials. Field trials were conducted at the East Lansing Crops and Soils Teaching
and Research Field Lab, East Lansing, MI, in 2001 and 2002. The experimental design
was a randomized complete block. Soybean was seeded in 38-cm wide rows at 444,800
seeds/ha (2001 — ‘Pioneer 92336,’ 2002 - ‘Mycogen 5251RR’). The dominant weed
species each year was common lambsquarters. Herbicides were applied when weeds
were 15-20 cm tall (13-16 June 2001, 22-25 June 2002). Tank-mixture treatments were a
factorial of one formulation of glyphosate-IPA at 0.63 kg/ha, four Mn formulations (Mn-
EDTA at 0.74 kg/ha, Mn-LS at 0.59 kg/ha, and Mn-EAA at 0.54 kg/ha, and MnSOa at
2.5 kg/ha), and AMS at 0 and 20 g/L. In an application timing component of the study,
Mn-EAA was applied 2 d, 1 d, and l h before glyphosate, and 1 h and 1 d aﬁer
glyphosate. Treatments were applied using a backpack sprayer (XR8002VS ﬂat-fan

nozzles”) at a volume of 190 L/ha, pressure of 290 kPa, and speed of 4.8 kph. Plots were

 

1° TeeJetQ, Spraying Systems Co., PO. Box 7900, Wheaton, IL 60189

evaluated for weed control 14 and 28 d after treatment, and at pre-harvest. Soybean grain
was harvested for yield.

Statistical Analysis. Data were tested against the assumptions of the analysis of variance,
and data from repeated experiments were combined if they passed the Levene test for
homogeneity of variance (SAS 2001). Data were analyzed using PROC MIXED and
PROC GLM of SAS. Treatment means were separated using the Tukey adjustment in
PROC MIXED. The data reported in this paper are not transformed. In experiments
where two glyphosate-IPA formulations were used, the data were combined for analysis
because they responded to the Mn fertilizers similarly. All means reported from those

experiments represent the average of the two glyphosate-IPA formulations.

RESULTS AND DISCUSSION
Tank-mixture antagonism. Mn-EAA and Mn-LS reduced control of velvetleaf, giant
foxtail, and common lambsquarters in glyphosate tank-mixtures without AMS, (Figures 1
and 2). MnSO4 only reduced control of common lambsquarters (Figure 2), and Mn-
EDTA increased control of giant foxtail and velvetleaf (Figure 1). In glyphosate tank-
mixtures with AMS, Mn-EAA and MnSO4 reduced control of all three weed species, but
Mn-LS only reduced control of giant foxtail. Mn-EDTA did not reduce control of any of
the weed species. Control of common lambsquarters was greater in 2001 than in 2002
(Figure 2). In 2001, treatments were applied following a 2.3 cm rain. Conditions at the
time of treatment in 2002 were very dry. Adding AMS overcame the negative effect of
each Mn fertilizer in 2001, but in 2002, Mn-EAA and MnSO4 reduced control in tank-

mixtures with AMS. This demonstrates that the risk of glyphosate efﬁcacy being reduced

 

in tank-mixtures with Mn is greater when environmental conditions favor poor weed
control.

The Mn formulations tested differed signiﬁcantly in the degree of antagonism they
caused, and the antagonism also varied depending upon the species treated (Figure 1).
Bailey et al. (2002) observed that the antagonism caused by a Mn-glucoheptonate

formulation was more severe than that caused by Mn lignin sulfonate, and that glyphosate

efﬁcacy was antagonized more by Mn on common lambsquarters than on large crabgrass. "‘
Thelen et al. (1995a) reported that hard-water cations, such as Ca2+, complex with .

glyphosate, causing reduced glyphosate absorption and the observed hard-water

antagonism. MnSO4 in solution dissociates into Mn“ and S042] and Mn” may complex i,

 

with glyphosate, similarly to Ca“. Manganese in Mn-LS and Mn—EAA formulations is
weakly chelated. In mixtures of glyphosate and Mn-LS or Mn-EAA, the Mn2+ may
dissociate from the LS or EAA, allowing glyphosate to preferentially complex with Mn”,
thereby reducing glyphosate efﬁcacy. The positive effect of Mn-EDTA in tank-mixtures
without AMS likely results from the chelating strength of the EDTA molecule (Figure 1).
The Mn” stability constant of EDTA, log K = 13.81, is much higher than that of
glyphosate, log K = 5.53 (Martell and Smith 1974; Lundager Madsen et al. 1978). EDTA
may bind Mn2+ tightly enough to prevent it from forming a complex with glyphosate. In
addition, EDTA is a hexavalent chelate with high stability constants for most di- and
trivalent metals, and each EDTA molecule may coordinate with multiple cations (Saito
1968; Martel] and Smith 1974). EDTA in the fertilizer may have complexed hard-water
ions present in tap water that otherwise would have complexed with glyphosate and

reduced efficacy to the level obtained in the no Mn treatment (Figure 1). When tank-

mixtures, without AMS, were prepared in distilled water, there was no difference in
velvetleaf control (p = 0.05) between the no Mn and Mn-EDTA treatments (data not
shown). The Mn-EDTA formulation reported here contained a 1:1 molar ratio of Mn and
EDTA.

The glyphosate-K formulation appeared more susceptible to antagonism by Mn than
the glyphosate-IPA formulations (Table 1). When data for all Mn treatments were
averaged together, velvetleaf control was greater for mixtures with glyphosate-IPA (53%)
than mixtures with glyphosate-K (40%) (p = 0.05). The reduced control of velvetleaf by
glyphosate-K may result from different adjuvant compositions of the glyphosate
formulations. The glyphosate by Mn fertilizer interaction was signiﬁcant because
velvetleaf control in the presence of Mn-EDTA was less for glyphosate-K than
glyphosate-IPA tank-mixtures. It is notable because it shows that even a strong chelate
does not eliminate the tank-mixture antagonism on some species. There was no
signiﬁcant antagonism for either glyphosate-IPA or glyphosate-K when tank-mixed with
Mn-EDTA on giant foxtail.

Some fertilizer labels advocate a speciﬁc order of adding Mn and herbicide to the
spray mixture. However, the control of giant foxtail and velvetleaf was equal regardless
of whether MnSO4 was added to the spray mixture before or after glyphosate (Table 2).
Szelezniak et al. (2001) reported that the order of adding AMS and glyphosate for three
waters of varying hardness did not affect glyphosate efficacy, and no change in efﬁcacy
was observed even when AMS was added 24 h aﬁer glyphosate. Thelen et al. (1995a)
showed that the Cay-glyphosate complex became more structured the longer Ca+2 and

glyphosate remained in solution together. Although glyphosate may form complexes of

10

 

varying stability with di- and trivalent metal cations, any binding between glyphosate and
these cations appears to reduce glyphosate efﬁcacy (N elawaja and Matysiak 1991;
Stahlman and Phillips 1979). Adding AMS to the spray mixture reverses much of the
glyphosate-divalent cation complex in a short time as ammonium ions compete for
binding sites on the glyphosate molecule (Thelen et al. 1995a).
Timing of separate applications. After establishing that there was a signiﬁcant
antagonism of glyphosate efﬁcacy in tank-mixtures with Mn, experiments were
conducted to determine if the antagonism occurred when Mn-EAA and glyphosate were
applied separately. Glyphosate efficacy on giant foxtail was not reduced when Mn-EAA
was applied 6, 4, or 2 d before (-6 d, -4 d, -2 d) and 2 d after (+2 d) glyphosate (Figure 3).
However, when velvetleaf was treated with Mn-EAA 2 d before (-2 d) glyphosate,
control was reduced, although the reduction was not as great as that caused by the Mn-
EAA tank-mixture. The effect of separate applications was investigated further by
narrowing the application interval between Mn and glyphosate to velvetleaf (Figure 4).
The antagonism increased as the period of time between Mn and glyphosate applications
was shortened. The antagonism was greatest when Mn-EAA was applied before
glyphosate (+2 (1, +1 (1, +1 h), and was evident when Mn-EAA was applied 1 h (+1 h)
after glyphosate. This phenomenon was species dependant and did not occur on giant
foxtail in greenhouse bioassays (Figure 3) or on common lambsquarters in ﬁeld trials
(data not shown).

Potential reasons why glyphosate efﬁcacy was affected by the timing of Mn
application in velvetleaf include the following. First, velvetleaf is pubescent and some

Mn may have adhered to the leaf hairs. Subsequent herbicide treatments would have

11

washed the suspended Mn along with glyphosate, allowing the glyphosate and Mn to
complex. Second, velvetleaf releases calcium rich substances to the leaf surface from
specialized trichomes known as chalk glands. Glyphosate absorption is reduced when
glyphosate binds to this ﬁ'ee Ca2+ (Hall et al. 2000), and the additive effect of glyphosate
binding to both Ca2+ and Mn2+ may have made the antagonism evident. Third, with only
a 1 h interval between applications, the Mn2+ and glyphosate may have penetrated the
cuticle together and complexed during that process. Finally, free Mn2+ in the cytoplasm
may have reacted with glyphosate, interfering with glyphosate efﬁcacy and translocation
(Nilsson 1985).

Tank-mixture adjuvants. We observed in the initial tank-mixture bioassays that AMS
increased the efﬁcacy of glyphosate and Mn fertilizer tank-mixtures, but did not
eliminate the antagonism for all fertilizers on all species (Figures 1 and 2). The adjuvants
EDTA and citric acid, which overcame the hard-water antagonism of glyphosate, were
tested to determine if they might overcome the Mn antagonism (Shea and Tupy 1984;
Thelen et al. 1995b). The antagonism was considered present in AMS, citric acid, and
EDTA tank-mixtures if control did not equal that obtained by glyphosate plus AMS
(Table 3).

Control of giant foxtail and velvetleaf obtained from tank-mixtures with no adjuvant
(None) was Similar to that observed earlier (Figure l, w/o AMS), with the exception of
MnSO4 reducing velvetleaf control in this experiment (Table 3). No tank-mixture
containing Mn-EDTA reduced glyphosate efﬁcacy on giant foxtail. However, adding
citric acid to the Mn-EDTA tank-mixture reduced velvetleaf control. On giant foxtail,

AMS did not overcome the antagonism caused by Mn-EAA, Mn-LS, or MnSO4, but

12

citric acid overcame the antagonism caused by MnSO4, and EDTA overcame the
antagonism caused by Mn-EAA. On velvetleaf, AMS overcame the antagonism caused
by Mn-LS, but neither citric acid nor EDTA overcame antagonisms caused by Mn-EAA,
Mn-LS, and MnSOa.

When included in glyphosate-Mn tank-mixtures, AMS was as effective, or was more
effective, than either citric acid or EDTA at increasing control of velvetleaf and giant
foxtail, with two exceptions — EDTA added to the Mn-EAA tank-mixture on giant
foxtail, and citric acid added to the MnSO4 tank-mixture on velvetleaf (p = 0.05). The
ability of AMS to increase glyphosate phytotoxicity or to overcome a Ca2+ antagonism of
glyphosate depends upon the species (Nalewaja and Matysiak 1992). Therefore, it is not
surprising that the Mn fertilizer by adjuvant by weed species interaction was signiﬁcant
in this experiment.

The chelate EDTA appeared to be more effective at increasing control of giant
foxtail than velvetleaf (Table 3). In tank-mixtures with glyphosate-K, Mn-EDTA
reduced control of velvetleaf but not giant foxtail (Table 1). Shea and Tupy (1984)
reported that glyphosate control of wheat was enhanced by including EDTA in the tank-
mixture. EDTA not only chelated hard-water cations, but also increased glyphosate
efficacy on wheat when mixtures were prepared in distilled water. It may be that EDTA
affects glyphosate absorption by broadleaf and grass species differently.

Variable rates of glyphosate and manganese fertilizer. In the work described above
relatively low rates of glyphosate were tank-mixed with relatively high rates of Mn
fertilizer to increase the likelihood of identifying any antagonisms. Bailey et al. (2002)

reported that when the amount of Mn relative to glyphosate reached a critical threshold,

13

 

control of large crabgrass decreased signiﬁcantly. Nilsson (1985) reported that the
phytotoxic effects of root-fed glyphosate decreased as the rate of Fe and Mn in the
nutrient solution increased.

Experiments were conducted to describe the relationship between Mn rate and
glyphosate rate on velvetleaf control. Regression equations were developed from visual
ratings of velvetleaf 14 d after treatments were applied (Table 4). The Mn-EAA, Mn-LS,
and MnSO4 terms were each negative and signiﬁcant in the equations. Mn-EDTA did
not affect glyphosate control. Results from this experiment suggest that a 10-25%

reduction in velvetleaf control may occur for every kg/ha of Mn” applied from Mn-EAA,

 

max» A)’ “xx-t 1.
\-

Mn-LS, or MnSO4. The tank-mixtures in this experiment were prepared in distilled
water. Additional antagonism may occur in water containing appreciable amounts of Ca
(>100 mg/L) or other hard-water cations (N elawaja and Matysiak 1993).

In summary, when glyphosate and Mn are tank-mixed, a fully chelated Mn-EDTA
formulation (1 :1 molar ratio) is the least likely to reduce glyphosate efﬁcacy. Glyphosate
Should be applied at the full-labeled rate for the weeds present and the Mn rate should
reﬂect crop needs (01-02 kg Mn/ha) — excess Mn (1-2 kg/ha) increases the likelihood of
reduced weed control. AMS should be added to all glyphosate-Mn tank-mixtures at 2%
(w/w); neither citric acid nor EDTA were sufficiently more effective than AMS to justify
the additional cost of using them as adj uvants. When environmental conditions favor
poor weed control (e.g., large weeds, drought, etc.), consideration should be given to

applying glyphosate and Mn on separate days.

14

LITERATURE CITED

Bailey, W. A., D. H. Poston, H. P. Wilson, and T. E. Hines. 2002. Glyphosate
interactions with manganese. Weed Technol. 16:792-799.

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

Buhler, D. D. and O. C. Burnside. 1983b. Effect of water quality, carrier volume, and
acid on glyphosate phytotoxicity. Weed Sci. 31:163-169.

Carnberato, J. J. 2001. Manganese deﬁciency and fertilization of soybeans [Online].
Available at http://www.clemson.edu/edisto/soybean/manganese.pdf (veriﬁed 7 May
2004).

Gettier, S. W., D. C. Martens, D. L. Hallock, and M. J. Stewart. 1984. Residual Mn and
associated soybean yield response from MnS04 application on a sandy loam soil.
Plant Soil 81:101-110.

Gettier, S. W., D. C. Martens, and T. B. Brumback. 1985. Timing of foliar manganese

application for correction of manganese deﬁciency in soybean. Agron. J. 77:627-
630.

Glass, R. L. 1984. Metal complex formation by glyphosate. J. Agric. Food. Chem.
32:1249-1253.

Hall, G. J ., C. A. Hart, and C. A. Jones. 2000. Plants as sources of cations antagonistic to
glyphosate activity. Pest Manag. Sci. 56:351-358.

Heckman, J. R. 2000. Manganese: Needs of soils and crops in New Jersey. Rutgers
Cooperative Extension Fact Sheet 973. (Available online at
http://www.rce.rutgers.edu/pubs/pdfs/fs973.pdf.) (Veriﬁed 7 May 2003.)

Heckman, J. R., B. A. Majek, and E. P. Prostko. 1999. Application of manganese
fertilizer with postemergence soybean herbicides. J. Prod. Agric. 12:445-448.

Lundager Madsen, H. E., H. H. Christensen, and C. Gottlieb-Petersen. 1978. Stability
constants of copper(II), zinc, manganese(ll), calcium, and magnesium complexes of
N-(phosphonomethyl)glycine (glyphosate). Acta Chem. Scand. A 32:79-83.

Martel], A. E. and R. M. Smith. 1974. Critical stability constants. Vol. 1. New York:
Plenum Press.

Mengel, K. and E. A. Kirkby. 1987. Principles of plant nutrition. 4‘h ed. Bern,
Switzerland: International Potash Institute.

15

Nalewaja, J. D. and R. Matysiak. 1991. Salt antagonism of glyphosate. Weed Sci.
39:622-628.

Nalewaja, J. D. and R. Matysiak. 1992. Species differ in response to adjuvants with
glyphosate. Weed Technol. 6:561-566.

Nalewaja, J. D. and R. Matysiak. 1993. Optimizing adjuvants to overcome glyphosate
antagonistic salts. Weed Technol. 7:337-342.

Nilsson, G. 1985. Interactions between glyphosate and metals essential for plant growth.
In E. Grossbard and D. Atkinson, eds. The Herbicide Glyphosate. London:
Butterworths. pp. 35-47.

Saito, Y. 1968. X-ray and neutron diffraction. In K. Nakarnoto and P. J. McCarthy, eds.
Spectroscopy and structure of metal chelate compounds. New York: John Wiley and
Sons, Inc. pp. 1-72.

SAS Institute. 2001. The SAS system, version 8.2. The SAS Institute, Cary, NC.

Shea, P. J. and D. R. Tupy. 1984. Reversal of cation-induced reduction in glyphosate
activity with EDTA. Weed Sci. 32:803-806.

Stahlman, P. W. and W. M. Phillips. 1979. Effects of water quality and spray volume on
glyphosate toxicity. Weed Sci. 27:38-41.

Szelezniak, E., Z. Woznica, and J. D. Nalewaja. 2001. Glyphosate efﬁcacy as affected by
mixing sequence with adjuvants. North Central Weed Sci. Soc. Abstr. 56. [CD-Rom
Computer File]. North Central Weed Sci. Soc., Champaign, IL. (Dec. 2001).

Thelen, K. D., E. P. Jackson, and D. Penner. 1995a. The basis for the hard-water
antagonism of glyphosate activity. Weed Sci. 43:541-548.

Thelen, K. D., E. P. Jackson, and D. Penner. 1995b. Utility of nuclear magnetic
resonance for determining the molecular inﬂuence of citric acid and an

organosilicone adjuvant on glyphosate activity. Weed Sci. 43:566-571.

Tisdale, S. L., W. L. Nelson, J. D. Beaton, and J. L. Havlin. 1993. Soil Fertility and
Fertilizers. 5th ed. New York: Macmillan Publishing Co.

16

Table I. The effect of Mn formulation, glyphosate (GLY) salt“, and ammonium
sulfate (AMS) on control of giant foxtail and velvetleaf 14 d after treatment in a
greenhouse bioassay.

 

 

 

 

Control
Mn-EAA Mn-EDTA Mn-LS MnSO4
Species GLY salt AMS GLY” + GLY + GLY + GLY + GLY
%
Giant foxtail IPA 0 g/L 84 a 26 b 91 a 13 b 61 ab
IPA 20 g/L 93 a 47 ab 87 a 50 a 81 a
K Og/L 51b 38ab 83a 26b 50b
K 20 g/L 98 a 53 a 84 a 58 a 68 ab
Velvetleaf IPA 0 g/L 49 b 5 b 69 ab 16 b 16 b
IPA 20 g/L 98 a 60 a 86 a 84 a 46 a
K 0 g/L 28 b 20 b 31 c 9 b 18 b
K 20 g/L 88 a 50 a 54 be 74 a 26 ab

 

" Two formulations of glyphosate, one containing an isopropylamine (IPA) salt and the
second a potassium (K) salt, were used. Treatment rates were glyphosate at 0.28 kg
ae/ha, Mn-EAA at 0.54 kg Mn/ha, Mn-EDTA at 0.74 kg/ha, Mn-LS at 0.59 kg/ha, and
MnSO4 at 2.5 kg/ha. Spray volume was 190 L/ha.

b Means within a column for each species followed by the same letter are not
statistically different, p = 0.05, Tukey adjusted. The standard errors were 4.9
(velvetleaf) and 5.5 (giant foxtail).

17

 

Table 2. The effect of mixing order in preparing glyphosate-Mn tank-mixturesa on
control of giant foxtail and velvetleaf 14 d aﬁer treatment in a greenhouse bioassay.

 

 

 

 

Control
Glyphosate Mn before
Species AMS b Glyphosate before Mn glyphosate
%
Giant foxtail 0 g/L° 63 a 53 a 55 a
20 g/L 97 a 71 b 73 b
Velvetleaf 0 g/L 28 a 23 a 21 a
20 g/L 88 a 59 b 55 b

 

a Treatment rates were glyphosate at 0.28 kg ae/ha, and MnSO4 at 2.5 kg Mn/ha.
Spray volume was 190 L/ha.

b When AMS was included in the tank-mixture, it was added ﬁrst.

° Means within a row followed by the same letter are not statistically different, p =
0.05, Tukey adjusted. The standard errors were 2.6 (velvetleaf) and 3.2 (giant foxtail).

18

.u‘.”

 

F” “4.”; V."ud|‘.$" I. . . '1’.- VJ .1 7

Table 3. The effect of adjuvant and Mn formulation on velvetleaf and giant foxtail
control 14 d after glyphosate (GLY) applicationa in a greenhouse bioassay.

 

Control

 

Mn-EAA Mn-EDTA Mn-LS MnSO4
Species Adjuvant GLY + GLY + GLY + GLY + GLY

 

Giant foxtail Noneb 62 b 10 c 92 a 8 c 56 b
AMS” 98 a 61 c 90 a 70 bc 83 ab
Citric acid° - 65 b 92 a 48 c 89 a
EDTA“ - 93 ab 99 a 81 bc 71 c

Velvetleaf Noneb 58 b 2 c 81 a 11 c 13 c
AMS” 93 a 64 b 88 a 83 a 54 b
Citric acidc - 46 c 66 b 51 c 74 b
EDTAc - 55 b 81 a 45 bc 36 c

 

3 Application rates were glyphosate at 0.45 kg ae/ha (14 mM), Mn-EAA at 0.54 kg
Mn/ha (52 mM), Mn-EDTA at 0.74 kg/ha (72 mM), Mn-LS at 0.59 kg/ha (57 mM),
MnSO4 at 2.5 kg/ha (244 mM), AMS at 20 g/L (154 mM), citric acid at 21 g/L (100
mM), and EDTA at 38 g/L (100 mM). Spray volume was 190 L/ha.

b Means within these rows followed by the same letter are not signiﬁcantly different, p
= 0.05, Tukey adjusted.

c Means within these rows are compared to the glyphosate plus AMS treatment within
a species. Means followed by the same letter are not signiﬁcantly different, p = 0.05,
Tukey adjusted.

19

Table 4. The effect of glyphosate rate, Mn rate“, and ammonium sulfate (AMS) on
control of velvetleaf 14 d after treatment in greenhouse bioassays.

 

 

Mn AMS Regression equationb adj R2

Mn-EAA 0 g/L 23 + 27(glyphosate) — 11(Mn-EAA) 0.6774
Mn-EAA 20 g/L 55 + 21(glyphosate) — 23(Mn-EAA) 0.5221
Mn-EDTA 0 g/L 24 + 26(glyphosate) + 4*(Mn-EDTA) 0.6888
Mn-EDTA 20 g/L 46 + 21(glyphosate) — 2*(Mn-EDTA) 0.4476
Mn-LS 0 g/L 20 + 29(glyphosate) - 10(Mn-LS) 0.7060
Mn-LS 20 g/L 55 + 21(glyphosate) — 16(Mn-LS) 0.4612
MnSO4 0 g/L 12 + 30(glyphosate) — 12(MnSO4) 0.8133
W80; 20 g/L 45 + 24(glyphosate) — 16(MnSO4) 0.5649

 

a Six rates of glyphosate, 0, 0.2, 0.4, 0.8, 1.7, and 3.3 kg ae/ha, and six rates of each
Mn fertilizer, 0, 0.1, 0.2, 0.5, 0.9, 1.8 kg Mn/ha were used. Treatment solutions were
prepared in distilled water. Spray volume was 190 L/ha.

b Regression equations were developed from visual ratings. Glyphosate represents kg
ae/ha, and Mn represents kg Mn/ha. Equation terms marked with an “*” are not
signiﬁcantly different from 0, p = 0.05.

20

A .. sz‘ h—j‘ﬂ‘

 

Figure 1. Giant foxtail and velvetleaf control 14 d after treatment with glyphosate-Mn
fertilizer tank-mixtures in greenhouse bioassays. Glyphosate was applied at 0.28 kg
ae/ha, Mn-EAA at 0.54 kg Mn/ha, Mn-EDTA at 0.74 kg/ha, Mn-LS at 0.59 kg/ha, and
MnSO4 at 2.5 kg/ha. Spray volume was 190 L/ha, ammonium sulfate (AMS) was added
at 20 g/L, and mixtures were prepared in tap water. Error bars represent the standard
error. Columns of the same shading with the same letter are not statistically different, p =

0.05, Tukey adjusted.

21

Control (%)

Control (%)

100

 

90—
80—
70—
60—
504
40-
30«
20-
104

 

Giant foxtail

 

b

I]: w/JMS f
‘!W//:M§ 1
be

 

 

108

 

90 ,
80

70 7
60 —
50 7
40 ~
30 ~
20 —
10 _

 

a Velvetleaf

bc

 

a

a

ab

 

 

 

 

f

No Mn Mn-EAA Mn-EDTA

Mn-LS

MnSO4

Mn fertilizer in tank-mixture with glyphosate

22

Figure 2. Common lambsquarters control 28 d after treatment with glyphosate-Mn
fertilizer tank-mixtures in a soybean ﬁeld in East Lansing, MI. Glyphosate was applied
at 0.63 kg ae/ha, Mn-EAA at 0.54 kg Mn/ha, Mn-EDTA at 0.74 kg/ha, Mn-LS at 0.59
kg/ha, and MnSO4 at 2.5 kg/ha. Spray volume was 190 L/ha and ammonium sulfate
(AMS) was added at 20 g/L. In 2001 treatments were applied the day after a 2.3 cm
rainfall event. In 2002 treatments were applied when conditions were very dry. Error
bars represent the standard error. Within a year, columns of the same shading with the

same letter are not statistically different, p = 0.05, Tukey adjusted.

23

Control (%)

Control (%)

100

90*

80

70*
60*

50

40.
3o-
20.

100
90

80«

70
60

50,
4o.

30

20-

10

 

   

 

 

 

 

 

 

 

2001
a a
b b
2002 inmmaAMSi
a a a l!![AMSW

ab

 

 

 

bc
rbLI b

 

No Mn Mn-EAA Mn-EDTA Mn-LS MnSO4

Mn fertilizer in tank-mixture with glyphosate

24

Figure 3. The effect of application timing on glyphosate efﬁcacy when glyphosate and
Mn were applied separately. Giant foxtail and velvetleaf control 13 d aﬁer treatment.
Mn-ethylaminoacetate (Mn-EAA) was applied to plants 6, 4, or 2 d before glyphosate (- 6
d, -4 d, -2 d), 2 d after glyphosate (+2 d), and in tank-mixtures (Tank-mix) with
glyphosate. Glyphosate was applied at 0.28 kg ae/ha, and Mn-EAA at 0.54 kg Mn/ha.
Spray volume was 190 L/ha and ammonium sulfate (AMS) was added at 20 g/L. Error
bars represent the standard error. Columns of the same shading with the same letter are

not statistically different, p = 0.05, Tukey adjusted.

25

 

90 - Giant foxtail lEIw/o Ailvléi
Iw/AMS ;
80 , a

70 ,
60 -
50 J a
40
30
20 -
10
0

100
90 _ Velvetleaf

Control (%)

 

80

70

60 ,
50

40 -
30 ,
20 a
10 -
o -

No Mn -6 -2 d Tank-mix +2 d

Control (%)

 

 

 

Timing of Mn application

26

Figure 4. The effect of separate applications of Mn and glyphosate on velvetleaf control
14 d after treatment. Mn-ethylaminoacetate (Mn-BAA) was applied to plants 3, 2, or 1 d
before (-3 d, -2 d, -1 d), 1 h before (-1 h), 1 h after (+1 h), and 1 and 2 d aﬁer (+1 (1, +2 (1)
glyphosate, and in tank-mixtures (T. mix) with glyphosate. Error bars represent the
standard error. Columns of the same shading with the same letter are not statistically
different, p = 0.05, Tukey adjusted. Glyphosate was applied at 0.28 kg ae/ha and Mn-

EAA at 0.54 kg Mn/ha. Spray volume was 190 L/ha, and ammonium sulfate (AMS) was

added at 20 g/L.

27

 

a Velvetleaf 1E1w/o‘ AMS} ab
ab bc ilwﬂMS bC
cd
d

 

a.
I 0 DC

NoMn -3d -2d -1d

-1h T.mix +1h

Timing of Mn application

28

+1

 

CHAPTER 2
GLYPHOSATE COORDINATION WITH MANGANESE IN SOLUTION AND

ITS EFFECT ON GLYPHOSATE ABSORPTION AND TRANSLOCATION

Abstract: Recent reports indicate that manganese (Mn) applied as a foliar fertilizer in
tank-mixtures with glyphosate has the potential to antagonize glyphosate efﬁcacy and
reduce weed control. It was hypothesized that Mn2+ complexed with glyphosate in a
similar manner to Ca2+, forming salts that were not readily absorbed and thereby reducing
glyphosate efficacy. This study was conducted to show how Mn complexes with
glyphosate, and to measure the effect of Mn on glyphosate absorption and translocation
in velvetleaf (Abutilon theophrastz).

Electron spin resonance (ESR), or electron paramagnetic resonance (EPR), is a
spectrosc0pic tool used to analyze transition metal behavior in solution. EPR spectra
show that Mn2+ complexes with glyphosate. In aqueous solutions Mn forms six
coordination bonds and preferentially coordinates with oxygen. EPR spectra show that
water is present in the inner coordination sphere.

Growth chamber bioassays were conducted to measure absorption and translocation
of l4C-labeled glyphosate in solution with four Mn fertilizers: Mn-ethylaminoacetate
(Mn-BAA), Mn-ethylenediaminetetraacetate (Mn-EDTA), Mn-lignin sulfonate (Mn-LS),
and MnSO4. Glyphosate efﬁcacy, absorption and translocation in velvetleaf depended
upon the fertilizer formulation included in the tank-mixture. Mn-EDTA did not interfere
with glyphosate efficacy, absorption or translocation. Both MnSO4 and Mn-LS reduced

glyphosate efﬁcacy, absorption and translocation. Mn-EAA severely antagonized

29

glyphosate efﬁcacy even though glyphosate in tank-mixtures with Mn-EAA was
absorbed rapidly by the treated leaf. However, little of the absorbed glyphosate was
translocated from the treated leaf. Adding ammonium sulfate (AMS) increased the
efﬁcacy, absorption, and translocation of glyphosate for each Mn fertilizer tank-mixture.
The Mn-EAA fertilizer contained approximately 0.5% iron (Fe) not reported on the
fertilizer label. Iron is presumed to be partially responsible for the very limited
translocation of glyphosate from the treated leaf in Mn-EAA tank-mixtures.
Nomenclature: Glyphosate; velvetleaf, Abutilon theophrasti Medicus. # ABUTH.
Additional Index Words: hard water antagonism, micronutrient complex, EPR.
Abbreviations: AMS, ammonium sulfate; D20, deuterium oxide; DTPA,
diethylenetriarninepentaacetate; EPR, electron paramagnetic resonance; ESR, electron
spin resonance; glyphosate, isopropylamine salt of glyphosate; Mn-EAA, manganese
sulfate with ethylaminoacetate chelate; Mn-EDTA, manganese chelated by
ethylenediaminetetraacetate; Mn-LS, manganese sulfate with lignin sulfonate chelate;

MnSO4, manganese sulfate monohydrate.

INTRODUCTION
There are numerous reports in the literature of divalent, trivalent, and sometimes
monovalent cations found in hard water antagonizing glyphosate efficacy (N elawaja and
Matysiak 1991; Stahlman and Phillips 1979; Thelen et al. 1995a). This occurs because
glyphosate, like many other phosphonic acids, acts as a chelating agent and forms stable
complexes with di- and tri-valent metal cations (Glass 1984; Lundager Madsen et al.

1978; Motekaitis and Martell 1985; Subramaniam and Hoggard 1988). Glyphosate

30

efﬁcacy is reduced because the glyphosate-metal complexes reduce absorption into or
translocation within the treated tissue (Hall et al. 2000; Nalewaja et a1. 1992; Nilsson
1985; Thelen et al. 19953; Wills and McWhorter 1985).

Manganese (Mn) deﬁciency is common in soybean (Glycine max L.) grown in humid
areas east of the Mississippi River, especially around the Great Lakes (Tisdale et al.

1993). Because Mn deﬁciency symptoms frequently appear near the time of post-

emergence glyphosate applications in glyphosate-resistant soybean, producers have I
expressed a preference for tank-mixing glyphosate and foliar Mn fertilizer to reduce

application costs. Recent research has shown that glyphosate efficacy is antagonized

when it is tank-mixed with some Mn fertilizers that are not fully chelated by EDTA i

 

(Bailey et al. 2002; Bemards et al. 2004).

Glyphosate is a weak acid and at pH 6.0 has a -2 charge (Figure l). The neutral acid
is dipolar with positive amine and negative phosphonate groups. As a ligand (L), it forms
stable complexes with di- and tri-valent metal cations (M) in 1:1 and 2:1 ratios (ML and
MLz), and also forrnsa protonated species (MHL) (Motekaitis and Martell 1985). The
2:1 complexes form primarily in alkaline solutions. Work has been done with several
cations to understand how they interact with glyphosate. NMR (nuclear magnetic
resonance) spectra show that glyphosate complexes with Ca2+ and Mg2+ via the
phosphonyl and carboxyl oxygen of glyphosate, but in solutions with excess Ca2+, spectra
indicate that Ca2+ associates with multiple oxygen from the phosphonyl group (Thelen et
al. 1995a). Iron as F e3+ forms 1:1 complexes with glyphosate at pH levels below 4.3 that
are stable enough to suppress Fe hydrolysis (McBride and Kung 1989). Infrared

spectroscopy (IR) does not show evidence for coordination of aqueous Fe3+ through the

31

amino or carboxyl groups (Barja and Dos Santos Afonso 1998), but electron spin
resonance (ESR) spectroscopy spectra shows that the carboxyl group associated with F e3 +
and contributes to stability (McBride and Kung 1989). Similar interactions likely occur
between glyphosate and F e-oxides and hydroxides present at higher pH levels (Barja and
Dos Santos Afonso 1998; Glass 1984). Copper (012+) interacts with glyphosate
differently from Fe3+ and Ca2+. In 1:1 and 2:1 glyphosatezCu2+ complexes, the amine
nitrogen and carboxyl oxygen coordinate in the equatorial positions instead of the
phosphonyl oxygen (McBride 1991). This behavior is expected — stronger acids like F e3 +
preferentially coordinate with hard ligands like oxygen present on the phosphonyl and
carboxyl groups, but weaker acids like Cu2+ preferentially coordinate with soft ligands
like nitrogen.

While some information is available on the stability of Mn-glyphosate complexes,
there has only been speculation on the structure of those complexes. In aqueous solution
Mn2+ is a strong acid like F e3 +, and may be expected to coordinate with glyphosate
similarly to Fe”. EPR is a tool used to analyze transition metal behavior in solution
(Atkins and de Paula 2002). Protons and electrons each have spin and when placed in a
magnetic ﬁeld, electrons or protons whose axis of rotation aligns with the magnetic ﬁeld
are in a lower energy state. Irradiating a molecule or metal in a magnetic ﬁeld can induce
transitions between energy states. The resulting ESR or EPR spectra may be used to
show how metals with unpaired electrons, in this case Mn”, interact with ligands like
glyphosate.

The hard water antagonism of glyphosate is affected by the cation(s) present and

their salts, the pH of the solution, the volume of the carrier, the ratio of glyphosate to

32

cation, and the presence of adjuvants, surfactants, chelates or AMS (Buhler and Burnside
1983; Hall et al. 2000; Nelawaja and Matysiak 1991; Shea and Tupy 1984; Stahlman and
Phillips 1979; Thelen et al. 1995a, 1995b). The effect of Mn on glyphosate efﬁcacy
depends upon the glyphosate salt, Mn formulation, and the weed species treated (Bailey
et al. 2000; Bemards et al. 2004). The compound included in the fertilizer formulation to
chelate or deliver Mn to the soybean plant may be the most important factor to
determining the degree of interaction with glyphosate. If the chelate has a higher stability
constant for the Mn than glyphosate, like EDTA, the degree of antagonism is less than if

it has a stability constant similar or less than glyphosate, like citric acid (N ilsson 1985).

17—13—_—‘ _""" ‘ "’

Movement into the cells is the major barrier to glyphosate absorption and
translocation by plants (Hall et al. 2000; Nilsson 1985). Glyphosate is absorbed by cells
through passive diffusion or via a phosphate transporter energized by the plant
plasmalemma ATPase (Hetherington et al. 1998). Glyphosate is continually cycled
through the phloem, moves throughout the plant (for at least 72 h), and accumulates in
young leaves, roots, and meristems (Bromilow et al. 1993). Foliar absorption is reduced
when ”C-labeled glyphosate is applied in solution with Ca, Fe, Mg, Mn, and Zn (Nilsson
1985 ; Thelen et al. 1995a; Wills and McWhorter 1985). Calcium in the membrane,
cuticle, and cell walls of velvetleaf complexes with glyphosate and prevents movement of
glyphosate into the symplast (Hall et al. 2000). However, CaClz in solution increases
glyphosate absorption in maize and soybean cell cultures (Hetherington et al. 1998), and
adding Fe and Mn increases translocation of l4C-glyphosate from root-fed solutions to
wheat leaves (N ilsson 1985). Although Fe and Mn increase glyphosate concentrations in

the wheat leaves, the phytotoxic effects are reduced, indicating that the metal-glyphosate

33

complex remains intact through import into the plant cell and translocation to the
growing point. Some of the negative effects of di- and tri-valent cations on glyphosate
absorption and translocation are reduced when glyphosate is applied in solution with
AMS (Hall et al. 2000).

The objectives of this study were to determine how Mn interacts with the glyphosate
molecule using EPR spectroscopy, and to compare the absorption and translocation of
l4C-glyphosate in tank mixtures with four commercial Mn fertilizer formulations to

determine the basis for their differential effects on glyphosate efﬁcacy.

MATERIALS AND METHODS

EPR analysis. Samples were prepared in either H20 or D20 (deuterium oxide). Stock
solutions were prepared to the following concentrations and ratios: 0.75 mM MnSOn;
1.125 mM glyphosatelz0.75 mM MnSO4; and 1.125 mM DTPA:0.75 mM MnSO4. The
stock solution was then mixed with an equal volume of 0.4 M sucrose. The sample
solutions were transferred to 4 mm EPR quartz tube and were stored in liquid nitrogen
until they were used.

The continuous wave-EPR spectra were obtained at X-band using a Bruker ER 300E
spectrometer outﬁtted with a TE102 EPR cavityz. Liquid helium temperature
measurements were carried out using an Oxford ESR 900 cryostat and ITC 502

temperature controller3 . The microwave frequency was determined with an EIP 258

 

1 Glyphosate acid (95% pure), AdjuVants Plus Agro-Chemicals, 1338 Lincoln Rd, Kingsville,
Ontario, Canada N9Y2$5
2 Bruker BioSpin Corporation, 19 Fortune Drive, Manning Park, Billerica, MA 01821

3 Oxford Instruments Inc., 130A Baker Ave, Concord, MA 01742

34

frequency counter4. The external magnetic ﬁeld strength was measured with a Bruker
035M NMR gaussmeter.

Pulsed EPR experiments were performed on a laboratory-built spectrometer (Singh
et al. 2000). ESEEM data were collected at X—band using a reﬂection cavity with a
folded strip-line element serving as the resonant element. A 3-pulse (90-1-90-T-90)
sequence was used. Dead-time reconstruction was performed prior to Fourier
transformation as described (Mims 1984). Analysis software for the experimental
ESEEM data was written in MATLABS.
Glyphosate-Mn tank-mixture efficacy. Velvetleaf was seeded in potting mix6 in 0.9 L
plastic pots and grown in greenhouses located on the Michigan State University campus
in East Lansing, MI. Natural light was supplemented by high-pressure sodium lights that
produced a photosynthetic photon ﬂux density of 200 llE/mzls. The photoperiod was
16/8 h light/dark, and the temperature was 23 :1: 3 °C. Plants were thinned to one
velvetleaf plant per pot before treatment, and at the time of treatment had 6 leaves
(approximately 14 cm tall).

Glyphosate-Mn tank-mixtures were prepared in distilled water to eliminate the effect
of other hard water cations. Four commercial Mn fertilizers were used: Mn-

ethylaminoacetate7 (Mn-BAA), Mn-ethylenediaminetetraacetate8 (Mn-EDTA), Mn-lignin

 

4 EIP Microwave Inc., 1589 Centre Pointe Drive, Milpitas, CA 95035

5 The MathWorks, 3 Apple Hill Drive, Nantick, MA 01760

6 Baccto" High Porosity Professional Potting Mix, Michigan Peat Company, PO. Box 980129,
Houston, TX 77098

7 POST-MAN Liquid Mn (5% Mn), Traylor Chemical & Supply Co., Inc., 1911 Traylor Boulevard,

Orlando, FL 32804

35

sulfonate9 (Mn-LS), and manganese sulfate monohydratelo (MnSO4). Low glyphosate
and high Mn rates increased the likelihood of identifying any antagonism that existed and
were consistent with rates reported in Bemards et al. (2004). Labels for the three liquid
Mn formulations recommended the following foliar rates for ﬁeld crops: Mn-EAA, 2.3
to 4.7 L/ha; Mn-EDTA, 2.3 to 23 L/ha; Mn-LS, 4.7 to 9.4 L/ha. In all experiments
reported below, the liquid fertilizers were applied at 9.4 L/ha. Because the fertilizer
analyses were not identical, this equaled 0.54 kg Mn/ha for Mn-EAA, 0.74 kg/ha for Mn-
EDTA, and 0.59 kg/ha for Mn-LS. The dry formulation, MnSO4, was applied at 2.5 kg
Mn/ha (equal to 7.8 kg MnSOa/ha). Extension publications recommend 0.1-0.2 kg
Mn/ha, although rates of 1-2 kg/ha are not uncommon (Camberato 2001). The fertilizers
were tank-mixed with glyphosate at 0.28 kg ae/ha (the labeled glyphosate rate for most
annual weeds is 0.84 kg/ha), both with AMS at 20 g/L and without AMS. Velvetleaf
plants were treated with glyphosate-Mn tank-mixtures on a single-tip track sprayer (8001
ﬂat fan nozzle, pressure 170 kPa, 187 L/ha). The experiment contained four replications
and was conducted twice. Plants were rated visually 7 and 14 d after treatment on a scale
of 0 (untreated and healthy) to 10 (dead plant). Ratings were converted to percentages
for analysis.

14C-glyphosate absorption and translocation bioassays. Velvetleaf was planted as
described above, and grown for 19-20 d in the greenhouse before being moved into an

environmental growth chamber (day length of 16 h and day/night temperatures of 22/16

 

’ TRACO 6% Mn, Traylor Chemical & Supply Co., Inc., 1911 Traylor Boulevard, Orlando, FL 32804
9 METAGRO Liquid Mn (5% Mn), Traylor Chemical & Supply Co., Inc., 1911 Traylor Boulevard,
Orlando, FL 32804

'° Tecmangam (32% Mn), Tetra Micronutrients, PO. Box 73087, Houston, TX 77273

36

°C and a photosynthetic photon ﬂux density of 300 umol/mz/s). At the time of the
transfer the 5th leaf was opening. Prior to being moved into the growth chamber, the
second oldest leaf was wrapped in foil and plants were treated on a single-tip track
sprayer with the same glyphosate-Mn tank-mixtures as described for the glyphosate-Mn
tank-mixture efﬁcacy bioassays. Plants were then moved to the growth chamber and the
foil was removed. Two l-uL drops of the treatment solutions, spiked with 1000 Bq/ 1.1L of
l4C-glyphosate, labeled at the methyl carbon of the N-phosphonomethyl glycine, were
placed on the adaxial side of the second oldest leaf. This leaf was excised and rinsed at 4,
24, or 48 h using 4 mL of an acid solution (1 part 0.1 M HCl and 1 part methanol) (Hall
et al. 2000). The rinsate was radioassayed by liquid scintillation spectrophotometryl 1.
The quantity of l4C-glyphosate in the rinsate was compared to the amount of 14C-
glyphosate applied for each treatment. At the time of rinsing, plants were harvested and
divided into four parts: 1) treated leaf, 2) stem and leaves above treated leaf, 3) stem and
leaves below treated leaf, and 4) roots. All parts were dried, weighed, ground,
combusted”, and analyzed for l4C-glyphosate by liquid scintillation. The percentage of
l4C-glyphosate present in each portion of the plant was calculated by dividing the 14C
DPM for that portion of the plant by the sum of 14C DPM recovered ﬁ'om combusted
tissue. The sum of 14C glyphosate recovered from rinsing and combusting plant tissue
was approximately 85% of what was applied to the leaves.

Mn fertilizer analysis. Dilutions of each fertilizer were analde for Mn, Fe, Al, Zn,

Cu, and Ca content using DCP (direct current plasma) spectroscopy.

 

‘1 Beckman LS6500 Scintillation System, Beckman Instruments, Inc 2500 Harbor Blvd, Fullerton,

CA 92634

12 OX-300 Biological Material Oxidizer, RJ. Harvey Instrument Corporation, Hillsdale, NJ

37

Statistical analysis. Data were tested against the assumptions of the analysis of
variance, and data from repeated experiments were combined if variances were similar

(SAS 2001). Data were analde using PROC MIXED and PROC GLM of SAS.

RESULTS AND DISCUSSION
EPR analysis. The continuous wave EPR spectra of the 1.125 mM glyphosate + 0.75
mM Mn complex in H2O at room temperature is presented in Figure 2. The spectra
contained six peaks and showed the presence of Mn (the number of peaks in a spectra is
21+1, and for Mn I=5/2). Because the peaks were not subdivided, the spectra may be
interpreted as indicating that only one species of the glyphosate-Mn complex was
present. The random orientation and rotation of molecules in solution at room
temperature prevented the spectra from showing that Mn was interacting with glyphosate.
The sample was then frozen and additional spectra were obtained (Figure 3). Six peaks
were again evident, but were subdivided unevenly. In a frozen sample if Mn2+ were
interacting only with water, the subdivisions would have been even. Because they are
not, we interpret the spectra to indicate that Mn2+ is complexed by glyphosate.

Electron Spin Echo Envelope Modulation (ESEEM) is a pulsed EPR technique used
to measure electron-nuclear hyperﬁne couplings. In this experiment, electron spin echo
was generated by the application of a sequence of resonant microwaves to the sample.
The electron spin echo decay envelope, which was modulated by electron-nuclear
hyperﬁne interactions, was obtained by plotting the integrated intensity of the spin echo
as a function of time between two of the pulses in the sequence (Hoogstraten and Britt

2002). To isolate the effect of water in the inner coordination sphere, a ratio of the

38

spectra of four different samples was derived (Figure 4). Data from the region marked by
the arrow (2400-2450 ns) was used to approximate a distance between Mn2+ and D20 of
3.0 A, and indicated that water was present in the inner coordination sphere.

In aqueous solutions, Mn2+ forms six coordination bonds in an octahedral structure.
Glyphosate has the potential to form four coordination bonds, although two or three are
most common. In solution with glyphosate, Mn2+ shows intermediate binding strength
and there is always free Mn2+ (Motekaitis and Martell 1985). Glyphosate likely interacts
with Mn2+ through oxygen atoms of the carboxyl or phosphonyl groups, or both, similar
to F e3+ and Ca2+ (McBride and Kung 1989; Thelen et al. 1995a). The sample solutions
prepared for EPR analysis were acidic. In glyphosate-Mn solutions with pH levels less
than 7, the equilibrium data shows a 1:1 coordination ratio of glyphosate and Mn”
(Motekaitis and Martell 1985).

Glyphosate-Mn tank-mixture efﬁcacy. Mn-EAA, Mn-LS, and MnSOa reduced
glyphosate efﬁcacy in tank-mixtures without AMS, but Mn-EDTA did not (Table 1).
Adding AMS increased glyphosate efﬁcacy, but did not eliminate the tank-mixture
antagonism caused by the four Mn fertilizers. The efficacy of Mn-LS and Mn-EDTA
tank-mixtures were equal, and the antagonism caused by Mn-EAA, a chelated
formulation, was more severe than the antagonism caused by MnSO4,

Other reports have shown an interaction on glyphosate efﬁcacy between Mn
formulations and adjuvants included in the tank-mixture (Bailey et al. 2002; Bemards et
al. 2004). AMS eliminated the antagonism caused by some hard water cations, such as
Ca2+, but only reduced the antagonism caused by others, like Fe3+ (N alewaja and

Matysiak 1991; Thelen et al. 1995a). At the Mn and glyphosate application rates used in

39

this study, Mn” behaved more like Fe3+, as the antagonism was evident for all
glyphosate-Mn tank-mixtures containing AMS. Mn-EDTA did not antagonize
glyphosate efﬁcacy when tank-mixtures were prepared in tap water, and in tank-mixtures
without AMS, Mn-EDTA signiﬁcantly increased glyphosate efﬁcacy (Bemards et al.
2004). This effect was presumed to be the result of EDTA chelating hard water cations
present in tap water.

l"C-glyphosate absorption and translocation bioassays. Mn-LS and MnSO4
signiﬁcantly reduced 14C-glyphosate absorption, and Mn-EDTA increased l4C-glyphosate
absorption in tank-mixtures without AMS (Table 2). Glyphosate in solutions with Mn-
EAA was absorbed by the leaf more rapidly than in the no Mn control, but at 48 h the
amount of glyphosate absorbed was similar. The addition of AMS signiﬁcantly increased
glyphosate absorption by velvetleaf ﬁom all tank-mixtures. However, each Mn fertilizer
either delayed or reduced glyphosate absorption relative to the no Mn control. In tank-
mixtures with and without AMS, most of the glyphosate was absorbed within the ﬁrst 24
h.

Mn-EAA, Mn-LS, and MnSO4, reduced glyphosate translocation in tank-mixtures
without AMS, but Mn-EDTA did not (Table 2). Nearly all of the glyphosate applied in
tank-mixtures with Mn-EAA remained in the treated leaf. This extremely limited
translocation explained why tank-mixtures with Mn-EAA antagonized glyphosate
efﬁcacy even though glyphosate absorption was high (see Table 1). Translocation of
absorbed glyphosate was similar for Mn-LS and MnSO4 tank-mixtures, with more than
half of the absorbed glyphosate remaining in the treated leaf. The antagonism caused by

Mn-LS and MnSO4 is therefore attributable to both reduced absorption and reduced

40

translocation. In Mn-LS and MnSO4 tank-mixtures we estimate that less than 10% of the
applied glyphosate entered the plants and moved toward actively growing tissue where
glyphosate has its phytotoxic effect.

Adding AMS increased the percentage of absorbed glyphosate that was translocated
from the treated leaf for Mn-EAA, Mn-LS, and W304 tank-mixtures (Table 4). Of
these three fertilizers, the increase in glyphosate absorbed (Table 2) and translocated
(Table 3) was greatest for Mn-LS. The percentage of glyphosate translocated from
Mn804 tank-mixtures was less than the no Mn control, but was signiﬁcantly greater than
from Mn-EAA tank-mixtures (Table 3). The percentage of glyphosate translocated
reﬂects the differences in velvetleaf control, where control was highest for Mn-LS,
intermediate for MnSO4, and lowest for Mn-EAA (Table 1). Adding AMS never
completely eliminated the antagonism because the sum of the glyphosate absorbed plus
glyphosate translocated to the growing point for the Mn fertilizers never matched that of
the no Mn control. When Mn2+ was not present in the tank-mixture (no Mn), or was
sequestered by EDTA (Mn-EDTA), adding AMS did not have a large effect on the
percentage of absorbed glyphosate translocated ﬁ'om the treated leaf. However, the total
amount of glyphosate translocated was greater because AMS signiﬁcantly increased the
amount of glyphosate absorbed (see Table 2), thereby illustrating how glyphosate
efﬁcacy was increased.

Glyphosate absorption in these experiments was similar to that reported elsewhere
(Hall et al. 2002). Foliar applied glyphosate-Mn tank-mixtures reduced, but did not
eliminate, the absorption and translocation of glyphosate (N ilsson 1985), similar to the

effect caused by Ca2+ in hard water (Thelen et al. 1995a). Adding AMS overcame some

41

of the Mn antagonism of glyphosate and increased absorption, but the reversal was not
complete (Nelawaja and Matysiak 1992; Thelen et al. 1995a). AMS increased glyphosate
absorption 4 h after treatment, earlier than the 24-72 h reported by Hall et al. (2000), and
also increased the amount absorbed and translocated (Wills and McWhorter 1985).
Glyphosate was actively translocated for at least 48 h, and the greatest portion of
translocated glyphosate accumulated in the tissue above the treated leaf (Bromilow et al.
1993)

Glyphosate must pass through the plasmalemma and enter the symplast to be
phytotoxic. Calcium, Mn, and Fe did not block the movement of glyphosate into the cell
and throughout the symplast (Hetherington et al. 1998; Nilsson 1985). In fact, di- and tri-
valent cations increased glyphosate movement into and through the symplast, possibly
because coordination with glyphosate formed a non-ionic molecule that more easily
diffused through the cell membrane (Hetherington et al. 1998). However, when
glyphosate complexed with Mn was applied foliarly, as was done in this study, less
glyphosate appeared able to reach the cell membrane. Two scenarios may account for
this occurrence.

First, the glyphosate-Mn complex may be unable to penetrate the epicuticular wax.
In spray solutions containing Ca2+, glyphosate was trapped in an amorphous spray
deposit on the leaf surface if CaSO4 salts did not crystallize during the drying process
(Nalewaja et al. 1992). Glyphosate trapped in a similar spray deposit with Mn would
have been rinsed off in our experiments and shown reduced absorption. This may be the
type of effect we observed with Mn804 and Mn-LS where glyphosate absorption was

decreased (Table 2). However, when AMS was added to the tank mixture, free Mn2+

42

may have re-crystallized with SO42' during spray droplet drying, thereby allowing
glyphosate to pass through the epicuticular wax and increasing measured absorption.

Second, the Mn-glyphosate complex may have passed through the epicuticular wax,
but if Mn2+ was coordinated by only one glyphosate molecule it may have adhered to a
negative charge in the cuticle, cell wall or on the cell membrane. Hall et al. (2000)
attributed the reduced absorption and efﬁcacy of glyphosate on velvetleaf to glyphosate
binding to Ca2+ in the apoplast. They report that adding AMS increased the amount of
glyphosate translocated, presumably because it prevented glyphosate from interacting
with the free Ca2+.
Mn fertilizer analysis. The absorption and translocation of glyphosate in solutions with
Mn-EDTA, Mn-LS, and MnSOa was similar to what was expected based on efficacy
data. However, the high absorption and low translocation of glyphosate in Mn-EAA
tank-mixtures was puzzling. Glyphosate solubility, absorption, and efﬁcacy are affected
differently by different cations, and Fe, Al, Zn, and Cu are generally more antagonistic
than Ca (Nalewaja and Matysiak 1991; Nilsson 1985; Stahlman and Phillips 1979;
Sundaram and Sundaram 1997; Wills and McWhorter 1985). Dilutions of each fertilizer
dilutions were analyzed for the presence of cations (other than Mn) that might antagonize
glyphosate. The data showed that Mn-EAA was unique from the other Mn formulations
because it contained 0.5% Fe not reported on the fertilizer label (Table 5).

The extremely limited translocation of glyphosate in Mn-EAA fertilizers is likely
caused by the Fe”. The glyphosate-Fe3+ complex was rapidly absorbed into the treated
leaf (Table 2), but was bound tightly in the apoplast, which reduced glyphosate

translocation (Tables 3 and 4). Autoradiographs of glyphosate-micronutrient tank

43

mixtures on pea (Pisum sativum L.) showed little glyphosate movement from the treated
leaf when glyphosate was tank-mixed with Fe (N ilsson 1985). When glyphosate was
applied in solutions with FeC13 to purple nutsedge, greater than 80% of the glyphosate
was absorbed, but less than 15% of the absorbed glyphosate was translocated from the
treated tissue (Wills and McWhorter 1985).

Iron is the cation that caused the greatest antagonism of glyphosate in several
studies (Stahlman and Phillips 1979; Sundaram and Sundaram 1997). Glyphosate has a
higher chelation constant for F e3+ than for Mn2+ and preferentially coordinates with F e3 I
in solution, forming a very stable complex or precipitate, depending on the Fe
concentration (Motekaitis and Martell 1985). The solubility of the Fe-glyphosate
complex is much lower than that of the Mn-glyphosate complex at pH 7.0 (Sundaram and
Sundaram 1997). Consequently, there may be less glyphosate in solution to act on the
plant in solutions containing Fe”.

EDTA is a stronger chelate than citric acid, lignin sulfonate, glucoheptonate, and
other chelates commonly used in micronutrient fertilizers. Fully chelated Mn-EDTA
fertilizer formulations are less likely to antagonize glyphosate efﬁcacy because EDTA
has a higher chelate stability constant for Mn2+ (13.8) than glyphosate (5.5), and will
retain more Mn2+ in tank-mixtures (Martell and Smith 1974). However, Mn fertilizers
containing chelates with stability constants lower than glyphosate, such as citric acid
(4.2) or lignin sulfonate, will lose Mn2+ to glyphosate, and glyphosate efficacy will be
antagonized (Martell and Smith 1974; Nilsson 1985).

In summary, EPR spectra showed that glyphosate complexed with MnSO4. The

glyphosate-Mn complex reduced glyphosate absorption and translocation. The effect of

44

four Mn formulations on glyphosate absorption and translocation explained their
differential effects on glyphosate efﬁcacy. Mn chelated by EDTA was the least likely to
reduce glyphosate absorption, translocation, and efﬁcacy. Similar formulations should be
preferred for use in glyphosate-Mn tank-mixtures where that is a necessary or desirable
practice. Adding AMS to the tank-mixture increased glyphosate absorption and
translocation, but did not eliminate the antagonism caused by Mn fertilizers. The
presence of other micronutrient cations in the fertilizer exacerbated the antagonistic
effects of Mn. Further studies evaluating the relative effect of other micronutrients and
micronutrient formulations to glyphosate efficacy would be useful to identifying

micronutrient formulations that will not antagonize glyphosate in tank-mixtures.

45

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dehydrogenase by electron spin-echo envelope modulation spectroscopy. J. Am.
Chem. Soc. 122:931-938.

Stahlman, P. L. and W. M. Phillips. 1979. Effects of water quality and spray volume on
glyphosate phytotoxicity. Weed Sci. 27:38-41.

Subramaniam, V. and P. E. Hoggard. 1988. Metal complexes of glyphosate. J. Agric.
Food Chem. 36:1326-1329.

47

Sundaram, A. and K.M.S. Sundaram. 1997. Solubility products of six metal-glyphosate
complexes in water and forestry soils, and their inﬂuence on glyphosate toxicity to
plants. J. Environ. Sci. Health B32:583-598.

Thelen, K. D., E. P. Jackson, and D. Penner. 1995a. The basis for the hard-water
antagonism of glyphosate activity. Weed Sci. 43:541-548.

Thelen, K. D., E. P. Jackson, and D. Penner. 1995b. Utility of nuclear magnetic
resonance for determining the molecular inﬂuence of citric acid and an
organosilicone adjuvant on glyphosate activity. Weed Sci. 43:566-571.

Tisdale, S. L., W. L. Nelson, J. D. Beaton, and J. L. Havlin. 1993. Soil fertility and
fertilizers. Fifth ed. Macmillan Publishing Co., New York.

Wills, G. D. and C. G. McWhorter. 1985. Effect of inorganic salts on the toxicity and

translocation of glyphosate and MSMA in purple nutsedge (Cyperus rotundus).
Weed Sci. 33:755-761.

48

Table I . The interaction of Mn fertilizer formulation and ammonium

sulfate (AMS) on velvetleaf control in glyphosate tank-mixtures“.

 

 

 

 

 

Velvetleaf control
Mn fertilizer none” AMS
%
No Mn 37.5 a 96.3 a
Mn-EAA 1.3 b 35.0 d
Mn-EDTA 43.8 a 68.8 b
Mn-LS 8.8 b 68.8 b
MnSO4 11.3b 51.3c

 

" Tank-mixtures were prepared in distilled water. Glyphosate was applied
at 0.28 kg ae/ha, Mn-EAA at 0.54 kg Mn/ha, Mn-EDTA at 0.74 kg/ha,
Mn-LS at 0.59 kg/ha, and Mn804 at 2.5 kg/ha. Spray volume was 190
L/ha.

b Plants were rated for control 14 d after treatments were applied, 0 = no
control, 100 = dead plant. Means within a column marked by the same
letter are not signiﬁcantly different, p = 0.05, Tukey adjusted. The standard
error was 2.41.

49

Table 2. The effect of Mn formulation, ammonium sulfate (AMS), and time on

absorption of l4C-glyphosate by velvetleaf“.

 

l4C-glyphosate absorptionb

 

Effect of time

 

 

 

AMS Mn fertilizer 4 hc 24 h 48 h mean separationd
%

0 g/L No Mn 18.7 be 43.3 b 45.9 b 2, y, y
Mn-EAA 34.7 a 41.6 b 43.2 b 2, z, z
Mn-EDTA 29.6 ab 56.2 a 67.6 a z, y, y
Mn-LS 14.7 c 23.2 c 21.6 c z, z, z
MnSO4 10.4 c 23.9 c 25.5 c z, z, z

20g/L NoMn 41.6a 91.5a 91.8a z,y,y
Mn-EAA 35.5 ab 72.2 b 77.2 b 2, y, y
Mn-EDTA 39.8 a 74.1 b 80.8 ab 2, y, y
Mn-LS 22.0 c 46.4 c 58.2 c z, y, x
MnSOa 23.4 bc 43.1 c 47.3 c z, y, y

 

“ The second oldest leaf of plants in the 4-leaf stage was covered with aluminum foil,
and plants were treated as described in Table 1. The foil was removed and two 1 11L
drops of the treatment solutions, spiked with 1000 Bq/uL of l4C-glyphosate were
placed on the adaxial side of the second oldest leaf. This leaf was excised and rinsed
at 4, 24, or 48 h using 4 mL of an acid solution.

b The percent absorption was calculated by dividing the quantity of l4C-glyphosate in
the rinsate by the amount of l4C-glyphosate applied.

° Means within a column marked by the same letter are not signiﬁcantly different, p =
0.05. The standard error for tank-mixtures without AMS (0 g/L) was 4.2. The
standard error for tank-mixtures with AMS (20 g/L) was 4.3.

d Represents the statistical differences between the means for 4 h, 24 h, and 48 h,
respectively. Means with the same letter are not signiﬁcantly different, p = 0.05.

50

Table 3. The effect of Mn formulation and time on translocation of l4C-glyphosate
from tank-mixtures prepared without ammonium sulfate (AMS)“.

 

 

 

 

l4C-glyphosate translocationb Effect of time
Plant section Mn fertilizer 4 h0 24 h 48 h mean separationd
------ % of recovered ------
Above no Mn 14.8 a 41.5 a 52.4 a z, y, x
Mn-EAA 1.4 b 2.1 c 4.6 c z, z, z
Mn-EDTA 11.8 a 44.5 a 45.9 a z, y, y
Mn-LS 9.3 a 25.8 b 27.5 b 2, y, y 11‘
W80, 10.8 a 21.5 b 22.2 b 2, y, y
Treated leaf no Mn 75.9 c 33.8 c 27.4 d z, y, x P,
Mn-EAA 97.8 a 96.7 a 93.8 a z, z, z
Mn-EDTA 79.5 be 29.5 c 27.7 d z, y, y
Mn-LS 82.8 b 63.1 b 63.5 c z, y, y 5'
MnSOa 79.5 be 66.6 b 71.8 b 2, y, y
Below no Mn 6.8 a 15.6 a 11.8 b 2, x, y
Mn-EAA 0.5 b 0.8 c 0.7 d z, z, z
Mn-EDTA 6.3 a 17.1 a 15.9 a z, y, y
Mn-LS 4.9 a 6.7 b 5.4 c z, z, z
MnSO4 5.8 a 8.1 b 3.5 c z, z, 2
Roots no Mn 2.5 a 9.1 a 8.3 b 2, y, y
Mn-EAA 0.3 b 0.4 c 0.9 d z, z, z
Mn-EDTA 2.3 a 8.9 a 10.5 a z, y, y
Mn-LS 3.0 a 4.5 b 3.6 c z, z, 2
W804 3.8 a 3.8 b 2.4 cd 2, z, z

 

8 Plants were treated as described in Table 2. After the treated leaf was rinsed, the plant was divided
into four sections 1) stem and leaves above treated leaf (Above), 2) Treated leaf, 3) stem and leaf
below treated leaf (Below), and 4) Roots. Plant tissue was dried, weighed, ground, combusted, and
analyzed for I‘C-glyphosate by liquid scintillation spectrophotometry.

b The percentage of 4C-glyphosate present in each section of the plant was calculated by dividing the
1‘C DPM for that section of the plant by the sum of 1‘C DPM recovered from combusted tissue.

° Means within a column marked by the same letter are not signiﬁcantly different, p = 0.05. The
standard error for Above was 2.4, for Treated leaf 2. l , for Below 1.0, and for Roots 0.6.

d Represents the statistical differences between the means for 4 h, 24 h, and 48 h, respectively. Means
with the same letter are not signiﬁcantly different, p = 0.05.

51

Table 4. The effect of Mn formulation and time on translocation of l4C-glyphosate
from tank-mixtures prepared with ammonium sulfate (AMS)“.

 

Plant section Mn fertilizer

l[CI-glyphosate translocationb

Effect of time

 

4h°

24h

48h

mean separationd

 

Above

no Mn

------ % of recovered in tissue-----

9.5 ab 30.4 a 40.5 a z, y, x

Mn-EAA 4.2 c 13.3 b 12.8 c z, y, y

Mn-EDTA 12.0 a 31.8 a 42.2 a z, y, x

Mn-LS 9.3 ab 31.8 a 39.5 a z, y, x

Mn804 7.1 bc 28.3 a 34.3 b 2, y, x

Treated leaf no Mn 79.4 be 38.5 c 28.7 c x, y, z

Mn-EAA 92.2 a 77.7 a 80.1 a y, z, z

Mn-EDTA 77.5 c 38.8 c 31.6 c x, y, z

Mn-LS 82.6 bc 43.4 bc 31.1 c x, y, z

MnSO.t 83.2 b 46.6 b 39.8 b x, y, 2

Below no Mn 8.4 a 22.6 a 21.2 a z, y, y
Mn-EAA 2.7 c 6.3 d 4.2 e Z, y. Yz

Mn-EDTA 7.9 ab 21.7 ab 17.7 b 2, x, y

Mn-LS 6.1 ab 15.9 c 18.1 b 2, y, y

MnSO4 5.7 b 19.5 b 17.3 b 2, y, y

Roots no Mn 2.7 ab 8.5 a 9.5 ab 2, y, y

Mn-EAA 0.9 b 2.8 c 2.9 c z, z, z

Mn-EDTA 2.6 ab 7.7 ab 8.5 b 2, y, y

Mn-LS 2.0 ab 9.0 a 11.4 a z, y, x

MnSO4 4.0 a 5.6 b 8.6 b 2, z, y

 

‘ Plants were treated as described in Table 2. After the treated leaf was rinsed, the plant was divided
into four sections 1) stem and leaves above treated leaf (Above), 2) Treated leaf, 3) stem and leaf
below treated leaf (Below), and 4) Roots. Plant tissue was dried, weighed, ground, combusted, and
analyzed for l‘C-glyphosate by liquid scintillation spectrophotometry.

b The percentage of 4C-glyphosate present in each section of the plant was calculated by dividing the
MC DPM for that portion of the plant by the sum of MC DPM recovered from combusted tissue.

° Means within a column marked by the same letter are not signiﬁcantly different, p = 0.05. The
standard error for Above was 1.7, for Treated leaf 2.0, for Below 0.9, and for Roots 0.8.

d Represents the statistical differences between the means for 4 h, 24 h, and 48 h, respectively. Means
with the same letter are not signiﬁcantly different, p = 0.05.

52

Table 5. Analysis of Mn fertilizer formulations for cations antagonistic to

 

 

 

 

 

glyphosate efﬁcacy“.
Cation concentration
Mn fertilizer Fe Ca Zn Al Cu
lie/L

Mn-EAA 4550 742 1 1 8 10 trace
Mn-EDTA trace 303 3 7 trace
Mn-LS 217 600 79 74 8
MnSO4 33 530 296 8 5

 

a Dilutions of each of the fertilizers were analyzed using DCP (direct current
plasma) spectroscopy.

53

‘0 -
pKa1 2.2 O

pKa2 5.5 /C_C_
pKa3 10.1 /

Figure I. A depiction of glyphosate acid (N-phosphonomethylglycine). The log
protonation constants for three coordinating groups are given (Motekaitis and

Martell 1985).

54

2500

 

2000
1500
1000*
500*
o 2
-500
-1000.
-1500
-2000~

 

 

 

-2500 t l
3000 3200 3400 3600 3800

gauss

Figure 2. Continuous wave EPR spectra of 1.125 mM glyphosate + 0.75 mM Mn

solution at room temperature.

55

 

100000
80000 ,
60000
40000 a
20000 a

0

-20000
-40000 ,
-60000 ,
-80000

-100000 1. t .

3000 3200 3400 3600 3800

 

 

l

 

gauss

Figure 3. Continuous wave EPR spectra of 1.125 mM glyphosate + 0.75 mM Mn

solution frozen to the temperature of liquid helium.

56

 

0.9 1

0.8 ,

 

 

0.7 l 1 r
0 1000 2000 3000 4000

 

Time (ns)

Figure 4. The ratio of four 3—pulsed ESEEM EPR spectra, tau = 204 us. The ratio
[glyphosate-Mn (D2O)/(H20)]/[DTPA-Mn (D2O)/(H2O)] allowed us to isolate the
effect of D20 in the inner coordination sphere. The arrow points to the region of

the spectra used to calculate the distance between Mn2+ and D20 of 3.0 A.

57

CHAPTER 3
IRRIGATION SCHEDULING TO INCREASE SOYBEAN YIELD

IN THE GREAT LAKES REGION

Abstract: Soybean (Glycine max L.) producers with center-pivot irrigation systems in
the Great Lakes region have reported inconsistent yield response to irrigation. Little
research has been reported on the effect of irrigation scheduling for soybeans in this
region. Our objective was to evaluate ﬁve irrigation schedules, based on soybean growth
stage and the soil moisture, on soybean yield. A three year ﬁeld study was conducted on
a Spinks loamy ﬁne sand (sandy, mixed, mesic Psammentic Hapludalfs) near Benton
Harbor, MI. Soil moisture was measured weekly to a depth of 0.9 m using TDR (time
domain reﬂectometry) technology. Five irrigation treatments were evaluated. Deﬁcit
and full season treatments were irrigated to maintain volumetric soil moisture (V SM) at
25% and 50% of capacity, respectively, for the entire growing season. The growth stage
treatments of ﬂowering, pod development, and seed ﬁll were irrigated to maintain VSM
at 25% of capacity before reaching the designated growth stage, and at 50% of capacity
thereafter.

Soybean yield of the full season, ﬂowering, and pod development irrigation
treatments were statistically equal each year. Yields of soybeans irrigated beginning at
seed ﬁll were less than the full season but greater than the deﬁcit irrigation treatments.
Soybean yield increases were related to increased numbers of seeds and pods per plant,
and increased seed size. In 2003, the yield response of eight cultivars to the ﬁve

irrigation schedules was similar. Plant height and lodging increased as the rate of

58

irrigation applied increased. The irrigation schedule did not affect full season weed
control when glyphosate was applied twice, but when glyphosate was applied only once,
weed control was reduced for treatments that were not irrigated until after ﬂowering.
Grain yield of soybean plots receiving one or two applications of glyphosate were equal.
Soybean aphid (Aphis glycines) were present in 2003 and aphid populations were
statistically higher in irrigated plots, although the difference may not be practically
signiﬁcant. To maximize yield, soybean producers in the Great Lakes Region should
plant high-yielding varieties, and maintain VSM at greater than 50% of capacity
beginning at ﬂowering (R1-R2).

Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. #1
CHEAL; large crabgrass, Digitaria sanguinalis L. # DIGSA; soybean, Glycine max L.;
soybean aphid, Aphis glycines.

Additional Index Words: weed control, soybean aphid.

Abbreviations: AMS, ammonium sulfate; TDR, time domain reﬂectometry; VSM,

volumetric soil moisture.

INTRODUCTION
Seed corn producers near the Michigan-Indiana border have been encouraged by
seed companies to include soybean in their crop rotation. In this region, seed corn is
often produced on sandy soils, and most growers have installed center pivot irrigation

systems. However, these growers have reported inconsistent soybean yield responses to

 

1 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 10'” Street, Lawrence, KS 66044-

8897.

59

 

irrigation. Little research has been reported from northern states east of the Mississippi
River on the effect of irrigation in soybeans. In studies conducted in the South and Great
Plains soybean yield generally increased with irrigation, although the percent increase
varied signiﬁcantly depending upon the year and weather, cultivar, soybean growth stage
at irrigation, planting date, quantity and timeliness of irrigation, soil type, and incidence
of lodging and disease (Ashley and Ethridge 1978; Boquet 1989; Camp and Sadler 2002;
Daniels and Scott 1991; Eck et al. 1987; Heatherly 1985; Kadhem et al. 1985a; Specht et
a1. 1989). Irrigation schedules are usually based on critical soil moisture thresholds
and/or soybean growth stage (Brady et al. 1974; Elmore et al. 1988). The objective of
this research was to evaluate the effect of ﬁve irrigation schedules on soybean yield in the
Great Lakes region.

Soybean yield response to irrigation is affected by the growth stage at which the
irrigation is applied. Soybean yields are more tolerant of water deﬁcits during vegetative
growth than during reproductive growth (Kirda 2002; Moutonnet 2002). Irrigation
applied during the vegetative growth stage does not consistently increase yield, except
when water availability is extremely restricted or irrigation is needed to enable
emergence (Ashley and Ethridge 1978; De Costa and Shanmugathasan 2002; Doss and
Thurlow 1974; Martin et al. 1979; Neyshabouri and Hatﬁeld 1986; Scott 1985).
Irrigation beginning at ﬂowering, pod development, or seed ﬁll grth stages each
increase yield, although soil water levels must remain plentiful until physiological
maturity to maximize yield increases (Doss and Thurlow 1974; Eck et al. 1987; Heatherly
and Spurlock 1993; Korte et al. 1983a). When irrigation water availability is restricted,

the maximum yield increase may be obtained by a single irrigation at the pod

60

 

development stage (Kadhem et al. 1985a). Although yield response is affected by the
grth stage when irrigation is applied, soybean yield and dry matter accumulation are
linearly related to the amount of water transpired (Lawn 1982; Ray and Sinclair 1998;
Santa Olalla et al. 1994; Scott 1985; Specht et al. 1986). Peak water use rates occur
during ﬂowering and pod development (Benham et al. 1999).

Soybean yield is determined by the number of pods per plant, seeds per pod, and
seed size. The grth stage when irrigation begins affects the way yield is partitioned
into those components. Irrigation during vegetative growth may increase plant height
and the number of reproductive nodes (De Costa and Shanmugathasan 2002; Elmore et
al. 1988). However, it also may lead to more lodging, create an environment favorable
for some diseases, and allow the roots to remain shallow, exposing plants to greater risk
if moisture is deﬁcient later in the growing season (Ashley and Ethridge 1978; Boquet
1989; Brady et al. 1974; Scott 1985). Irrigation during ﬂowering (RI-R2), pod
development (RB-R4), and early seed ﬁll (R5) increases the number of pods and seeds
per plant because the soybean continues to ﬂower through these stages, but the number of
pods per plant declines as irrigation is delayed (Heatherly 1985; Korte et al. 1983b;
Kadhem et al. 1985b). Delaying irrigation until late seed ﬁll when the plant is no longer
ﬂowering increases seed size (Ashley and Ethridge 1974; Heatherly 1985; Kadhem et al.
1985b; Korte et al. 1983b).

Many researchers recommend that soil moisture deﬁcits not exceed 50% in the upper
60-90 cm of soil to avoid yield loss (Benham et al. 1999; Brady 1974; Elmore et al.
1988). However, yield is not always reduced when soil moisture deﬁcits exceed that

threshold (Brady et al. 1974; Doss and Thurlow 1974; Eck et a1. 1987). Soybean

61

transpiration does not decline until the fraction of transpirable soil water reaches 35%,
and yields should remain constant until the transpirational ﬂux declines (Ray and Sinclair
1998). Center pivot irrigation systems restrict the amount of water that can be applied
during peak consumptive use because their capacity is limited. On coarse textured soils
where soil water storage is limited this requires careful management of soil moisture
levels to ensure that adequate moisture is available during peak consumptive use
(Benham et al. 1999; Elmore et al. 1988).

Soybean varieties show a broad spectrum of responsiveness to irrigation, ranging
from substantial yield increases to signiﬁcant yield decreases relative to non-irrigated
checks (Ashley and Etheridge 1978; Korte et al. 1983a; Kadhem et al. 1985a; Specht et
al. 1986). High-yielding varieties usually respond more positively to irrigation than
average- or poor-yielding varieties (Kirda 2002; Sneller and Dombek 1997; Specht et al.
2001). When high-yielding, lodging resistant varieties are grown, varieties with
determinate and indeterminate growth habits yield similarly (Cooper et al. 1991). A
second objective to this research was to evaluate eight high-yielding varieties adapted to
the Great Lakes Region to determine if they responded differently to ﬁve irrigation
schedules.

Studies measuring weed control under irrigated systems show that the irrigation
treatment does not affect weed populations as much as the weed control treatment.
Heatherly et al. (1994) tested three levels of weed control and reported that weed
populations increased as the level of control decreased in both irrigated and non-irrigated
environments. In a later study, Heatherly et al. (2003) compared glyphosate and non-

glyphosate cultivars and weed control treatments in irrigated environments. Two POST

62

applications of glyphosate controlled weeds as well as conventional POST herbicides and
conventional PRE+POST herbicide programs. The third objective in this study was to
evaluate the effect of irrigation schedules on weed control using glyphosate.

Soybean aphid was ﬁrst identiﬁed in North America in 2000. When soybean ﬁelds
are heavily infested with soybean aphid, grain yield is reduced (DiFonzo and Hines
2002). Aphids are susceptible to fungal diseases, and a high moisture environment in an
irrigated canopy may favor the spread of fungus that kills aphids. The fourth objective of

this study was to evaluate the effect of irrigation schedule on soybean aphid populations.

MATERIALS AND METHODS

The experiment was conducted during the 2001-2003 growing seasons on a Spinks
frne sandy loam (sandy, mixed, mesic Psammentic Hapludalfs) at Michigan State
University’s Southwest Michigan Research and Extension Center (SWMREC), near
Benton Harbor, MI. In the spring of 2001, the soil was limed to a pH of 6.5. It was
fertilized each year with K20 (2001, 80 kg/ha; 2002, 120 kg/ha; 2003 120 kg/ha), chisel
plowed and disked. Daily maximum and minimum air and soil temperatures, solar
radiation, wind, and precipitation were recorded by the Automated Weather Station
located on the research station and operated by the Michigan State University
Agricultural Weather Office.

Five irrigation treatments were evaluated. Deﬁcit and full season treatments were
irrigated to maintain VSM at 25% and 50% of capacity, respectively, for the entire
growing season. The growth stage treatments of ﬂowering (Rl-R2), pod development

(R3-R4), and seed ﬁll (RS-R6) were irrigated to maintain VSM at 25% of capacity before

63

 

reaching the designated growth stage, and at 50% of capacity thereafter (Ritchie et al.,
1997). No irrigation was applied after soybean reached physiological maturity (R7).

Irrigation was applied using Wobbler® sprinklers2 from 2 m tall risers. The nozzle
size was 5.6 mm and pressure was regulated at 104 kPa. Risers were spaced 18.3 m apart
and were mounted on 7.6 cm aluminum solid-set pipe. The radius of the spray circle was
approximately 8.2 m. Irrigation distribution was measured by attaching 0.95 L beverage
cups to 1.3 cm PVC pipes 1.1 m above the soil surface. The cups were spaced 1.4 m
apart along the north/south axis of the plot for all treatments, and the east/west axis in the
full season treatment. The water in each cup was measured using a graduated cylinder
shortly after the irrigation was completed and the irrigation amount was calculated.

One (2001) or two (2002, 2003) tecanat plastic tubes were inserted in the soil 4.6 m
away from each sprinkler riser after planting. Volumetric soil moisture was measured
weekly in 18 cm intervals to a depth of 0.9 m using a TRIME-T3 tube access probe and
FM3 moisture meter3, except for a six week period in 2002 when the moisture meter was
being repaired. The TRIME-method is a TDR-technology modiﬁed to determine the
TDR-curve by time measurements at distinct voltage levels3. The system allowed easy
monitoring of soil moisture at ﬁve depths up to 0.9 m during the growing season. The
volumetric soil moisture capacity was estimated at 16% based on measurements taken on
2 June 2001 following a 22 mm rain event. A VSM of 8% was designated as 50% of

capacity, and a VSM of 4% was designated as 25% of capacity.

 

2 Senninger Irrigation Inc., 6416 Old Winter Garden Road, Orlando, FL 23 835.

3 MESA Systems Co., 6 West Mill St., Unit 3, Medﬁeld, MA 02052

64

The experimental design was a split-plot in 2001, and a split-split plot in 2002 and
2003. In all years, the main plot was irrigation treatment. In 2001, the sub-plot was
weed control treatment. In 2002 the sub-plot was variety, and the sub-sub plot was weed
control treatment. In 2003, the sub-plot was herbicide tolerance, and the sub-sub plot
was variety. Three replications of the test site were planted with a grass cover crop in
2000, the fourth replication was used for tomato and potato experiments. Soybeans were
grown in rotation with corn in 2001 and 2002, and treatments were replicated four times.
In 2003, irrigation treatments were replicated eight times (four replications were soybean
following corn, and four were soybean following soybean).

Soybeans were planted on 26 April 2001, 7 June 2002, and 3 May 2003 in 38 cm
rows using planters equipped with John Deere 7200 MaxEmerge Plus planting units. The
late planting date in 2002 was a second planting; the ﬁrst planting was infected by
pythiurn rot and emergence was poor. The years and varieties planted were: 2001,
Asgrow 2703RR; 2002, Asgrow 2703RR, and ‘Stout;’ 2003, glyphosate resistant Asgrow
2703RR, D.F. Seeds 8316RR, DynaGro 3200RR, and Garst 2502RR and non-glyphosate
resistant Asgrow 2553, Dairyland DSR 300, Garst D308, and Stout. The seeding rate
was 400,000 seeds/ha, with the exception of Stout in 2002, which was seeded at 640,000
seeds/ha as recommended by R. Cooper (personal correspondence).

Four weed control treatments using glyphosate were evaluated in 2001 and 2002 in
Asgrow 2703RR plots: 1) no control (weedy), 2) one application of glyphosate +
ammonium sulfate (AMS), 3) two applications of glyphosate + AMS, and 4) unlimited

applications of glyphosate until canopy closure (weed free). Glyphosate4 (0.84 kg/ha)

 

4 Roundup UltraMAX", Monsanto Company, 800 Lindbergh Boulevard, St. Louis, MO 63167

65

was applied in solution with 2% AMS (w/w) when weeds were 5-10 cm tall. Only two
applications of glyphosate were necessary in treatment #4 so herbicide applications for
this treatment were identical to treatment #3 and data were combined for analysis.
Herbicides were applied using a backpack sprayer with CO2 propellant at 187 L/ha.
Nozzle spacing was 51 cm, and the nozzles were TeeJet 8002XR Flat Fanss. Control
ratings were made on a scale of 0-99 where 0 represents no control and 99 represents
complete control 14 and 28 d after application, and prior to harvest. The dominant weed
species were common lambsquarters and large crabgrass.

In 2002, four weed control treatments were used in Stout soybean: 1) no control; 2)
tl'lifensulfuron-methyl6 (4.4 g/ha) + ﬂuazifop-P-butyl7 (140 g/ha) + fenoxaprop-P—ethyl7
(39 g/ha) + 1.25% non-ionic surfactant (v/v) applied POST; 3) S-metolochlor8 (1.1 kg/ha)
applied PRE, and bentazon9 (1.1 kg/ha) + sethoxydirnlo (0.21 kg/ha) + crop oil
concentrate (2.3 L/ha) applied POST; and 4) S-metolochlor (1.1 kg/ha) applied PRE, and
thifensulfuron-methyl (4.4 g/ha) + ﬂuazifop-P-butyl (140 g/ha) + fenoxaprop-P-ethyl (39
g/ha) + 1.25% non-ionic surfactant (v/v) applied POST. All treatments were applied

using a backpack sprayer as described above.

 

5 TeeJet”, Spraying Systems Co., PO. Box 7900, Wheaton, IL 60189

6 Harmony GT“, [5.1. du Pont de Nemours and Company, Crop Protection, Wilmington, DE 19898

7 F usion", Syngenta Crop Protection, Inc., Greensboro, NC 27409

8 Dual 11 Magnumo, Syngenta Crop Protection, Inc., Greensboro, NC 27409

9Basagran", BASF Corporation, Agricultural Products, PO. Box 13528, Research Triangle Park, NC
27709

‘0 Poast", BASF Corporation, Agricultural Products, PO. Box 13528, Research Triangle Park, NC

27709

66

In 2003, glyphosate resistant beans received two POST applications of glyphosate
(0.84 kg/ha) + 2% AMS (w/w), and non-glyphosate resistant beans received S-
metolachlorll (1.4 kg/ha) + metribuzin12 (0.35 kg/ha) PRE, and thifensulfuron-methyl
(4.4 g/ha) + 1.25% non-ionic surfactant (v/v) POST.

Soybean aphids were present in the ﬁeld in 2001 and 2002, but their numbers were
extremely low. In 2003 aphid populations were much higher and counts were made
according to the MSU scale (DiFonzo and Hines 2002) on 6 and 14 August 2003. We
selected a leaf at the mid-canopy height from 10 plants in each of the Asgrow 2703RR
and Garst D308 plots. Shortly after the count on 14 August the aphid population crashed
because of a pathogenic fungus, and no further counts were made.

Plots were 2.3 m wide and 6.1 m long, and were trimmed to 4.3 m for harvesting.
Prior to harvest, plant heights were obtained by measuring three plants in each plot to the
top of the stem. Lodging ratings were made on a scale of 1 to 5, where 1 represents no
plants lodged, and 5 represents all plants lodged. Plots were harvested with a small plot
combine equipped to measure grain weight and moisture. Ten plants were hand-
harvested from each plot and the number of pods and seeds per plant was counted. In
2003 seed weight was also measured.

Data were tested to determine if they met the assumptions for analysis of variance.
Data from different years were combined when treatments were identical and variances

were similar. All statistical analysis was done using PROC MIXED and PROC GLM of

 

” Dual Magnum", Syngenta Crop Protection, Inc., Greensboro, NC 27409
'2 Sencor" DF, Bayer CropScience LP, PO. Box 12014, 2 T.W. Alexander Drive, Research Triangle

Park, NC 27709

67

 

SAS (SAS 2001). Contrasts were evaluated for signiﬁcance using Bonferroni

simultaneous conﬁdence intervals.

RESULTS AND DISCUSSION
Weather and irrigation. Monthly precipitation in 2001 was near normal with the
exception of July when it was 3.7 cm less than normal (Table 1). Temperatures
throughout the growing season were normal or slightly cooler. The critical period for
moisture stress occurred from mid-June to mid-August when average weekly rainfall was
less than 1.25 cm and soybeans were near their peak transpiration rates. The VSM in the
pod development, seed ﬁll, and deﬁcit treatments fell below 50% of capacity the second
week of July and neared 25% of capacity the third week of July (Figure 1). At 25% VSM
capacity plants wilted at mid-day, but recovered turgor at night. Irrigation was applied to
keep those treatments above the 25% VSM capacity threshold until they reached the
appropriate growth stage for full irrigation. VSM was not allowed to go below 50% of
capacity in the full season and ﬂowering treatments. The last irrigation was applied 9
August because plentiﬁrl rainfall the second half of August raised available moisture
above the 50% threshold for the remainder of the season (Table 2).

In 2002, May temperatures were cooler than normal and contributed to the Pythium
spp. infection that reduced emergence in the ﬁrst planting (Table 1). Temperatures aﬁer
the soybeans were replanted June 7 were warmer than normal. Growing season
precipitation after June 7 was less than average, but was distributed more evenly across
the growing season than in 2001. Plentiful rain during the critical pod development and

seed ﬁll stages in August bolstered yields. Soil moisture levels were similar for the

68

irrigation treatments during the ﬁrst quarter and last half of the growing season when the
TDR equipment was repaired and available (Figure 2).

Temperatures in 2003 were cooler than normal every month but August, slowing
crop. development and reducing yield potential (Table 1). Growing season precipitation
was less than normal. It was particularly dry during the critical reproductive period of
mid-July through mid-September. Average VSM for all treatments fell below 50% of
capacity in mid-August because of a missed irrigation (Figure 3). More irrigation was
applied in 2003 than the previous two years, and the deﬁcit irrigation treatment was
irrigated three times in August to keep VSM above 25% of capacity (Table 2).

Soybean yield. Irrigation signiﬁcantly increased average soybean yield each year of the
study (Table 3). The full season irrigation treatment had the highest grain yield, but was
statistically equal with irrigation treatments initiated at ﬂowering and pod development
each year. Waiting until seed ﬁll to irrigate reduced seed yield relative to the full season
irrigation treatment each year, but increased yield relative to the deﬁcit irrigation
treatment in 2001 and 2003.

The largest yield increases from irrigation were obtained in 2001 when full season
irrigation increased yields 90%, irrigation begun at ﬂowering increased yields 80%, at
pod development 70%, and at seed ﬁll 50%, relative to the deﬁcit irrigation treatment.
Soybean yields beneﬁted from early planting, warm growing conditions in May, and
plentiﬁrl moisture the second half of August (Comis 2001). In 2002, full season
irrigation only increased yields 25% because plentiful precipitation during the pod
development and seed ﬁll growth stages enabled soybeans to compensate for earlier

water stress (Table 1). In 2003, full season irrigation increased seed yield 55%, but the

69

 

average seed yields were much lower than in 2001 or 2002 (Table 3). Soybean yields
were lower than average throughout the Midwest in 2003. Factors that contributed to
lower yields were cool temperatures during vegetative growth, soybean aphid damage,
and the missed irrigation during early seed ﬁll. The resulting water stress caused some of
the older leaves to senesce, reducing leaf area, radiation use, and the plants’ ability to ﬁll
seeds (Sionit and Kramer 1977), and may have caused embryo abortion and reduced seed
size (Eck et al. 1987; Shaw and Laing 1966).

Full season irrigation and irrigation initiated at ﬂowering produced statistically equal
grain yield. The critical period for beginning irrigation was during the ﬂowering or pod
development growth stages (Ashley and Ethridge 1978; Benharn et al. 1999; Brady et al.
1974; Doss et al. 1974; Elmore et al. 1988). Elmore et al. (1988) suggested that in dry
years irrigation should begin at ﬂowering, and in wetter years may be delayed until pod
development without reducing yield. In this experiment, yield of the pod development
treatment was statistically equal to yield of the ﬁrll season irrigation — but in 2001 and
2002 yields were 10-12% lower, similar to the yield loss reported for moisture stress
during ﬂowering (Eek et al. 1987). In 2001 and 2002 partial irrigations were applied to
the pod development treatment before it reached R3 to prevent the VSM from falling
below 25% of capacity (Figures 1 and 2). Yield of the pod development treatment might
otherwise have been signiﬁcantly lower.

The irrigation treatment by variety interaction was not signiﬁcant in 2002 or 2003 (p
= 0.05), but in 2002 ‘Stout’ yields responded more to increasing irrigation than Asgrow

2703RR (Table 3). In 2003 yields of each variety were increased by irrigation. DynaGro

70

3200RR yielded less (p = 0.05) than the other varieties because poor seedling emergence
caused very low plant populations.

Most studies evaluating multiple cultivars have reported that varieties respond
differently to irrigation treatments (Ashley and Ethridge 1978; Kadhem et al. 1985a;
Korte et al. 1983a), and variety response is not always consistent across years (Elmore et
al. 1988). However, Cooper et al. (1991) reported that ﬁve high-yielding varieties all
responded positively to irrigation. The varieties grown in this experiment were among
the highest yielding in Michigan State University variety trials. High-yielding varieties
are more responsive to favorable growing conditions than low- or moderate-yielding
varieties. Thus, for a ﬁeld where irrigation is an option and the environment is favorable
for high yields, it is important to plant a high-yielding cultivar (Kirda 2002; Sneller and
Dombek 1997; Specht et al. 2001).

Irrigation water use efﬁciency (IWUE) is a measure of yield gained per cm of water
applied in excess of the deﬁcit irrigation treatment. The IWUE was much greater in 2001
than in 2002 or 2003 (Table 4). The ﬂowering and pod development treatments appeared
to be more efﬁcient times to begin irrigating. The IWUE may not change drastically
between irrigation treatments because yield is linearly related to transpiration. Elmore et
al. (1988) reported similar IWUE for full season, ﬂowering, and pod development
irrigation treatments. However, treatments can signiﬁcantly affect IWUE, and Heatherly
(1988) reported a higher IWUE for early planted soybeans. Water use efﬁciency (WUE)
values are provided in Table 4 for comparison with IWUE.

Yield components. Yield components of the varieties tested responded to irrigation

treatments similarly, and the irrigation treatment by variety interaction was not

71

signiﬁcant, p = 0.05. However, there were signiﬁcant differences between the irrigation
treatments (Table 5) and between the varieties (Table 6).

The number of seeds per plant and pods per plant responded to the irrigation
treatments similarly (Table 5), but the response was not consistent across years. In 2001
the number of pods per plant and seeds per plant increased as the irrigation applied
increased, and differences in seed and pod numbers paralleled yield differences between
the treatments (compare Table 3). In 2002 the number of pods per plant and seeds per
plant were greater for the full season and ﬂowering treatments than for the other three
treatments. In 2003 the number of pods per plant and seeds per plant was equal for the
full season, ﬂowering, and pod elongation treatments, and was signiﬁcantly greater than
the seed ﬁll or deﬁcit irrigation treatments. Other research shows that the number of
pods and seeds per plant increases with irrigation during reproductive growth stages. The
largest increases occur during ﬂowering and pod development, and the increase declines
as irrigation is delayed (Ashley and Ethridge 1978; Elmore et al. 1988; Kadhem et al.
1985b; Korte et al. 1983b). However, in dry years, irrigation at pod development may
not increase pods per plant, similar to what was observed in 2002 (Elmore et al. 198 8).

The number of seeds per pod was affected by irrigation treatment only in 2003, and
was greatest for early reproductive treatments. Seeds per pod declined when irrigation
was withheld until seed ﬁll and plants experienced water stress in early August when
many of the pods and embryos were being set. The water stress may have caused more

embryo abortion in this treatment than treatments irrigated earlier in the growing season

(Shaw and Laing 1966).

72

Average seed weight was measured in 2003. The largest seed size was in the seed
ﬁll treatment. Seed size was intermediate for full season and pod development, and
smallest for ﬂowering and deﬁcit irrigation. Irrigation during pod development and seed
ﬁll increases seed size (Ashley and Ethridge 1978; Elmore et al. 1988; Kadhem et al.
1985b; Korte et al. 1983b). The greatest increases in seed size usually occurs during R5-
R6 because the primary sink of the plant is seeds that are already formed (Elmore et al.
1988). No stress during early reproductive growth stages may allow the plants to set
more seeds than they can adequately ﬁll, resulting in smaller seeds (Kadhem et al.
1985b)

Yield of seven of the varieties was equal, but the way it was partitioned between the
yield components differed (Table 6). In 2002, yields and yield component values of
Asgrow 27 03RR were greater than those of Stout. Stout is a semi-determinate variety.
Some reports showed that the yield components of determinate varieties increased less in
response to irrigation than those of indeterminate varieties (Kadhem et al. 1985b; Korte et
al. 1983b). However, the determinate varieties were planted at a higher population than
the indeterminate varieties, and the lower response is likely the result of the higher plant
populations. Stout was planted at higher populations than Asgrow 2703RR in 2002.

In 2003, DynaGro 3200RR produced more seeds and pods per plant than the other
varieties and had the smallest seed — all the result of very low plant populations. Stout
had the most pods per plant, the fewest seeds per pod, the fewest seeds per plant, and the
largest seeds. Dairyland DSR 300 had the fewest pods per plant, but the greatest number

of seeds per pod. Although there were signiﬁcant differences in pods per plant, seeds per

73

pod, and seed weight in 2003, the number of seeds per plant was similar for most of the
varieties.

Plant height and lodging. Plant height increased as irrigation applied increased and as it
was applied earlier in the season (Table 7). Plants in the full season and ﬂowering
treatments grew to similar heights, except in 2002. Waiting until seed ﬁll to irrigate did
not increase plant height signiﬁcantly above the deﬁcit irrigation treatment. Plants in the
pod development treatment increased in height compared to deﬁcit irrigation, but the
increase in available water occurred too late in the season for the plants in that treatment
to reach the same height as plants in the full season treatment.

The incidence of lodging was inﬂuenced by plant height, irrigation quantity and
timing (Table 8). In 2001 and 2003, lodging ratings and plant heights in the full season
and ﬂowering treatments were similar, but in 2002 when plants in the ﬂowering treatment
were signiﬁcantly shorter, less lodging occurred. There was little lodging in the pod
elongation, seed ﬁll, or ﬂowering treatments. Others have shown that plant height and
lodging both increase with increased irrigation application, when irrigation is applied
earlier in the growing seasons, and when irrigation is applied at lower soil moisture
depletion levels (Brady et al. 1974; Doss and Thurlow 1974; Specht et al. 1986).

The cultivars differed in plant height and susceptibility to lodging (Tables 7 and 8).
DynaGro 3200RR was signiﬁcantly shorter than all other varieties and lodged the least,
traits again partially attributable to low plant populations. Stout lodged more than any
other variety. This was surprising because Stout was promoted as a semi-determinate
soybean adapted for high-yielding, high-moisture environments (Cooper et al. 2001). In

general, varieties that were taller were more susceptible to lodging. With the exception

74

 

of Stout in the full season treatment in 2002, the majority of the lodging consisted of
leaning plants which did not affect harvestability of the crop.

Irrigation and weed control. The irrigation treatment did not affect control of common
lambsquarters or large crabgrass when glyphosate was applied twice (Table 9). However,
when glyphosate was applied only once, control of large crabgrass declined with
decreasing irrigation, and control of lambsquarters in the deﬁcit irrigation treatment was
less than in the other irrigation treatments. When glyphosate was applied only once, new
weeds may have germinated before canopy closure. The soybeans under irrigation were
taller and canopy closure occurred earlier in the season and improved the competitiveness
of the crop. However, weeds that grew above the canopy increased in size and fecundity
with increasing irrigation, and created the potential for greater weed pressure in the
future. Two applications of glyphosate were as effective as conventional POST and
PRE+POST herbicide program (Heatherly et al. 2003) and provided excellent season-
long control (Table 9). One application of glyphosate may provide adequate weed
control if irrigation begins at the ﬂowering growth stage, depending on row spacing and
the weed spectrum present.

Although more weeds grew in plots that received one application of glyphosate
instead of two, they did not reduce crop yield (Table 9). This contrasts with reports
which show that seed yield in irrigated environments increases as weed control increases,
but in non-irrigated environments medium and high weed control yields are equal
(Heatherly et al. 1994). Plots that were not treated with glyphosate yielded less than plots
that were treated, and the number of pods per plant, seeds per pod, and seeds per plant

were lower for the weedy treatment (Table 10).

75

 

We observed no differences in yield between glyphosate-resistant and conventional
varieties in 2003 (Table 3). Heatherly et al. (2003) reported that grain yield of glyphosate
resistant cultivars under irrigation was lower than non-glyphosate resistant cultivars in
two of three years.

Irrigation and soybean aphids. Soybean aphids were present at very low population
levels in 2001 and 2002, and at moderate population levels in 2003. Counts were made
on two dates in early August 2003 when the aphid populations were at their highest levels
(Table 11). ANOVA analysis showed that the irrigation treatments were signiﬁcant, p =
0.10. Contrasts were made to compare treatments that were being irrigated with those
that were not. Count data from the two dates were combined for the contrasts. Aphid
populations were statistically higher in treatments that were receiving irrigation (full
season, ﬂowering, and pod development) than those that were not (seed ﬁll and deﬁcit), p
= 0.05 (Table 12). When the full season treatment, which had been receiving irrigation
for over one month at the time of the aphid counts was contrasted with the two treatments
that had not been irrigated, the difference in the estimates was larger (Table 12). The
difference between the estimates in both contrasts is small, and may be of little practical
signiﬁcance. However, the difference in aphid populations between irrigation treatments
was opposite of our initial hypothesis -— that being that populations would be lower in
more humid soybean canopies. Further research on the effect of irrigation on soybean
aphid populations, and the effect of soybean aphid on yield in irrigated soybeans would
be useful.

The Great Lakes Region often experiences extended periods of time with inadequate

precipitation during critical growth stages, e. g., 2001 and 2003. Crops grown on sandy

76

 

soils are particularly susceptible to water stress and concomitant yield loss. Soybean
yield was increased by irrigation in all three years of this study. Grain yield was not
statistically increased by applying irrigation before pod development, as long as VSM did
not fall below 25% of capacity prior to the pod development growth stage (R3). Eight
high-yielding varieties adapted to the Great Lakes Region responded to the irrigation
treatments similarly. TDR technology was an effective tool for monitoring soil moisture
for irrigation scheduling, and affordable, related technologies should be developed for r
producer use. Soybean producers in the Great Lakes Region may effectively increase ’

soybean yield under irrigation by planting high-yielding varieties, and irrigating to

 

maintain soil moisture levels above 50% of capacity, beginning at ﬂowering or pod

development.

77

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Boquet, D. J. 1989. Sprinkler irrigation effects on determinate soybean yield and lodging
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Comis, D. 2001. Pushing the yield limits. Agricultural Research 49(6):l3-15.

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Cooper, R. L., T. Mendiola, R. J. Fioito, A. F. Schmitthenner, and A. E. Dorrance. 2001.
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Doss, D. B., R. W. Pearsons, and M. T. Rogers. 1974. The effect of soil water stress at
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Heatherly, L. G. 1985. Irrigation management for soybean yield enhancement. p. 980-
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Heatherly, L. G. 1988. Planting date, row spacing, and irrigation effects on soybean
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Heatherly, L. G. and SR. Spurlock. 1993. Timing of furrow irrigation termination for
determinate soybean on clay soil. Agron. J. 85:1103-1108.

Heatherly, L. G., S. R. Spurlock, and K. N. Reddy. 2003. Inﬂuence of early-season
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Kadhem, F. A., J. E. Specht, and J. H. Williams. 1985a. Soybean irrigation serially timed
during stages R1 to R6. 1. Agronomic responses. Agron. J. 77:291-298.

Kadhem, F. A., J. E. Specht, and J. H. Williams. 1985b. Soybean irrigation serially timed
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80

Table 1. Weather conditions during the 2001, 2002, and 2003 growing seasons.

 

 

 

Average daily max air Total precipitation Total radiation
Dates ‘01“ ‘02 ‘03 30 yr” ‘01 ‘02 ‘03 30yr ‘01 ‘02 ‘03
---------- °C ------- ----------- mm ---------- --- 105 kJ/m2 --

Apr 16-30 18.1 16.5 17.2 15.0 30.2 27.7 20.1 49.0 3.23 2.65 2.85
May 1-15 23.6 16.3 18.3 18.3 29.2 57.4 83.8 42.2 3.00 2.82 2.40
May 16-31 19.7 19.0 18.5 21.4 39.6 28.4 25.7 42.4 2.97 3.47 3.39
Jun 1-15 21.7 24.2 21.8 23.9 69.4 59.7 7.4 44.5 2.59 2.93 3.02
Jun 16-30 26.8 29.5 27.6 25.9 20.3 11.2 8.1 45.2 3.60 3.95 4.08
Jul 1-15 25.7 29.4 28.2 27.1 17.8 34.8 66.3 41.7 4.00 4.16 3.72
Jul 16-31 30.0 30.3 26.2 27.4 28.7 6.1 12.7 40.6 3.55 3.55 3.97
Aug 1-15 29.4 28.7 27.2 27.0 19.6 44.7 24.1 39.4 3.56 3.52 3.24
Aug 16-31 25.6 26.5 29.2 25.8 79.0 87.6 5.1 48.8 2.67 3.05 3.53
Sep 1-15 25.2 27.4 25.1 23.8 28.9 3.6 25.1 54.6 2.92 3.00 2.76
Sep 16-30 18.5 23.8 19.1 21.2 39.9 25.9 51.1 51.3 1.85 2.36 2.26
Oct 1-15 17.8 19.0 18.1 18.2 98.3 27.7 45.5 40.4 1.45 1.87 2.22

 

a Daily maximum temperatures, precipitation, and solar radiation were recorded by
the Automated Weather Station located on the research station and operated by the
Michigan State University Agricultural Weather Ofﬁce. Averages were calculated
from that data.

b The 30 year average (30 yr) represents values interpolated from daily and monthly
normals for the Benton Harbor airport (1971-2000), and were obtained at
http://lwf.ncdc.noaa.gov/oa/ncdc.htrnl.

 

81

I! P‘nfll'u)! VJ.“ "

 

Table 2. Date and amount of irrigation events during the 2001, 2002, and 2003
growing seasons.

 

Irrigation appliedll
Year Date Full Season Flowering Pod Dev. Seed Fill Deﬁcit

 

 

 

 

 

mm
2001 19 May 26
29 June 17
6 July 17 17
11 July 17 17
16 July 25 25
20 July 22 22 11 11 11
23 July 8 8 8 8 8
24 July 22
30 July 20 20 20 20
3 Aug 25
9 Aug 22 22 22 22 11
Total 174 131 83 86 30
2002 28 June 7
2 July 5
3 July 15
5 July 18
15 July 28
19 July 20
22 July 18 20 10 10 10
26 July 30 33
9 Aug 12 12 11
6 Sept 11 12 10 11
10 Sept 21 24 24 22
Total 185 101 55 43 10
2003 24 June 9
2 July 19
29 July 13 12
1 Aug 19 17 18
8 Aug 12 11 11
18 Aug 35 33 34 34
21Aug 28 27 28 29 30
25 Aug 12 12 11 11 11
27Aug 36 36 36 37 8
5 Sept 24 23 24 25
Total 207 171 162 136 49

 

" The depth of irrigation water was calculated by averaging the depth of water
collected in 10 cups spaced 1.4 m apart along the north/south axis of all plots within
an irrigation treatment.

82

Table 3. Soybean yield as affected by year, irrigation treatment, and variety.

 

 

 

 

 

Yielda
Year Cultivar Full Flower. Pod Seed Deﬁcit Cultivar
Season Dev. Fill Averageb
kg/ha
2001 Asgrow 2703RRc 5055 a 4684 ab 4566 ab 4055 b 2635 c 4199
2002d Asgrow 2703RR 4670 4362 4441 4037 4214 4344 y
Stout 3884 3952 3115 2883 2630 3293 2
average 4277 a 4157 a 3778 ab 3460 be 3422 c
2003 Asgrow 2703RR 3289 3110 3276 2800 2316 2958 y
Stout 3143 2797 3490 2757 1954 2828 y
Asgrow 2553 3139 2965 3195 2802 2077 2835 y
Dairyland DSR 300 3573 3026 3159 3145 1959 2972 y
D.F. Seeds 8316RR 3174 3304 3079 2967 2190 2943 y
DynaGro 3200RR 2306 2390 2134 1932 1497 2052 z
Garst 2502RR 3252 3283 3349 2926 2100 2982 y
Garst D308 3169 3115 3146 2846 1813 2817y
average 3131 a 2999 ab 3104 a 2772 b 1988 c

 

" Data represent the average of all plots treated with herbicide.

b Means within this column within a year marked by the same letter are not
statistically different, p = 0.05, Tukey adjusted. Standard error terms were 100 (2002),
and 88 (2003).

c Means within a row marked by the same letter are not statistically different, p = 0.05,
Tukey adjusted. Standard error terms were 274 (2001), 158 (2002), and 70 (2003).

d The irrigation x variety interaction was not signiﬁcant (p = 0.05) in 2002 or 2003.
The standard error terms were 223 (2002) and 197 (2003).

83

Table 4. Water use efficiency and irrigation water use efﬁciency.

 

Water use efﬁciency Irrigation water use efﬁciency

 

Irrigation treatment 2001 2002 2003 2001 2002 2003

 

----------- kg/cm/ha----------- ------------kg/cm/ha-----------
Full Season 89 1 14 60 169 49 72
Flowering 89 142 62 203 81 83
Pod Development 95 154 66 364 79 99
Seed Fill 74 148 62 254 12 90
Deﬁcit 62 1 70 55 -- -- --

 

" Water use efficiency (WUE) was calculated as follows: WUE = [yield]/ [cm water
(precipitation + irrigation)].

b Irrigation water use efficiency (IWUE) was calculated as follows: IWUE = [yield
(irrigation treatment — deﬁcit irrigation)]/ [cm water (irrigation treatment — deﬁcit
irrigation)].

84

Table 5. Soybean yield components as affected by irrigation treatment.

 

Yield componenta

 

 

Year Irrigation treatment pods/plant” seeds/pod seeds/plant g/ 100 seed
2001 Full Season 53.4 a 1.60 a 85.8 a
Flowering 51.2 ab 1.59 a 81.8 ab
Pod Development 46.1 ab 1.56 a 71.1 b
Seed Fill 43.7 b 1.59 a 70.8 b
Deﬁcit 30.5 c 1.53 a 49.2 c
2002 Full Season 27.2 a 2.05 a 55.5 a
Flowering 26.0 a 2.06 a 53.6 a
Pod Development 20.1 b 2.10 a 42.3 b
Seed Fill 19.3 b 2.15a 41.6b
Deﬁcit 17.7 b 2.05 a 37.1 b
2003 Full Season 30.2 a 1.78 b 53.1 a 16.9 b
Flowering 31.2 a 1.84 ab 56.4 a 16.1 c
Pod Development 28.2 a 1.87 a 51.6 a 17.0 b
Seed Fill 23.5 b 1.70 c 39.4 b 18.3 a
Deﬁcit 23.5 b 1.61 d 37.2 b 15.8 c

 

“ Data represent the average of 10 plants from each plot treated with two applications

of herbicide.

b Means within a year within a column marked by the same letter are not statistically

different, p = 0.05.

85

Table 6. Soybean yield components as affected by variety.

 

Yield componenta

 

 

Year Cultivar pods/plant” seeds/pod seeds/plant g/ 100 seed

2001 Asgrow 2703RR 45.8 1.58 71.5

2002 Asgrow 2703RR 23.3 a 2.12 a 49.7 a
Stout 20.8 b 2.04 b 42.4 b

2003 Asgrow 2703RR 25.7 cd 1.73 c 49.3 b 15.9 e
Stout 31.7b 1.15 e 36.8c 18.4a
Asgrow 2553 25.5 cd 1.80 c 46.2 b 16.5 d
Dairyland DSR 300 22.4 d 2.14 a 48.1 b 18.2 ab
D.F. Seeds 8316RR 25.5 cd 1.82 c 46.3 b 17.7 b
DynaGro 3200RR 37.3 a 1.58 d 59.9 a 13.8 f
Garst 2502RR 25.7 cd 1.91 b 49.4 b 17.1 bc
Garst D308 22.7 d 1.93 b 44.3 b 17.0 c

 

a Data represent the average of 10 plants from each plot treated with two applications
of herbicide.
b Means within a year within a column marked by the same letter are not statistically
different, p = 0.05.

86

Table 7. Plant height as affected by irrigation and variety.

 

 

 

 

 

Plant heighta
Year Cultivar Full Flower. Pod Seed Deﬁcit Cultivar
Season Dev. Fill averageb
cm

2001 Asgrow 2703RRc 84 a 89 a 76 ab 71 ab 66 b

2002cl Asgrow 2703RR 85 72 64 59 61 68 a
Stout 61 56 53 49 48 53 b
Irrigation Average 73 a 64 b 58 c 54 d 55 cd

2003 Asgrow 2703RR 68 66 63 61 59 63 a
Stout 57 56 62 54 55 57 b
Asgrow 2553 59 55 58 54 51 55 b
Dairyland DSR 300 71 66 64 62 56 64 a
D.F. Seeds 8316RR 75 75 69 61 60 68 a
DynaGro 3200RR 47 46 41 46 42 45 c
Garst 2502RR 69 67 65 63 57 64 a
Garst D308 65 62 59 55 42 58 b

Irrigation trt. avg. 64 a 62 ab 60 be 57 cd 54 d

 

8 Plants were measured to the top of the stem prior to harvest. Data represent the
average of two or three plants from each plot.

b Means within this column within a year marked by the same letter are not
statistically different, p = 0.05, Tukey adjusted. Standard error terms were 0.69
(2002), and 1.18 (2003).

° Means within a row marked by the same letter are not statistically different, p = 0.05,
Tukey adjusted. Standard error terms were 4.01 (2001), 1.09 (2002), and 0.94 (2003).
d The irrigation X variety interaction was signiﬁcant (p = 0.05) in 2002 but not in
2003. The standard error terms were 0.61 (2002) and 2.63 (2003).

87

Table 8. Lodging as affected by irrigation and variety.

 

 

Lodging ratinga
Year Cultivar Full Flower. Pod Seed Deﬁcit Cultivar
Season Dev. Fill averageb

 

2001 Asgrow 2703RR° 1.7 a 1.6 ab 1.2 c 1.3 bc 1.0 c

2002‘l Asgrow 2703RR 1.2 1.2 1.0 1.0 1.0 1.1 b
Stout 2.5 1.8 1.3 1.1 1.0 1.5 a
Irrigation Average 1.9 a 1.5 b 1.2 c 1.1 c 1.0 c

2003 Asgrow 2703RR 1.13 1.06 1.10 1.00 1.05 1.07 c
Stout 2.00 1.75 2.16 1.79 1.16 1.77 a
Asgrow 2553 1.14 1.13 1.13 1.19 1.13 1.140

Dairyland DSR 300 1.56 1.25 1.31 1.23 1.06 1.28 b
D.F. Seeds 8316RR 1.63 1.63 1.19 1.06 1.19 1.34 b
DynaGro 3200RR 1.00 1.06 1.00 1.06 1.00 1.03 c
Garst 2502RR 1.56 1.50 1.31 1.25 1.13 1.35 b
Garst D308 1.25 1.06 1.06 1.13 1.06 1.11 c
Irrigation trt. avg. 1.41 a 1.31 ab 1.28 b 1.21 b 1.10 c

 

“ Plots were rated prior to harvest on a scale of 1 to 5, where 1 = no lodged plants, and
5 = all plants lodged.

b Means within this column within a year marked by the same letter are not
statistically different, p = 0.05. Standard error terms were 0.04 (2002) and 0.05
(2003)

° Means within a row marked by the same letter are not statistically different, p = 0.05.
Standard error terms were 0.12 (2001), 0.06 (2002), and 0.04 (2003).

d The irrigation X variety interaction was signiﬁcant (p = 0.05) in 2002 and in 2003.
The standard error terms were 0.08 (2002) and 0.11 (2003).

88

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Table 10. Yield components as affected by weed control treatment“.

 

Yield component

 

2001 2002

 

 

Treatment pod/plantb seed/pod seed/plant pod/plant seed/pod seed/plant

 

Weedy 27.9 b 1.45 b 41.2 b 19.6 b 2.04 b 39.9 b
Treated 50.6 a 1.62 a 82.0 a 24.6 a 2.12 a 52.2 a

 

a For statistical analysis, the weed control treatments were weedy (no glyphosate
applied) and treated (glyphosate applied). There were no differences between plots
that received one or two applications of glyphosate, so data from these treatments
were combined for analysis. Data represent the average of 10 plants from each plot.

b Means within a column marked by the same letter are not statistically different, p =
0.05.

Table 11. Soybean aphid count averages based on the MSU scalea in 2003.

 

 

 

Aphid count
Irrigation treatment 6 Aug 14 Aug
Full Season 2.0 2.4
Flowering 2.0 2.1
Pod Development 1.8 2.2
Seed Fill 1.7 2.1
Deﬁcit 1.5 1.8

 

a The rating scale is 0-4, and estimates the number of aphids on one leaﬂet of a
trifoliate in the middle of the plant. Values given are 0 = no aphids, 1 = 1-10 aphids,
2 = 11-25 aphids, 3 = 26-99 aphids, and 4 = 100+ aphids.

90

Table 12. Estimates of soybean aphid population differences between irrigation
treatments.

 

 

Contrast Estimate Standard Error LL, UL Pr>t
dry v. irrigated8| 0.27 0.1054 0.02, 0.52 0.0169
dry v. full season 0.39 0.1407 0.06, 0.73 0.0093

 

“ ‘dry’ included seed ﬁll and deﬁcit irrigation treatments, ‘irrigated’ included ﬁill
season, ﬂowering, and pod development irrigation treatments.

91

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