INTEGRATING A CEREAL RYE COVER CROP AND SOYBEAN ROW WIDTH WITH
HERBICIDES TO MANAGE PALMER AMARANTH IN MICHIGAN
By
Kelsey Marie Rogers

A THESIS
Submitted to
Michigan State University
In partial fulfillment of the requirements
for the degree of
Crop and Soil Sciences – Master of Science
2017

ABSTRACT
INTEGRATING A CEREAL RYE COVER CROP AND SOYBEAN ROW WIDTH WITH
HERBICIDES TO MANGAGE PALMER AMARANTH IN MICHIGAN
By
Kelsey Marie Rogers
Glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Wats.) is an emerging concern
for Michigan farmers. Integrating the use of cultural practices such as cover crops and narrow
soybean row width may improve the control of herbicide-resistant Palmer amaranth in Michigan.
In 2015 and 2016, field experiments investigated the effects of a cereal rye cover crop, including
termination method, and soybean row width as cultural practices to improve Palmer amaranth
control with herbicides. Additional pot experiments were conducted to evaluate the effects of
cereal rye stage, biomass, and termination method on Palmer amaranth emergence. Cereal rye
biomass was greater in 2016 compared with 2015 due to the greater accumulation of growing
degree days prior to cereal rye termination. Winter annual weeds were suppressed by more than
75% with cereal rye. However, overall Palmer amaranth emergence and control was not effected
by cereal rye. In outdoor pot experiments, cereal rye suppression of Palmer amaranth emergence
varied by year with the most consistent suppression with the later stage (Feekes 10.1 or later)
cereal rye. In greenhouse pot experiments, total Palmer amaranth emergence was only reduced
by cereal rye terminated by cutting compared with chemical termination. Planting soybean in
narrow rows reduced the emergence period of Palmer amaranth in both years. Palmer amaranth
was effectively controlled by both herbicide management strategies in 2015, while only the high
management strategy provided season-long control in 2016. Utilizing an intensive herbicide
management program and planting soybean in narrow rows provided the greatest impact on
Palmer amaranth management in Michigan soybean production.

Dedicated to my parents, Pat and Deb Rogers

iii

ACKNOWLEDGEMENTS

I would like to thank Dr. Christy Sprague for giving me the opportunity to pursue my Master
of Science degree at Michigan State University and for serving as my major advisor. Her
guidance, patience, and support has been invaluable to my educational and professional
development and without her I wouldn’t be where I am today. I would also like to thank Dr.
Karen Renner and Dr. Chris DiFonzo for serving on my guidance committee and providing
valuable insight.
I would also like to give a special thanks to Gary Powell, Erin Hill, and Brian Stiles for their
assistance in the design, setup, maintenance, and harvest of my field experiments. They are an
essential part of the weed science program, and I would not have been able to complete my
research without them. This research would have been difficult to conduct without the help of
my fellow graduate students: Amanda Goffnett, Amanda Harden, Dr. Jonathon Kohrt, Daniel
Wilkinson, Michael Probst, and Aaron Brooker. Additionally, thank you to all the undergraduate
workers: Cole Ragon, Kalvin Canfield, Mike Schulze, Jake Hall, Jerrid Smith, Dennis Erickson,
Haonan Qi, Matt Singer, Kirk Hautau, Zach Montney, and Adam Constine. I appreciate all the
help with seed counting, plant pulling, and Palmer amaranth harvesting. I would also like to
thank Dr. James Kells and Dr. Thomas Nikolai for providing me with the opportunity to serve as
their teaching assistant. Special thanks to the Michigan Soybean Promotion Committee and
Project GREEEN for the funding to conduct this research.
Finally, I would like to extend a special thank you to my parents, grandparents, and all my
additional family and friends who have supported and encouraged me though all my endeavors.

iv

TABLE OF CONTENTS

LIST OF TABLES ......................................................................................................................... vi
LIST OF FIGURES ...................................................................................................................... vii
CHAPTER 1 ....................................................................................................................................1
REVIEW OF LITERATURE ..........................................................................................................1
Introduction ..........................................................................................................................1
Weed Competition in Soybean ............................................................................................1
Palmer amaranth ..................................................................................................................2
Managing Palmer amaranth with Herbicides .......................................................................7
Management in glufosinate-resistant soybean .........................................................8
Cultural Weed Control Practices .........................................................................................9
Soybean row width ..................................................................................................9
Cover crops for weed suppression .........................................................................11
LITERATURE CITED ..................................................................................................................19
CHAPTER 2 ..................................................................................................................................30
INTEGRATING A CEREAL RYE COVER CROP AND SOYBEAN ROW WIDTH WITH
HERBICIDES TO MANAGE PALMER AMARANTH IN MICHIGAN ...................................30
Abstract ..........................................................................................................................................30
Introduction ....................................................................................................................................31
Materials and Methods ...................................................................................................................36
Field Experiment ................................................................................................................36
Palmer amaranth measurements ............................................................................38
Soybean measurements ..........................................................................................39
Effects of Cereal Rye on Palmer amaranth Emergence – Pot Experiments ......................40
Outdoor ..................................................................................................................40
Greenhouse ............................................................................................................41
Data Analysis .....................................................................................................................42
Results and Discussion ..................................................................................................................43
Field Experiment ................................................................................................................43
Cereal rye cover, biomass, and effects on early-season weeds..............................43
Palmer amaranth emergence ..................................................................................45
Palmer amaranth control, biomass, and density.....................................................46
Soybean yield .........................................................................................................48
Effects of Cereal Rye on Palmer amaranth Emergence – Pot Experiments ......................49
Outdoor ..................................................................................................................49
Greenhouse ............................................................................................................49
APPENDIX ....................................................................................................................................53
LITERATURE CITED ..................................................................................................................68

v

LIST OF TABLES

Table 2.1. Herbicide application timings, active ingredients, and product information for the
three different herbicide strategies used for management of glyphosate-resistant Palmer amaranth
in Middleton, MI (2015-2016) .......................................................................................................54
Table 2.2. Monthly precipitation and the 30-year average for Middleton, MI in 2015 and 2016 .55
Table 2.3. Cereal rye and weed dry biomass at cover crop termination and cover crop regrowth
from the cereal rye plots in Middleton, MI (2015-2016). Cereal rye biomass was analyzed for
C:N ratio.........................................................................................................................................56
Table 2.4. Soil moisture at 7.6 cm and 11.9 cm depths measured at the time of soybean planting
in the cereal rye and no cover plots in 2015 and 2016...................................................................57
Table 2.5. Main effects of cereal rye cover crop, soybean row width, and herbicide management
strategy on Palmer amaranth control in 2015 and 2016 at Middleton, MI ....................................58
Table 2.6. Main effects of cover crop and soybean row width on Palmer amaranth density and
biomass in the no herbicide management plots at peak biomass in 2015 and 2016 at Middleton,
MI...................................................................................................................................................59
Table 2.7. Main effects and interactions of cereal rye cover crop, soybean row width, and
herbicide management strategy on soybean yield in 2015 and 2016 in Middleton, MI ................60
Table 2.8. Effect of cover crop by herbicide management strategy interaction on soybean yield in
2015 and 2016 ................................................................................................................................61
Table 2.9. Effect of a cereal rye or non-rye cover on the initial and total emergence of Palmer
amaranth in outside pots in 2015 and 2016 ...................................................................................62
Table 2.10. Effect of termination method and cereal rye cover on initial and total Palmer
amaranth emergence in the greenhouse .........................................................................................63

vi

LIST OF FIGURES

Figure 2.1. Cumulative Palmer amaranth emergence as a percent of total emergence combined
across all soybean row widths with cereal rye terminated by glyphosate (RG) (♦), cereal rye
terminated by mowing (RM) (■) and no cover (NC) (●) in 2015 (a) and 2016 (b). There was no
difference between the treatments in 2015; however, RM was significantly different from RG
and NC in 2016. Fitted lines were calculated with the Gompertz equation: NC, y=100*exp(8.25*exp(-0.07*x)), R2 = 0.53; RG, y=100*exp(-10.57*exp(-0.09*x)), R2 = 0.74; RM,
y=100*exp(-26.87*exp(-0.12*x)), R2 = 0.78 ................................................................................64
Figure 2.2. Cumulative Palmer amaranth emergence in 2015 (a) and 2016 (b) as a percent of
total emergence combined across all cover crop treatments in 19 (●) and 76 cm (▲) rows.
Soybean row widths were significantly different from each other in both years. Fitted lines were
calculated with the Gompertz equation: 76 cm rows, y=100*exp(-12.67*exp(-.08*x)), R2 = 0.74;
19 cm rows, y=100*exp(-33.93*exp(-0.14*x)), R2 = 0.86 ............................................................65
Figure 2.3. Percent soybean canopy closure in 19 (●) and 76 cm (▲) rows in 2015 (a) and 2016
(b). Soybean row widths were significantly different from each other in both years ....................66
Figure 2.5. Effect of rainfall on cumulative Palmer amaranth emergence in the low and high
herbicide management strategies in 2015 (a) and 2016 (b). POST applications were made on July
15, 2015 and June 10, 2016. LPOST application was applied on July 8, 2016 .............................67

vii

CHAPTER 1
REVIEW OF LITERATURE

Introduction
Soybean (Glycine max (L.) Merr.) is an annual broadleaf crop that is grown primarily for
soybean oil and meal. Soybean accounts for about 90% of the oilseed production in the United
States with 88% of oil used for human consumption, and 98% of soybean meal used for animal
feed (ERS 2016; Wills 2013). After corn (Zea mays L.), soybean is the second most planted field
crop grown in the United States with the main soybean producing areas in the Midwest and the
lower Mississippi Valley (USDA 2010, ERS 2016). Michigan ranks 12th in soybean production
of the 31 states that produce soybean in the United States (USDA 2016). In 2016, more than 206
million ha-1 of soybean were planted in the United States with over 5 million ha-1 planted in
Michigan alone (NASS 2016). In 2015, the total soybean production value for the United States
was over 34 billion dollars, while Michigan production was over 851 million dollars (NASS
2016).
Weed Competition in Soybean
Weeds are considered the number one production problem in soybean (Vivian et al. 2013). If
not managed they influence soybean growth, development, and yield. Broadleaf weeds tend to be
more competitive with soybean than grass species. Cowen et al. (1998) reported that broadleaf
weeds, Powell amaranth (Amaranthus powellii S. Wats.) and redroot pigweed (Amaranthus
retroflexus L.), were more competitive than barnyardgrass (Echinochloa crus-galli (L.) Beauv.)
in soybean across all locations, years, and time of weed emergence relative to crop emergence.
Similarly, Weaver (2001) observed that green foxtail (Setaria viridis (L.) Beauv.) at low

densities was less competitive and had a lower impact on soybean yield when compared with
low densities of common lambsquarters (Chenopodium album L.) and common ragweed
(Ambrosia artemisiifolia L.). Weeds not only impact crop yield but also reduce harvest
efficiency by plugging up combines resulting in lower yields from harvest loss. Burnside (1973)
observed that in addition to reduced seed quality from plant debris, a predominance of broadleaf
weeds at the time of harvest caused 19% greater harvest losses compared with plots that had
predominantly grass species. On average, weeds reduce soybean yield almost 50% when no
weed control is used (Dille et al. 2016). In the United States, the potential economic loss from
weed competition in soybean is over $15 billion dollars annually (Dille et al 2016). One of the
most competitive annual broadleaf weeds is Palmer amaranth (Amaranthus palmeri S. Wats.),
which was ranked as the most troublesome weed in the United States in a recent survey
conducted by the Weed Science Society of America (Van Wychen 2016).

Palmer amaranth
Palmer amaranth is a dioecious summer annual weed species that is a member of the
Amaranthus family. Palmer amaranth is native to the Sonoran Desert region that spans portions
of Mexico, southern Arizona, and California (Ehleringer 1983). It thrived in this region by being
able to germinate quickly after a major rainfall and complete its lifecycle under limited soil
moisture available at the time of germination (Ehleringer 1983). However, Palmer amaranth
populations can now be found in the previously uninhabited areas of the Southern United States
and has recently spread into the Midwest (Culpepper et al. 2010; Heap 2016). Palmer amaranth
was first identified in Michigan in 2010 in a southwest Michigan county (Sprague 2011). Since
2010, it has spread to eleven counties in the southern half of Michigan’s Lower Peninsula (C

2

Sprague, Weed Scientist, Michigan State University, personal communication). Although Palmer
amaranth is native to the southwest region of the United States, research has shown that Palmer
amaranth has been able to successfully compete its lifecycle, produce seed that can survive
Michigan’s cold winters, and become a weed problem the following season (Powell 2014).
Palmer amaranth has become a major weed problem in field crops, due to its rapid and
continued emergence, high photosynthetic capacity, and prolific seed production (Keeley et al.
1987; Ward et al. 2013). Palmer amaranth can grow up to 0.21 cm per growing degree day and
reach mature heights of over 2 meters (Horak and Loughin 2000). Compared with other
Amaranthus species, Palmer amaranth produces more biomass at temperatures above 25 C and
has up to a 62% greater growth rate per growing degree day (Guo and Al-Khatib 2003;
Culpepper et al. 2006). Palmer amaranth that emerges with corn can reduce yield 91% with only
8 plants per meter of corn row (Massinga et al. 2001). In fact, the presence of only one plant per
meter of row reduced corn yield by as much as 40%. In a similar study, just one Palmer amaranth
plant per meter of row reduced soybean yield by 32% (Klingaman and Oliver 1994). These
studies show the importance of managing Palmer amaranth early to prevent serious yield loss.
Palmer amaranth emergence in Michigan typically begins in late May to early June with
continued emergence through mid-September (Powell 2014; Kohrt 2017). Depending on
soybean planting date, Palmer amaranth emergence occurs two to three weeks after soybean
planting, unless weather conditions delay soybean planting until late June (USDA 2010).
Reduced soybean growth and yield may occur if Palmer amaranth are not controlled. Monks and
Oliver (1988) observed a reduction in soybean biomass when Palmer amaranth was within 50 cm
of soybean. Additionally, Palmer amaranth has been documented to reduce soybean canopy
closure 55% when 10 plants were present within a meter of row (Klingaman and Oliver 1994).

3

The rapid and erect growth of Palmer amaranth allows it to compete effectively with soybean for
light. Klingaman and Oliver (1994) reported that Palmer amaranth reached heights 60 cm taller
than surrounding soybean plants and lead to a soybean yield reduction of up to 64% with just
3.33 plants m-1. Palmer amaranth leaves move with the sun and stay perpendicular to the solar
rays (Ehleringer and Forseth 1980). This allows the plant to maintain a high photosynthetic rate
throughout the day, increasing the plants growth. Palmer amaranth also has an extensive root
system. In comparison to soybean, Palmer amaranth has been reported to produce 3.7 times more
roots that were 5 times longer (Wright et al. 1999). This root system allows Palmer amaranth to
more effectively penetrate compacted soils and provide greater access to water and nutrients,
giving it a more competitive edge against the crop, especially in dry conditions (Place et al.
2008).
The presence of large Palmer amaranth seedbanks in agricultural fields can be attributed to
its prolific seed production. Keeley et al. (1987) examined individual female Palmer amaranth
plants that produced as many as 1 million seeds each and averaged between 200,000 and 600,000
seeds per female plant when no crop competition was present. In Michigan soybean systems,
Kohrt (2017) observed individual Palmer amaranth plants that emerged with soybean and grew
to maturity produced over 350,000 seeds. Seed production was reduced when Palmer amaranth
emergence occurred after crop emergence. However, even Palmer amaranth plants that emerged
in August in Michigan were able to produce viable seed prior to a killing frost. Massinga et al.
(2001) found that seed production from 0.5 plant per meter of row that emerged with corn
produced 140,000 seeds m-2, while plants at the same density that emerged at the four-leaf stage
of corn produced only 1,800 seeds m-2. Due to this high seed production from female plants,

4

Klingaman and Oliver (1994) stated that, control as great as 99% may not be sufficient in
reducing the Palmer amaranth population below the economic threshold.
Palmer amaranth seeds are a smooth, round or disk shape with a black seed coat measuring
1-2 mm in diameter (Sauer 1955). Since these seeds are so small, they must be in a relatively
shallow position within the soil profile or on the soil surface in order to establish. Palmer
amaranth seeds buried at depths greater than 3.8 cm will not emerge (Ward et al. 2013; Menges
1974). Palmer amaranth seeds fall a short distance from the mother plant by gravity but can be
dispersed to greater distances by external forces such as water movement, contaminated farm
equipment, manure, seed sources, as well as by birds and other animals (Costea et al. 2004,
2005). It is speculated that Palmer amaranth seed was introduced into Michigan agricultural
fields by the spreading of dairy manure from cattle that were fed contaminated cotton seed
(Sprague 2013). Contaminated cotton seed and gin trash have been identified as sources of the
spread to new areas in the southern United States. Norsworthy et al. (2009) reported that Palmer
amaranth was the most prevalent viable broadleaf weed seed in cotton by-products. More
recently, Palmer amaranth has spread to additional counties in Illinois, Iowa, and Ohio when
seed mixes for Conservation Reserve Program (CRP) fields were found to be contaminated with
Palmer amaranth seed (Hager 2016; Hartzler 2016).
In addition to its competitive nature and prolific seed production, Palmer amaranth has
shown a remarkable ability to become resistant to several different herbicide sites of action.
Palmer amaranth populations have been confirmed resistant to acetolactate synthase inhibitors
(ALS) (Group 2); microtubule inhibitors (Group 3); photosystem II inhibitors (PSII) (Group 5);
the 5-enolpyruvyl-shikimate-3-phosphate inhibitor (EPSP), glyphosate (Group 9);
protoporphyrinogen oxidase inhibitors (PPO) (Group 14); and 4-hydroxyphenylpyruvate

5

dioxygenase inhibitors (HPPD) (Group 27) (Gossett et al. 1992; Horak and Peterson 1995; Heap
2016). Palmer amaranth populations have also been discovered to be resistant to multiple
herbicide sites of action groups. One of the first known multiple-resistant Palmer amaranth
populations was identified in Georgia in 2008 with resistance to glyphosate (Group 9) and the
ALS-inhibiting (Group 2) herbicide, pyrithiobac (Sosnoskie et al. 2011). The first three-way
resistant Palmer amaranth population was confirmed in Kansas in 2009 with resistance to ALS(Group 2), photosystem II- (Group 5), and HPPD-inhibitors (Thompson 2009). Since then,
several other states have reported three-way resistant Palmer amaranth populations, including
Michigan, where a population with resistance to glyphosate (Group 9), thifensulfuron an ALSinhibiting herbicide (Group 2), and atrazine (Group 5) was confirmed in 2013 (Kohrt et al.
2017).
While Palmer amaranth has developed resistance to several different herbicide sites of action,
glyphosate resistance has perhaps caused the greatest issues with weed management. The first
confirmed case of glyphosate-resistant Palmer amaranth was identified in Georgia in 2005
(Culpepper et al 2006). This population survived glyphosate applications up to 12 times the
normal field use rate and was found in a cotton field that relied solely on glyphosate for weed
control for seven years. The continuous use of glyphosate throughout the growing season, year
after year as the only method of weed control, has led to increased selection pressure for weeds
with glyphosate resistance. These resistant weed populations can quickly multiply if left without
proper management. Glyphosate-resistant Palmer amaranth has since spread across 23 states
including Michigan (Sprague 2011; Heap 2016). The mechanism of glyphosate resistance in
Palmer amaranth is by gene amplification of EPSP synthase (Powles 2010; Gaines et al. 2011).
Several greenhouse studies have documented that glyphosate-resistant Palmer amaranth

6

populations have been able to survive glyphosate applications over 10 times the normal field use
rate (Culpepper et al. 2006; Norsworthy et al. 2008). Palmer amaranth’s ability to develop
resistance to glyphosate and multiple herbicide sites of action, along with its physical
characteristics makes it challenging for growers to effectively manage this weed in glyphosateresistant soybean.

Managing Palmer amaranth with Herbicides
Glyphosate-resistant Palmer amaranth can be difficult to manage in soybean due to the
limited number of effective herbicide site of action groups. Several studies have found that in
order to manage glyphosate-resistant Palmer amaranth, an effective preemergence (PRE)
herbicide with residual activity needs to be included in the herbicide program (Culpepper et al.
2008; Whitaker et al. 2008; Whitaker et al. 2011; Powell 2014; Bell et al. 2016; Norsworthy et
al. 2016). Flumioxazin (Group 14) has been observed to be one of the most effective PRE
herbicides used to control Palmer amaranth with observed control of up to 100, 99, and 98%, 20,
40, and 60 days after application (Whitaker et al. 2011). Similarly, Powell (2014) observed good
control (>85%) of Palmer amaranth with flumioxazin in Michigan; however, the amount of
rainfall received after application impacted the herbicide’s effectiveness. Powell (2014) also
stated that in optimal environmental conditions, soil-applied herbicides may only provide 28
days of control before new Palmer amaranth seedlings emerge.
This lack of season-long control, requires the use of an effective postemergence (POST)
herbicide application to manage weeds that are not controlled by the PRE herbicide application
alone. Additionally, PRE followed by POST herbicides have been shown to reduce Palmer
amaranth population density and seed production when compared to a POST only application

7

(Norsworthy et al. 2016). Powell (2014) examined the effects of POST applied lactofen,
fomesafen, and glufosinate for control of 8 and 15 cm Palmer amaranth. Results of this
experiment suggested an inconsistency in Palmer amaranth control with the use of lactofen and
fomesafen. Control with fomesafen was only 56% when applied to 15 cm Palmer amaranth,
while control with lactofen was 90%. In drought conditions, glufosinate provided the greatest
control of Palmer amaranth with up to 86 and 80%, 7 and 14 days after treatment (DAT),
respectively. Fomesafen provided less control with only 68% by 14 DAT and lactofen had the
least control with 54%. Lastly, Powell (2014) concluded that if Palmer amaranth was greater
than 8 cm tall, glufosinate applied at 0.75 kg ha-1 may provide the greatest control compared with
lactofen and fomesafen.
Glufosinate has been of particular interest as it is an effective POST herbicide for control of
glyphosate-resistant Palmer amaranth. Glufosinate is a nonselective herbicide that inhibits
glutamine synthase (Group 10) and results in an accumulation of ammonium in the plant
(Wendler et al. 1990). Braswell et al. (2016) reported that significant increases in Palmer
amaranth control were observed when PRE herbicides were followed with glufosinate POST.

Management in glufosinate-resistant soybean. Glufosinate can only be applied POST in crops
that are glufosinate-resistant. Glufosinate-resistant soybean (LibertyLink®) were first released to
the public in 1998 (Carpenter and Gianessi 1999); however, due to the lack of market because of
the wide-spread popularity of glyphosate-resistant soybean they were not commercially grown
until 2009. Glufosinate-resistant soybean allows growers to use a broad-spectrum herbicide
POST with a different site of action than glyphosate. Results from a study conducted by Norris et
al. (2002) found that broadleaf weed control with glufosinate was equal to or better than

8

conventional treatments with sequential high rate applications providing the most consistent
control. Control of the Amaranthus species, Palmer amaranth, redroot pigweed, and common
waterhemp (Amaranthus rudis Sauer), with glufosinate was found to be poor with one
application. However, with sequential applications the weed population decreased and control
exceeded 80% (Coetzer et al. 2002). Similar results were observed by Hoffner et al. (2012) who
reported consistent control of Palmer amaranth with the use of an effective PRE followed by
POST or sequential POST applications of glufosinate in glufosinate-resistant soybean. The lack
of control with one application of glufosinate may be in part due to new weed emergence
following the initial POST application, since glufosinate does not provide residual control.
However, sequential applications of glufosinate has provided similar control to the use of a PRE
herbicide followed by a POST glufosinate application in common waterhemp (Beyers et al.
2002). Additionally, acetochlor (Group 15) has been shown to aid glufosinate in Palmer
amaranth control when tank-mixed POST due to its residual properties. Glufosinate alone
provided only 85 and 86% control when applied to Palmer amaranth less than 10 cm tall early
POST and mid-season POST, while the combination of glufosinate and acetochlor provided 94
and 91% control, respectively (Cahoon et al. 2015).

Cultural Weed Control Practices
Soybean row width. Soybean are grown in different row widths and populations depending on
the preference and farming practices of the grower. Typically, soybean row widths grown in
Michigan include: 76, 38, and 19 cm. Harder et al. (2007) reported weed control was improved
in soybean planted in 19 cm rows compared with 76 cm rows. Soybean planted in 38 cm has also
been reported to reduce Palmer amaranth biomass and seed production 38 and 65%, respectively,

9

when compared with soybean planted in 76 cm rows (Butts et al. 2016). This may be due to
earlier canopy closure in soybean planted in narrow rows that prevents light from reaching the
soil surface earlier than in wide rows, aiding in the prevention of late-season weed emergence
(Wax and Pendleton 1968; Murphy and Gossett 1981). Légère and Schreiber (1989) found that
by 50 days after emergence, soybean canopy closure in 25 cm row widths progressed rapidly and
was complete, while canopy closure in the 76 cm row widths was never quite achieved. Due to
the earlier canopy closure, soybean planted in 19 cm narrow rows have been shown to reduce
total weed biomass by 30% compared with 76 cm wide rows (Mickelson and Renner 1997).
Another study observed that soybean planted in 25 cm rows, reduced the total pigweed biomass
by almost 20% and produced significantly more yield than soybean planted in 76 cm rows
(Légère and Schreiber 1989).
Jha and Norsworthy (2009) reported Palmer amaranth emergence was reduced in the
presence of a soybean canopy compared with plots without soybean. Similarly, DeVore et al.
(2013) observed Palmer amaranth emergence was greatly reduced once the soybean canopy
closed. Amaranthus species display a phytochrome-controlled germination response in which the
presence of red light stimulates germination (Gallagher and Cardina 1998; Leon and Owen
2003). An inhibitory effect of far-red light on Palmer amaranth germination was observed by Jha
et al. (2010), which explains why germination and emergence primarily occurs early in the
season prior to canopy closure and supports previous findings of emergence declining with the
presence of a soybean canopy. However, Holden et al. (2015) reported that herbicide programs
provided more of distinguishable impact on Palmer amaranth control, density and seed
production than soybean row widths.

10

Cover crops for weed suppression. “A cover crop is any living ground cover that is planted into
or after a main crop and is then commonly killed before the next crop is planted” (Hartwig and
Ammon 2002). In essence, cover crops are short term rotations between cash crops (Reeves
1994). There are three broad categories of cover crops: brassicas, legumes and grasses. These
cover crops can be summer annual plants that die during the winter months, winter annuals or
perennials. There are many options, combinations and mixtures of cover crops that can be grown
depending on the objectives of the grower, the region the cover crops will be grown in, and the
time of year the cover crops will be planted. Many benefits can be obtained from the use of cover
crops, although these vary between the different crops. These benefits can include nitrogen
fixation, prevention of soil erosion and nutrient loss, improved soil conditions, weed and disease
suppression, as well as providing a favorable environment for beneficial predators (Unger and
Vigil 1998; Hartwig and Ammon 2002; Gallandt et al. 2005; Larkin et al. 2010; Blanco-Canqui
et al. 2015).
Winter annual cover crops are established in the late summer or early fall to provide cover
during the winter (Teasdale 1996). In the early spring, these cover crops experience rapid growth
which produces most of the final biomass. Winter annual cover crops are especially beneficial
for preventing erosion on sandy soils that are erodible when left bare during the winter months
(Snapp et al. 2005). These cover crops can also be used for weed suppression as reported by
Moore et al. (1994) and Korres and Norsworthy (2015), who found that small grain cover crops,
such as cereal rye, reduced weed size and biomass. Price et al. (2006) also observed the effect of
small grain cover crops on weed suppression and reported 69-73% control on multiple weed
species, including Palmer amaranth, in plots with a cereal rye or black oat cover crop that were
terminated in early May with a glyphosate application and roller-crimped 3 d later. This

11

vegetative cover reduces the quantity of light available and heat adsorbed by the soil, which can
cause less favorable conditions for weed germination and emergence (Teasdale 1996). Putnam
and DeFrank (1983) examined vegetable seed growth in grass cover crop residue and observed
that in general larger seeded plants grew normally and sometimes benefited from the residue,
while small seeded plants were severely injured when compared with wood shavings as a cover.
Cereal rye (Secale cereal L.) is a winter annual grass that provides many benefits that
include, but are not limited to, reduced nitrogen loss, erosion control, improved soil quality, and
weed suppression (Kinyangi et al. 2001). Debany et al. (2001) stated that, “rye is the most cold
tolerant, easiest to establish, most productive, and the earliest to head among temperate region
cereal crops.” Cereal rye is planted in the fall, left to overwinter, then terminated in the spring
prior to crop planting. Cereal rye has the ability to produce more biomass the farther south it is
grown. Cereal rye produced 3600, 5100, and 11,000 kg ha-1 of biomass when terminated in the
early summer in Ohio and the early spring in North Carolina and Alabama, respectively (Yenish
et al. 1995; Akemo et al. 2000; Price et al. 2012). In Michigan, cereal rye has been shown to
produce between 800 and 2900 kg ha-1 of shoot biomass (Snapp et al. 2005). However, Hill
(2014) reported cereal rye biomass in Michigan ranged from 784 to 12,777 kg ha-1 when shoots
and roots were harvested. Cereal rye planting and termination date is a significant factor in
determining how much cover crop biomass will be produced (Mirsky et al. 2009; Webster et al.
2016; Hill 2014). Hill (2014) observed that the lowest amount of biomass was produced when
cereal rye was planted in early November and terminated in mid-May, while rye planted in midSeptember and terminated in early-June produced the greatest amount of biomass. An earlier
planting date provides an extended growing season and increases growing degree day
accumulation resulting in greater biomass production prior to termination.

12

The speed at which cereal rye residue is broken down is influenced by the carbon to nitrogen
ratio (C:N) of the residue. The ideal C:N ratio for microbes to consume is 24:1, residue
containing a higher ratio will be degraded more slowly compared with ratios below the ideal
(Eiland 2001; NRCS 2011). Cereal rye, at or close to maturity, will have higher C:N ratios that
allows the residue to remain longer on the soil surface compared with cereal rye in the vegetative
stages. Additionally, C:N ratios 25:1 or greater can immobilize soil N preventing the nutrient
from being readily available for the subsequent crop and potentially reducing yields (Schomberg
et al. 2007; Pantoja et al. 2016).
Cereal rye has a fibrous root system that can help alleviate compacted soil from the root
channels left behind after decomposition, while a rye cover left on the soil surface can aid in the
conservation of water early in the season (Williams and Weil 2004). A cereal rye cover crop
included in a corn/soybean no-till rotation was shown to improve water-aggregate stability and
was effective in trapping soil phosphorous compared with a winter fallow (Villamil et al. 2006).
When using a cover crop that leaves residue in the spring, growers need to consider the
possibility of an increase in the presence of damaging insects along with those that are beneficial.
Hammond (1990) found that seed corn maggot (Delia platura Miegen) impacted soybean stands
when soil contained decaying cereal rye, while Reddy (2001) found that soybean stands
displayed better establishment in cover crop residue that had begun to breakdown compared with
stands in a fresh layer of residue. This was further demonstrated by Liebl et al. (1992) where
yields of soybean planted into cereal rye terminated 2 weeks prior to planting were significantly
higher compared with soybean planted into rye terminated at planting. Armyworm (Psudaletia
unipuncta), a major pest in field corn, lays its eggs on the leaves of small grains. No-till corn
planted into cereal rye has been reported to increase armyworm damage compared with

13

conventionally grown corn (Untung 1978). This pest may be managed by mowing cereal rye
residue due to physically harming the insect larvae. Laub and Luna (1991) reported reduced
armyworm densities in early stage corn when cereal rye was terminated by mowing compared
with chemical termination.
Cover crops have historically been plowed under or terminated with burndown herbicides
prior to planting a row crop (Price et al. 2009). There are three common methods used to
terminate cover crops, they include: winterkill, mechanical, or herbicide application (Legleiter et
al. 2012). Since cereal rye is a winter annual it can only be terminated mechanically or
chemically. Tillage is a mechanical termination method that incorporates the rye into the soil
profile before planting, however this will not work for no-tillage systems. One method of
termination for a no-till system is to mow the standing cereal rye; however, it is suggested to
wait until head emergence (Feekes 10.1 or later) for the greatest amount of biomass and minimal
regrowth (Wilkins and Bellinder 1996). A flail mower is the recommended tool when mowing
cereal rye as it will leave the cover crop residue more uniformly distributed on the soil surface
than other mowers; however, this also leaves the residue in smaller pieces that can break down
more rapidly (Creamer and Dabney 2002). Another no-till method of termination is the use of a
roller-crimper which cuts or crushes the plants stem and lays the residue flat on the ground
creating a longer lasting cover compared with mowing. The effectiveness of roller-crimper
termination increases as cereal rye matures with consistent control achieved at anthesis (Feekes
10.5) or later (Mirsky et al. 2009; Wayman et al. 2014).
Of the different termination methods, an application of herbicide alone or in combination
with mechanical termination is the most commonly used method of terminating cereal rye in
studies (Moore et al. 1994; DeVore et al. 2013; Wiggins et al. 2016). Cereal rye termination with

14

a roller-crimper or glyphosate was 85% effective while adding glyphosate with the roller-crimper
provided greater than 97% termination (Price et al. 2009). In the presence of weed interference,
cereal rye termination method, whether it was rolled or chemically terminated, had little effect on
no-till soybean yield (Davis 2010). Other studies have reported varying effects on soybean yield
grown with a cereal rye cover crop when compared with no cover crop controls. Results ranged
from increased yield (Atech and Doll 1996), decreased yield (Reddy 2001), to having no effect
on yield (Liebl et al. 1992; Koger et al. 2002).
The effect of weed suppression by a cereal rye cover crop can be due to a combination of
physical and chemical factors (Kruse et al. 2000). The presence of a cereal rye cover crop mulch
has been shown to reduce weed density by 50% and weed biomass by 75% without the use of
herbicides (Malik et al. 2008; Bernstein et al. 2011). However, the degree of weed suppression
depends upon the amount of biomass that can be produced. Increases in total cereal rye biomass
have been shown to correlate with greater weed suppression and reduced weed biomass (Ryan et
al. 2011; Smith et al. 2011; Finney et al. 2016). Early suppression of Amaranthus species has
been observed with the presence of high cereal rye residue levels on the soil surface,
demonstrating the potential benefit of using the cover crop for early-season weed control (Price
et al. 2006, 2012; Mirsky et al. 2011; Saini et al. 2006). DeVore et al. (2013) observed in
Arkansas, as much as a 51% reduction in Palmer amaranth emergence with the presence of a
cereal rye cover crop terminated with glyphosate two weeks prior to soybean planting in a fullseason soybean production system. A study conducted by Price et al. (2006) found that a cereal
rye cover crop in conjunction with a PRE herbicide was able to provide similar weed control to a
PRE followed by POST herbicide management system with no cereal rye. The use of this
management strategy may allow for a reduction in the number of herbicide applications made

15

during the growing season for weed control depending upon the amount of rye biomass
produced.
In addition to high biomass production, cereal rye produces allelochemicals that are toxic to
certain plant species reducing plant germination, growth, and development. This chemical
interference of other plants is known as allelopathy (Barnes and Putnam 1983; Kruse et al.
2000). Allelopathic effects have been of great interest to researchers and growers for the
possibility of aiding in weed management while also providing other cover crop benefits. The
concentration of allelochemicals depend on the density and age of the allelopathic plant along
with soil factors such as pH, organic matter content, moisture content and availability of other
carbon sources to microorganisms (Blum et al. 1993; Blum 1996; Kruse et al. 2000). Cereal rye
residues have been shown to reduce the emergence of lettuce (Latuca sativa L.) by 58% and
proso millet (Panicum miliaceum L.) by 35% when compared with a control of a wood shavings
mulch (Barnes and Putnam 1986). Cereal rye shoot tissue inhibited lettuce germination more
than root tissue. This demonstrates that there can be more that may inhibit the germination and
growth of plants when using a cereal rye cover crop than just a physical barrier.
The effects of weed suppression has been shown to last as long as 63 days after cereal rye
was terminated, depending on the amount of biomass produced and environmental conditions
(Masiunas et al. 1995). However, the maximum concentration of allelochemicals from cereal rye
occurs within 20 days of rye decomposition (Chou and Patrick 1976). Cereal rye residues have
been shown to inhibit early-season weed growth by at least 75%, while cereal rye residues with
no-tillage reduced weed growth by 63% (Putnam and DeFrank 1983).
Several studies have observed the effect of integrating a cereal rye cover crop into the crop
rotation to try and suppress Palmer amaranth. In Arkansas, Bell et al. (2016) reported that the

16

combination of moldboard plowing in the fall followed by a cereal rye cover crop terminated
with glyphosate two weeks prior to soybean planting, reduced Palmer amaranth emergence and
density. Palmer amaranth and redroot pigweed suppression, from a cereal rye cover crop
terminated with a roller-crimper and glyphosate application, was observed in Alabama with
reported rye biomass of 4177 kg ha-1 from a late planting date and up to almost 11,000 kg ha-1
from an early planting date (Price et al. 2012). Norsworthy et al. (2011) collected cereal rye
biomass terminated with glyphosate that averaged from 7880 to 8460 kg ha-1 in Arkansas, which
provided up to 91% control of Palmer amaranth. These studies along with another study
conducted by Aulakh et al. (2012) who terminated cereal rye with a roller crimper and
glyphosate application, observed that the presence of a cereal rye cover crop reduced Palmer
amaranth density in no-till cotton with no negative effect on crop yield. Culpepper et al. (2010b)
similarly reported a reduction in Palmer amaranth emergence from cereal rye by 94% in the
middle of the cotton row and an improvement of late-season control by 18% compared with no
cereal rye. In addition to reduced Palmer amaranth emergence, cotton yield was 15% higher with
the use of a cereal rye cover crop (Culpepper et al. 2010b). Suppression of Palmer amaranth
depends on the amount of cover crop biomass produced. Webster et al. (2013) predicted Palmer
amaranth control in cotton would be 25, 50, and 75% with a cereal rye biomass of 2950, 4900,
and 8600 kg ha-1. Wiggins et al. (2016) reported that 2440 kg ha-1 of cereal rye biomass in
combination with a PRE herbicide, provided 87% control of Palmer amaranth in cotton.
However, the use of a cover crop with no herbicide application provided less than 65% control of
Palmer amaranth.
Although cover crops may contribute to weed control during the early spring, additional
management practices, including herbicides, are still needed to provide optimum season-long

17

control (Teasdale 1996; Masiunas et al. 1995; Aulakh et al. 2012; Wiggins et al. 2016). Since
Palmer amaranth is a relatively new weed to Michigan, we wanted to determine if seeding a
cereal rye cover crop and planting soybean in narrow rows would suppress Palmer amaranth
emergence and improve control compared with a herbicide management strategy alone.

18

LITERATURE CITED

LITERATURE CITED
Akemo MC, Regnier EE, Bennett MA (2000) Weed suppression in spring-sown rye (Secale
cereale)-pea (Pisum satvium) cover crop mixtures. Weed Technol 14:545-549
Ateh CM, Doll JD (1996) Spring-planted winter rye (Secale cereale) as a living mulch to control
weeds in soybean (Glycine max). Weed Technol 10:347-353
Aulakh JS, Price AJ, Enloe SF, van Santon E, Wehtje G, Patterson MG (2012) Integrated Palmer
amaranth management in glufosinate-resistant cotton: I. Soil-inversion, high-residue cover
crops and herbicide regimes. Agronomy 2:295-311
Barnes, JP, Putnam AR (1983) Rye residues contribute weed suppression in no-tillage cropping
systems. J Chem Ecol 9:1045-1057
Barnes JP, Putnam AR (1986) Evidence for allelopathy by residues and aqueous extracts of rye
(Secale cereale). Weed Sci 34:384-390
Bell HD, Norsworthy JK, Scott RC (2016) Integrating cereals and deep tillage with herbicide
programs in glyphosate- and glufosinate-resistant soybean for glyphosate-resistant Palmer
amaranth management. Weed Technol 30:85-98
Bernstein ER, Posner JL, Stoltenberg DE, Hedtcke JL (2011) Organically managed no-tillage
rye-soybean systems: agronomic, economic, and environmental assessment. Agron J
103:1169-1179
Beyers JT, Smeda RJ, Johnson WG (2002) Weed management programs in glufosinate-resistant
soybean (Glycine max) 1. Weed Technol 16:267-273
Blanco-Canqui H, Shaver TM, Lindquist JL, Shapiro CA, Elmore RW, Francis CA, Hergert GW
(2015) Cover crops and ecosystem services: insights from studies in temperate soils. Agron J
107:2449-2474
Blum U, Gerig TM, Worsham AD, King LD (1993) Modification of allelopathic effects of pcoumaric acid on morning-glory seedling biomass by glucose, methionine, and nitrate. J
Chem Ecol 19:2791-2811
Blum U (1996) Allelopathic interactions involving phenolic acids. J Nematol 28(3):259
Butts TR, Norsworthy JK, Kruger GR, Sandell LD, Young BG, Steckel LE, Loux MM, Bradley
KW, Conley SP, Stoltenberg DE, Arriaga FJ, Davis VM (2016) Management of pigweed
(Amaranthus spp.) in glufosinate-resistant soybean in the Midwest and mid-south. Weed
Technol 30:355-365
Burnside OC (1973) Influence of Weeds on Soybean Harvesting Losses with a Combine. Weed
Sci 29:520-523

20

Braswell LR, Cahoon CWJr., York AC, Jordan DL, Seagroves RW (2016) Fluridone and
encapsulated acetochlor reduce protoporphyrinogen oxidase inhibitor use in a glufosinatebased Palmer amaranth management program for cotton. Weed Technol 30:838-847
Carpenter J, Gianessi L (1999) Herbicide tolerant soybean: Why growers are adopting Roundup
Ready varieties. J Environ Econ Manage 2:65-72
Chou, Chang-Hung, Patrick ZA (1976) Identification and phytotoxic activity of compounds
produced during decomposition of corn and rye residues in soil. J Chem Ecol 2:369-387
Coetzer E, Al-Khatib K, Peterson DE (2002) Glufosinate efficacy on Amaranthus species in
glufosinate-resistant soybean (Glycine max) 1. Weed Technol 16:326-331
Costea M, Weaver SE, Tardif FJ (2004) The biology of Canadian weeds. 130. Amaranthus
retroflexus L., A. powellii S. Watson and A. hybridus L." Can J Plant Sci 84:631-668
Costea M, Weaver SE, Tardif FJ (2005) The biology of invasive alien plants in Canada. 3.
Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea & Tardif. Can J Plant
Sci 85:507-522
Cowan P, Weaver SE, Swanton CJ (1998) Interference between Pigweed (Amaranthus spp.),
Barnyardgrass (Echinochloa crus-galli), and Soybean (Glycine max). Weed Sci 46:533-539
Creamer NG, Dabney SM (2002) Killing cover crops mechanically: Review of recent literature
and assessment of new research results. Amer J Altern Agric 17:32-40
Culpepper AS, Grey TL, Vencill WK, Kichler JM, Webster TM, Brown SM, York AC, Davis
JW, Hanna WW, (2006) Glyphosate-resistant Palmer amaranth (Amaranthus palmeri)
confirmed in Georgia. Weed Sci 54:620-626
Culpepper AS, York AC, MacRae AW, Kichler J (2008) Glyphosate-resistant Palmer amaranth
response to weed management programs in Roundup Ready and Liberty Link cotton. Page
169 in Proceedings of the Beltwide Cotton Conferences Memphis, TN: National Cotton
Council
Culpepper AS, Webster TM, Sosnoskie LM, York AC (2010a) Glyphosate-resistant Palmer
amaranth in the United States. Pages 195-212 in Nandula VK, ed. Glyphosate resistance in
crops and weeds: history, development, and management. New Jersey: John Wiley and Sons
Inc
Culpepper AS, Kichler J, Sosnoskie L, York A, Sammons D, Nichols R (2010b) Integrating
cover crop residue and moldboard plowing into glyphosate-resistant Palmer amaranth
management programs. 1531 Proceedings in Beltwide Cotton Conference New Orleans, LA:
National Cotton Council of America
Dabney SM, Delgado JA, Reeves DW (2001) Using winter cover crops to improve soil and
water quality. Commun Soil Sci Plant Anal 32:1221-1250

21

Davis AS (2010) Cover-crop roller-crimper contributes to weed management in no-till soybean.
Weed Sci 58:300-309
DeVore JD, Norsworthy JK, Brye KR (2013) Influence of deep tillage, a rye cover crop, and
various soybean production systems on Palmer amaranth emergence in soybean. Weed
Technol 27:263-270
Dille AJ, Sikkema PH, Everman WJ, Davis VM, Burke IC (2016) Perspectives on soybean yield
losses due to weeds in North America. Page 36 in Proceedings of 56th annual meeting of the
Weed Science Society of America San Juan, PR: Weed Science Society of America
Ehleringer J, Forseth IN (1980) Solar tracking by plants. Science 210:1094-1098
Ehleringer J (1983) Ecophysiology of Amaranthus palmeri, a Sonoran Desert summer
annual. Oecologia 57:107-112
Eiland F, Klamer M, Lind AM, Leth M, Baath E (2001) Influence of initial C/N ratio on
chemical and microbial composition during long term composting of straw. Microb Ecol
41:272-280
[ERS] US Department of Agriculture- Economic Research Service (2016) Soybean and oil
crops: background. http://www.ers.usda.gov/topics/crops/soybean-oil-crops/background.aspx
Accessed September 26, 2016
Finney DM, White CM, Kaye JP (2016) Biomass production and carbon/nitrogen ratio influence
ecosystem services from cover crop mixtures. Agron J 108:39-52
Gaines TA, Shaner DL, Ward SM, Leach JE, Preston C, Westra P (2011) Mechanism of
resistance of evolved glyphosate-resistant Palmer amaranth (Amaranthus palmeri). J Agri
Food Chem 59:5886-5889
Gallagher RS, Cardina J (1998) Phytochrome-mediated Amaranthus germination, I: effect of
seed burial and germination temperature. Weed Sci 46:48-52
Gallandt ER, Molloy T, Lynch RP, Drummond FA (2005) Effect of cover-cropping systems on
invertebrate seed predation. Weed Sci 53:69–76
Gossett BJ, Murdock EC, Toler JE (1992) Resistance of Palmer amaranth (Amaranthus palmeri)
to dinitroaniline herbicides. Weed Technol 6:587-591
Guo P, Kassim Al-Khatib (2003) Temperature effects on germination and growth of redroot
pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp
(A. rudis). Weed Sci 51:869-875
Hager A (2016) Remain vigilant for Palmer amaranth. http://bulletin.ipm.illinois.edu/?p=3700.
Accessed March 21, 2017

22

Hammond, RB (1990) Influence of cover crops and tillage on seedcorn maggot (Diptera:
Anthomyiidae) populations in soybean. Environ Entomol 19:510-514
Harder DB, Sprague CL, Renner KA (2007) Effect of soybean row width and population on
weeds, crop yield, and economic return. Weed Technol 21:744-752
Hartwig NL, Ammon HA (2002) Cover crops and living mulches. Weed Sci 50:688-699
Hartzler B (2016) Palmer amaranth in Iowa-what we know.
http://crops.extension.iastate.edu/cropnews/2016/08/palmer-amaranth-iowa-what-we-know.
Accessed March 21, 2017
Heap I (2016) The international survey of herbicide resistant weeds. www.weedscience.org.
Accessed September 26, 2016
Hill EC (2014) Cover crop influence on nitrogen availability, weed dynamics, and dry bean
(Phaseolus vulgaris L.) characteristics in an organic system. Ph.D dissertation. East Lansing,
Michigan: Michigan State University 224 p
Hoffner AE, Jordan DL, Chandi A, York AC, Dunphy EJ, Everman WJ (2012) Management of
Palmer amaranth (Amaranthus palmeri) in glufosinate-resistant soybean (Glycine max) with
sequential applications of herbicides. ISRN Agron 2012:131650. doi:10.5402/2012/131650
Holden BD, Norsworthy JK, Scott RC, Popp M (2015) Effect of row spacing, seeding rate, and
herbicide program in glufosinate-resistant soybean on Palmer amaranth management. Weed
Technol 29:390-404
Horak MJ, Loughin TM (2000) Growth analysis of four Amaranthus species. Weed Sci 48:347355
Horak MJ, Peterson DE (1995) Biotypes of Palmer amaranth (Amaranthus palmeri) and
common waterhemp (Amaranthus rudis) are resistant to imazthapyr and thifensulfuron.
Weed Technol 9:192-195
Jha P, Norsworthy JK (2009) Soybean canopy and tillage effects on emergence of Palmer
amaranth (Amaranthus palmeri) from a natural seed bank. Weed Sci 57:644-651
Jha P, Norsworthy JK, Riley MB, Bridges W and Jr (2010) Annual changes in temperature and
light requirements for germination of Palmer amaranth (Amaranthus palmeri) seeds retrieved
from soil. Weed Sci 58:426-432
Klingaman TE, Oliver LR (1994) Palmer amaranth (Amaranthus palmeri) interference in
soybean (Glycine max). Weed Sci 42:523-527
Keeley PE, Carter CH,Thullen RJ (1987) Influence of planting date on growth of Palmer
amaranth (Amaranthus palmeri). Weed Sci 35:199-204

23

Kinyangi JM, Smucker AJM, Mutch DR, Harwood RR (2001) Managing cover crops to recycle
nitrogen and protect groundwater. East Lansing, Michigan: Extension Bulletin E-2763
Koger CH, Reddy KN, Shaw DR (2002) Effects of rye cover crop residue and herbicides on
weed control in narrow and wide row soybean planting systems. Weed Biol Manag 2:216224
Kohrt JR (2017) Biology and management of multiple- (glyphosate ALS and atrazine) resistant
Palmer amaranth in Michigan. Ph.D dissertation. East Lansing, Michigan: Michigan State
University 153 p
Kohrt JR, Sprague CL, Nadakuduti SS, Douches D (2017) Confirmation of a three-way
(glyphosate, ALS, and atrazine) herbicide-resistant population of Palmer amaranth
(Ameranthus palmeri) in Michigan. Weed Sci 70:1-12
Korres NE, Norsworthy JK (2015) Influence of a rye cover crop on the critical period for weed
control in cotton. Weed Sci 63:346-352
Kruse M, Strandberg M, Strandberg B (2000) Ecological effects of allelopathic plants-a
review. NERI Technical Report 315
Laub CA, Luna JM (1991) Influence of winter crop suppression practices on seasonal abundance
of armyworm (Lepidoptera: Noctuidae), cover crop regrowth, and yield in no-till corn.
Entomol Soc Am 20:749-754
Larkin RP, Griffin TS, Honeycutt CW (2010) Rotation and cover crop effects on soilborne
potato diseases, tuber yield, and soil microbial communities. Plant Dis 94:1491-1502
Légère A, Schreiber MM (1989) Competition and canopy architecture as affected by soybean
(Glycine max) row width and density of redroot pigweed (Amaranthus retroflexus). Weed Sci
37:84-92
Legleiter T, Johnson B, Jordan T, Gibson K (2012) Successful cover crop termination with
herbicides. West Lafayette, Indiana: Extension Bulletin WS-50-W
Leon RG, Owen MDK (2003) Regulation of weed seed dormancy through light and temperature
interactions. Weed Sci 51:752-758
Liebl R, Simmons FW, Wax LM, Stoller EW (1992) Effect of rye (Secale cereale) mulch on
weed control and soil moisture in soybean (Glycine max). Weed Technol 6:838-846
Malik MS, Norsworthy JS, Culpepper AS, Riley MB, Bridges WJr (2008) Use of wild radish
(Raphanus raphanistrum) and rye cover crops for weed suppression in sweet corn. Weed Sci
56:588-595
Masiunas JB, Weston LA, Weller SC (1995) The impact of rye cover crops on weed populations
in a tomato cropping system. Weed Sci 43:318-323

24

Massinga RA, Currie RS, Horak MJ, Boyer Jr J (2001) Interference of Palmer amaranth in
corn. Weed Sci 49:202-208
Menges RM (1974) Distribution of viable weed seeds in soil profiles at various locations in the
lower Rio Grande Valley of Texas. J of The Rio Grande Valley Hortic Soc 28:50-56
Michael HJ, Loughin T (2000) Growth analysis of four amaranthus species. Weed Sci 48:347355
Mickelson JA, Renner KA (1997) Weed control using reduced rates of postemergence herbicides
in narrow and wide row soybean. J Prod Agri 10:431-437
Mirsky SB, Curran WS, Mortensen DM, Ryan MR, Shumway DL (2009) Control of cereal rye
with a roller/crimper as influenced by cover crop phenology. Agron J 101:1589-1596
Mirsky SB, Curran WS, Mortensen DM, Ryan MR, Shumway DL (2011) Timing of cover crop
management effects on seed suppression in no-till planted soybean using a roller-crimper.
Weed Sci 59:380-389
Monks DW, Oliver LR (1988) Interactions between soybean (Glycine max) cultivars and
selected weeds. Weed Sci 36:770-774
Moore MJ, Gillespie TJ, Swanton CJ (1994) Effect of cover crop mulches on weed emergence,
weed biomass, and soybean (Glycine max) development. Weed Technol 8:512-518
Mullen MD, Melhorn CG, Tyler DD, Duck BN (1998) Biological and biochemical soil
properties in no-till corn with different cover crops. J Soil Water Conserv 53:219-224
Murphy TR, Gossett BJ (1981) Influence of shading by soybean (Glycine max) on weed
suppression. Weed Sci 29:610-615
[NASS] US Department of Agriculture- National Agricultural Statistics Service (2016) Crop
production 2016 summary. https://quickstats.nass.usda.gov/. Accessed: September 21, 2016
Norris JL, Shaw DR, Snipes CE (2002) Influence of row spacing and residual herbicides on
weed control in glufosinate-resistant soybean (Glycine max) 1. Weed Technol 16:319-325
Norsworthy JK, Griffith GM, Scott RC, Smith KL, Oliver LR (2008) Confirmation and control
of glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in Arkansas. Weed
Technol 22:108-113
Norsworthy JK, Smith KL, Steckel LE, Koger CH (2009) Weed seed contamination of cotton
gin trash. Weed Technol 23:574-580
Norsworthy JK, McClelland M, Griffith G, Bangarwa SK, Still J (2011) Evaluation of cereal and
brassicaceae cover crops in conservation-tillage, enhanced, glyphosate-resistant cotton. Weed
Technol 25:6-13

25

Norsworthy JK, Korres NE, Walsh MJ, Powels SB (2016) Integrating herbicide programs with
harvest weed seed control and other fall management practices for the control of glyphosateresistant Palmer amaranth (Amaranthus palmeri). Weed Sci 64:540-550
[NRCS] US Department of Agriculture- Natural Resource Conservation Service (2011) Carbon
to nitrogen ratios in cropping systems.
https://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download?cid=nrcs142p2_052823
&ext=pdf. Accessed: March 20, 2017
Pantoja JL, Woli KP, Sawyer JE, Barker DW (2016) Winter rye cover crop biomass production,
degredation, and nitrogen recycling. Agron J 108:841-853
Place G, Bowman D, Burton M, Rutty T (2008) Root penetration through a high bulk density
soil layer: differential response of a crop and weed species. Plant Soil 307:179-190
Powles SB (2010) Gene amplification delivers glyphosate-resistant weed evolution. Proceedings
of the National Academy of Sciences 107:955-956
Powell DK (2014) Biology and management of multiple (glyphosate/ALS)-resistant Palmer
amaranth in Michigan soybean. Ph.D dissertation. East Lansing, MI: Michigan State
University. 122p
Price AJ, Reeves DW, Patterson MG (2006) Evaluation of weed control provided by three winter
cereals in conservation-tillage soybean. Renew Agr Food Syst 21:159-164
Price AJ, Arriaga FJ, Raper RL, Balkcom KS, Kornecki TS, Reeves DW (2009) Comparison of
mechanical and chemical winter cereal cover crop termination systems and cotton yield in
conservation agriculture. J Cotton Sci 13:238-245
Price AJ, Balkcom KS, Duzy LM, Kelton JA (2012) Herbicide and cover crop residue
integration for Amaranthus control in conservation agriculture cotton and implications for
resistance management. Weed Technol 26:490-498
Putnam AR, DeFrank J (1983) Use of phytotoxic plant residues for selective weed control. Crop
Protect 2:173-181
Reddy KN (2001) Effects of cereal and legume cover crop residues on weeds, yield, and net
return in soybean (Glycine max) 1. Weed Technol 15:660-668
Reeves DW (1994) Cover crops and rotations. Advances in Soil Science: Crops Residue
Management CRC Press 125-172.
Ryan MR, Mirsky SB, Mortensen DA, Teasdale JR, Curran WS (2011) Potential synergistic
effects of cereal rye biomass and soybean planting density on weed suppression. Weed Sci
59:238-246

26

Saini M, Price AJ, van Santen E (2006) Cover crop residue effects on early-season weed
establishment in a conservation-tillage corn-cotton rotation. Pages 175-178 in Proceedings of
28th Southern Conservation Tillage Systems Conference Amarillo, TX
Sauer JD (1955) Revision of the dioecious amaranths. Madrono 13:5-46
Schomberg HH, Martini NL, Diaz-Perez JC, Phatak SC, Balkcom KS, Bhardwaj HL (2007)
Potential for using sunn hemp as a source of biomass and nitrogen for the Piedmont and
Coastal Plains regions of the southeastern USA. Agron J 106:1041-1050
Smith AN, Reberg-Horton SC, Place GT, Meijer AD, Arellano C, Mueller JP (2011) Rolled rye
mulch for weed suppression in organic no-tillage soybean. Weed Sci 59:224-231
Snapp SS, Swinton SM, Labarta R, Mutch D, Black JR, Leep R, O'Neil K (2005) Evaluating
cover crops for benefits, costs and performance within cropping system niches. Agron
J 97:322-332
Sosnoskie LM, Kichler JM, Wallace RD, Culpepper AS (2011) Multiple resistance in Palmer
amaranth to glyphosate and pyrithiobac confirmed in Georgia. Weed Sci 59:321-325
Sprague CL (2011) Glyphosate-resistant Palmer amaranth confirmed in Southwest Michigan.
http://msue.anr.msu.edu/news/glyphosate_resistant_palmer_amaranth_confirmed_in_southw
est_michigan. Accessed September 26, 2016
Sprague CL (2013) Scouting Michigan Fields for Palmer amaranth.
http://msue.anr.msu.edu/news/scouting_michigan_fields_for_palmer_amaranth. Accessed
September 26, 2016
Teasdale JR (1996) Contribution of cover crops to weed management in sustainable agricultural
systems. J Prod Agri 9:475-479
Thompson C (2009) Multiple resistant Palmer amaranth. International survey of herbicide
resistant weeds. www.weedscience.org. Accessed September 26, 2016
Unger PW, Vigil MF (1998) Cover crop effects on soil water relationships. J Soil Water
Conserv 53:200-207
Untung K (1978) The distribution and bionomics of the armyworm, Pseudaletia unipuncta
(Haw.) in Michigan. Ph.D dissertation. East Lansing, MI: Michigan State University.
[USDA] US Department of Agriculture (2010) Field crops usual planting and harvesting dates.
http://usda.mannlib.cornell.edu/usda/current/planting/planting-10-29-2010.pdf Accessed
September 26, 2016
[USDA] US Department of Agriculture (2016) Crop production 2015 summary.
https://www.usda.gov/nass/PUBS/TODAYRPT/cropan16.pdf. Accessed March 14, 2017

27

Van Wychen, L (2016) WSSA Survey Ranks Palmer Amaranth as the Most Troublesome Weed
in the U.S., Galium as the Most Troublesome in Canada. http://wssa.net/2016/04/wssasurvey-ranks-palmer-amaranth-as-the-most-troublesome-weed-in-the-u-s-galium-as-themost-troublesome-in-canada/. Accessed April 10, 2017
Villamil MB, Bollero GA, Darmody RG, Simmons FW, Bullock DG (2006) No-till
corn/soybean systems including winter cover crops. Soil Sci Soc Am J 70:1936-1944
Vivian R, Reis A, Kálnay PA, Vargas L, Ferreira ACC, Mariani F (2013) Weed management in
soybean–issues and practices. Pages 47-84 in El-Shemy HA, ed. Soybean-pest resistance.
InTech DOI:10.5772/54595
Ward SM, Webster TM, Steckel LE (2013) Palmer amaranth (Amaranthus palmeri): a review.
Weed Technol 27:12-27
Wax LM, Pendleton JW (1968) Effect of row spacing on weed control in soybean. Weed
Sci 16:462-465
Wayman S, Cogger C, Benedict C, Burke I, Collins D, Bary A (2014) The influence of cover
crop variety, termination timing and termination method on mulch, weed cover and soil
nitrate in reduced-tillage organic systems. Renew Agr Food Syst 30:450-460
Weaver SE (2001) Impact of lamb’s-quarters, common ragweed and green foxtail on yield of
corn and soybean in Ontario. Can J Plant Sci 81:821-828
Webster TM, Nichols RL (2012) Changes in the prevalence of weed species in the major
agronomic crops of the Southern United States: 1994/1995 to 2008/2009. Weed Sci 60:145157
Webster TM, Scully BT, Grey TL, Culpepper AS (2013) Winter cover crops influence
Amaranthus palmeri establishment. Crop Protect 52:130-135
Webster TM, Grey TL, Simmons DB, Culpepper AS, Scully BT (2016) Suppression of Palmer
amaranth (Amaranthus palmeri) with high-biomass rye (Secale cereal). Pages 60-61 in
Proceedings of 56th annual meeting of the Weed Science Society of America San Juan, PR:
Weed Science Society of America
Wendler C, Barniske M, Wild A (1990) Effect of phosphinothricin (glufosinate) on
photosynthesis and photorespiration of C3 and C4 plants. Photosyn Res 24:55-61
Whitaker JR, York AC, Jordan DL, Culpepper AS, Sosnoskie LM (2011) Residual herbicides for
Palmer amaranth control. J of Cotton Sci 15:89-99
Wiggins MS, Hayes RM, Steckel LE (2016) Evaluating cover crops and herbicides for
glyphosate-resistant Palmer amaranth (Amaranthus palmeri) control in cotton. Weed Technol
30:415-422

28

Williams SM, Weil RR (2004) Cover crop channels may alleviate soil compaction effects on
soybean crop. Soil Sci Soc J 68:1403-1409
Wills K (2013) Where do all these soybeans go?
http://msue.anr.msu.edu/news/where_do_all_these_soybeans_go. Accessed March 20, 2017
Wilkins ED, Bellinder RR (1996) Mow-kill regulation of winter cereals for spring no-till crop
production. Weed Technol 10:247-252
Wright SR, Jennette MW, Coble HD, Rufty TW (1999) Root morphology of young Glycine max,
Senna obtusifolia, and Amaranthus palmeri. Weed Sci 47:706-711
Yenish JP, Worsham AD, Chilton WS (1995) Disappearance of BIBOA-GLUCOSIDE, DIBOA,
and BOA from rye (Secale cereale L.) cover crop residue. Weed Sci 43:18-20

29

CHAPTER 2
INTEGRATING A CEREAL RYE COVER CROP AND SOYBEAN ROW WIDTH
WITH HERBICIDES TO MANAGE PALMER AMARANTH IN MICHIGAN

Abstract
Integrating the use of narrow row widths and cover crops may improve management of
herbicide-resistant Palmer amaranth in Michigan soybean production systems. A field
experiment was established in the fall of 2014 and 2015 near Middleton, Michigan in a field with
a glyphosate-resistant Palmer amaranth population. This experiment determined if a cereal rye
cover crop in conjunction with a standard or high herbicide program improved control of Palmer
amaranth in soybean planted in 76- and 19-cm rows. Additionally, pot experiments determined
the effects of cereal rye stage, biomass, and termination method on Palmer amaranth emergence.
Cereal rye biomass was 1200 and 2186 kg ha-1 at spring termination in 2105 and 2016,
respectively. Cereal rye reduced winter and early summer annual weed biomass by 77 and 84%
in 2015 and 2016, respectively, compared with the no cover control. Cereal rye was successfully
terminated in plots terminated with glyphosate. However, cereal rye was not completely
terminated by flail mowing in either year and produced an additional 580 to 1291 kg ha-1 of dry
biomass, before being terminated with glyphosate prior to soybean planting. Palmer amaranth
emergence began in early- to mid-June, 2-3 weeks after soybean planting. Cereal rye had little
effect on Palmer amaranth emergence in 2015; however, a delay in emergence was observed
with mowing in 2016 that may affect the timing of postemergence herbicide applications.
Soybean canopy closure affected Palmer amaranth emergence in both years, with prolonged
emergence in soybean planted in 76 cm rows compared with 19 cm rows. In 2015, Palmer
amaranth was controlled with both the high and standard herbicide management programs;

30

however, a lack of precipitation in 2016 resulted in poor control in the standard management
program. Cereal rye and soybean row width had little effect on soybean yield. However, the use
of a herbicide management strategy significantly increased yield in both years compared with no
herbicide management. In 2016, soybean yield was 574 kg ha-1 higher in the high herbicide
management compared with the standard management program. In outside pot experiments,
cereal rye biomass at Feekes 10.1 or later suppressed Palmer amaranth emergence in both years,
while all covers, regardless of cereal rye stage and biomass, suppressed Palmer amaranth
emergence in 2015. In the greenhouse, cereal rye terminated by cutting reduced Palmer amaranth
emergence compared with termination by glyphosate. Soybean planted in narrow rows aided in
preventing late-season Palmer amaranth emergence. Planting a cereal rye cover crop that had a
maximum biomass of 2180 kg ha-1 was not effective in suppressing Palmer amaranth in
Michigan. However, applying a PRE followed by a POST herbicide program and planting
soybean in narrow rows reduced the duration of Palmer amaranth emergence and provided
effective control.
Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats.; soybean, Glycine max (L.)
Merr.; cereal rye, Secale cereal L.; acetochlor; flumioxazin; glufosinate; glyphosate.
Key words: Glyphosate-resistant; biomass; cover crop; row width; emergence; herbicide
management.
Introduction
Palmer amaranth (Amaranthus palmeri S. Wats.) is a dioecious summer annual weed species
that exhibits season-long emergence, rapid biomass accumulation and prolific seed production
(Ehleringer 1983; Keeley et al. 1987; Culpepper et al. 2010a; Ward et al. 2013). Native to the
southwest region of the United States, Palmer amaranth populations can now be found in

31

previously uninhabited areas of the southern and Midwest United States (Culpepper et al. 2010a;
Heap 2016). First identified in Michigan in 2010 (Sprague 2011), Palmer amaranth can
successfully complete its lifecycle and produce seed that survives Michigan’s cold winters, thus
becoming a weed problem the following season (Kohrt 2017; Powell 2014). In Michigan, Palmer
amaranth typically emerges from late-May through September (Powell 2014). In soybean, Kohrt
(2017) observed individual plants that emerged in May, grew to maturity, and produced over
350,000 seeds per plant. When Palmer amaranth emerged after the crop, seed production was
reduced. However, even Palmer amaranth plants that emerged in August were able to produce
viable seed prior to a killing frost.
Managing Palmer amaranth with herbicides poses several challenges. Kohrt (2017) reported
that Palmer amaranth emergence in Michigan continued through mid-September. Continued
emergence throughout the season makes Palmer amaranth difficult to control with preemergence
(PRE) herbicide applications alone. Palmer amaranth can grow up to 0.21 cm per growing degree
day and reach a mature height over 2 meters (Horak and Loughin 2000). Compared with other
Amaranthus species, Palmer amaranth can produce more biomass at temperatures above 25 C,
and its growth rate is 62% greater per growing degree day (Gou and Al-Khatib 2003; Culpepper
et al. 2006). Rapid growth makes timing of postemergence (POST) herbicide applications
difficult for effective Palmer amaranth control. Finally, Palmer amaranth has a remarkable ability
to develop resistance to herbicides (Klingaman and Oliver 1994; Heap 2016). To date,
populations are confirmed are resistant to acetolactate synthase inhibitors (ALS) (Group 2);
microtubule inhibitors (Group 3); photosystem II inhibitors (PSII) (Group 5); the 5-enolpyruvylshikimate-3-phosphate inhibitor (EPSP), glyphosate (Group 9); protoporphyrinogen oxidase

32

inhibitors (PPO) (Group 14); and 4-hydroxyphenylpyruvate dioxygenase inhibitors (HPPD)
(Group 27) (Gossett et al. 1992; Horak and Peterson 1995; Heap 2016).
While Palmer amaranth has developed resistance to multiple herbicide sites of action,
glyphosate resistance has perhaps caused the greatest issues with weed management. The first
confirmed case of glyphosate-resistant Palmer amaranth was identified in 2005 in a Georgia
cotton field that was sprayed with glyphosate for seven straight years as the sole means of weed
control (Culpepper et al. 2006). This population survived glyphosate applications up to 12 times
the normal field use rate. Glyphosate-resistant Palmer amaranth has since spread across 23 states,
including Michigan (Sprague 2011; Heap 2016).
The use of an effective herbicide management program is important in controlling Palmer
amaranth and reducing future emergence. Flumioxazin (Group 14) is one of the most effective
PRE herbicides, with observed control of up to 98% at 60 days after application (Whitaker et al.
2011). In Michigan, Powell (2014) observed good control (>85%) of Palmer amaranth with
flumioxazin 28 days after planting; however, the amount of rainfall after application impacted
herbicide effectiveness. Flumioxazin alone does not provide season-long control. Thus, an
effective POST herbicide application is needed to manage weeds that are not controlled by the
PRE herbicide application alone. Glufosinate is an effective POST herbicide for control of
glyphosate-resistant Palmer amaranth. Barnett et al. (2013) observed over 90% control when
glufosinate was applied to Palmer amaranth under 13 cm in height. When glufosinate was
applied to 26 cm tall Palmer amaranth, control was reduced to 59%, demonstrating the
importance of timely POST applications. Acetochlor (Group 15) aids glufosinate in Palmer
amaranth control when tank-mixed POST due to its residual properties. Glufosinate alone
provided only ~86% control when applied to Palmer amaranth less than 10 cm tall early POST

33

and mid-season POST, while the combination of glufosinate and acetochlor provided 94 and
91% control, respectively (Cahoon et al. 2015).
Planting soybean in narrow row widths is a cultural practice that many growers use to
improve weed suppression. Narrow row soybean forms a closed canopy earlier than soybean
planted in wide rows, reducing the amount and quality of light that reaches the soil surface and,
in turn reducing late-season weed germination and emergence (Wax and Pendleton 1968;
Murphy and Gossett 1981). By 50 days after emergence, soybean canopy closure in 25 cm rows
was complete, while canopy closure in the 76 cm rows was never quite achieved (Légère and
Schreiber 1989). Amaranthus species display a phytochrome-controlled germination response in
which the presence of red light stimulates germination (Gallagher and Cardina 1998; Leon and
Owen 2003). Since the soil surface under a plant canopy receives more far-red light than red
light due to adsorption by the plant, soybean planted in narrow rows reduces late-season Palmer
amaranth emergence. An inhibitory effect of far-red light on Palmer amaranth germination was
observed by Jha et al. (2010). This explains why the majority of germination occurs prior to crop
canopy closure, and supports observations that emergence declines in the presence of a soybean
canopy.
DeVore et al. (2013) reported Palmer amaranth emergence was greatly reduced once the
soybean canopy closed, providing late-season control. Jha and Norsworthy (2009) similarly
reported that Palmer amaranth emergence was reduced by 73% from the presence of a soybean
canopy. Soybean planted in narrow rows also reduced the growth of weeds after emergence
compared with wide rows. Mickelson and Renner (1997) reported that total weed biomass in 19
cm rows was reduced by 30% compared with biomass in 76 cm rows, due to earlier canopy
closure. Another study observed that planting soybean in 25 cm rows, reduced total pigweed

34

biomass by almost 20% and produced more yield than soybean planted in 76 cm rows (Légère
and Schreiber 1989).
Cover crops are an additional tool to aid in weed management and reduce herbicide use for
weed control (Mortensen et al. 2012). Cereal rye in particular, has been of interest as it provides
ground cover during the winter months, produces high amounts of biomass, reduces soil erosion
and has the potential to provide early-season weed control (Kinyangi et al. 2001). Cereal rye is
terminated chemically with herbicides or mechanically prior to the planting of a cash crop in the
spring. Flail mowing is one mechanical termination method used in no-till systems to leave
residue more uniformly distributed across the soil surface. However, it is suggested to wait until
head emergence (Feekes 10.1 or later) for the greatest amount of biomass and minimal regrowth
(Wilkins and Bellinder 1996).
When using a cover crop that leaves residue in the spring, growers need to consider the
possibility of an increase in the presence of damaging insects along with those that are beneficial.
Crop damaging insects such as armyworm (Psudaletia unipuncta) and black cutworm (Agrotis
ipsilon) lay their eggs on spring plant vegetation, such as cover crops and weeds, while seed corn
maggot (Delia platura Miegen) feeds on plant residues in the soil. Earlier termination of the
cover crop compared with crop planting may reduce the presence of these pests due to the early
removal of their food source which forces them to find alternative sources. Reddy (2001)
observed better establishment of soybean stands in cover crop residue that had begun to
breakdown compared with stands in a fresh layer of residue. Physical termination of the cover
crop, such as mowing, may kill armyworm and black cutworm larvae reducing the population
(Laub and Luna 1991).

35

Cereal rye mulches left on the soil surface reduce the quality of available light and heat
adsorbed by the soil, causing less favorable conditions for germination and a reduction of earlyseason weed growth (Moore et al. 1994; Teasdale 1996). DeVore et al. (2013) reported that in
Arkansas, a cereal rye cover crop reduced Palmer amaranth emergence in soybean by up to 71%.
In no-till cotton production, cereal rye reduced Palmer amaranth emergence by 94% and
improved late-season control by 18%, compared with no cover crop (Culpepper et al. 2010b).
However, cover crop residues typically do not provide season-long weed control, so additional
cultural methods or herbicide applications are needed.
The search for management practices that can be integrated with herbicides to improve the
management of Palmer amaranth in Michigan soybean production led to the following research
objectives: 1) determine if a cereal rye cover crop and planting soybean in narrow rows reduce
Palmer amaranth emergence and growth in Michigan, and 2) determine the effect of above- and
belowground cereal rye biomass and termination method on Palmer amaranth emergence.

Materials and Methods
Field Experiment. Field experiments were conducted in 2015 and 2016 in a commercial field
near Middleton, MI (43.2616°N; -84.7609°W). This field was in a corn-soybean rotation. In
2013, two years following a dairy cattle manure application to this field, Palmer amaranth was
identified and confirmed glyphosate-resistant. The soil type at this location was a Metea loamy
sand (loamy, mixed, active, mesic Arenic Hapludalfs) with pH of 7.0 and 2.5% soil organic
matter.
The experiment was conducted as a split-split plot arranged in a randomized complete block
design with 18 treatments, each replicated four times. Each plot measured 3 m wide by 9 m long.

36

The main plot factor was a cereal rye cover crop, the subplot factor soybean row width, and the
sub-subplot factor was herbicide management strategy. The main plots consisted of three cover
crop factors: 1) a cereal rye cover crop terminated with 1.26 kg a.e. ha-1 glyphosate (Roundup
PowerMAX, Monsanto Co., St. Louis, MO) + 2% w w-1 spray grade ammonium sulfate (AMS)
(Actamaster, Loveland Products Inc., Loveland, CO) (RG), 2) a cereal rye cover crop terminated
by a flail mower (RM), and 3) no rye cover crop (NC). The subplots consisted of two row-width
factors, soybean planted in 76- and 19-cm rows. The sub-subplot factors consisted of three
herbicide management factors: 1) high herbicide management (HH), 2) standard herbicide
management (SH), and 3) no herbicide management (NH) (Table 2.1).
‘Wheeler’ cereal rye was sown in 19 cm rows at a rate of 100 kg ha-1 on October 23, 2014
and September 30, 2015 using a no-till drill (Great Plains, Salina, KS). Cereal rye was
terminated on May 14, 2015 and May 11, 2016 when it was 30 cm tall at Feekes 6 and 58 cm tall
at Feekes 9, respectively. Prior to termination, percent ground cover was measured using linetransects (Laflen et al. 1981) laid diagonally across each main cereal rye and no cover crop plots.
Incidents of cereal rye, weed or no vegetation were recorded every 30 cm along a 23 m transect.
Aboveground cereal rye and weed biomass was harvested at this time from three random 0.25 m2
subsamples in each main plot. Annual bluegrass (Poa annua L.), common chickweed (Stellaria
media (L.) Vill.), and shepherd’s purse (Capsella bursa-pastoris (L.) Medik.) were present at the
time of biomass collection in both years. Common lambsquarters (Chenopodium album L.) and
field pennycress (Thlaspi arvense L.) were also present in 2015. Biomass samples were dried for
approximately 7 d at 65 C and weighed. Seven to 10 d after termination, cereal rye was evaluated
for percent control from termination method. Cereal rye regrowth occurred in the plots that were
terminated with the flail mower and regrowth biomass was harvested, dried, and weighed from

37

three 0.25 m2 samples. Cereal rye subsamples of initial and regrowth biomass were analyzed for
C:N ratios using a ECS 4010 CHNSO Analyzer (Costech Analytical Technologies Inc.,
Valencia, CA). After sampling, the RM and NH plots were treated with 1.26 kg a.e. ha-1
glyphosate + 2% w w-1 AMS.
Glufosinate-resistant soybean ‘DF 9221 LL’ (D.F. Seeds, Dansville, MI) was planted on May
27, 2015 and May 24, 2016, approximately 2 weeks after initial rye termination, in 76 and 19 cm
rows. Soybean seeding rates for the 76 and 19 cm row widths were based on local MI
recommendations of 370,500 and 494,000 seeds ha-1, respectively. At planting, percent ground
cover of terminated vegetation was reassessed in the main plots using the line-transect method
described above. Soil moisture was also measured at this time with a FieldScout TDR 300 Soil
Moisture Meter (FieldScout, Spectrum Technologies, Aurora, IL) by collecting nine
measurements per main plot at depths of 7.6 and 11.9 cm.
Preemergence (PRE) herbicides in the SH and HH plots were applied immediately after
planting (Table 2.1). Postemergence (POST) and late postemergence (LPOS) herbicide
applications were made when emerged Palmer amaranth was approximately 7.5 cm tall. All
herbicide applications were made using a tractor-mounted, compressed air sprayer or CO2
pressurized backpack sprayer calibrated to deliver 177 L ha-1 at 207 kPa of pressure through
11003 AIXR flat-fan nozzles (TeeJet, Spraying Systems Co., Wheaton, IL).

Palmer amaranth measurements. Palmer amaranth control was evaluated at the POST
application, 21 d after POST treatment (DAT), and at harvest. Evaluations were based on a scale
of 0 to 100% with 0 representing no control and 100 indicating complete control. Two permanent
0.25 m2 quadrats were established in each plot to measure Palmer amaranth emergence

38

throughout the growing season. Each week, newly emerged Palmer amaranth seedlings were
counted and removed from these quadrats. At Palmer amaranth maturity when peak biomass
occurred, plants were counted and aboveground biomass was harvested from two random 0.25
m2 subsamples in the NH plots. Palmer amaranth biomass was dried and weighed to assess
biomass reduction from cover crop and row width treatments. The number of Palmer amaranth
plants from the middle 76 cm by 9 m area in the SH and HH plots was also recorded at the time
of peak biomass.

Soybean measurements. Light interception was measured weekly or biweekly only in the HH
plots using a SunScan Canopy Analysis system (Delta-T Devices Ltd, Burwell, Cambridge, UK).
Three measurements were taken above and below the soybean canopy at corresponding
locations. Soybean canopy closure was calculated using Equation 1.
Canopy closure (%) = [(above canopy measurement – below canopy measurement) /
(above canopy measurement)] × 100

[Eq.1]

Final soybean populations were assessed in all plots at maturity. Soybean was harvested by
hand from a 1.5 m wide by 3 m long area in the center of each plot on October 19, 2015 and
October 21, 2016. Harvested plants were threshed through an ALMACO stationary thresher
(Allan Machine Company, Nevada, IA) to separate the beans from other plant material and
debris. Cleaned samples were weighed, and grain moisture and test weight were measured using
a Grain Analysis Computer (GAC®) 2100 Agri (Dickey-john, Auburn, IL). Yields were
calculated by adjusting to 13% moisture.
Precipitation and temperature data was obtained from Michigan State University Enviroweather Automated Weather Station Network located in Ithaca, MI (MSU Enviro-weather 2016).

39

Effects of Cereal Rye on Palmer amaranth Emergence – Pot Experiments. Outdoor. A pot
experiment was conducted outside in the summers of 2015 and 2016 to examine the mulching
effects of a cereal rye cover on Palmer amaranth emergence. The experimental setup was a
completely randomized design with eight treatments in 2015 and five treatments in 2016. All
treatments were replicated four times. The treatments consisted of, cereal rye at two different
growth stages (early and late; Feekes 6 and 10.5 in 2015 and Feekes 9 and 10.1 in 2016,
respectively) applied at three mulching rates, a non-cereal rye cover, and a no cover control.
‘Wheeler’ cereal rye aboveground biomass for the two different growth stages was collected
from separate fields. Cereal rye fresh weight per 0.25 m2 was taken to determine the amount of
biomass needed per pot. Dry biomass was 1200 and 3750 kg ha-1 for the early and late cereal rye
stages, respectively. Mulching rates were categorized as low, medium, and high which was
equivalent to 1200, 2400 and 3750 kg ha-1, respectively. The low rate was the 1X rate for the
early stage cereal rye, the medium rate was 2X the rate of the early stage cereal rye and
approximately 0.5X the rate of the late stage rye, and the high rate was equivalent to 1X the late
stage cereal rye. In 2016, only the low and medium rates were examined for the early stage
cereal rye and only the low rate was examined for the late stage rye. In both years, the non-cereal
rye cover, a dried raffia palm (Ashland, Irving, Texas), was applied at the low mulching rate.
Each treatment was applied to the surface of 10 x 10 cm pots planted with 50 Palmer amaranth
seeds 0.75 cm deep in a sandy loam sterilized field soil with a pH of 7.4 and 3% soil organic
matter. Pots were watered immediately after planting and were kept moist by rainfall or overhead
watering on a daily basis. Emergence counts were recorded twice a week with new seedlings
removed at each count until germination ceased. Emergence counts were recorded for 45 d.

40

Greenhouse. A greenhouse experiment determined the effects of cereal rye termination method
and biomass on Palmer amaranth emergence. Ten seeds of ‘Wheeler’ cereal rye were planted at a
depth of 1.4 cm in 10 x 10 cm pots filled with a sandy loam sterilized field soil with a pH of 7.4
and 3% soil organic matter. Approximately two weeks later, additional pots of cereal rye were
planted to ensure two levels of cereal rye biomass. The first planting date was used for the high
biomass treatments, while the second planting date was used for the low biomass treatments.
Cereal rye was grown in the greenhouse at 25 ± 5 C and sunlight was supplemented to provide a
total midday light intensity of 1,000 µmol m-2 s-1 photosynthetic photon flux at plant height in a
16 h day. Plants were watered and fertilized to promote optimum plant growth.
Cereal rye was terminated one month after the initial planting when plants were 36 and 25
cm tall. The average aboveground dry biomass weighed 3.5 and 0.4 g pot-1 for the high and low
biomass treatments, respectively. The high biomass weight was equivalent to 3500 kg ha-1 while
the low rate was equivalent to 400 kg ha-1 of dry biomass. Treatments for the two different sizes
of cereal rye included: 1) cereal rye terminated with glyphosate with the biomass left in place
(roots and shoots), 2) cereal rye terminated with glyphosate and aboveground biomass harvested
3 d later to leave only the roots in place (roots only), 3) aboveground cereal rye biomass from the
previous treatment where only the roots were left in place (shoots only), 4) cereal rye terminated
by cutting with scissors to simulate mowing, and 5) a non-rye cover. A dried raffia palm
(Ashland, Irving, Texas) was used as the non-rye cover, applied at weights that matched the
average dry weight of the high and low aboveground cereal rye biomass treatment. A no cover
treatment was also included as a control. Cereal rye that was terminated by cutting regrew and
was subsequently treated with glyphosate 3 d after cutting. Glyphosate applications applied at

41

1.68 kg a.e. ha-1 + 2% w w-1 AMS were made using a single nozzle track sprayer calibrated to
deliver 187 L ha-1 at 193 kPa of pressure through an 8001E nozzle (TeeJet Technologies,
Wheaton, IL). Five days after initial cereal rye termination, aboveground biomass was removed
and 50 Palmer amaranth seeds pot-1 were planted at a depth of 0.75 cm before the mulch was
replaced. Glyphosate treatments containing above- and belowground biomass had seeds planted
on the soil surface and 0.75 cm of soil added to cover the seeds so as to not disturb the cereal rye
residue. Pots were watered daily. Emergence was recorded every 2-3 d for 14 d from each pot,
with the newly emerged seedlings removed. All treatments were replicated six times and the
experiment was repeated in time.

Data Analysis. Statistical analysis was conducted using SAS® 9.4 (SAS Institute Inc., Cary,
NC). Assumptions of normality or residuals and homogeneity of variances were confirmed using
PROC UNIVARIATE. Analysis of variance was conducted using PROC MIXED. The statistical
model for the field experiment included the main effects of cereal rye cover crop, soybean row
width, herbicide management system, and their interactions as fixed effects and replication was
considered a random effect. Due to significant year interactions, data were analyzed separately
by year. Data were combined over main effects when significant interactions did not occur. The
statistical model for both pot experiments, included treatment and year or repetition in time as
fixed effects and replication as a random effect. Data were analyzed separately by year for the
outside experiments due to differences in treatment structure and differences between the
growing seasons. Data were combined over repetition in time for greenhouse experiment. For all
experiments, treatment means were separated using Fisher’s protected LSD at α ≤ 0.05 level of
significance.

42

Cumulative weekly Palmer amaranth emergence from no herbicide management plots was
regressed against days after planting (DAP) using the Gompertz equation (Equation 1; Forcella
et al. 2000) in SigmaPlot version 11.0 (Systat Software Inc., San Jose, CA).
Y = 100 × exp[-B × exp(-K × X)]

[Eq.2]

Where Y is the cumulative emergence, B is the DAP prior to emergence, K is the rate of
emergence, and X is d accumulations. Curves were separated using the extra sum-of-squares
principle for non-linear regression analysis (Lindquist et al. 1996).

Results and Discussion
Field Experiment. Cereal rye cover, biomass, and effects on early-season weeds. In early May,
at the time of cover crop termination, cereal rye provided 88 and 80% ground cover in 2015 and
2016, respectively (Table 2.3). Cereal rye dry biomass was greater in 2016 compared with 2015
with 2180 and 1250 kg ha-1, respectively (Table 2.3) due to an earlier cereal rye sowing date (~3
weeks) and warmer spring temperatures in 2016. Growing degree day (GDD) accumulations
between cereal rye planting and termination were 400 and 640 GDD (base 4.4 C) for the
2014/2015 and 2015/2016 growing seasons, respectively. An earlier planting date resulted in an
increase in cereal rye growth. The variability in cereal rye biomass at the time of termination was
similar to findings by Snapp et al. (2005) who reported fall-sown cereal rye in Michigan
produced between 800 and 2900 kg ha-1 of dry aboveground biomass; however, Hill (2014)
reported up to 12,777 kg ha-1 of dry above- and belowground biomass when rye was planted in
mid-September and terminated in early-June. Cereal rye planting and termination date is a
significant factor in determining how much cover crop biomass will be produced (Mirsky et al.
2009; Hill 2014).
43

Warmer temperatures in 2016 also resulted in greater winter and early-summer annual weed
growth in the no cover controls. Weed ground cover and biomass were 66% and 1315 kg ha-1 in
2016, compared with only 33% and 104 kg ha-1 in 2015 (Table 2.3). Early-season weeds present
at the time of termination included annual bluegrass, common chickweed, and shepherd’s purse
in 2015 and 2016. Common lambsquarters and field pennycress were also present in 2015.
Cereal rye reduced early-season weed growth by 74 and 84% in 2015 and 2016, respectively,
compared with the no cover control (Table 2.3). Putnam and DeFrank (1983) reported similar
early-season weed growth reductions (75%) in the presence of a cereal rye cover crop.
Cereal rye was terminated at Feekes stage 6 in 2015 and Feekes stage 9 in 2016. The carbon
to nitrogen (C:N) ratios for cereal rye were 12:1 and 31:1 at these stages (Table 2.3). Flail
mowing did not effectively terminate cereal rye. Cereal rye regrew in both years due to the early
stage at which the rye was terminated and produced an additional 1290 kg ha-1 of dry biomass
one week after mowing in 2015 and 580 kg ha-1 of dry biomass two weeks after mowing in 2016.
Mowed plots were then treated with glyphosate to terminate regrowth. Cereal rye should be at
head emergence or near anthesis (Feekes 10 or later) for effective termination; mowing at earlier
growth stages results in significant regrowth (De Bruin et al. 2005; Wilkins and Bellinder 1996).
Cereal rye terminated by mowing provided 12% more ground cover compared with termination
by glyphosate alone in 2016 (Table 2.3). This was most likely due to the additional 580 kg ha-1 of
cereal rye regrowth plus a more uniform distribution of the cover created by the flail mower.
Soil moisture at soybean planting was greater at the 11.9 cm compared with the 7.6 cm depth
in 2015 (Table 2.4). Cereal rye had no effect on soil moisture in 2015, but increased soil
moisture at both 11.9 and 7.6 cm in 2016 (Table 2.4). In 2015, a lower amount of rainfall in the
26 days prior to soybean planting, combined with the low amount of cereal rye biomass, was not

44

sufficient to prevent soil moisture evaporation from the rye treatments. In 2016, the 71 cm of
rain accumulated in the month of May prior to soybean planting, in conjunction with greater
cereal rye biomass, resulted in higher soil moisture retention in the rye cover treatments.

Palmer amaranth emergence. Palmer amaranth emergence in the no herbicide management (NH)
plots began two to three weeks after soybean planting. Initial emergence was on June 16, 2015
(392 GDD) and June 6, 2016 (306 GDD) (base 10 C) and ceased by July 21, 2015 and August
10, 2016. Similar Palmer amaranth emergence dates were reported by Kohrt (2017) in Michigan
in 2014 and 2015. The cereal rye cover crop, regardless of termination method, did not affect
cumulative Palmer amaranth emergence in 2015 (Figure 2.1a). However, cereal rye terminated
by mowing significantly delayed weed emergence in 2016 when compared with rye terminated
by glyphosate or no cover crop (Figure 2.1b). The delayed emergence in mowed plots may be
attributed to more evenly distributed ground cover created by mowing as well as higher residue
levels produced by the rye regrowth. The result was a reduction in the amount of sunlight
reaching the soil surface to delay initial emergence. Although there was a delay in cumulative
emergence in the mowed plots, there was no difference in total Palmer amaranth emergence by
the end of the season (data not shown). In fact, neither cereal rye nor soybean row width had any
effect on total Palmer amaranth emergence. Only herbicide management significantly reduced
total emergence.
Soybean planted in wide rows extended Palmer amaranth emergence in both years (Figure
2.2). Emergence lasted an additional 10 d in 2015 and 17 d in 2016, for Palmer amaranth to
reach 100% emergence when soybean was planted in 76 versus 19 cm rows. The extended
emergence period in 76 cm rows corresponded with a delay in reaching 80% soybean canopy

45

closure compared with 19 cm rows of 7 d in 2015 and 30 d in 2016 (Figure 2.3). The yearly
difference in the amount of time it took to reach 80% soybean canopy closure may be explained
by the early-season drought conditions experienced in 2016 (Table 2.2). The reduced canopy in
2016 encouraged an environment for additional weed emergence later in the season. An open
soybean canopy increases red light hitting the soil surface and promotes Palmer amaranth
germination, while a closed canopy increases the amount of far-red light which inhibits
germination (Jha et al 2010). In 2016 soybean planted in the 76 cm rows never exceeded 80%
canopy closure because of a lack of soil moisture in the six weeks after soybean planting, similar
to what Bell et al. (2015) reported; under dry conditions soybean planted in wide rows never
reached 90% ground cover, while soybean planted in narrow rows reached 90% cover in 85 d.

Palmer amaranth control, biomass, and density. Planting a cereal rye cover crop, regardless of
termination method, had no influence on Palmer amaranth control at any evaluation timing,
except at the POST application timing in 2016 (Table 2.5). Palmer amaranth control was greater
at this point in time where cereal rye was terminated by mowing compared with cereal rye
terminated with glyphosate or no cover. There was a delay in Palmer amaranth emergence when
cereal rye was terminated by mowing in 2016 (Figure 2.1b); greater ground cover and cereal rye
biomass in this treatment may have resulted in slower degradation of the cereal rye residue
(Table 2.3). A C:N ratio of 24:1 is ideal for a microbial diet (NRCS 2011); the C:N ratios were
31:1 and 23:1 for the initial and regrowth cereal rye in 2016, respectively (Table 2.3). Residues
that have ratios higher than 24:1 will decompose more slowly, whereas residues with C:N ratios
lower than 24:1 will decompose more rapidly (Odhiambo and Bomke 2001). The C:N ratio in

46

2015 for both the initial and regrowth cereal rye was 12:1 because the cereal rye was terminated
at an earlier vegetative stage (Feekes 6).
In the absence of herbicides, cereal rye did not reduce Palmer amaranth density or biomass in
2015 compared with no cover crop (Table 2.6). In 2016 the number, but not the biomass, of
Palmer amaranth plants m-2 was lower when cereal rye was terminated by mowing compared
with no cover (Table 2.6). Creamer and Dabney (2002) reported that flail mowing cereal rye
provided more uniform distribution of residue, and more rapid break down, which could account
for the lack of significant effects on Palmer amaranth biomass and control later in the season.
In the absence of herbicides, Palmer amaranth density and biomass was lower in soybean
planted in 76 compared with 19 cm rows in 2015 (Table 2.6). However, in 2016 soybean planted
in 19 cm rows reduced Palmer amaranth biomass by 46%, even though there was no difference
in weed density between the two row widths. Narrow row soybeans effectively suppressed
Palmer amaranth in 2016 due to the quicker canopy closure, reducing light from reaching the soil
surface earlier in the season (Figure 2.3).
Herbicides had the greatest impact on Palmer amaranth control in both years. In 2015, timely
rainfall (Table 2.2) after the PRE application effectively incorporated flumioxazin into the soil
resulting in early-season Palmer amaranth control for up to 42 DAP at the time of POST
application (Table 2.5). Palmer amaranth control 21 d after POST (DAT) and at harvest was
similar between the SH and HH herbicide programs. At peak biomass, Palmer amaranth density
was <1 plant m-2 in these treatments (data not shown). Since Palmer amaranth control was
greater than 95% 21 DAT, the LPOS treatment of glufosinate was not applied.
In contrast, in 2016 low rainfall (Table 2.2) following soybean planting resulted in only 14 d
of Palmer amaranth control with flumioxazin PRE in 2016 (Table 2.5). Therefore, POST

47

glufosinate and glufosinate plus acetochlor applications in the SH and HH strategies,
respectively, were applied earlier in 2016 compared with 2015. Flumioxazin provides good
control (>85%) of Palmer amaranth (Powell 2014). However, rainfall after application can
impact the effectiveness and length of weed control, which helps explain the differences in PRE
herbicide activity between 2015 and 2016. Palmer amaranth control was 67% and 95% in the SH
and HH plots, respectively, 21 DAT in 2016 (Table 2.5). Others reported consistent Palmer
amaranth control with the use of an effective PRE herbicide followed by POST application of
glufosinate (Hoffner et al. 2012). However, these treatments are not always consistent over
years. The addition of acetochlor to the POST glufosinate treatment in the HH strategy was
beneficial for late-season Palmer amaranth control. Palmer amaranth emergence continued 30
DAT in the HH plots, so an additional application of glufosinate was needed for season-long
weed control (Figure 2.4). The benefit of the HH strategy was also evident at the time of soybean
harvest; Palmer amaranth control was 94% in the HH plots and only 62% in the SH plots. An
extended emergence period due to the continuing rain events following POST application caused
difficulty in controlling Palmer amaranth with a single POST herbicide application.

Soybean yield. Row spacing had no effect on soybean yield in either year, when averaged over
cover crop and herbicide treatment (Table 2.7). Cereal rye had no effect on soybean yield in
2015; soybean yield was greater in 2016 in cereal rye treatments when herbicides were not
applied (Table 2.8). Some studies have reported increases (Ateh and Doll 1996), decreases
(Reddy 2001), or no effect (Liebl et al. 1992; Koger et al. 2002; Pantoja et al. 2015) on soybean
yield in the absence of herbicides when compared with no cover crop. Soybean yield was
reduced in the absence of a herbicide in both years (Tables 2.7; 2.8). The greatest difference

48

between herbicide management strategies occurred in 2016. Soybean yield was 36% higher in
the HH plots than the NH control, but only 23% higher in the SH plots. This was likely due to
the lower Palmer amaranth control in the SH system.

Effects of Cereal Rye on Palmer amaranth Emergence – Pot Experiments. Outdoor. Any
cover applied to the soil surface, whether it was early or late-stage cereal rye or the non-rye
cover, reduced initial and total Palmer amaranth emergence in 2015 (Table 2.9). Only the latestage cereal rye and the non-rye cover reduced Palmer amaranth emergence in 2016; however,
the non-rye cover was not different in total emergence from the early-stage cereal rye treatments
(Figure 2.9). The high C:N ratio (34:1) may be the reason Palmer amaranth emergence was
reduced with the late-stage cereal rye compared with a no cover control. These results mirror
those observed in the field study when cereal rye had a greater amount of biomass and a high
C:N ratio that may have reduced the rate at which cereal rye decomposed.

Greenhouse. Significant reductions (at least 70%) in initial and total Palmer amaranth emergence
occurred in pots where cereal rye was terminated by cutting to simulate mowing (Table 2.10)
The high rate of the non-rye cover also effectively reduced Palmer amaranth emergence, but only
at initial emergence. Again, this may be due to the presence of both above- and belowground
cereal rye biomass in addition to a uniform residue distribution over the soil surface provided by
the cutting. From this study, we can conclude that cereal rye terminated by cutting was more
effective at reducing Palmer amaranth emergence than a non-rye cover and termination with
glyphosate. This suggests that physically injuring the cereal rye by cutting may release
allelochemicals, such as the benzoxazinoid 2,4-dihydroxy-1,4 (2H)-

49

benzoxazin-3-one (DIBOA), that suppressed Palmer amaranth emergence. Weed suppression
from cereal rye containing DIBOA was studied by Tabaglio et al. (2008) who reported up to
52% suppression of redroot pigweed from seven cereal rye cultivars compared with a no cover
control.
Cereal rye had little impact on Palmer amaranth emergence or control in either year of our
field research, with the exception of delayed emergence in 2016 when cereal rye was terminated
by mowing. The lack of cover crop effect may be due to the low (less than 3000 kg ha-1) amounts
of cereal rye biomass produced at termination in addition to the amount of time between rye
termination and Palmer amaranth emergence. Palmer amaranth suppression with a cereal rye
cover crop has been reported by Price et al. (2012) and Norsworthy et al. (2011) in cotton.
However, the cereal rye biomass in these studies ranged from 4177 to almost 11,000 kg ha-1, 2-5
times more than the highest amount of biomass produced in this study. Furthermore, multiple
studies found that while cover crops contributed to weed control in early spring, additional
management practices, including herbicides, are needed for effective season-long control
(Teasdale 1996; Masiunas et al. 1995; Aulakh et al. 2012; Wiggins et al. 2016).
Soybean planted in narrow rows reduced the duration of Palmer amaranth emergence
compared with wide rows, likely due to the earlier canopy closure. When no herbicide was
applied, and in the presence of a high Palmer amaranth population, narrow row soybean reduced
weed growth and biomass, while having no negative effect on soybean yield.
Herbicides had the greatest impact on Palmer amaranth management in both years. The high
and standard management strategies performed similarly in 2015, in Palmer amaranth control
and soybean yield due to the consistent and prolonged weed control from PRE flumioxazin.
Under drought conditions in 2016, the effectiveness of PRE flumioxazin was less, resulting in an

50

earlier POST application timing. Later-season rainfall in 2016 also promoted late Palmer
amaranth emergence that was controlled with the LPOS application in the high management
strategy. As a result, there was an increase of 574 kg ha-1 in soybean yield with the use of the HH
strategy compared with the SH strategy in 2016.
The goal of this study was to determine which (if any) cultural practices to recommend with
herbicides to manage Palmer amaranth in Michigan soybean production. The use of a cereal rye
cover crop terminated at Feekes stage 9 with 2180 kg ha-1 of biomass, regardless of termination
method, had less of an impact on Palmer amaranth emergence and growth than planting soybean
in narrow rows. In Mississippi, Koger et al. (2002) also observed that cereal rye had no effect on
Palmer amaranth control and that planting soybean in narrow rows was a more effective cultural
method for reducing weed pressure. However, other studies have reported that cereal rye
suppressed and helped provide control of Palmer amaranth (Norsworthy et al. 2011; DeVore et
al. 2012; Korres and Norsworthy 2015). Cereal rye biomass in these studies ranged from 5352 to
8460 kg ha-1.
In our study, flail mowing did not effectively terminate cereal rye. Previous studies have
reported that cereal rye should be terminated at Feekes stage 10.1 or later to produce the most
biomass while having minimal regrowth (Wilkins and Bellinder 1996; Mirsky et al.2009;
Wayman et al. 2014). Planting cereal rye earlier in the fall in combination with a later cereal rye
stage at the time of termination, may improve Palmer amaranth suppression by increasing
biomass and providing a longer lasting mulch. However, this is difficult to accomplish in a
Michigan corn-soybean rotation due to the late timing of corn harvest in October through
November. Additionally, corn is planted in late-April and soybean in May which leads to earlier
rye termination. Palmer amaranth emergence in Michigan began four weeks after cereal rye

51

termination and two weeks after soybean planting. During this time, the cereal rye may lose
some its suppressive effect due to decomposition of the residue by the time Palmer amaranth
emergence would begin. One way to potentially increase biomass and extend the cover crop
growing season is to interseed into the cash crop. Growers may also to choose to terminate cereal
rye closer to or after cash crop planting. Even though cereal rye did not impact Palmer amaranth
control in this study, winter annual weeds were suppressed and there is the potential for
additional ecosystem services (Finney et al. 2016). In Michigan, the recommended method of
controlling Palmer amaranth and preventing late-season emergence is planting soybean in
narrow rows to reduce the period of Palmer amaranth emergence in addition to an intensive
herbicide management program.

52

APPENDIX

53

APPENDIX
CHAPTER 2 TABLES AND FIGURES

Table 2.1. Herbicide application timings, active ingredients, and product information for three different herbicide strategies used for
management of glyphosate-resistant Palmer amaranth in Middleton, MI (2015-2016).
Management
Timingsa
Active ingredient
Rate
Trade name
Manufacturerb
kg ai ha-1
High
PRE
flumioxazin
0.07
Valor
Valent Co.
POSTc
glufosinate + acetochlor
0.6 + 1.26
Liberty 280SL + Warrant
Bayer CropScience +
Monsanto
LPOSd
glufosinate
0.6
Liberty
Bayer CropScience
Standard

PRE
flumioxazin
0.07
Valor
Bayer + Monsanto
POST
glufosinate
0.6
Liberty
Bayer CropScience
No herbicide
-----a
Abbreviations: PRE, preemergence application, POST, postemergence application, LPOS, late-postemergence application.
b
Manufacturer information: Bayer CropScience, Research Triangle Park, NC, www.cropscience.bayer.com; Monsanto Co., St. Louis,
MO, www.monsanto.com; Valent Co., Walnut Creek, CA, www.valent.com.
c
Spray grade ammonium sulfate (AMS) (Actamaster, Loveland Products Inc., Loveland, CO, www.lovelandproducts.com) at 2% w
w-1 was added to all POST and LPOS applications.
d
The LPOS herbicide application was if needed and was only used in 2016

54

Table 2.2. Monthly precipitation and the 30-year average for Middleton, MI in 2015 and 2016.
Precipitationa (mm)
Month
2015
2016
30 yr.c
April
68
41
77
b
b
May
60 (35)
78 (71)
88
June
78
16
87
July
56
95
70
August
93
127
87
September
82
72
87
a
Precipitation data collected from the Enviro-weather Automated Weather Station Network
(https://mawn.geo.msu.edu/)
b
Rainfall accumulation up to the date of soybean planting.
c
30-year average precipitation data collected from the National Oceanic and Atmospheric
Administration (https://www.ncdc.noaa.gov/cdo-web/datatools/normals)

55

Table 2.3. Cereal rye and weed dry biomass at cover crop termination and cover crop regrowth from cereal rye plots in Middleton, MI
(2015-2016). Cereal rye biomass was analyzed for C:N ratio.
2015
2016
Ground cover
Dry biomass
Ground cover
Dry biomass
Cover crop
Cereal rye Weedb Cereal rye Weed
Cereal rye Cereal rye Weed
Cereal rye Weed
Cereal rye
a
_________
_________
______
-1 ______
_________
_________
______
-1 ______
At termination
%
kg ha
C:N ratio
%
kg ha
C:N ratio
Cereal rye
88
4 bd
1249
27 b
12:1
80
9b
2186
206 b
31:1
No cover
-33 a
-104 a
--66 a
-1315 a
-Cereal rye regrowthc
Cereal rye regrowth
______
At planting
kg ha-1 ______
C:N ratio
kg ha-1 ______
C:N ratio
Rye glyphosate
92 a
0b
0b
--71 b
3b
0b
--Rye mowedc
83 a
4b
1291 a
-12:1
83 a
2b
582 a
-23:1
No cover
-34 a
----43 a
---a
Cereal rye was terminated at Feekes stage 6 (30 cm) on May 14, 2015 and at Feekes stage 9 (58 cm) on May 11, 2016.
b
Weed species present were: annual bluegrass, common chickweed, and shepherd’s purse. Common lambsquarters and field
pennycress were also present in 2015.
c
Cereal rye regrowth occurred in plots terminated by flail mowing. Regrowth biomass was collected one week after mowing in 2015
and two weeks after mowing in 2016. Regrowth was then terminated with glyphosate at 1.26 kg ae ha-1 + 2% w w-1 spray grade
ammonium sulfate (AMS).
d
Means followed by the same letter at each evaluation timing within a column are not statistically different at α ≤ 0.05.
______

56

Table 2.4. Soil moisturea at 7.6 and 11.9 cm depths measured at the time of soybean planting in
the cereal rye and no cover plots in 2015 and 2016.
Prong length
Cover crop termination
2015
2016
___________________
% moisture ___________________
7.6 cm
Cereal rye – glyphosate
10.4 ab
16.8 a
Cereal rye – mowed
10.3 a
18.0 a
No cover
9.2 a
12.3 b
11.9 cm
Cereal rye – glyphosate
21.3 b
17.9 a
Cereal rye – mowed
20.7 b
18.3 a
No cover
20.4 b
13.7 b
a
Soil moisture reported as volumetric water content and measured with a TDR 300 Soil
Moisture Meter (FieldScout, Spectrum Technologies, Aurora, IL).
b
Means followed by the same letter within a column are not statistically different at α ≤ 0.05.

57

Table 2.5. Main effects of cereal rye cover crop, soybean row width and herbicide management
strategy on Palmer amaranth control in 2015 and 2016 at Middleton, MI.
Main effects

at POST
___________

2015
21 DAT

at Harvest

% control ___________

at POST
___________

2016
21 DAT

at Harvest

% control ___________

Cover crop
Rye glyphosate
Rye mowed
No cover

88
89
86

93
96
97

95
97
96

83 ba
98 a
93 b

80
80
83

77
73
84

Row width
76 cm
19 cm

87
89

94
97

95
98

93
89

78
84

76
80

Herbicide strategy
High management
Standard management

86
90

96
95

97
95

88
95

95 a
67 b

94 a
62 b

p-value _____________________________________
0.5215
0.1906
0.5030
0.0158
0.7829
0.2410
0.4602
0.1759
0.0568
0.3067
0.1802
0.4051
0.0891
0.5267
0.1666
0.0824
<0.0001
<0.0001
a
Means followed by the same letter for each main effect within a column are not statistically
different at α ≤ 0.05.
Effects
Cover crop
Row width
Herbicide management

____________________________________

58

Table 2.6. Main effects of cover crop and soybean row width on Palmer amaranth density and
biomass in the no herbicide management plots at peak biomass in 2015 and 2016 at Middleton,
MI.
Density
Biomass
Main effects
2015
2016
2015
2016
____________
-2 ____________
____________
-2 ____________
plants m
gm
Cover crop
Rye glyphosate
14.7
38.5 ab
13.9
89.9
Rye mowed
14.0
20.0 b
17.4
76.2
No cover
13.2
57.4 a
30.9
123.8
Row width
76 cm
19 cm

9.5 ba
18.5 a

38.1
39.1

11.4 b
30.1 a

125.8 a
67.5 b

____________________________________
Effects
p-value __________________________________
Cover crop
0.9055
0.0288
0.1547
0.2192
Row width
0.0022
0.9329
0.0187
0.0183
Cover crop*Row width
0.3521
0.6570
0.9153
0.7347
a
Means followed by the same letter for each main effect within a column are not statistically
different at α ≤ 0.05.

59

Table 2.7. Main effects and interactions of cereal rye cover crop, soybean row width, and
herbicide management strategy on soybean yield in 2015 and 2016 at Middleton, MI.
Yield
Main effects
2015
2016
________________
-1 ________________
kg ha
Cover crop
Rye glyphosate
3324
2739
Rye mowed
3419
3037
No cover
3235
2502
Row width
76 cm
19 cm

3321
3331

2616
2902

Herbicide management
High herbicide
Standard herbicide
No herbicide

3718 aa
3607 a
2649 b

3358 a
2784 b
2135 c

________________
Effects
p-value ________________
Cover crop
0.6856
0.4234
Row width
0.9126
0.1542
Herbicide management
<0.0001
<0.0001
Cover crop*Row width
0.1436
0.2400
b
Cover crop*Herbicide management
0.0018
0.1964
Row width*Herbicide management
0.1352
0.3728
Cover crop*Row width*Herbicide management
0.8258
0.9687
a
Means followed by the same letter for each main effect within a column are not statistically
different at α ≤ 0.05.
b
The cover crop by herbicide management interaction data is presented in Table 2.8.

60

Table 2.8. Effect of cover crop by herbicide management strategy interaction on soybean yield in
2015 and 2016.
Soybean yield
Cover crop - Termination
Herbicide strategy
2015
2016
_______________
-1 _______________
kg ha
Cereal rye - glyphosate
High
3570 aa
3453
Standard
3574 a
2724
None
2829 b
2039
Cereal rye - mowed

High
Standard
None

3780 a
3532 a
2945 b

3364
2960
2786

No cover

High
Standard
None

3809 a
3718 a
2178 c

3258
2667
1581

_______________
Effects
p-value _______________
Cover crop
0.6856
0.4234
Herbicide management
<0.0001
<0.0001
Cover crop*Herbicide management
0.0018
0.1964
a
Means followed by the same letter are not statistically different at α ≤ 0.05.

61

Table 2.9. Effect of a cereal rye or non-rye cover on the initial and total emergence of Palmer amaranth in outside pots in 2015 and
2016.
2015
2016
Treatment
Rated
Initial emergence
Total emergence
Initial emergence
Total emergence
________________
-1 ________________
________________
plants pot
plants pot-1 ________________
a
c
Early stage- cereal rye
Low
8 bc
14 bc
36 a
40 ab
Medium
5 cd
13 bc
37 a
40 ab
High
4 cd
16 bc
b
Late stage- cereal rye
Low
10 bc
21 b
19 b
28 c
Medium
6 cd
12 c
High
1d
9c
Non-rye covere
14 b
21 b
25 b
31 bc
No cover
21 a
32 a
41 a
43 a
a
Early stage cereal rye was Feekes 6 and 9 in 2015 and 2016.
b
Late stage cereal rye was Feekes 10.5 and 10.1 in 2015 and 2016.
c
Means followed by the same letter within a column are not statistically different at α ≤ 0.05.
d
Rates of cereal rye biomass were 1200 kg (low), 2400 kg (medium), and 3750 kg (high) ha-1.
e
Non-rye cover was raffia palm (Ashland, Irving ,Texas).

62

Table 2.10. Effect of termination method and cereal rye cover on initial and total Palmer
amaranth emergence in the greenhouse.
Initial
Total
Cover
Termination method
emergence
emergence
__________
plants pot-1 _________
a
b
Cereal rye - High biomass
Glyphosate - above & below
16 bcf
18 cde
c
Glyphosate – above
21 a
24 a
Glyphosate – belowd
19 ab
19 bcde
e
Cutting
3e
3f
Cereal rye - Low biomassa
Glyphosate - above & below
15 cd
17 de
Glyphosate - above
21 a
24 ab
Glyphosate - below
21 a
23 abc
Cutting
3e
5f
Non-rye cover - High biomass
11 d
15 e
Non-rye cover - Low biomass
21 a
22 abcd
No cover
17 abc
17 de
a
Rates of cereal rye biomass were 3500 kg (high) and 400 kg (low) ha-1.
b
The above- and belowground treatment included cereal rye shoots and roots.
c
The aboveground treatment included only cereal rye shoots.
d
The belowground treatment included only intact cereal rye roots where aboveground shoot
biomass was removed.
e
The cutting treatment contained cereal rye shoots cut into pieces and spread out on soil surface
and roots left intact.
f
Means followed by the same letter within a column are not statistically different at α ≤ 0.05.

63

Figure 2.1. Cumulative Palmer amaranth emergence as a percent of total emergence combined
across all soybean row widths with cereal rye terminated by glyphosate (RG) (♦), cereal rye
terminated by mowing (RM) (■) and no cover (NC) (●) in 2015 (a) and 2016 (b). There was no
difference between the treatments in 2015; however, RM was significantly different from RG
and NC in 2016. Fitted lines were calculated with the Gompertz equation: NC, y=100*exp(8.25*exp(-0.07*x)), R2 = 0.53; RG, y=100*exp(-10.57*exp(-0.09*x)), R2 = 0.74; RM,
y=100*exp(-26.87*exp(-0.12*x)), R2 = 0.78.

64

Figure 2.2. Cumulative Palmer amaranth emergence in 2015 (a) and 2016 (b) as a percent of
total emergence combined across all cover crop treatments in 19 (●) and 76 cm (▲) rows.
Soybean row widths were significantly different from each other in both years. Fitted lines were
calculated with the Gompertz equation: 76 cm rows, y=100*exp(-12.67*exp(-.08*x)), R2 = 0.74;
19 cm rows, y=100*exp(-33.93*exp(-0.14*x)), R2 = 0.86.

65

Figure 2.3. Percent soybean canopy closure in 19 (●) and 76 cm (▲) rows in 2015 (a) and 2016
(b). Soybean row widths were significantly different from each other in both years.

66

5

40
35

4

30
25

3

20
2

15

Rainfall (mm)

Cumulative emergence (m-2)

2015 (a)

10

1

5
0

40

2016 (b)

35

30

30

25

25

20

20
15

15

10

Rainfall (mm)

Cumulative emergence (m-2)

35

0

10

5

5

0

0

Rainfall

SH

HH

Figure 2.4. Effect of rainfall on cumulative Palmer amaranth emergence in the standard and high
herbicide management strategies in 2015 (a) and 2016 (b). POST applications were made on July
15, 2015 and June 10, 2016. LPOST application was applied on July 8, 2016.

67

LITERATURE CITED

68

LITERATURE CITED
Ateh CM, Doll JD (1996) Spring-planted winter rye (Secale cereale) as a living mulch to control
weeds in soybean (Glycine max). Weed Technol 10:347-353
Aulakh JS, Price AJ, Enloe SF, van Santon E, Wehtje G, Patterson MG (2012) Integrated Palmer
amaranth management in glufosinate-resistant cotton: I. Soil-inversion, high-residue cover
crops and herbicide regimes. Agronomy 2:295-311
Barnett KA, Culpepper AS, York AC, Steckle LE (2013) Palmer amaranth (Amaranthus
palmeri) control by glufosinate plus fluometuron applied postemergence to WideStrike®
cotton. Weed Sci 27:291-297
Bell HD, Norsworthy JK, Scott RC, Popp M (2015) Effect of row spacing, seeding rate, and
herbicide program in glufosinate-resistant soybean on Palmer amaranth management. Weed
Technol 29:390-404
Cahoon CW, York AC, Jordan DL, Seagroves RW (2015) Cotton response and Palmer amaranth
control with mixtures of glufosinate and residue herbicides. J of Cotton Sci 19:622-630
Creamer NG, Dabney SM (2002) Killing cover crops mechanically: Review of recent literature
and assessment of new research results. Amer J Altern Agri 17:32-40
Culpepper AS, Grey TL, Vencill WK, Kichler JM, Webster TM, Brown SM, York AC, Davis
JW, Hanna WW, (2006) Glyphosate-resistant Palmer amaranth (Amaranthus palmeri)
confirmed in Georgia. Weed Sci 54:620-626
Culpepper AS, Webster TM, Sosnoskie LM, York AC (2010a) Glyphosate-resistant Palmer
amaranth in the United States. Pages 195-212 in Nandula VK, ed. Glyphosate resistance in
crops and weeds: history, development, and management. New Jersey: John Wiley and Sons
Inc
Culpepper AS, Kichler J, Sosnoskie L, York A, Sammons D, Nichols R (2010b) Integrating
cover crop residue and moldboard plowing into glyphosate-resistant Palmer amaranth
management programs. 1531 Proceedings in Beltwide Cotton Conference New Orleans, LA:
National Cotton Council of America
De Bruin JL, Porter PM, Jordan NR (2005) Use of a rye cover crop following corn in rotation
with soybean in the upper Midwest. Agron J 97:587-598
DeVore JD, Norsworthy JK, Brye KR (2012) Influence of deep tillage and a rye cover crop on
glyphosate-resistant Palmer amaranth (Amaranthus palmeri) emergence in cotton. Weed
Technol 26:832-838
DeVore JD, Norsworthy JK, Brye KR (2013) Influence of deep tillage, a rye cover crop, and
various soybean production systems on Palmer amaranth emergence in soybean. Weed
Technol 27:263-270
69

Ehleringer J (1983) Ecophysiology of Amaranthus palmeri, a Sonoran Desert summer
annual. Oecologia 57:107-112
Finney DM, White CM, Kaye JP (2016) Biomass production and carbon/nitrogen ratio influence
ecosystem services from cover crop mixtures. Agron J 108:39-52
Forcella F, Bench-Arnold RL, Sanchez R, Ghersa CM (2000) Modeling seedling emergence.
Field Crops Res 67:123-139
Gallagher RS, Cardina J (1998) Phytochrome-mediated Amaranthus germination, I: effect of
seed burial and germination temperature. Weed Sci 46:48-52
Gossett BJ, Murdock EC, Toler JE (1992) Resistance of Palmer amaranth (Amaranthus palmeri)
to dinitroaniline herbicides. Weed Technol 6:587-591
Guo P, Kassim Al-Khatib (2003) Temperature effects on germination and growth of redroot
pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp
(A. rudis). Weed Sci 51:869-875
Heap I (2016) The international survey of herbicide resistant weeds. www.weedscience.org.
Accessed September 26, 2016
Hill EC (2014) Cover crop influence on nitrogen availability, weed dynamics, and dry bean
(Phaseolus vulgaris L.) characteristics in an organic system. Ph.D dissertation. East Lansing,
Michigan: Michigan State University 224 p
Hoffner AE, Jordan DL, Chandi A, York AC, Dunphy EJ, Everman WJ (2012) Management of
Palmer amaranth (Amaranthus palmeri) in glufosinate-resistant soybean (Glycine max) with
sequential applications of herbicides. ISRN Agron 2012:131650. doi:10.5402/2012/131650
Horak MJ, Loughin TM (2000) Growth analysis of four Amaranthus species. Weed Sci 48:347355
Horak MJ, Peterson DE (1995) Biotypes of Palmer amaranth (Amaranthus palmeri) and
common waterhemp (Amaranthus rudis) are resistant to imazthapyr and thifensulfuron.
Weed Technol 9:192-195
Jha P, Norsworthy JK (2009) Soybean canopy and tillage effects on emergence of Palmer
amaranth (Amaranthus palmeri) from a natural seed bank. Weed Sci 57:644-651
Jha P, Norsworthy JK, Riley MB, Bridges W and Jr (2010) Annual changes in temperature and
light requirements for germination of Palmer amaranth (Amaranthus palmeri) seeds retrieved
from soil. Weed Sci 58:426-432
Keeley PE, Carter CH,Thullen RJ (1987) Influence of planting date on growth of Palmer
amaranth (Amaranthus palmeri). Weed Sci 35:199-204

70

Kinyangi JM, Smucker AJM, Mutch DR, Harwood RR (2001) Managing cover crops to recycle
nitrogen and protect groundwater. East Lansing, Michigan: Extension Bulletin E-2763
Klingaman TE, Oliver LR (1994) Palmer amaranth (Amaranthus palmeri) interference in
soybeans (Glycine max). Weed Sci 42:523-527
Koger CH, Reddy KN, Shaw DR (2002) Effects of rye cover crop residue and herbicides on
weed control in narrow and wide row soybean planting systems. Weed Biol Manag 2:216224
Kohrt JR (2017) Biology and management of multiple- (glyphosate, ALS, and atrazine)-resistant
Palmer amaranth in Michigan. Ph.D dissertation. East Lansing, MI: Michigan State
University. 153 p
Korres NE, Norsworthy JK (2015) Influence of a rye cover crop on the critical period for weed
control in cotton. Weed Sci 63:346-352
Laflen JM, Amaemiya M, Hintz EA (1981) Measuring crop residue cover. J Soil Water Conserv
36:341-343
Laub CA, Luna JM (1991) Influence of winter crop suppression practices on seasonal abundance
of armyworm (Lepidoptera: Noctuidae), cover crop regrowth, and yield in no-till corn.
Entomol Soc Am 20:749-754
Légère A, Schreiber MM (1989) Competition and canopy architecture as affected by soybean
(Glycine max) row width and density of redroot pigweed (Amaranthus retroflexus). Weed Sci
37:84-92
Leon RG, Owen MDK (2003) Regulation of weed seed dormancy through light and temperature
interactions. Weed Sci 51:752-758
Liebl R, Simmons FW, Wax LM, Stoller EW (1992) Effect of rye (Secale cereale) mulch on
weed control and soil moisture in soybean (Glycine max). Weed Technol 6:838-846
Lindquist JL, Mortensen DA, Clay SA, Schmenk, Kells JJ, Howatt K, Westra P (1996) Stability
of corn (Zea mays)-velvetleaf (Abutilon theophrasti) interference relationships. Weed Sci
44:309-313
Masiunas JB, Weston LA, Weller SC (1995) The impact of rye cover crops on weed populations
in a tomato cropping system. Weed Sci 43:318-323
Michigan State University Enviro-weather (2016) weather station network.
https://www.enviroweather.msu.edu Accessed: October 12, 2016
Mickelson JA, Renner KA (1997) Weed control using reduced rates of postemergence herbicides
in narrow and wide row soybean. J Prod Agri 10:431-437

71

Mirsky SB, Curran WS, Mortensen DM, Ryan MR, Shumway DL (2009) Control of cereal rye
with a roller/crimper as influenced by cover crop phenology. Agron J 101:1589-1596
Moore MJ, Gillespie TJ, Swanton CJ (1994) Effect of cover crop mulches on weed emergence,
weed biomass, and soybean (Glycine max) development. Weed Technol 8:512-518
Mortensen DA, Egan JF, Maxwell BD, Ryan MR, Smith RG (2012) Navigating a critical
juncture for sustainable weed management. BioSci 62:75-84
Murphy TR, Gossett BJ (1981) Influence of shading by soybeans (Glycine max) on weed
suppression. Weed Sci 29:610-615
[NCRS] US Department of Agriculture- Natural Resources Conservation Service (2011) Carbon
to nitrogen ratios in cropping systems.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8
&ved=0ahUKEwj40cLWs_fSAhUq5IMKHSCGAeYQFggfMAA&url=https%3A%2F%2Fw
ww.nrcs.usda.gov%2Fwps%2FPA_NRCSConsumption%2Fdownload%3Fcid%3Dnrcs142p
2_052823%26ext%3Dpdf&usg=AFQjCNHYBo0n19DN8sgEtqMIwKw4X62pjg&sig2=mB
xb-SM2uY5ymw1S6uqJIQ&bvm=bv.150729734,d.amc. Accessed: March 8, 2017
Norsworthy JK, McClelland M, Griffith G, Bangarwa SK, Still J (2011) Evaluation of cereal and
brassicaceae cover crops in conservation-tillage, enhanced, glyphosate-resistant cotton. Weed
Technol 25:6-13
Odhiambo JJO, Bomke AA (2001) Grass and legume cover crop effects on dry matter and
nitrogen accumulation. Agron J 93:299-307
Pantoja JL, Woli KP, Sawyer JE, Barker DW (2015) Corn nitrogen fertilizer requirement and
corn-soybean productivity with a rye cover crop. Soil Sci Soc Am j 79:1482-1495
Price AJ, Balkcom KS, Duzy LM, Kelton JA (2012) Herbicide and cover crop residue
integration for Amaranthus control in conservation agriculture cotton and implications for
resistance management. Weed Technol 26:490-498
Powell DK (2014) Biology and management of multiple (glyphosate/ALS)-resistant Palmer
amaranth in Michigan soybean. Ph.D dissertation. East Lansing, MI: Michigan State
University. 122 p
Putnam AR, DeFrank J (1983) Use of phytotoxic plant residues for selective weed control. Crop
Prot 2:173-181
Reddy KN (2001) Effects of cereal and legume cover crop residues on weeds, yield, and net
return in soybean (Glycine max) 1. Weed Technol 15:660-668
Snapp SS, Swinton SM, Labarta R, Mutch D, Black JR, Leep R, O'Neil K (2005) Evaluating
cover crops for benefits, costs and performance within cropping system niches. Agron
J 97:322-332

72

Sprague CL (2011) Glyphosate-resistant Palmer amaranth confirmed in Southwest Michigan.
http://msue.anr.msu.edu/news/glyphosate_resistant_palmer_amaranth_confirmed_in_southw
est_michigan. Accessed September 26, 2016
Tabaglio V, Gavazzi C, Schulz M, Maracco A (2008) Alternative weed control using the
allelopathic effect of natural benzoxazinoids from rye mulch. Agron Sustain Dev 28:397-401
Teasdale JR (1996) Contribution of cover crops to weed management in sustainable agricultural
systems. J Prod Agri 9:475-479
Ward SM, Webster TM, Steckel LE (2013) Palmer amaranth (Amaranthus palmeri): a review.
Weed Technol 27:12-27
Wax LM, Pendleton JW (1968) Effect of row spacing on weed control in soybeans. Weed
Sci 16:462-465
Wayman S, Cogger C, Benedict C, Burke I, Collins D, Bary A (2014) The influence of cover
crop variety, termination timing and termination method on mulch, weed cover and soil
nitrate in reduced-tillage organic systems. Renew Agr Food Syst 30:450-460
Wiggins MS, Hayes RM, Steckel LE (2016) Evaluating cover crops and herbicides for
glyphosate-resistant Palmer amaranth (Amaranthus palmeri) control in cotton. Weed Technol
30:415-422
Whitaker JR, York AC, Jordan DL, Culpepper AS, Sosnoskie LM (2011) Residual herbicides for
Palmer amaranth control. J of Cotton Sci 15:89-99
Wilkins ED, Bellinder RR (1996) Mow-kill regulation of winter cereals for spring no-till crop
production. Weed Technol 10:247-252

73