1 a Systems Approach to Conyza Canadensis Management Thesis

A Systems Approach to Conyza canadensis Management

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in

the Graduate School of The Ohio State University

Bryan P. Reeb, B.S.

Graduate Program in Horticulture and Crop Science

The Ohio State University

2018

Thesis Committee

Dr. Mark M. Loux, Advisor

Dr. Kent Harrison

Dr. Laura Lindsey

Copyrighted by

Bryan P. Reeb

2018

Abstract

In Ohio, no-tillage soybean production, glyphosate, and acetolactate synthase

inhibitor (ALS)-inhibiting herbicides are common components for management of horseweed. However, resistance to glyphosate was confirmed in Ohio in 2002, followed

by multiple resistance to both ALS inhibitors and glyphosate in 2003. Field studies were conducted from the fall of 2009 through the summer of 2016 to develop strategies for management of horseweed populations with these types of resistance in no-tillage

soybeans. The objectives were to: 1) determine the efficacy of fall-applied herbicides for

management of horseweed emerging in the spring; 2) determine the effect of spring

application timing and herbicide on the residual control of horseweed through mid-

summer; and 3) determine the most effective herbicide program for soybeans resistant to

glufosinate, glyphosate/dicamba, or to glyphosate/glufosinate/2,4-D.

Among the fall-applied herbicides, only chlorimuron and flumioxazin have

potential to control spring-emerging horseweed when applied the previous fall.

Chlorimuron controlled ALS-sensitive horseweed only, and flumioxazin provided control

into spring only at one site. Overall, this study showed low and variable potential for fall

herbicide treatments to provide control of spring-emerging horseweed that are resistant to

ALS inhibitors. Spring preplant application of a single active ingredient did occasionally

provide effective residual control of horseweed through the time of a POST application,

but this occurred most consistently with sequential preplant applications or multiple-

herbicide treatments. Combinations of metribuzin and either flumioxazin, saflufenacil, or

sulfentrazone were needed to provide 90% or greater control through soybean harvest.

The results from these studies show that horseweed populations can be controlled using

residual herbicides exclusively, applied either in the fall or in spring prior to planting.

However, there was considerable variability in effectiveness among herbicides and

environments, and differences among herbicides were not consistent among studies.

Studies with glufosinate-resistant soybeans showed the effectiveness of several

approaches. Failure to use fall herbicide treatments or spring herbicides with residual

activity on horseweed can result in inadequate late-season control, even where it is possible to use glufosinate POST. The ability to use 2,4-D more intensively in a 2,4-D- resistant soybean can result in effective late-season control of glyphosate-resistant horseweed. For a number of the treatments that relied primarily on 2,4-D for control of emerged plants though, effective control required both at plant and POST applications of

2,4-D. The inclusion of other effective herbicides, such as paraquat or saflufenacil, in the spring treatment, or glufosinate in the POST, can improve control and reduce reliance on

2,4-D. Dicamba was more effective than 2,4-D on emerged horseweed, especially in the absence of fall-applied herbicides. Dicamba herbicide systems adequately controlled horseweed if dicamba was included in the POST, or if used exclusively in the preplant application in combination with effective residual herbicides.

iii

Vita

December 2006…………………………………… A. Science, The Ohio State University

June 2008…………………………………………. B.S. Agriculture, The Ohio State

University

February 2009 to present…………………………. Research Associate, The Ohio State

University

January 2012 to present…………………………... Graduate Research Associate,

Department of Horticulture and Crop

Science, The Ohio State University

Fields of Study

Major Field: Horticulture and Crop Science

Specialization: Weed Science

Table of Contents

Abstract ...... ii Vita ...... iv List of Figures ...... vi List of Tables ...... vii Chapter 1. Literature Review ...... 1 Bibliography ...... 10 Chapter 2: Residual herbicide horseweed control systems...... 15 Introduction ...... 15 Materials and Methods ...... 15 Results and Discussion ...... 18 Bibliography ...... 23 Chapter 3: Herbicide-resistant soybean horseweed control systems...... 30 Introduction ...... 30 Materials and Methods ...... 30 Results and Discussion ...... 34 Thesis Bibliography ...... 54

List of Figures

Figure 2.1 Field site, herbicide application, and soybean planting information……...….24

Figure 3.1 Field site, herbicide application, and soybean planting information………....42

List of Tables

Table 2.1 Effect of fall applied residual herbicide(s) on horseweed population densities the following spring (Study 1). ………………………………………………………….25

Table 2.2 Effect of herbicide rate with single and split-spring application timings on residual horseweed control (Study 2)………………………………………..………...... 26

Table 2.3 Effect of herbicide(s) applied 30 days prior to soybean planting on residual horseweed control (Study 3)………………………………………………….……...... 28

Table 3.1 Effect of herbicide system on control of horseweed in glufosinate-resistant soybeans (Study 4).…………………………………………………………………..…..44

Table 3.2 Effect of herbicide system on control of horseweed in soybeans resistant to glyphosate, glufosinate, and 2,4-D – Ohio (Study 5)………………………………………………………..46

Table 3.3 Effect of herbicide system on control of horseweed in soybeans resistant to glyphosate, glufosinate, and 2,4-D – Indiana (Study 5)……………………………….……………48

Table 3.4 Effect of herbicide system on control of horseweed in soybeans resistant to glyphosate and dicamba – Ohio (Study 6………………………………..………………50

Table 3.5 Effect of herbicide system on control of horseweed in soybeans resistant to glyphosate and dicamba – Indiana (Study 6)……….……………………………………52

vii

Chapter 1. Literature Review

Conyza canadensis or Erigeron canadensis, known also by the common names of horseweed, marestail, and Canada fleabane, is a member of the Asteraceae (composite) family with a robust taproot (Tilley, 2012; Brown and Whitwell 1988). Native to most of

North America, reproduction is by seed only, which spread by wind and water (Weaver,

2001). Horseweed is described as both a summer annual and winter annual, with seed germinating at any time with sufficient moisture and warmth (Waggoner et al., 2011).

Main et al. (2006) conducted field studies over a two-year period in Tennessee that proved horseweed has multiple germination periods as well as a long in-season germination period. At one location emergence was recorded in 8 of 12 months, overall two distinct germination periods were noted, spring and fall. In winter annual forms, the seed germinates in the fall and the plant overwinters as a rosette. It then bolts in the spring and produces flowers in mid to late summer. Spring germinating horseweed spends relatively little time as a rosette before bolting (Loux et al., 2006).

Panicles of flower heads develop from axillary buds of the upper leaves, and the resulting seeds will mature approximately three weeks after fertilization (Weaver, 2001).

The plants produce a large quantity of windborne seed, which makes it an effective colonizer of disturbed soils (Regehr and Bazzaz, 1979). Seed production can exceed

200,000 seeds per plant in a no-till field without crop (Bhowmik and Bekech, 1993).

Shields et al. (2006) first confirmed horseweed seeds found in the planetary boundary layer and concluded seeds could easily move up to 500 km from a single dispersal event.

The timing of seed shed and availability of resources, rather than the need for

vernalization to induce flowering (Bhowmik and Bekech, 1993) may regulate the horseweed life cycle. Horseweed is adapted to a broad range of conditions and soil types; it prefers slightly acid to neutral soil pH (Tilley, 2012). The over-wintering rosettes allow

horseweed to initiate growth early in the spring, gain a competitive advantage over spring-germinating plants, and possibly reach sufficient size to tolerate herbicides applied

at the time no-tillage crops are planted (Buhler and Owen, 1997). The longevity of seeds

has not been established (Weaver, 2001), but viable seeds have been found in the

seedbank of a 20-year-old abandoned pasture despite its absence in the above ground

vegetation (Tsuyuzaki and Kanda, 1996). Nandula et al. (2006) reported emergence was

at its maximum for seeds planted on the soil surface and no seedlings emerged from

seeds placed at a depth of 0.5 cm or deeper, while Owen et al. (2009) reported that lack

of plant residue on the soil surface creates more light exposure to facilitate horseweed

seed germination. While horseweed is adapted to a wide range of soil types and

conditions, it is considered intolerant of flooding (Stoecker et al., 1995).

Horseweed has become an increasingly common and problematic weed in no-till

soybean production in Ohio and the eastern cornbelt due to the frequent occurrence of

biotypes resistant to glyphosate (Davis et al., 2009). Horseweed populations were

influenced by tillage, or lack of tillage, and became established in typical no-till field

situations. Shallow disking virtually eliminates horseweed (Brown and Whitwell, 1988).

Around 40 to 80% of Ohio soybean production is under no-till practices on a given year,

and this lack of soil disturbance favors horseweed seed germination (Grassini et al.,

unpublished, 2016). When fields are managed under conservation tillage, it can create

environments in which herbicide-resistant weeds are likely to develop. This is because practitioners rely primarily and in some cases solely on herbicides for weed control,

thereby imposing a consistent and uniform resistance selection pressure on weeds

(Vencill et al., 2012).

In Ohio, no-till soybean production glyphosate and acetolactate synthase (ALS) inhibiting herbicides are commonly used for burndown and residual efficacy of glyphosate-resistant horseweed. A few studies have looked at the competitive advantage of glyphosate-resistant horseweed versus susceptible populations and their phenological

differences. Typically, there is no noticeable difference in the appearance of a resistant

weed biotype versus a susceptible one. Shrestha et al. (2010) compared the competition

between glyphosate-resistant and susceptible biotypes and concluded that a reason

resistance may spread fast was due to the accelerated phenological development of the

resistant biotype. However, in the case of glyphosate-resistant horseweeds biotypes from

Mississippi, Nandula et al. (2015) reported phenological differences in growth over an

eleven-week period. The glyphosate-resistant biotype was larger, had more leaves,

bolted, flowered, and senesced later than the susceptible biotype. Davis et al. (2009)

observed no significant differences in the seed production across four horseweed populations: susceptible, ALS, glyphosate, and the combination of ALS and glyphosate-

resistance. These populations also exhibited similar phenotypic habits, such as growth

rate. In field studies in Tennessee on glyphosate-resistant horseweed by Ye et al. (2016),

their population had a long pollen shed period—almost 45 days—which is similar to the

phenological characteristics described by Weaver et al. (2001). Due to the long distance

3 of seed dispersal using wind tunnel experiments (Shields et al., 2006), and modeling

Dauer et al. (2006); estimated seed could travel up to 100 meters. This suggests that an area-wide management plan may be one possible way to slow the spread of horseweed seed, which is not a typical method of weed management.

The Weed Science Society of America (WSSA) describes herbicide resistance and herbicide tolerance as, “The inherent ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type. Resistance may be naturally occurring, induced by genetic engineering, or selections of variants from tissue cultures. Herbicide tolerance is described as the inherent ability of a species to survive and reproduce after herbicide treatment. This implies there was no selection or genetic manipulation to make the plant tolerant; it is naturally tolerant” (12, (4), 789).

Burgos et al. (2013) observed that ideally, growers should be managing crop production fields to delay resistance to herbicides or avert weed population shifts, but the most convenient and economical choice is not always best, and could lead to weed resistance and forces changes in growing practices. Soteres and Peterson (2018) state it takes up to four years to notice resistance. Resistance starts with a lag phase characterized by scattered plants with low-level resistance, followed by the exponential phase where resistance levels and distribution grows exponentially.

Kniss (2017), concluded from his analysis of herbicide resistance evolution, that glyphosate is relatively low in the number of resistant weed species reported in the

United States, between 1990 and 2015—only 17 had evolved resistance. In comparison to

27 and 51, weed species for photosystem II inhibitors and ALS inhibitors, respectively. 4

Both photosystem II and ALS inhibitors have been applied about the same number of

times (approximately 3 billion) between 1990 and 2015. Synthetic auxins are close to

glyphosate (40) in the current number of resistant weeds worldwide at 36 reported to date

(Heap, 2017). This number may rise with the addition of auxin-tolerant crops that are

being developed and released for commercial use. While synthetic auxins can provide

effective control of horseweed, 2,4-D seems to be more variable in effectiveness versus

dicamba (Reeb unpublished data). Currently, there are horseweed populations with

resistance to EPSP synthase inhibitors (group 9), ALS inhibitors (group 2), photosystem I

inhibitors (group 22), or photosystem II Inhibitors (group 7) (Heap, 2017). Group

numbers are current WSSA site of action classification codes. According to WSSA’s

weed survey of common and troublesome weeds in 2016; horseweed was the second

most common weed in soybeans, along with waterhemp as the most troublesome.

Horseweed also ranked in the top ten as most common and troublesome across all

broadleaf crops, fruits, and vegetables (Wychen, 2017).

A glyphosate-resistant horseweed population was first reported in 2000

(VanGessel, 2001). Resistance to glyphosate was confirmed in Ohio in 2002 and then in

2003 resistance to both ALS inhibitors and glyphosate (Heap, 2017). Trainer et al. (2005)

surveyed a large portion of western Ohio and found multiple fields that exhibited

glyphosate and ALS resistant horseweed populations. An OSU greenhouse study

(unpublished 2011) confirmed 25% of the 45-horseweed populations tested had multiple

resistance to both glyphosate and ALS inhibiting herbicides. The frequency in Ohio of this type of resistance is currently assumed even higher. Dinelli et al. (2006) concluded

that Ohio populations along with Delaware, Virginia, and Arkansas exhibited the

common non-target site resistance mechanism of reduced translocation. However, they

did not share a common evolutionary or geographic origin. Page et al. (2017) was the

first to provide results on target site-mediated resistance in horseweed populations

(Canadian accessions), possibly due to the horseweed being self-pollinated and being able to disperse seeds over very long distances. Several unanswered questions still remain and need to be addressed to fully understand the origins and relationships of target and non-target site resistance in horseweed (Page et al., 2017).

Horseweed remains problematic because of the frequent occurrence of biotypes resistant to glyphosate and ALS inhibiting herbicides (Davis et al., 2010). Outcrossing has been documented with horseweed, increasing the potential for spreading resistant genes both among and between populations (VanGessel, 2001). Yuan et al. (2010) suggests focusing on mechanism and not pollen or seed spread is crucial for management of resistance. Glyphosate is one of the most important and widely used herbicides due to its versatility and broad spectrum of weeds controlled. Because of this, glyphosate plays a pivotal role in the world of agriculture; therefore, it is important to preserve its efficacy on non-resistant weed species (Powles and Preston, 2006). Shergill et al. (2017) conducted a five-year study looking at herbicide strategies on efficacy and evolution of resistant weeds and concluded that over the span horseweed did show more variable population densities at harvest due the selection of more tolerant populations.

When considering horseweed control measures, application timing, and crop rotation are important factors to consider. Davis et al. (2007) found that spring herbicide 6 applications reduced early and midseason densities more than fall applications. A corn- soybean rotation will consistently lower the population while a continuous soybean rotation will likely elevate populations (Davis et al., 2009). Resistance to glyphosate is a common occurrence in horseweed populations, but more and more farmers are complaining of 2,4-D being less effective. Due to this, the efficacy of 2,4-D and its influence on seed production of surviving horseweed plants has come into question.

Kruger et al. (2010) showed that 2,4-D can be a very effective tool in controlling horseweed at traditional use rates. Field experiments in Mississippi proved that in a glyphosate-resistant soybean program, effective horseweed control could be obtained with glufosinate, paraquat, dicamba, and 2,4-D preplant programs when residual herbicides such as metribuzin or sulfentrazone are added (Eubank et al., 2008). However,

Steckel et al. (2006) concluded that only glufosinate tank mixes with high rates of dicamba and 2,4-D provided above 90% control one month after application. Steckel et al. (2006) also notes that single glufosinate application timing is important. When this study was conducted in Tennessee and under early spring application conditions when it is cool, horseweed control can vary greatly. According to Take Action (2017), liberty link soybean systems are the most effective strategy for horseweed control from burndown to in season applications. They highlight the continued need for multiple sites of action and inclusion of residual herbicides applied preplant-burndown and note dicamba, as currently labeled for dicamba-tolerant crops, as another viable option for preplant and post emergence control. In tank mix situations, the addition of glyphosate to dicamba

does not enhance the control of glyphosate-resistant horseweed (Flessner et al., 2015).

This implies that dicamba is carrying the majority of the load for control.

Most studies today focus on residual herbicide programs for control of

horseweed; one of the most common additions is saflufenacil. Saflufenacil applied early

preplant resulted in control 30 days after application that was as good as dicamba while

also providing residual control through the time of planting (Owen et al., 2011). When

combined with glyphosate, saflufenacil has significant foliar activity on horseweed also

providing broad-spectrum control on many other weed species. This also allows for

application to a larger range of horseweed plant heights (Mellendorf et al., 2013). The

soundest approach to glyphosate-resistant horseweed control is to make herbicide

applications to relatively small plants. This reduces the risk of the evolution of protoporphyrinogen oxidase inhibitor (PPO) resistance says Mellendorf et al. (2013). In addition to saflufenacil, metribuzin is a common additional site of action that aides in glyphosate-resistant horseweed control as well as provide additional residual control.

Budd et al. (2016) reported that the addition of metribuzin to glyphosate plus saflufenacil provided nearly 98% control. This also added an additional site of action. Adjuvants must be considered when combining multiple herbicides. Mellendorf et al. (2015) reported that control was greater with crop oil concentrate versus non-ionic surfactant.

Overlooking application equipment, specifically nozzle size and type, may lead to reduced efficacy of herbicides on glyphosate-resistant horseweed. In year two of a two- year study using an ultra-coarse TTI11004 nozzle; control of glyphosate-resistant horseweed was significantly reduced in comparison to medium-sized droplet nozzles 8 when glyphosate plus 2,4-D was applied (Legleiter et al., 2017). While glyphosate- resistant palmer amaranth, tall waterhemp, and giant ragweed showed no significant difference in control across years or nozzle type. The same scenario was repeated using dicamba plus glyphosate. Results showed no significant difference in horseweed control across years or nozzle type (Legleiter et al., 2017). This again shows the difficulty and variability in horseweed control from year to year and across differing herbicide resistant crops.

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Yuan, J., Abercrombie, L., Cao, Y., Halfhill, M., Zhou, X., Peng, Y., Stewart, C. (2010). Functional Genomics Analysis of Horseweed (Conyza canadensis) with Special Reference to the Evolution of Non–Target-Site Glyphosate Resistance. Weed Science. 58: 109-117.

Chapter 2: Residual herbicide horseweed control systems.

Introduction

In Ohio, no-till soybean production, glyphosate and ALS inhibiting herbicides are common components for management of glyphosate-resistant horseweed. However, resistance to glyphosate was confirmed in Ohio in 2002, followed by multiple resistance to both ALS inhibitors and glyphosate in 2003 (Heap, 2017). Trainer et al. (2005) surveyed a large portion of western Ohio and found multiple fields with populations resistant to both glyphosate and ALS inhibitors. The objective of these studies was to determine the efficacy of fall- and spring-applied herbicides for residual control of glyphosate-resistant horseweed, while deemphasizing the use of ALS-inhibiting herbicides.

Materials and Methods

Three different field studies were conducted over a five–year period, involving horseweed management with fall- or spring-applied residual herbicides. The initial study

(Study 1) focused on the efficacy of fall-applied herbicides for management of horseweed emerging the following spring. It was conducted once on a grower field in Mt. Orab, OH with horseweed resistant to ALS-inhibiting herbicides, from the fall of 2009 through the summer of 2010. The study was conducted twice at the OARDC Western Agricultural

Research Station in South Charleston, OH, from fall of 2009 through summer of 2011, with horseweed sensitive to ALS-inhibiting herbicides. Horseweed populations at both sites were resistant to glyphosate. Various residual herbicides and herbicide

combinations were applied with 1.1 kg ae ha-1 of 2,4-D ester and 0.84 kg ae ha-1 of glyphosate to control emerged weeds (Figure 2.1 and Table 2.1). A treatment consisting of glyphosate and 2,4-D alone was considered to be the non-residual control, by which to assess the residual control from other treatments. Herbicides were applied in a volume of

190 l ha-1 with a CO2-pressurized backpack sprayer, using Spraying Systems DG 8002

nozzles at 360 kPa. The study was conducted in the absence of a crop at both sites.

Additional herbicide was not applied between the November application and final

assessment of control and population density the following June.

Treatments were arranged in a randomized complete block design with three

replications. Plots were 3 m wide by 9 m long. Two 0.5 m2 quadrats were established

within each plot in mid-March for the purpose of assessing population density. Data

were averaged between quadrats for analysis. Measurements included a visual

assessment of efficacy and enumeration of horseweed population density bi-weekly from

mid-March through early June. Efficacy was assessed based on a scale of 0 to 100,

where 0 was no control and 100 complete control. Statistical analysis was performed on

date from mid-April, mid-May, and early June assessments using PROC Mixed in SAS

9.3. Data from each site was analyzed separately, and year was considered to be a

random effect at South Charleston. Population density data were subjected to square root

transformation for analysis using the capability procedure. Means were separated using

Fisher’s protected LSD at the 95% level of probability. Results were then back-

transformed for presentation.

A second field study was conducted in 2012 and 2013 in South Charleston, Ohio

to determine the effect of application timing and rate on the residual control of

glyphosate-resistant horseweed from metribuzin, flumioxazin, and sulfentrazone (Study

2). The use of ALS-inhibiting herbicides was deemphasized in these studies due to the

prevalence of horseweed populations in Ohio that are resistant to both glyphosate and

ALS-inhibiting herbicides. Herbicides were applied 30 days prior to soybean planting, 7

days prior to planting, or sequentially – 30 days preplant followed by at planting (Figure

2.1 and Table 2.2). Soybeans were planted on April 30, 2012 and April 27, 2013 at a

density of 550,000 seeds per hectare in rows spaced 38 cm apart. The study received a

post emergence application of 870 g ae ha-1 of glyphosate approximately 4 weeks after

planting. Treatments were arranged in a randomized complete block design with three

replications, and individual plots were 3 m wide by 9 m long. Herbicides were applied in

a volume of 140 liters per hectare with a CO2-pressurized backpack sprayer, using

Spraying Systems AIXR 110015 nozzles at 300 kPa. Visual assessment of horseweed

control occurred at the time of the POST glyphosate application, 30 days after POST, and just prior to soybean harvest. Population density was measured at these same intervals.

Density was measured in two 0.5 m quadrats placed arbitrarily in each plot, which was then averaged for the purpose of analysis. Data were averaged over years, and subjected to statistical procedures similar to those previously described. Results were then back- transformed for presentation.

A third field study was conducted in 2013 and 2014 in South Charleston, OH

(Study 3). The objective of this study was to determine the optimum combination of

residual herbicides for control of glyphosate-resistant horseweed, when applied in early

spring. Use of ALS-inhibiting herbicides was de-emphasized for the reason stated

previously. Various combinations and rates of metribuzin, flumioxazin, saflufenacil,

sulfentrazone, and dicamba were applied in early April, 30 days prior to soybean planting

(Figure 2.1 and Table 2.3). All treatments included 1.3 kg ae ha-1 of glyphosate, and 0.56

kg ae ha-1 of 2,4-D ester was included in all non-dicamba treatments. Soybeans were

planted on April 27, 2013 and May 5, 2014 at a density of 550,000 seeds per hectare in

rows spaced 38 cm apart. The study received a POST application of 0.87 kg ae ha-1 of glyphosate approximately 4 weeks after planting. Study design, assessments, and data analysis were similar to those described for Study 2.

Results and Discussion

Efficacy of the fall treatments in Study 1 varied between the two research sites

(Table 2.1). At South Charleston, treatments containing chlorimuron provided the most effective control of ALS-sensitive horseweed through early June, when the last assessment occurred. Population density for these treatments ranged from 0 to 630 plants m-2 in mid-April versus more than 1000 plants m-2 for almost all other treatments. In

early June, population density for chlorimuron-containing treatments ranged from 20 to

315 plants m-2 versus 390 to 720 for all other treatments. Higher chlorimuron rates often resulted in lower population, but not consistently among all herbicide combinations and assessment timings. The higher rate of flumioxazin, 71 g ha-1, was the only other

treatment that controlled horseweed into the spring, and only through mid-May. This

treatment resulted in 0, 59, and 420 plants m-2 in mid-April, mid-May, and early June,

respectively. Aside from control of plants emerged at the time of application, fall

treatments at Mt. Orab did not appear to provide any residual control of ALS-resistant

horseweed into the following spring. The population density in spring, which was

variable across the experimental area, was not affected by herbicide treatment at any

assessment timing. Results would appear to indicate that among the herbicides used in

this study, only chlorimuron and flumioxazin have potential to control spring-emerging

horseweed when applied the previous fall. Chlorimuron controlled ALS-sensitive

horseweed only, and flumioxazin provided control into spring only at one site. Overall,

this study showed low and variable potential for fall herbicide treatments to provide

control of spring-emerging horseweed that are resistant to ALS inhibitors. Most Ohio

populations of horseweed are considered to be ALS-resistant at this point, which would

indicate a relatively higher importance of residual herbicides applied in spring versus fall in horseweed management programs.

Residual control of horseweed in study 2 varied among herbicides, herbicide rates, and application timing (Table 2.2). Among the single treatments applied 30 or 7 days prior to planting (DPP), only metribuzin provided control that exceeded 80%, and

630 g ha-1 was overall more effective than 420 g ha-1. Control among all evaluation timings ranged from 53 to 90% for 420 g ha-1 and 68 to 100% for 630 g ha-1, and from 91

to 100% for the higher rate applied 7 DPP. Control from single-application sulfentrazone

and flumioxazin treatments did not exceed 57%. The combination of flumioxazin and chlorimuron applied once controlled up to 81% applied 30 DPP, but only 69% applied 7

DPP. For the sequential-application treatments, 30 DPP followed by PRE, control among

all evaluation timings ranged from 65 to 100%. Metribuzin was most effective in this

approach, controlling 100 and 91% of horseweed in June and at harvest, respectively.

Control did not exceed 83% for the other herbicides applied sequentially.

Population density generally reflected the differences in control. For the at POST

and 30 days after POST evaluations, single application densities were overall lowest for

metribuzin and flumioxazin plus chlorimuron. Population densities for all treatments

ranged from 0 to 44 plants m-2, but from 0 to 10 plants m-2 for all metribuzin- and

chlorimuron-containing treatments except the low rate of metribuzin applied 30 DPP.

However, density did not exceed 10 plants m-2 for any of the sequential treatments,

indicating this may be a more effective method of managing herbicides with limited

residual on horseweed. There appeared to be natural mortality of horseweed occurring

in the latter part of the growing season, since densities were generally lowest at harvest,

ranging from only 0 to 8 plants m-2. Differences among treatments were also less

apparent then, but the most effective treatments were still those where metribuzin was

applied at 630 g ha-1 or sequentially. While glyphosate and 2,4-D were applied with all of the treatments to try to ensure that the study measured only residual activity on

horseweed, control of larger emerged plants at the later application timing was influenced

by residual herbicides also. For example, the lower control with chlorimuron plus

flumioxazin at the 7 DPP versus 30DPP was probably due the to combination of larger

plants and the presence of ALS resistance in the horseweed population, which would

have reduced the contribution of chlorimuron to control. Among the other residual

herbicides, only metribuzin has activity and contributed to the control of emerged plants.

This activity could be part of the reason that metribuzin was more effective than flumioxazin and sulfentrazone, especially for 7 DPP applications. Nonetheless, in this study, the residual activity of metribuzin persisted longer than the other herbicides.

In Study 3, where herbicides were applied as a single treatment 30 DPP, residual control of horseweed varied among herbicides and herbicide rates (Table 2.3). A number of treatments provided 90% or greater control at the time of the POST application, but overall evaluations, most effective control occurred with treatments that included multiple residual herbicides. At the early evaluation, sulfentrazone and saflufenacil controlled at least 93% of horseweed, but control from flumioxazin or metribuzin did not exceed 83% in the absence of another residual herbicide. Dicamba applied alone at 280 or 560 g ha-1 resulted in control ranging from only 35 to 62% among all evaluation timings. Mixtures of dicamba with other herbicides did not improve control compared with the other herbicide alone, with one exception. Adding dicamba to metribuzin improved control at the early evaluation from 68 to 77% to 97 to 100%, and 280 g ha-1 of dicamba was sufficient to do so. Adding metribuzin and/or saflufenacil to flumioxazin- improved control to 96 to 100% at time of POST application, and adding saflufenacil to

530 g ha-1 of metribuzin improved control from 77 to 100%.

Differences among treatments became more evident at the later evaluation timings. At the time of soybean harvest, control exceeded 80% only where saflufenacil was included and where two or three residual herbicides were combined, with two exceptions. Saflufenacil alone controlled 85% of horseweed, and flumioxazin plus the higher rate of metribuzin controlled 90%. The population density was also lowest for

these same treatments, at 2 or less plants m-2. Density in other treatments ranged from 3

to 15 plants m-2. These densities were still considerably lower than densities measured at

30 days after POST application, which as mentioned previously, reflects natural mortality

of horseweed over time due presumably to competition from the soybeans.

The results from all of these studies show that horseweed populations can be controlled using residual herbicides exclusively, applied either in the fall or in spring prior to planting. However, there was considerable variability in effectiveness among herbicides and environments, and differences among herbicides were not consistent among studies. Adequate residual activity into spring from fall-applied herbicides occurred primarily only for chlorimuron, and only in a population that was still sensitive to ALS-inhibiting herbicides. While spring preplant application of a single active ingredient did occasionally provide effective control of horseweed through the time of a

POST application, this occurred most consistently with sequential preplant applications or multiple-herbicide treatments. Dicamba provided some control of horseweed, and substantially improved control provided by metribuzin in mixtures, but not for other herbicides. In one study, saflufenacil was most effective for control when applied alone or with other herbicides, especially later in the season. Results generally show a need for growers to adopt a more complex approach to residual herbicide management, in soybean systems where there are not options for POST control.

Bibliography

Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. Tuesday, November 7, 2017. Available www.weedscience.com

Trainer, G. D., Loux, M. M., Harrison, S. K. and Regnier, E. 2005. Response of horseweed biotypes to foliar applications of cloransulam-methyl and glyphosate. Weed Technology. 19: 231-236.

Figure 2.1 Field site, herbicide application, and soybean planting information.

Study/Test Site Year Texture %OM pH CEC Herb App Nozzle Pressure Spay Vol. Soybean Var. Plant Date Seeds/Ha Study 1 South 2010 Silty Clay 1.8 6.2 8.6 11/29/2009 DG 8002 360 kPa 190 l/ha N/A N/A N/A Charleston, OH 2011 Silty Clay 1.8 6.2 8.6 11/10/2010 DG 8002 360 kPa 190 l/ha N/A N/A N/A Mt. Orab, OH 2010 Silt Loam 1.3 6.4 8.6 11/13/2009 DG 8002 360 kPa 190 l/ha N/A N/A N/A Study 2 South 2012 Silt Loam 1.5 5.8 13 4/2/2012 AIXR 110015 300 kPa 140 l/ha Pioneer 93Y51 RR 4/30/2012 550,000 Charleston, OH 4/25/2012 AIXR 110015 300 kPa 140 l/ha 5/2/2012 AIXR 110015 300 kPa 140 l/ha 5/31/2012 AIXR 110015 300 kPa 140 l/ha

2013 Silty Clay 2 6.2 9.7 4/3/2013 AIXR 110015 300 kPa 140 l/ha Asgrow 3533 RR 4/27/2013 550,000

24 Loam 4/22/2013 AIXR 110015 300 kPa 140 l/ha 4/27/2013 AIXR 110015 300 kPa 140 l/ha 5/29/2013 DG 11002 230 kPa 140 l/ha Study 3 South 2013 Silty Clay 2 6.2 9.7 4/3/2013 AIXR 110015 330 kPa 140 l/ha Asgrow 3533 RR 4/27/2013 550,000 Charleston, OH Loam 5/29/2013 DG 11002 230 kPa 140 l/ha 2014 Silty Clay 2 6.2 9.7 4/9/2014 AIXR 110015 330 kPa 140 l/ha Asgrow 3533 RR 5/5/2014 550,000 Loam 6/4/2014 AI 11002 190 kPa 140 l/ha

Abbreviations: % OM, percent organic matter; CEC, cation exchange capacity; Herb App, herbicide application timing;

Ha,hectare.

Table 2.1. Effect of fall applied residual herbicide(s) on horseweed population densities the following spring (Study 1). Mt. Orab (ALS‐R) South Charleston (ALS‐S) Herbicide(s) Rate 16‐Apr 19‐May 8‐Jun 16‐Apr 19‐May 8‐Jun g ai or ae ha‐1 plants m‐2 Check 1170 480 300 1340 300 520 Dicamba 280 950 650 640 1410 350 430 Dicamba 560 530 370 260 1270 510 590 Metribuzin 210 890 520 490 1910 460 620 Metribuzin 420 430 330 250 1800 450 530 Metribuzin 630 94 370 220 1720 420 670 Chlorimuron + tribenuron 18 + 4 620 600 280 380 41 210 Chlorimuron + tribenuron 35 + 7 1010 740 470 390 32 100 Chlorimuron + metribuzin 17 + 100 240 770 450 630 25 140 Chlorimuron + metribuzin 33 + 200 380 450 360 45 18 220 Chlorimuron + metribuzin 17 + 420 500 460 430 320 39 320 25 Chlorimuron + flumioxazin 15 + 42 370 390 340 15 4 97 Chlorimuron + flumioxazin 29 + 84 150 380 340 0 6 20 Flumioxazin 36 270 310 210 520 150 390 Flumioxazin 71 110 340 150 0 59 420 Saflufenacil 25 610 400 310 1040 360 620 Sulfentrazone + metribuzin 150 + 230 190 570 260 580 240 490 Chlorimuron + sulfentrazone 11 + 87 790 570 420 0 73 260 Chlorimuron + sulfentrazone 22 + 170 220 760 430 15 17 85 Imazaquin 140 770 1050 610 1140 260 720 Pyroxasulfone 150 250 590 220 1290 630 690 Pyroxasulfone 300 110 200 110 460 310 600 LSD (0.05) N.S. N.S. N.S. 230 97 76 Abbreviations: LSD, least significant difference; ALS-R, ALS Resistant; ALS-S, ALS Sensitive; N.S., Not Significant All treatments had glyphosate + 2,4-D at 1100 and 840 g ai or ae ha-1 respectively

Table 2.2. Effect of herbicide rate with single and split-spring application timings on residual horseweed control (Study 2).

Control Density Herbicide(s) Timing Rate % plants m‐2 g ai or ae ha‐1 ATPO 30 DAPO HVST ATPO 30 DAPO HVST Flumioxazin + Chlorimuron 30 DPP 84 + 29 81 75 80 3 3 3 Flumioxazin 30 DPP 72 53 52 49 8 22 5 Flumioxazin 30 DPP 110 53 53 53 10 32 5 Metribuzin 30 DPP 420 68 53 78 1 44 3 Metribuzin 30 DPP 630 93 68 89 1 8 1 Sulfentrazone 30 DPP 87 27 40 32 13 36 8 Sulfentrazone 30 DPP 180 40 52 48 15 20 6 Flumioxazin + Chlorimuron 7 DPP 84 + 29 53 69 65 3 7 4 Flumioxazin 7 DPP 72 45 51 57 11 15 8 Flumioxazin 7 DPP 110 45 55 51 13 21 6 26 Metribuzin 7 DPP 420 90 78 83 1 9 3 Metribuzin 7 DPP 630 100 91 97 1 3 0 Sulfentrazone 7 DPP 87 38 47 49 13 32 6 Sulfentrazone 7 DPP 180 30 37 49 7 16 6 Glyphosate + 2,4‐D 7 DPP 540 + 480 42 50 42 9 26 8 Flumioxazin + Chlorimuron 30 DPP 42 + 15 82 83 78 2 4 3 Glufosinate + Flumioxazin + Chlorimuron PRE 450 + 42 + 15 Flumioxazin 30 DPP 36 71 65 58 3 9 3 Glufosinate + Flumioxazin PRE 450 + 54 Metribuzin 30 DPP 210 100 86 91 0 2 1 Glufosinate + Metribuzin PRE 450 + 310 Sulfentrazone 30 DPP 53 68 65 66 4 10 3 Glufosinate + Sulfentrazone PRE 450 + 70 Glufosinate + metribuzin + Flumioxazin + Chlorimuron PRE 450 + 210 + 84 + 29 53 65 81 5 10 2 LSD (0.05) 26 22 26 4 9 1

Abbreviations: DPP, days prior to plant; PRE, preemergence; LSD, least significant difference; ATPO, at post; DAPO, Days after post; HVST, harvest Treatments not containing glufosinate were applied with glyphosate and 2,4-D at 870 and 560 g ai or ae ha‐1 respectively. Glufosinate-containing treatments were applied with 2,4-D only. All treatments received POST application of glyphosate at 870 g ae ha‐1.

Table 2.3. Effect of herbicide(s) applied 30 days prior to soybean planting on residual horseweed control (Study 3).

Control Density Herbicide(s) Rate % plants m‐2 g ai ha‐1 ATPO 30 DAPO HVST 30 DAPO HVST Flumioxazin 90 83 58 80 16 4 Sulfentrazone 200 93 67 71 10 5 Saflufenacil 37 100 73 85 15 2 Metribuzin 300 68 43 66 26 7 Metribuzin 530 77 45 63 30 6 Dicamba 280 60 35 50 50 15 Dicamba 560 62 38 56 40 14 Flumioxazin + Metribuzin 90 + 300 98 63 70 10 3 28 Flumioxazin + Metribuzin 90 + 530 96 68 90 7 1 Flumioxazin + Metribuzin + Saflufenacil 90 + 300 + 37 100 98 95 3 1 Flumioxazin + Saflufenacil 90 + 37 98 83 95 4 1 Flumioxazin + Dicamba 90 + 280 86 58 66 14 6 Flumioxazin + Dicamba 90 + 560 93 54 68 15 6 Sulfentrazone + Metribuzin 200 + 300 99 63 78 15 3 Sulfentrazone + Metribuzin 200 + 530 99 57 68 10 4 Sulfentrazone + Metribuzin + Saflufenacil 200 + 300 + 37 98 88 88 3 1 Sulfentrazone + Saflufenacil 200 + 37 100 100 96 1 1 Sulfentrazone + Dicamba 200 + 280 98 59 67 8 6 Sulfentrazone + Dicamba 200 + 560 95 59 78 13 4 Metribuzin + Saflufenacil 530 + 37 100 70 89 3 2 Metribuzin + Dicamba 530 + 280 100 47 72 19 6 Metribuzin + Dicamba 530 + 560 97 42 71 40 5 LSD (0.05) 16 16 20 4 1

Abbreviations: LSD, least significant difference; ATPO, at post; DAPO, Days after post; HVST, harvest Treatments not containing dicamba were applied with glyphosate and 2,4-D ester at 1270 g ae ha-1 and 560 g ai ha-1, respectively. Dicamba-containing treatments were applied with glyphosate only. All treatments received POST application of glyphosate of 870 g ae ha-1. 29

Chapter 3: Herbicide-resistant soybean horseweed control systems.

Introduction

An OSU greenhouse study (unpublished 2011) confirmed that 25% of the 45-

horseweed populations tested had multiple resistance to both glyphosate and ALS

inhibiting herbicides. The frequency in Ohio of this type of resistance is currently

assumed to be even higher. The following studies again deemphasize the use of ALS

inhibitors as well as glyphosate for management of horseweed. In the following studies,

the objective for horseweed control was to determine the most effective herbicide

application program for soybeans resistant to glufosinate, glyphosate/dicamba, or to

glyphosate/glufosinate/2,4-D.

Materials and Methods

Three different field studies were conducted over a seven-year period, involving

various systems for management of glyphosate-resistant horseweed in soybeans. The initial study (study 4) was conducted at the OARDC Western Agricultural Research

Station in South Charleston, OH, from the fall of 2009 through the fall of 2012 with the objective of determining the effectiveness of a multiple-application systems approach for control of horseweed in no-tillage glufosinate-resistant soybeans. This study was conducted a total of three times. Trials were conducted on a naturally occurring glyphosate-resistant horseweed population, which was still somewhat sensitive to ALS- inhibiting herbicides. Various residual and non-residual herbicides were applied in early

November, 7 days prior to planting, and at the time of planting (Figure 3.1 and Table

3.1). Glufosinate-resistant soybeans were planted on April 29, 2010, May 22, 2011, and

April 30, 2012 in rows spaced 38 cm apart at a density of 480,000 to 560,000 seeds per

hectare. The study received a POST application of 450 g ai ha-1 of glufosinate

approximately 5 weeks after planting. Herbicides were applied in a volume of 190 l ha-1

with a CO2-pressurized backpack sprayer, using Spraying Systems DG 8002, AI 11002

or AIXR 110015 nozzles at 200 or 300 kPa depending on the nozzle.

Treatments were arranged in a randomized complete block design with three

replications. Plots were 3 m wide by 9 m long. Visual assessment of horseweed control

occurred at the time of soybean planting, at the time of the POST glufosinate application,

30 days after POST, and just prior to soybean harvest. Efficacy was assessed based on a scale of 0 to 100, where 0 represented a complete lack of control and 100 represented complete control. Population density was measured just prior to soybean harvest. Density was measured in two 0.5 m quadrats placed arbitrarily in each plot, which was then

averaged for the purpose of analysis. Statistical analysis was performed with PROC

Mixed in SAS 9.3, with year considered to be a random effect. Population density data

was subjected to square root transformation for analysis using the capability procedure.

Means were separated using Fisher’s protected LSD at the 95% level of probability.

A second field study was conducted in South Charleston, Ohio, and in Butlerville,

Indiana from the fall of 2014 through 2016 (study 5). The objective of these studies was

to determine the most effective herbicide program to control glyphosate-resistant

horseweed in soybeans resistant to 2,4-D, glyphosate, and glufosinate. The soybeans

were planted in Ohio on May 7, 2015 a density of 450,000 seeds per hectare in rows

spaced 38 cm apart. The study was conducted in the absence of a crop in the following

year. Soybeans were planted May 22, 2015 and May 26, 2016 in Indiana, at a density of

350,000 seeds per hectare in rows spaced 76 cm apart. Herbicides were applied in early

November, 7 days prior to planting, and at the time of planting (Figure 3.1 and Tables 3.2

and 3.3). Fall-applied herbicide or combinations of herbicides included: 2,4-D choline,

2,4-D, dicamba, glyphosate, and saflufenacil. Spring treatments applied 7 days prior to planting and at the time of planting had the following herbicide or combinations of herbicides: glyphosate, dicamba, 2,4-D, 2,4-D choline, saflufenacil, paraquat,

sulfentrazone plus cloransulam, and metribuzin. POST treatments included the following

herbicides or combinations of herbicides: glyphosate plus 2,4-D choline, glyphosate, glufosinate, and 2,4-D.

Treatments were arranged in a randomized complete block design with three

replications. Individual plots were 3 m wide by 9 m long. Herbicides were applied in a

volume of 140 l ha-1 with a CO2-pressurized backpack sprayer, using Spraying Systems

AIXR 110015, XR 11002 nozzles at 300 and 120 kPa depending on the nozzle. Control of horseweed was determined using a scale of 0 to 100, where 0 corresponded to a lack of

control and 100 corresponded to complete control. Control was evaluated at the time of

planting or PRE herbicide, 14 days after plant, at the time of post application, and 30

days after POST application in Ohio, and at the time of POST application, 14 days after

POST, and just prior to harvest in Indiana. Population density of horseweed was

measured at these same times. Density was measured in two 0.5 m quadrats placed

arbitrarily in each plot, and then averaged for the purpose of analysis. Statistical analysis

was performed for each site separately with PROC Mixed in SAS 9.3, with year

considered to be a random effect. Population density data was subjected to square root

transformation for analysis using the capability procedure.

A third field study was conducted in South Charleston, Ohio, and in Butlerville,

Indiana from the fall of 2014 through 2016 (Study 6). The objective of these studies was

to determine the most effective herbicide program to control glyphosate-resistant

horseweed in fallow or in soybeans resistant to dicamba and glyphosate. Soybeans were

not planted at the South Charleston location either year. Soybeans were planted at the

Butlerville location on May 22, 2015 and May 26, 2016 at a density of 350,000 seeds per

hectare in rows spaced 76 cm apart. Herbicides were applied in early November, 7 days

prior to planting, and at the time of planting (Figure 3.1 and Tables 3.4 and 3.5). Fall applied herbicide or combinations of herbicides included: 2,4-D, dicamba, glyphosate,

and saflufenacil. Spring treatments applied 7 days prior to planting and at the time of

planting had the following herbicide or combinations of herbicides: glyphosate, dicamba,

2,4-D, saflufenacil, paraquat, sulfentrazone plus cloransulam, and metribuzin. Post

applied treatments included the following herbicides or combinations of herbicides:

glyphosate and dicamba.

Treatments were arranged in a randomized complete block design with three

replications. Individual plots were 3 m wide by 9 m long. Herbicides were applied in a

volume of 140 l ha-1 with a CO2-pressurized backpack sprayer, using Spraying Systems

AIXR 110015, XR 11002 nozzles at 300 and 120 kPa depending on the nozzle. Control of horseweed was determined using a scale of 0 to 100, where 0 corresponded to a lack

33 of control and 100 corresponded to complete control. Control was evaluated at the time of planting or PRE herbicide, 14 days after plant, at the time of post application, and 30 days after POST application in Ohio, and at the time of POST application, 14 days after

POST, and just prior to harvest in Indiana. Population density of horseweed was measured at these same times. Density was measured in two 0.5 m quadrats placed arbitrarily in each plot, and then averaged for the purpose of analysis. Statistical analysis was performed for each site separately with PROC Mixed in SAS 9.3, with year considered to be a random effect. Population density data was subjected to square root transformation for analysis using the capability procedure.

Results and Discussion

In study 4, the glufosinate-resistant soybean study, most effective end of season horseweed control occurred where herbicides were applied the previous fall or where the

7 DPP treatment contained chlorimuron and flumioxazin (Table 3.1). Application of glyphosate/2,4-D or 2,4-D/chlorimuron/metribuzin in the fall, followed by any of the 7

DPP herbicide treatments resulted in 93 to 100% control at the time of soybean planting, versus 32 to 58% control in the absence of fall herbicides. The only exception to this was where glufosinate was applied in spring following glyphosate/2,4-D in the fall, for which control was only 66%. Among the spring-only treatments, only glyphosate/2,4-

D/chlorimuron/flumioxazin provided effective control, 90 to 98% through the rest of the season. For the others, control was 24 to 69% at the time of POST glufosinate application, and the POST activity further improved control to 85 to 89% at harvest.

Differences in control among the fall plus spring treatments occurred by the time of POST application, due to differences in the level of residual herbicide included in the fall and spring. Control did not exceed 53% where the spring treatment completely lacked residual. The inclusion of saflufenacil, metribuzin, or chlorimuron/flumioxazin resulted in 79, 78, and 97% control, respectively. The latter herbicide combination was expected to contribute the most residual activity on horseweed among these. POST glufosinate application increased horseweed control for the non-residual treatments only to 82 to 89% but did improve control of the metribuzin and saflufenacil treatments to at least 92%. Application of 2,4-D/chlorimuron/metribuzin in the fall without a 7 DPP treatment resulted in 69 and 83% control at POST and harvest, respectively. Use of this fall treatment followed by any spring treatment resulted in greater than 90% control at harvest. Where the spring treatment lacked residual, control was as low as 79% following this fall treatment, but the POST glufosinate improved control to the higher end of season levels. The end of season population densities reflected the differences in control discussed previously. Densities were lowest where fall treatments containing chlorimuron were used, and where fall-applied glyphosate/2,4-D was followed with spring treatments that had at least some residual activity on horseweed.

Results of this study indicate that failure to use fall herbicide treatments or spring herbicides with residual activity on horseweed can result in inadequate late-season control, even where it is possible to use glufosinate POST. It should be considered that most Ohio horseweed populations are resistant to ALS-inhibiting herbicides, including chlorimuron, so the results shown here for the fall treatments containing this herbicide are

probably optimistic overall. Results, where glyphosate/2,4-D were applied in fall, are

probably more representative in many Ohio soybean fields.

In the study that utilized soybeans resistant to 2,4-D, glyphosate, and glufosinate, there were considerable differences in results between the Ohio and Indiana site (Tables

3.2 and 3.3). Control in Indiana exceeded 90% at all evaluations regardless of herbicide management strategy, with three exceptions (Table 3.3). The combination of fall glyphosate/2,4-D, spring 2,4-D/paraquat/cloransulam/sulfentrazone/metribuzin, and

POST glyphosate resulted in 80% control at the end of the season, and spring followed by

POST application of glyphosate/2,4-D resulted in 87% control. Two applications of glyphosate resulted in only 68% control, but this level of control indicates that the

Indiana horseweed probably had incomplete resistance to glyphosate. There were no significant differences at harvest in horseweed population density among treatments in

Indiana. Density measurements earlier in the season were generally low and did not appear to yield additional insight on relative effectiveness of treatments.

At the Ohio site, late-season control also exceeded 90% for most treatments, but there were more differences among treatments for the earlier evaluation at the time of

POST herbicide application (Table 3.2). Control at POST application was as low as 37 to

52%, where glyphosate/2,4-D was applied at planting without fall treatment or inclusion of residual herbicides. This was improved to only 85% when followed with POST application of the same herbicides, but to 94% when followed with glufosinate/2,4-D.

This low level of initial control reflects the inadequate activity of glyphosate/2,4-D on

overwintered horseweed plants, along with the lack of post-plant residual control.

Control ranged from 71 to 78% for a number of other treatments at this time, although

POST application of 2,4-D and/or glufosinate improved late-season control to more than

90% for these. Where the POST herbicide used was glyphosate only, control remained at the lower levels into late season. Most effective treatment strategies at the time of POST application, where control exceeded 90%, included the following: fall-applied herbicides followed by spring treatment of paraquat/2,4-D/residual herbicides; and spring-only treatment that included 2,4-D/glyphosate/saflufenacil/residual herbicides. These results are indicative of the lack of sensitivity of horseweed in spring to 2,4-D, unless paired with another herbicide that also has substantial activity on emerged horseweed. Here this was demonstrated by the need for paraquat in the spring treatment, even following a fall treatment, which could be considered unusual. The need for saflufenacil to be included is not unexpected in the absence of fall treatment.

The range in density among treatments was considerable at the time of POST herbicide application, ranging from 0.7 to 4.3 plants m-2. Density was lowest, ranging

from 0.7 to 1.4 plants m-2, for those treatments where control at this time exceeded 90%,

as discussed above. The late-season population densities were generally low, ranging

from 0.7 to 1.4 plants m-2, and the few significantly higher numbers tended to occur

where control was below 90%. Overall, these results show that the ability to use 2,4-D

more intensively in a 2,4-D resistant soybean can result in effective late-season control of

glyphosate-resistant horseweed. For a number of the treatments that relied primarily on

2,4-D for control of emerged plants though, effective control required both at plant and

POST 2,4-D application. The POST application was required to control plants not killed

by the first application, and this might be expected to exert selection for plants with

resistance to 2,4-D. The inclusion of other effective herbicides, such as paraquat or saflufenacil, in the spring treatment, can improve control and reduce reliance on 2,4-D.

Similarly, including glufosinate in the POST treatment to supplement or replace 2,4-D

could accomplish the same.

In the studies that utilized dicamba herbicide-resistant soybeans, there were

minimal differences in results between the Ohio and Indiana sites (Tables 3.4 and 3.5).

Ohio site (Table 3.4) control was 92% or better for all fall applied treatments at the time

of planting. Control at POST timing was variable across treatments depending on

herbicide combinations and application timings. When no fall application was made

control at time of POST was 84 to 99%, which all contained glyphosate plus dicamba,

with either saflufenacil or metribuzin added. The effectiveness of dicamba on emerged

horseweed control was evident when fall followed by spring application did not include

dicamba as results ranged from 76 to 80%. Fall applied saflufenacil followed by spring

application of saflufenacil was the lone exception providing 98% control at POST. The

most effective control, 91 to 100% at POST for fall following spring applications, all

included dicamba applied 7 DPP or at plant. Regardless of fall application, this showed

the need for herbicides that effectively control spring emerging horseweeds. Where the

POST application was glyphosate only, control was reduced, compared to dicamba plus

glyphosate. This study showed the inclusion herbicides applied with dicamba, such as

saflufenacil and metribuzin, could help distribute the load of effective control on

glyphosate-resistant horseweed, reducing the reliance on dicamba solely. While using

dicamba multiple times in a season increases the selection pressure for resistance, it

shows how it can be an effective tool.

At the Indiana site (Table 3.5), differences in control of glyphosate-resistant

horseweed were minimal across treatments at POST timing. Excluding glyphosate

applied alone at 48%, control was 93% or greater for all treatments, regardless of

herbicide(s) and application timing. Typically, herbicides such as paraquat, saflufenacil,

and metribuzin added to glyphosate in the absence of dicamba when applied in the spring, are responsible for carrying the weight for horseweed control. However, in this

study control in these treatments ranged from 93 to 99%, while treatments where dicamba

was included ranged from 94 to 100% control. While all POST treatments provided

adequate control of horseweed, glyphosate applied alone was the lowest at 64 to 94%.

When dicamba was added to glyphosate POST, control increased and ranged from 99 to

100%. Results in this study again show the possible incomplete resistance to glyphosate

as noted in the previous study at the Indiana site.

The range in density at the Ohio site among treatments was minimal at the time of

POST herbicide application, ranging from 0.7 to 2.5 plants m-2. Density was lowest, ranging from 0.8 to 1.1 plants m-2, for those treatments where control at the time

exceeded 90%, as discussed above. Late-season densities were not significant and

generally low. This indicated there might have been some natural mortality of horseweed

by the end of season. The Indiana site showed minimal differences in horseweed densities

at POST. Population densities were generally low and ranged from 0.3 to 3 plants m-2 and

a significantly higher number occurred where control was below 90%. Density

39 measurements after POST were generally low and did not appear to yield additional insight on relative effectiveness of treatments. There was no noticeable decline in horseweed populations at the end of the season as seen at the Ohio site. Overall, these results show that the ability to use dicamba more intensively in a dicamba resistant soybean can result in effective late-season control of glyphosate-resistant horseweed.

In conclusion, while the use of ALS inhibiting herbicides such as chlorimuron were deemphasized throughout these studies, control provided from those treatments should be considered optimistic. Fall applied glyphosate/2,4-D is more representative of many Ohio soybean fields. Results from studies focusing on residual control of horseweed showed that fall applied herbicides followed by spring-applied herbicide combinations were the most effective and least variable amongst all rating timings.

Results from studies focusing on herbicide-resistant soybeans showed that with these systems, season long horseweed control could be accomplished. With the addition of residual herbicides such as sulfentrazone, saflufenacil, metribuzin, and flumioxazin, variability in control could be reduced. When the full weight of control was placed on

2,4-D the lack of consistency in controlling emerged horseweed at POST was evident compared to when dicamba was added. The soundest approach to full season horseweed control with the reduction in variability is to build a system of fall, spring, and in-crop applications that incorporate multiple sites of actions within each application timing. The addition of in-crop herbicides such as glufosinate, dicamba, and 2,4-D showed an increase in control and reduced variability after POST application. Emphasis should not rely solely on these post in-crop herbicides as a rescue option—especially when using the

40 same site of action that was used in the fall, spring, and/or then in-crop. This will likely put a strain on herbicide resistance and shorten the length of time before horseweed becomes resistant to a new site of action.

Figure 3.1. Field site, herbicide application, and soybean planting information.

Study Number/Site Year Texture %OM pH CEC Herb App Nozzle Pressure Spay Vol. Soybean Var. Plant Date Seeds/Ha Study 4 South 2010 Silt Loam 2 6.2 14 11/24/2009 DG 8002 360 kPa 190 l/ha S.C. 3330 LL 4/29/2010 480000 Charleston, OH 4/21/2010 DG 8002 360 kPa 190 l/ha 6/8/2010 DG 11002 360 kPa 190 l/ha South 2011 Silt Loam 1.7 6.1 12.8 11/9/2010 DG 8002 360 kPa 190 l/ha S.C. 3381 LL 5/22/2011 480000 Charleston, OH 4/21/2011 DG 8002 360 kPa 190 l/ha 6/28/2011 AI 11002 230 kPa 95 l/ha South 2012 Sandy Silt 2.2 5.9 22 11/17/2011 DG 110015 320 kPa 140 l/ha Pioneer 93L71 4/30/2012 560000 Charleston, OH Loam 4/25/2012 AIXR 110015 300 kPa 140 l/ha 6/15/2012 AI 11002 200 kPa 140 l/ha Study 5 South 2015 Silt Loam 1.5 5.8 13 11/10/2014 AIXR 110015 300 kPa 140 l/ha GC 16144857 5/7/2015 450000 Charleston, OH 4/24/2015 AIXR 110015 300 kPa 140 l/ha 4/29/2015 AIXR 110015 300 kPa 140 l/ha

42 6/3/2015 AIXR 110015 300 kPa 140 l/ha Butlerville, IN 2015 Silt Loam 1.8 5.6 6.7 11/12/2014 XR 11002 120 kPa 140 l/ha Dow Enlist 5/22/2015 350000 5/19/2015 XR 11002 120 kPa 140 l/ha 5/22/2015 XR 11002 120 kPa 140 l/ha 6/30/2015 XR 11002 120 kPa 140 l/ha South 2016 Silt Loam 1.5 5.8 13 11/3/2015 AIXR 110015 300 kPa 140 l/ha N/A N/A N/A Charleston, OH 4/20/2016 AIXR 110015 300 kPa 140 l/ha 4/26/2016 AIXR 110015 300 kPa 140 l/ha 6/28/2016 AIXR 110015 300 kPa 140 l/ha Butlerville, IN 2016 Silt Loam 1.8 5.6 6.7 11/10/2015 XR 11002 120 kPa 140 l/ha Enlist 27E15G9 5/26/2016 350000 5/13/2016 AIXR 110015 290 kPa 140 l/ha 5/27/2016 AIXR 110015 290 kPa 140 l/ha 6/24/2016 XR 11002 170 kPa 140 l/ha Study 6 South 2015 Silt Loam 1.5 5.8 13 11/10/2014 AIXR 110015 300 kPa 140 l/ha N/A N/A N/A Charleston, OH 4/24/2015 AIXR 110015 300 kPa 140 l/ha 5/7/2015 AIXR 110015 300 kPa 140 l/ha 6/3/2015 AIXR 110015 300 kPa 140 l/ha

Butlerville, IN 2015 Silt Loam 1.8 5.6 6.7 11/12/2014 XR 11002 120 kPa 140 l/ha Asgrow 2933 5/22/2015 350000 5/19/2015 XR 11002 120 kPa 140 l/ha 5/22/2015 XR 11002 120 kPa 140 l/ha 6/30/2015 XR 11002 120 kPa 140 l/ha South 2016 Silt Loam 1.5 5.8 13 11/3/2015 AIXR 110015 300 kPa 140 l/ha N/A N/A N/A Charleston, OH 4/20/2016 AIXR 110015 300 kPa 140 l/ha 4/26/2016 AIXR 110015 300 kPa 140 l/ha 6/28/2016 AIXR 110015 300 kPa 140 l/ha Butlerville, IN 2016 Silt Loam 1.8 5.6 6.7 11/10/2015 XR 11002 120 kPa 140 l/ha Asgrow 30X6 5/26/2016 350000 5/13/2016 AIXR 110015 290 kPa 140 l/ha 5/27/2016 AIXR 110015 290 kPa 140 l/ha 6/24/2016 XR 11002 170 kPa 140 l/ha

Abbreviations: % OM, percent organic matter; CEC, cation exchange capacity; Herb App, herbicide application timing; Ha, hectare.

Table 3.1. Effect of herbicide system on control of horseweed in glufosinate-resistant soybeans (Study 4). Control Density Herbicide(s) Timing Rate % plants m‐2 g ai or ae ha‐1 ATPL ATPO HVST HVST___ Glufosinate 7 DPP 660 33 24 83 2.3

Glufosinate + Metribuzin 7 DPP 660 + 420 32 69 88 1.8

Glyphosate + 2,4‐D 7 DPP 870 + 560 46 48 89 1.8

Glyphosate + Saflufenacil 7 DPP 870 + 25 58 61 85 1.9

Glyphosate + 2,4‐D + Chlorimuron + Flumioxazin 7 DPP 870 + 560 + 84 + 29 58 90 98 0.8

Glyphosate + 2,4‐D FALL 430 + 560 93 53 82 2.2

Glyphosate + 2,4‐D 7 DPP 870 + 560

44 Glyphosate + 2,4‐D FALL 430 + 560 94 79 92 1.1

Glyphosate + Saflufenacil 7 DPP 870 + 25

Glyphosate + 2,4‐D FALL 430 + 560 100 97 100 0.7

Glyphosate + 2,4‐D + Chlorimuron + Flumioxazin 7 DPP 870 + 560 + 84 + 29

Glyphosate + 2,4‐D FALL 430 + 560 66 36 83 2.2

Glufosinate 7 DPP 660

Glyphosate + 2,4‐D FALL 430 + 560 93 78 93 1.2

Glufosinate + Metribuzin 7 DPP 660 + 420

2,4‐D + Chlorimuron + Metribuzin FALL 560 + 33 + 200 76 69 83 2.1

2,4‐D + Chlorimuron + Metribuzin FALL 560 + 17 + 100 99 79 94 1.1

Glyphosate + 2,4‐D 7 DPP 870 + 560

2,4‐D + Chlorimuron + Metribuzin FALL 560 + 17 + 100 100 92 99 0.8

Glyphosate + Saflufenacil 7 DPP 870 + 25

2,4‐D + Chlorimuron + Metribuzin FALL 560 + 17 + 100 100 89 94 1.1

Paraquat + Metribuzin 7 DPP 700 + 420

2,4‐D + Chlorimuron + Metribuzin FALL 560 + 17 + 100 100 99 100 0.7

Glyphosate + 2,4‐D + Chlorimuron + Flumioxazin 7 DPP 870 + 560 + 84 + 29

2,4‐D + Chlorimuron + Metribuzin FALL 560 + 17 + 100 100 96 100 0.7

Glyphosate + 2,4‐D + Chlorimuron + Flumioxazin 7 DPP 870 + 560 + 42 + 15

2,4‐D + Chlorimuron + Metribuzin FALL 560 + 17 + 100 94 75 91 1.5

Glufosinate 7 DPP 660

45 2,4‐D + Chlorimuron + Metribuzin FALL 560 + 17 + 100 99 83 90 1.2

Glufosinate + Metribuzin 7 DPP 660 + 420

LSD (0.05) 24 16 10 0.91

Abbreviations: ATPL, At plant; DPP, days prior to plant; ATPO, at post; HVST, harvest; LSD, least significant difference All treatments received post application of glufosinate 450 g ai or ae ha‐1 Population densities are expressed in transformed means.

Table 3.2. Effect of herbicide system on control of horseweed in soybeans resistant to glyphosate, glufosinate, and 2,4-D – Ohio (Study 5). Control Density Herbicide(s) Timing Rate % plants m‐2___ g ai or ae ha‐ ATPL 14 DAPL ATPO 30 DAPO ATPO 30 DAPO 2,4‐D + Dicamba FALL 560 + 280 98 88 78 77 1.9 1.1 Glyphosate + 2,4‐D Choline + Metribuzin ATPL 1100 + 1100 + 310 Glyphosate POST 1700 2,4‐D + Glyphosate FALL 560 + 630 92 82 77 73 2.1 1.4 Glyphosate + 2,4‐D Choline + Metribuzin 7 DPP 1100 + 1100 + 310 Glyphosate POST 1700 2,4‐D + Glyphosate FALL 560 + 630 100 100 99 88 0.7 1.0 2,4‐D + Paraquat + Metribuzin 7 DPP 1100 + 840 + 310 Glyphosate POST 1700 2,4‐D + Glyphosate FALL 560 + 630 100 100 95 98 0.8 0.7 2,4‐D + Paraquat 7 DPP 560 + 840 Glufosinate POST 660 Glyphosate + Saflufenacil FALL 630 + 25 96 92 78 98 1.8 0.7 Glyphosate + Saflufenacil ATPL 840 + 25 Glyphosate + 2,4‐D Choline POST 950 + 900 2,4‐D + Glyphosate FALL 560 + 630 90 93 78 100 1.4 0.7 Glyphosate + Saflufenacil ATPL 630 + 25 Glyphosate + 2,4‐D Choline POST 950 + 900 2,4‐D + Glyphosate FALL 560 + 630 100 88 75 100 1.5 0.7 Paraquat + Saflufenacil ATPL 840 + 25 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + Saflufenacil FALL 630 + 25 98 83 74 100 3.1 0.7 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 Glyphosate + 2,4‐D Choline POST 950 + 900 Dicamba + Glyphosate FALL 280 + 630 95 91 77 98 2.1 0.7 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + Saflufenacil FALL 630 + 25 100 100 93 98 1.0 0.7 2,4‐D + Paraquat 7 DPP 1100 + 840 Glyphosate + 2,4‐D Choline POST 950 + 900 Dicamba + Glyphosate FALL 280 + 630 97 91 71 92 2.8 1.0 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 Glufosinate POST 660 46

Glyphosate + 2,4‐D Choline + Saflufenacil + Metribuzin ATPL 1100 + 1100 + 25 + 310 N/A 100 93 88 0.8 0.8 Glyphosate POST 1700 Glyphosate + 2,4‐D Choline + Saflufenacil ATPL 1100 + 1100 + 25 N/A 99 71 95 1.6 0.8 Glufosinate POST 660 Glyphosate + 2,4‐D Choline + Saflufenacil ATPL 1100 + 1100 + 25 N/A 100 91 100 1.4 0.7 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + 2,4‐D Choline + Saflufenacil ATPL 1100 + 1100 + 25 N/A 99 93 95 0.9 0.8 Glufosinate + 2,4‐D POST 660 + 900 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 N/A 74 76 96 1.7 0.7 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 N/A 62 37 85 4.3 1.4 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 N/A 64 52 94 3.2 1.1 Glufosinate + 2,4‐D POST 660 + 900 LSD(0.05) 2 4 6 4 .43 .14 Abbreviations: ATPL, At plant; DPP, days prior to plant; POST, Post Application; DAPL, days after plant; ATPO, at post; DAPO, days after post; N/A, Not Applicable; LSD, least significant difference. All 7 DPP and ATPL treatments had an application of sulfentrazone + cloransulam 200 + 25 g ai ha‐1 , except the last two treatments. Population densities are expressed in transformed means.

Table 3.3. Effect of herbicide system on control of horseweed in soybeans resistant to glyphosate, glufosinate, and 2,4-D – Indiana (Study 5). Control Density Herbicide(s) Timing Rate % plants m‐2 g ai or ae ha‐1 ATPO 14DAPO HVST ATPO 14DAPO HVST 2,4‐D + Dicamba FALL 560 + 280 83 96 97 1.0 1.2 1.6 Glyphosate + 2,4‐D Choline + Metribuzin ATPL 1100 + 1100 + 310 Glyphosate POST 1700 2,4‐D + Glyphosate FALL 560 + 630 97 97 93 0.8 1.1 2.3 Glyphosate + 2,4‐D Choline + Metribuzin 7 DPP 1100 + 1100 + 310 Glyphosate POST 1700 2,4‐D + Glyphosate FALL 560 + 630 99 89 80 1.4 2.1 2.9 2,4‐D + Paraquat + Metribuzin 7 DPP 1100 + 840 + 310 Glyphosate POST 1700 2,4‐D + Glyphosate FALL 560 + 630 94 99 91 1.5 1.4 2.5 2,4‐D + Paraquat 7 DPP 560 + 840 Glufosinate POST 660

48 Glyphosate + Saflufenacil FALL 630 + 25 99 100 100 0.7 0.8 1.6 Glyphosate + Saflufenacil ATPL 840 + 25 Glyphosate + 2,4‐D Choline POST 950 + 900 2,4‐D + Glyphosate FALL 560 + 630 98 99 98 0.7 0.8 2.1 Glyphosate + Saflufenacil ATPL 630 + 25 Glyphosate + 2,4‐D Choline POST 950 + 900 2,4‐D + Glyphosate FALL 560 + 630 98 99 96 0.9 0.8 1.1 Paraquat + Saflufenacil ATPL 840 + 25 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + Saflufenacil FALL 630 + 25 96 99 99 1.1 0.7 2.1 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 Glyphosate + 2,4‐D Choline POST 950 + 900 Dicamba + Glyphosate FALL 280 + 630 96 99 98 1.1 0.9 1.5 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + Saflufenacil FALL 630 + 25 95 98 98 1.5 1.0 2.4 2,4‐D + Paraquat 7 DPP 1100 + 840 Glyphosate + 2,4‐D Choline POST 950 + 900 Dicamba + Glyphosate FALL 280 + 630 97 94 99 1.1 1.4 2.0 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 Glufosinate POST 660 48

Glyphosate + 2,4‐D Choline + Saflufenacil + Metribuzin ATPL 1100 + 1100 + 25 + 310 100 100 96 0.7 1.2 2.2 Glyphosate POST 1700 Glyphosate + 2,4‐D Choline + Saflufenacil ATPL 1100 + 1100 + 25 100 99 99 0.7 1.6 1.7 Glufosinate POST 660 Glyphosate + 2,4‐D Choline + Saflufenacil ATPL 1100 + 1100 + 25 100 100 96 0.8 0.7 2.1 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + 2,4‐D Choline + Saflufenacil ATPL 1100 + 1100 + 25 98 100 99 0.7 0.8 1.7 Glufosinate + 2,4‐D POST 660 + 900 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 99 99 99 1.0 0.9 1.6 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 97 99 87 2.5 0.9 2.0 Glyphosate + 2,4‐D Choline POST 950 + 900 Glyphosate + 2,4‐D Choline ATPL 1100 + 1100 98 100 96 2.1 1.1 2.0 Glufosinate + 2,4‐D POST 660 + 900 Glufosinate ATPL 660 91 97 98 2.1 2.4 2.0 Glufosinate POST 660 Glyphosate ATPL 630 69 75 68 3.3 2.7 2.7 Glyphosate POST 630

49 LSD (0.05) 4 2 4 0.24 0.30 N.S.

Abbreviations: ATPL, At plant; DPP, days prior to plant; POST, Post Application; DAPL, days after plant; ATPO, at post; DAPO, days after post; HVST, Harvest; LSD, least significant difference; N.S. Not Significant All 7 DPP and ATPL treatments had an application of sulfentrazone + cloransulam 200 + 25 g ai ha‐1 , except the last four treatments. Population densities are expressed in transformed means.

Table 3.4. Effect of herbicide system on control of horseweed in soybeans resistant to glyphosate and dicamba – Ohio (Study 6).

Control Density Herbicide(s) Timing Rate % plants m‐2 g ai or ae ha‐1 ATPL 14DAPL ATPO 30DAPO ATPO 30DAPO 2,4‐D + Dicamba FALL 560 + 280 96 100 100 94 0.7 1.0 Glyphosate + Dicamba + Metribuzin ATPL 1100 + 560 +310 Glyphosate POST 1700 2,4‐D FALL 560 100 98 100 93 0.8 1.1 Glyphosate + Dicamba + Metribuzin ATPL 100 + 560 + 310 Glyphosate POST 1700 2,4‐D + Glyphosate FALL 560 + 630 92 100 97 88 0.7 1.3 Paraquat + Dicamba + Metribuzin ATPL 840 + 560 + 310 Glyphosate POST 1700 Glyphosate + Saflufenacil FALL 630 + 25 99 100 98 100 0.7 0.9 Glyphosate + Saflufenacil ATPL 840 + 25 Glyphosate + Dicamba POST 1100 + 560

50 2,4‐D + Glyphosate FALL 630 + 560 93 95 80 100 1.1 0.8 Glyphosate + Saflufenacil ATPL 840 + 25 Glyphosate + Dicamba POST 1100 + 560 2,4‐D + Glyphosate FALL 560 + 630 100 99 80 98 2.3 0.7 2,4‐D + Glyphosate 7 DPP 560 + 840 Glyphosate + Dicamba POST 1100 + 560 2,4‐D + Glyphosate FALL 560 + 630 100 93 76 97 2.5 0.9 2,4‐D + Paraquat 7 DPP 560 + 840 Glyphosate + Dicamba POST 1100 + 560 Glyphosate + Saflufenacil FALL 630 + 25 95 99 91 100 1.1 1.1 Glyphosate + Dicamba ATPL 1100 + 560 Glyphosate + Dicamba POST 1100 + 560 2,4‐D + Glyphosate FALL 560 + 630 94 100 98 100 0.7 0.8 Glyphosate + Dicamba ATPL 1100 + 560 Glyphosate + Dicamba POST 1100 + 560 Glyphosate + Saflufenacil FALL 630 + 25 99 98 100 97 0.7 0.7 Paraquat + Dicamba ATPL 840 + 560 Glyphosate + Dicamba POST 1100 + 560 Dicamba + Glyphosate + Saflufenacil + Metribuzin ATPL 560 + 1100 + 25 + 310 N/A 97 99 96 0.7 1.2 Glyphosate POST 1700

Dicamba + Glyphosate + Saflufenacil ATPL 560 + 1100 + 25 N/A 98 97 100 0.7 0.7 Glyphosate + Dicamba POST 1100 + 560 Dicamba + Glyphosate ATPL 560 + 1100 N/A 79 97 100 0.7 0.9 Glyphosate + Dicamba POST 1100 + 560 Dicamba + Glyphosate ATPL 560 + 1100 N/A 68 84 93 1.4 1.1 Glyphosate + Dicamba POST 1100 + 560 LSD (0.05) 2 2 4 2 0.13 N.S.

Abbreviations: ATPL, At plant; POST, POST Application; DAPL, days after plant; ATPO, at post; DAPO, days after post; N/A, Not applicable; LSD, least significant difference; N.S., Not Significant All 7 DPP and ATPL treatments had an application of sulfentrazone + cloransulam 200 + 25 g ai ha‐1, except the last treatment. Population densities are expressed in transformed means.

Table 3.5. Effect of herbicide system on control of horseweed in soybeans resistant to glyphosate and dicamba – Indiana (Study 6). Control Density Herbicide(s) Timing Rate % plants m‐2 g ai or ae ha‐1 ATPO 14 DAPO HVST ATPO 14 DAPO HVST 2,4‐D + Dicamba FALL 560 + 280 96 97 94 0.9 1.5 1.1 Glyphosate + Dicamba + Metribuzin ATPL 1100 + 560 + 310 Glyphosate POST 1700 2,4‐D FALL 560 96 95 91 0.9 1.8 1.6 Glyphosate + Dicamba + Metribuzin ATPL 100 + 560 + 310 Glyphosate POST 1700 2,4‐D + Glyphosate FALL 560 + 630 99 98 93 0.8 1.0 1.5 Paraquat + Dicamba + Metribuzin ATPL 840 + 560 + 310 Glyphosate POST 1700 Glyphosate + Saflufenacil FALL 630 + 25 99 99 99 0.7 0.8 1.5 Glyphosate + Saflufenacil ATPL 840 + 25 52 Glyphosate + Dicamba POST 1100 + 560 2,4‐D + Glyphosate FALL 630 + 560 93 99 99 0.9 1.1 1.1 Glyphosate + Saflufenacil ATPL 840 + 25 Glyphosate + Dicamba POST 1100 + 560 2,4‐D + Glyphosate FALL 560 + 630 94 96 99 1.3 1.2 1.3 2,4‐D + Glyphosate 7 DPP 560 + 840 Glyphosate + Dicamba POST 1100 + 560 2,4‐D + Glyphosate FALL 560 + 630 94 97 99 1.2 1.4 1.8 2,4‐D + Paraquat 7 DPP 560 + 840 Glyphosate + Dicamba POST 1100 + 560 Glyphosate + Saflufenacil FALL 630 + 25 100 100 100 0.8 0.7 1.6 Glyphosate + Dicamba ATPL 1100 + 560 Glyphosate + Dicamba POST 1100 + 560 2,4‐D + Glyphosate FALL 560 + 630 94 98 99 1.0 1.0 1.5 Glyphosate + Dicamba ATPL 1100 + 560 Glyphosate + Dicamba POST 1100 + 560 Glyphosate + Saflufenacil FALL 630 + 25 97 99 99 0.8 0.8 1.5 Paraquat + Dicamba ATPL 840 + 560 Glyphosate + Dicamba POST 1100 + 560 52

Dicamba + Glyphosate + Saflufenacil + Metribuzin ATPL 560 + 1100 + 25 + 310 100 99 100 0.7 3.0 1.1 Glyphosate POST 1700 Dicamba + Glyphosate + Saflufenacil ATPL 560 + 1100 + 25 98 98 100 0.8 3.4 1.1 Glyphosate + Dicamba POST 1100 + 560 Dicamba + Glyphosate ATPL 560 + 1100 94 96 100 1.3 3.1 0.9 Glyphosate + Dicamba POST 1100 + 560 Dicamba + Glyphosate ATPL 560 + 1100 99 97 99 1.2 4.5 1.1 Glyphosate + Dicamba POST 1100 + 560 Glyphosate ATPL 1700 48 58 64 3.0 4.4 2.2 Glyphosate POST 1700 LSD (0.05) 4 3 3 0.02 0.72 0.22

Abbreviations: ATPL, At plant; POST, Post Application, DAPL, days after plant; ATPO, at post; DAPO, days after post; HVST, harvest; LSD, least significant difference All 7 DPP and ATPL treatments had an application of sulfentrazone + cloransulam 200 + 25 g ai ha‐1, except the last two 53 treatments. Population densities are expressed in transformed means.

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