Evaluation of Topramezone for Use in Rice (Oryza Sativa L.) Production

University of Arkansas, Fayetteville ScholarWorks@UARK

Theses and Dissertations

12-2019

Evaluation of Topramezone for Use in Rice (Oryza sativa L.) Production

Matthew Moore University of Arkansas, Fayetteville

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Citation Moore, M. (2019). Evaluation of Topramezone for Use in Rice (Oryza sativa L.) Production. Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/3436

This Thesis is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of ScholarWorks@UARK. For more information, please contact [email protected]. Evaluation of Topramezone for Use in Rice (Oryza sativa L.) Production

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Crop, Soil, and Environmental Science

Matthew H. Moore University of Arkansas Bachelor of Science in Crop Science, 2016

December 2019 University of Arkansas

This thesis is approved for recommendation to the Graduation Council.

Robert Scott, Ph.D. Jason Norsworthy, Ph.D. Thesis Director Co-Director

Jarrod Hardke, Ph.D. Edward Gbur, Ph.D. Committee Member Committee Member

Trenton Roberts, Ph.D. Committee Member

Abstract

The rapid development of resistant weeds, particularly to postemergence-applied herbicides, is a growing concern to rice producers worldwide. Barnyardgrass, one of the most problematic weeds in rice cropping systems, alone has been found to be resistant to four herbicide sites of action (SOA) in certain populations, leaving growers no options for control.

This unsettling fact has led to research on other SOAs that have not previously been used in US rice. One SOA that has garnered interest is the 4-hydroxyphenylpyruvate dioxygenase (HPPD)- inhibiting herbicides. The only HPPD-inhibitor that will soon be labeled for use in US rice is benzobicyclon. The major disadvantage benzobicyclon has when compared to many other herbicides is that it must be applied directly into water, causing the cost of application to be much higher due to the need for an aerial application. Therefore, topramezone, another HPPD- inhibitor used in corn production was researched to evaluate the herbicide compatibility of mixtures containing topramezone, its efficacy on barnyardgrass and weedy rice, the effect of different mixtures, rates, and cultivar on rice tolerance, and the tolerance of soybean to low doses of topramezone. In field trials, topramezone applied alone provided comparable barnyardgrass control to quinclorac, fenoxaprop, cyhalofop, and imazethapyr. All mixtures containing topramezone resulted in additive effects with the exception of those containing saflufenacil or propanil, which had antagonistic effects. When tested in the greenhouse on accessions collected throughout Arkansas, topramezone exhibited varying levels of control when applied to most weedy rice but proved to be highly effective on barnyardgrass accessions. A wide level of variation in injury to rice occurred regardless of mixture, application timing, rate, or rice cultivar from site year to site year where some applications resulted in minimal injury and others resulted in near complete crop loss. Low doses of topramezone may also affect soybean yield potential

even when plants have visibly recovered from injury. These data signify that while topramezone may be used to effectively control barnyardgrass, the risk for severe crop injury is substantial, thereby casting doubt on its potential use in rice cropping systems.

Acknowledgments

First and foremost, I would like to thank my parents, Danny and Paula Moore, for pushing me to further my education and believing in me even in the times when I was unsure that I believed in myself. Their steadfast support and constant encouragement have helped me through this chapter of my life more than they will ever know. I would also like to thank my advisors Drs. Robert

Scott and Jason Norsworthy for giving me this opportunity and expecting the best from me day in, day out. I have learned more in the past few years from this program than I ever had in my entire educational career and for that, I am forever thankful. To the friends I have met along the way: Although I cannot say that I am going to miss living out of a hotel for three months of the year, I am going to miss you all. Thank you for the support, answered questions, and the frequent laughter each of you has given me. Without my parents, Drs. Scott and Norsworthy, and my weed science colleagues, none of this would have been possible. I cannot put into words how much I appreciate each and every one of you.

Table of Contents Chapter 1. General Introduction ...... 1 Literature Cited ...... 10 Chapter 2. Efficacy of Herbicide Mixtures Containing Topramezone ...... 14 Abstract ...... 14 Introduction ...... 16 Materials and Methods ...... 18 Results and Discussion ...... 20 Conclusions ...... 21 Literature Cited ...... 23 Tables and Figures ...... 25 Chapter 3. Efficacy of Topramezone on Weedy Rice and Barnyardgrass Populations in Arkansas ...... 30 Abstract ...... 30 Introduction ...... 32 Materials and Methods ...... 35 Results and Discussion ...... 37 Conclusions ...... 40 Literature Cited ...... 41 Tables and Figures ...... 44 Chapter 4. Influence of Mixture, Rate Timing, and Cultivar on Rice Tolerance to Topramezone ...... 55 Abstract ...... 55 Introduction ...... 57 Materials and Methods ...... 59 Results ...... 63 Discussion ...... 69 Conclusions ...... 73 Literature Cited ...... 75 Tables and Figures ...... 78 Chapter 5. Soybean Sensitivity to Low Rates of Topramezone Applied Early- Postemergence ...... 97 Abstract ...... 97

Introduction ...... 99 Materials and Methods ...... 101 Results and Discussion ...... 103 Conclusions ...... 105 Literature Cited ...... 106 Tables and Figures ...... 109 General Conclusions ...... 116

List of Tables Chapter 2 Table 1. Observed and expected barnyardgrass control 2 and 4 WAT (weeks after treatment) following early postemergence applications of common rice herbicides applied alone or with 12 or 24 g ai ha-1 of topramezone; average of means at the University of Arkansas—Pine Bluff Research Farm in 2016 and 2017 and the University of Arkansas—Rice Research and Extension Center in 2017 ...... 25 Chapter 3 Table 1. Weedy rice accessions as characterized by county, coordinates, hull color, and presence of awns as well as % mortality and standard errors (SE) 4 weeks after treatment following applications of topramezone (24 g ae ha-1) at the 2- to 3-leaf stage ...... 44 Table 2. Percentage mortality of 101 weedy rice accessions to topramezone at 24 g ae ha-1 grouped by hull color and presence of awns ...... 47 Table 3. Responses of 81 barnyardgrass accessions 3 weeks after treatment following applications of topramezone (24 g ae ha-1) at the 2- to 3-leaf stage ...... 48 Table 4. Barnyardgrass accession distribution of visible control with respect to corresponding herbicide application at 3 weeks after treatment ...... 51 Chapter 4 Table 1. P values for rice injury on 10 commonly planted rice cultivars from HPPD-inhibiting herbicides conducted at UAPB and RREC in 2016 and 2017 ...... 78 Table 2. P values for rice height and yield from HPPD-inhibiting herbicides (topramezone, tembotrione, or mesotrione) applied on rice cultivars conducted at UAPB and RREC in 2016 and 2017 ...... 79 Table 3. Rice injury (2 and 4 weeks after treatment), height (at flowering), and grain yield as influenced by cultivar and HPPD-inhibiting herbicide at UAPB in 2016 ...... 80 Table 4. Rice injury (2 and 4 weeks after treatment), height (at flowering), and grain yield as influenced by cultivar and HPPD-inhibiting herbicide at UAPB in 2017 ...... 82 Table 5. Rice injury (2 and 4 weeks after treatment), height (at flowering), and grain yield as influenced by cultivar and HPPD-inhibiting herbicide at RREC in 2017 ...... 84 Table 6. P values for rice injury, height, and yield from topramezone mixture trials conducted at UAPB in 2016 and 2017 ...... 86 Table 7. Rice injury (2 weeks after treatment) caused by herbicides applied alone or with topramezone at 12 or 24 g ae ha-1 at UAPB in 2017 ...... 87 Table 8. Main effects from mixture tolerance tests conducted at UAPB in 2017 for responses that did not produce a herbicide by topramezone rate interaction at P=0.05 ...... 88

Table 9. P values for rice injury, height, and yield from topramezone rate and timing tolerance trials conducted at UAPB and RREC in 2017 ...... 89 Table 10. Rice injury (2, 3, and 4 weeks after treatment), height (at flowering), and grain yield after applications of topramezone at PREa, DPRE, EPOST, PRFLD, and POSTFLD timings at UAPB in 2017 ...... 90 Table 11. Rice injury (2, 3, and 4 weeks after treatment), height (taken at flowering), and grain yield after applications of topramezone at PRE, DPRE, EPOST, PRFLD, and POSTFLD timings at RREC in 2017 ...... 91 Chapter 5

Table 1. Effect of low-rate applications of bispyribac, benzobicyclon, or topramezone on soybean injury, height, and yield at the University of Arkansas-Newport Extension Center near Newport, AR and the University of Arkansas-Agricultural Research and Extension Center in Fayetteville, AR...... 109 Table 2. Rates at which injury (2 and 4 WAT) or height (4 WAT) intersect and diverge following applications of low-rates of topramezone, benzobicyclon, or bispyribac on soybean at AREC and NEC in 2017 at the 95% confidence interval ...... 110

List of Figures Chapter 2 Figure 1. Environmental conditions at the University of Arkansas—Pine Bluff research farm near Lonoke, AR, in 2017 beginning at planting. Treatments for this herbicide mixtures efficacy trial were applied on June 3, 2016. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx) ...... 27 Figure 2. Environmental conditions at the University of Arkansas—Pine Bluff research farm near Lonoke, AR, in 2017 beginning at planting. Treatments for this herbicide mixtures efficacy trial were applied on June 9, 2017. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx) ...... 28 Figure 3. Environmental conditions at the University of Arkansas—Rice Research and Extension Center near Stuttgart, AR in 2017 beginning at planting. Treatments for this herbicide mixtures efficacy trial were applied on June 8, 2017. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx) ...... 29 Chapter 3 Figure 1. County map of Arkansas visualizing the percent mortality of 101 weedy rice accessions following treatments of topramezone at 24 g ha-1 applied at the 2- to 3-leaf stage. Applications contained 1% v/v of crop oil concentrate ...... 52 Figure 2. Number of barnyardgrass accessions with resistance to topramezone and other commonly applied rice herbicides in Arkansas. Individual ovals signify a single herbicide. Consequently, overlapping ovals indicate that accessions within overlapping areas are resistant to each corresponding herbicide ...... 53 Figure 3. Distribution map of location and control of 81 barnyardgrass accessions following applications of topramezone at 24 g ae ha-1. Applications contained 1% v/v of crop oil concentrate and were applied at the 2- to 3-lf stage ...... 54 Chapter 4

Figure 1. Photosynthetically active radiation (PAR) (µmol m-2 s-1) 24 hours after application at Lonoke following treatments applied June 3, 2016; June 7, 2017; June 8, 2017; June 9, 2017 for herbicide mixture and cultivar tolerance trials conducted in 2016 and 2017. Data collected via the Arkansas State Plant Board Weather Web in W m-2 and transformed using the equation (W m-2 × 4.57 × 45% = PAR) where W m-2 = total solar radiation, 4.57 = the constant to convert W m-2 to µmol m-2 s-1, and 45% = the usable amount of sunlight, and PAR = photosynthetically radiation. (http://170.94.200.136/weather/Default.aspx) ...... 92 Figure 2. Total W m-2 accumulated 8 and 24 hours after application at Lonoke following treatments applied June 3, 2016; June 7, 2017; June 8, 2017; June 9, 2017 for herbicide mixture and cultivar tolerance trials conducted in 2016 and 2017. Data collected via the Arkansas State Plant Board Weather Web in W m-2 (http://170.94.200.136/weather/Default.aspx) ...... 93

Figure 3. Environmental conditions at the University of Arkansas Pine Bluff research farm near Lonoke, AR, in 2016 beginning at planting. Treatments for both the cultivar tolerance and herbicide mixtures tolerance trial were applied on June 3, 2016. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx) ...... 94 Figure 4. Environmental conditions at the University of Arkansas—Pine Bluff research farm near Lonoke, AR in 2017 beginning at planting. Application Timings—Cultivar tolerance—June 7, 2017; Herbicide mixtures tolerance—June 9, 2017; Timing and rate tolerance: PRE—May 17, 2017, DPRE—May 22, 2017, 1- to 2-lf—June 2, 2017, 4- to 5-lf PRFLD—June 22, 2017, 1 WK PSTFLD—July 6, 2017. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx) ...... 95 Figure 5. Environmental conditions at the University of Arkansas—Rice Research and Extension Center near Stuttgart, AR in 2017 beginning at cultivar tolerance planting; timing and rate experiment planted on June 8, 2017. Application Timings—Cultivar tolerance: June 8, 2017; Timing and rate: PRE—June 8, 2017, DPRE—June 12, 2017, 1- to 2-leaf—June 16, 2017, 4- to 5-leaf PRFLD—June 29, 2017, 1 WK PSTFLD—July 6, 2017. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx) ...... 96 Chapter 5 Figure 1. Soybean injury 2 weeks after treatment following applications of low-doses of bispyribac, benzobicyclon or topramezone at the vegetative third trifoliate growth stage. Rates are proportional to a typical rate of each herbicide (bispyribac at 28 g ai ha-1 benzobicyclon at 369 g ai ha-1, or topramezone at 24 g ae ha-1). Three parameter Gompertz model y = a*exp(- exp(-b(x-c))) where y is the response, a is the asymptote, b is the growth rate, and c is the inflection point. Parameter estimates and standard errors: Bispyribac—a=86.19(11.17), b=22.68(4.39), c=0.03(0.01); benzobicyclon—a=44.39(20.77), b=15.22(10.39), c=0.04(0.03); topramezone—a=48.43(3.57), b=37.66(7.8), c=0.01(0.002) ...... 111 Figure 2. Soybean injury 4 weeks after treatment following applications of low-doses of bispyribac, benzobicyclon or topramezone at the vegetative third trifoliate growth stage. Rates are proportional to a typical rate of each herbicide (bispyribac at 28 g ai ha-1 benzobicyclon at 369 g ai ha-1, or topramezone at 24 g ae ha-1). Three parameter Gompertz model y = a*exp(- exp(-b(x-c))) where y is the response, a is the asymptote, b is the growth rate, and c is the inflection point. Parameter estimates and standard errors: Bispyribac—a=69.40(30.78), b=15.74(5.06), c=0.09(0.03); benzobicyclon—a=190.82(7080.53), b=3.61(31.54), c=0.46(5.93); topramezone—a=27.51(7.78), b=19.66(6.92), c=0.05(0.02) ...... 112 Figure 3. Soybean canopy height reduction at 4 weeks after treatment following applications of low-doses of bispyribac, benzobicyclon, or topramezone at the vegetative third trifoliate growth stage. Three parameter exponential model y = a*exp(b-x) where y is the response, a is the asymptote, and b is the growth rate. Parameter estimates and standard errors: Bispyribac—a=- 2.94.19(3.33), b=141.48(99.06), c=0.74(0.31); benzobicyclon—a=-71.95(1471.44), b=83.68(1460.1), c=0.03(0.58); topramezone—a=-6.68(6.14), b=71.93(36.48), c=0.49(0.29) ...... 113 Figure 4. Environmental conditions at the University of Arkansas—Agricultural Research and Extension Center in Fayetteville, AR in 2017 beginning at herbicide application (June 6) ...... 114

Figure 5. Environmental conditions at the University of Arkansas—Newport Extension Center near Newport, AR in 2017 beginning at herbicide application (July 6) ...... 115

Chapter 1.

General Introduction

Rice (Oryza sativa L.) is one of the most important agricultural products grown around the world. In 2015, China was the leading producer of rice with 144,560,000 metric tons produced, while India was second with 104,800,000 metric tons produced. This accounted for roughly 52% of the 478,138,000 metric tons produced globally. The same year, the United

States (US) was the eleventh largest producer of rice in the world with 7,106,000 metric tons produced (USDA-FAS 2016). Of the six states that grow rice, Arkansas is the largest producer, growing 45% of US total production (NASS 2017).

One of the most expensive and problematic issues facing rice producers is a lack of acceptable weed control. Studies conducted in 2016 indicated that the average cost for herbicides used for weed control in Arkansas rice production was $167.68 ha-1, the third most expensive input for growers behind irrigation and fertilizer costs (Flanders 2015; Mazzanti et al. 2016).

This cost does not include the potential crop yield loss a field may incur due to the presence of weeds prior to postemergence (POST) applications. The four most problematic weeds in

Arkansas rice are barnyardgrass (Echinochloa crus-galli L. Beauv.), sprangletops (Leptochloa ssp.), northern jointvetch (Aeschynomene virginica L.), and weedy rice (Oryza sativa L.) (Carey et al. 1994; Norsworthy et al. 2013; Ottis et al. 2005; Smith et al. 1977). A further complication is limited herbicide sites of action (SOAs) labeled for use in US rice, leading to overuse of the same effective herbicides each year which, in turn, further selects for resistance (Norsworthy et al. 2012). If left unsuppressed, weeds may cause 44 to 96% yield loss in commercial rice fields

(Ampong-Narko and De Datta 1991). This significant yield loss is directly attributed to weeds competing with rice plants for sunlight, nutrients, and water. In addition, weedy plants host

insects and diseases that may hinder grain yields within a rice crop. The amount of yield loss is determined by many factors including weed species, duration of competition, weed density, rice cultivar characteristics, and cultural management factors that affect rice growth (Scott et al.

2013).

The most effective way to combat weeds within a problematic field is to implement an integrated weed management (IWM) system into the cropping system which utilizes cultural, chemical, and mechanical weed control tactics. These methods have changed significantly over the past 20 years since the introduction of genetically modified glyphosate-resistant soybean

(Glycine max L. Merr.). Genetically modified soybeans were quickly adopted into US cropping systems and similar genetic modifications were developed for other crops such as cotton

(Gossypium hirsutum L.) and corn (Zea mays L.). Glyphosate provided a simple, convenient, and low-cost option for weed management due to the herbicide’s non-selective systemic nature.

Quickly after the implementation of glyphosate-resistant crops, many growers stopped using

IWM practices and replaced them with repeated applications of the herbicide to control weeds

(Owen et al. 2014). Overuse of one site of action (SOA) led to weeds that were once controlled by glyphosate to become resistant (Duke and Powles 2009). Although glyphosate was not the first herbicide that weeds in US fields became resistant to, it was the first herbicide to gain international attention for resistance because of its wide adoption by growers (Powles 2008).

Since the resistance issues in glyphosate have occurred in recent years, growers have started to implement more IWM strategies in all crops to prevent further resistance of weeds and to preserve the herbicide technologies that are in use today (Norsworthy et al. 2012; Owen et al.

2014). By using these methods, farmers have found more effective ways to manage most herbicide-resistant (HR) weeds in row crops.

Weeds in rice can often be more troublesome than in row crops such as cotton, corn, and soybean due to limited HR traits available and no option of in-crop tillage after planting (Riar et al. 2013). The first step in a rice IWM system is to plant into weed-free fields, using weed-free crop seeds. Prospective rice fields should be prepared before planting through cultivation or herbicide burndown application with residual effects to ensure that the soil is as free of weeds as possible for as long as possible. Following burndown, herbicides containing multiple SOAs with

POST and residual activity should be applied every 2 weeks to further prevent weeds from emerging. Regular scouting from a crop consultant at least once a week is recommended to determine the presence and species of undesirable plants in a field. Weeds should be controlled shortly after their emergence to prevent yield loss due to weed interference. Once a permanent flood is established, new weeds will typically not emerge; therefore, applications of herbicides with residual activity may be terminated (Scott et al. 2013; Smith and Fox 1973). Although an

IWM system is the most effective way to suppress weeds, a problem arises once weeds develop resistance to the herbicides available for use in rice production.

Herbicide usage and flooding are the most common techniques used to control weeds in rice production. There are several SOAs available for use to control common weeds in rice today. However, some weeds have become resistant to commonly used herbicides. These include some of the most problematic weeds across the world such as barnyardgrass, weedy rice, rice flatsedge (Cyperus iria L.), Amazon sprangletop (Leptochloa panicoides L. Beauv.), yellow nutsedge (Cyperus esculentus L.), and several others (Heap 2018; Norsworthy et al. 2006).

Propanil was the first photosystem II (PSII) (Group 7) herbicide introduced for rice weed control in 1959 (Smith 1961). After its introduction, propanil was quickly adopted and applied liberally every year, often two or more times in a single growing season (Carey et al. 1995). The

ease of use and effectiveness of this herbicide allowed growers to control many problematic weeds such as barnyardgrass, broadleaves, sedges, and several other grasses after the rice crop had emerged (Scott et al. 2013). The continual over-application of the herbicide led to the first confirmed instance of propanil-resistant barnyardgrass in 1990 near Harrisburg, AR, and propanil resistance quickly spread to almost all rice growing counties in the state (Baltazar and

Smith 1994).

The next herbicide that problematic weeds developed resistance to in rice production was quinclorac, a synthetic auxin (Group 4). Introduced in 1992, quinclorac quickly became the standard in Arkansas rice fields for control of propanil-resistant barnyardgrass (Talbert and

Burgos 2007). The systemic nature of quinclorac makes it readily absorbed by germinating seeds, roots, and leaves where it is then translocated throughout the plant (Grossmann and

Kwiatkowski 1999). It is often used in rice to control susceptible barnyardgrass, broadleaf signalgrass (Urochloa platyphylla Griseb.), large crabgrass (Digitaria sanguinalis L.), and fall panicum (Panicum dichotomiflorum Michx.). However, much like with propanil, repeated uses of quinclorac over the span of almost a decade gave way for synthetic auxin-resistant barnyardgrass in 1999 (Lovelace et al. 2007).

Cyhalofop and fenoxaprop are both acetyl CoA carboxylase (ACCase) inhibitors (Group

1) and are currently recommended in rice for control of barnyardgrass, broadleaf signalgrass,

Amazon sprangletop, and other grass weeds (Scott et al. 2018). Both can be effective in controlling rice weeds when applied alone; however, they are often used in mixtures with other herbicides to provide greater weed control in rice without sacrificing yield (Talbert and Burgos

2007). Although ACCase resistance is not currently widespread, there has been confirmed

ACCase-resistant barnyardgrass in Mississippi (Heap 2018).

The first imidazolinone herbicide-resistant rice, also known as Clearfield® (CL) rice, came into the US market in 2002. Clearfield® varieties were bred from mutated rice plants that were found to be resistant to the Group 2 acetolactate synthase (ALS) inhibitors, 5acunose and imazethapyr (Buehring 2008; Sanders et al. 1998). The introduction of imazethapyr and

5acunose gave growers an option to control red rice, barnyardgrass, sprangletop, and several other resistant weeds (Burgos et al. 2008; Scott et al. 2018).

Since 2001, CL rice has been widely adopted into the growing system. Weedy rice had been a major problem in Arkansas rice fields before the introduction of CL rice. Weedy rice has the potential to reduce rice yields by 100 to 755 kg ha-1 for every weedy rice plant m-2 (Ottis et al

2005). Controlling weedy rice within commercial rice fields is extremely difficult because both plants belong to the same species; therefore, herbicides that control weedy rice likewise control commercially grown rice cultivars (Gealy et al. 2003; Sudianto et al. 2013). Because of Group 2 herbicide efficacy, farmers grow many resistant cultivars each year. In 2012, 61% of the

Arkansas rice acreage was planted in CL cultivars (Wilson et al. 2013).

The introduction of Group 2 herbicide-resistant rice has created concern that weedy rice will cross with the resistant rice cultivars and cause red rice to also become resistant to ALS inhibitors. Because the crop and weed are the same species they can outcross with each other and create CL-resistant weedy rice (Gealy et al. 2003). Although instances of natural outcrossing are low, a study conducted in Stuttgart, AR in 2002 and 2003 by Rajguru et al.

(2005) confirmed that the ALS genes could be transferred from resistant cultivated rice to weedy rice. To date, resistant biotypes from both outcrossing and selection pressure have been documented with respect to weedy rice (Burgos et al. 2014). In the summer of 2010 in Stuttgart,

AR, weedy rice samples were collected in commercial fields historically planted in CL cultivars.

The offspring of these samples were tested, with the majority of these weedy rice plants producing at least 20% ALS-resistant offspring (Burgos et al. 2014). In addition, the use of CL rice on a large percentage of hectares for the past 14 years has resulted in ALS resistance in barnyardgrass, yellow nutsedge, and rice flatsedge (Norsworthy et al. 2014; Riar et al. 2015;

Tehranchian et al. 2015).

Overall, herbicide-resistant weeds are a significant threat to rice producers across the globe. In Arkansas alone, barnyardgrass has evolved resistance to five SOAs, including PSII- inhibitors, synthetic auxins, diterpene synthesis (DOXP) inhibitors, acetyl-CoA carboxylase

(ACCase) inhibitors, and acetolactate synthase (ALS) inhibitors (Lovelace et al. 2007;

Norsworthy et al. 2012; Wilson 2009). Of the current herbicides available for use, barnyardgrass resistant to these five SOAs are susceptible only to pendimethalin (microtubule assembly- inhibitor) and thiobencarb (lipid synthesis-inhibitor), both of which may be applied only after cultivated rice has germinated because of crop injury risks (Smith 1988; Smith and Khodayari

1985). Without new SOAs, control of HR weeds (especially barnyardgrass and weedy rice) in rice cropping systems is extremely difficult and in some cases near impossible. Therefore, research is being conducted on alternative SOAs not currently labeled for use in rice to determine if herbicides typically applied in other crops may successfully be used in rice production.

One SOA that has garnered interest are herbicides in the 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting family. HPPD inhibitors, also known as Group 27 herbicides, block the catabolic degradation of the amino acid tyrosine to plastoquinones used in photosynthesis and vitamin E by inhibiting the HPPD enzyme. Inhibition of the enzyme causes the plant to cease development of chloroplasts in susceptible plants. Therefore, the HPPD

inhibitor cuts off energy production from photosynthesis. HPPD inhibitors also prevent carotenoid biosynthesis, making the plant vulnerable to ultraviolet rays which results in sensitive plants turning white as chlorophyll is destroyed (van Almsick 2009). There are three distinct families that make up Group 27 herbicides: pyrazolone, isoxazoles, and triketones (Grossman and Ehrhardt 2007; Scott et al. 2018).

The first HPPD inhibitors, pyrazolynate (a pyrazolone) and pyrazoxyfen (also a pyrazolone), were developed in the 1970s in Japan for rice weed control. However, to control rice weeds effectively, growers needed to use high application rates of 3 of 4 kg ai ha-1 (Ahrens et al. 2013). There are no Group 27 herbicides labeled for use in rice production in the US today.

Mesotrione is an HPPD inhibitor in the triketone family. It is labeled mainly for use in corn, some turf species, and other vegetable crops. It can be used PRE or POST on most labeled crops (Senseman 2007). Mesotrione is typically used for broadleaf control; however, it also controls certain annual grasses such as barnyardgrass, smooth crabgrass [Digitaria ischaemum

(Schreb.)], fall panicum, and large crabgrass (Soltani et al. 2011). In turf, mesotrione has been known to cause unwanted bleaching to non-target grass species. However, in a study reported in

2007 (Goddard et al.), it was shown that with the addition of triclopyr, a Group 4 herbicide, the bleaching effect on turf was reduced without compromising weed control.

Tembotrione, like mesotrione, is an HPPD inhibitor in the triketone family. It is currently registered for use in corn production in the US in POST applications or control of broadleaves and several annual grasses (Senseman 2007). Tembotrione is typically formulated with the safener isoxadifen-ethyl which protects corn from excessive damage (van Almsick

2009).

Topramezone is an HPPD inhibitor in the pyrazolone family. It is currently labeled for use in corn production and some turf species for POST weed control (Senseman 2007).

Topramezone controls broadleaves extremely well and has a greater spectrum of control for annual grass weeds than other HPPD-inhibiting herbicides (Grossman and Ehrhardt 2007;

Soltani et al. 2011). Topramezone is also the most biologically active of the Group 27 HPPD inhibitors, controlling weeds in corn cropping systems with as little as 12.5 g ae ha-1 whereas mesotrione requires 100 g ai ha-1 for comparable control (Soltani et al. 2011; van Almsick 2009).

In a study conducted in Tennessee, tall fescue (Festuca arundinacea Schreb.) was safened with topramezone and applied with triclopyr without sacrificing weed control, much as seen with mesotrione (Bronson et al. 2014; Goddard et al. 2007).

In recent years, benzobicyclon, which is a member of the triketone family in the HPPD- inhibiting herbicides, has been developed and tested for control of common rice weeds (Davis et al. 2014; van Almsick 2009). Benzobicyclon provides excellent control of barnyardgrass,

Amazon sprangletop, and other rice weeds in flooded ecosystems (Davis et al. 2014). However, in a Korean study, HPPD-inhibiting herbicides including mesotrione and benzobicyclon caused severe injury to multiple rice varieties, with japonica-type rice being slightly more tolerant than indica-type rice (Kwon et al. 2012). With both mesotrione and benzobicyclon being part of the triketone family of HPPD-inhibitors, it is important to further research other families of HPPD- inhibiting herbicides. There has been no published research on topramezone (a pyrazolone) use in rice production to this date. HPPD inhibitors in the pyrazolone family have already been used in rice crops in Japan but were found to be unsuccessful due to their high application rate which makes them uneconomical for growers to use on a large scale (Soltani et al. 2011; van Almsick

2009). Therefore, its efficacy on rice weeds along with high herbicidal activity relative to application rate could indicate that topramezone may provide a new SOA for rice production.

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Chapter 2

Efficacy of Herbicide Mixtures Containing Topramezone

Abstract

Postemergence control of barnyardgrass is one of the most difficult challenges faced by rice producers worldwide. Rapid development of resistance to multiple herbicide sites of action

(SOA) in barnyardgrass has led to the need for research on other SOA not commonly used in rice production. Hence, three field experiments were conducted to evaluate the 4- hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor, topramezone, applied alone or in combination with other labeled herbicides on barnyardgrass control in rice. Treatments containing topramezone alone (12 or 24 g ae ha-1) at the 2- to 3-leaf rice growth stage were not significantly different with at least 77% barnyardgrass control, when compared to quinclorac

(75%), fenoxaprop (82%), cyhalofop (71%), or imazethapyr (84%) applied alone. No synergism occurred for any of the mixtures within these experiments. When either rate of topramezone was combined with quinclorac (90% and 91%), clomazone (92% and 94%), or florpyrauxifen-benzyl

(92% and 97%), barnyardgrass control was greater than topramezone or the other herbicides applied alone. Mixtures containing topramezone and saflufenacil (63% and 71%) or propanil

(60% and 62%) were the only treatments considered antagonistic. Both saflufenacil and propanil, in combination with topramezone, reduced barnyardgrass control when compared with topramezone applied alone. Although most mixture partners were compatible for use in combination with topramezone for barnyardgrass control, mixtures containing topramezone and saflufenacil or propanil should be avoided due to antagonism concerns.

Nomenclature: Clomazone; cyhalofop; fenoxaprop; florpyrauxifen-benzyl; imazethapyr; propanil; saflufenacil; topramezone; barnyardgrass, Echinochloa crus-galli L. Beauv.; rice,

Oryza sativa L.

Key words: herbicide mixtures, antagonism, additive, synergistic

Introduction

In years past, there have been many herbicides introduced which were highly effective at controlling the most troublesome weeds such as barnyardgrass and weedy rice in commercial rice fields. However, the time between releases of these herbicides for public use have been spread across many years. For example, propanil, a photosystem II (PSII) inhibitor, was labeled for use in the US in 1959 (Brandes 1962; Talbert and Burgos 2007). At the time of its release, propanil was exceptional at controlling barnyardgrass, one of the most problematic weeds for rice in the US. No other rice herbicide as effective in controlling barnyardgrass postemergence

(POST) was released until quinclorac (synthetic auxin) in the early 1990s (Brandes 1962;

Kießling and Pfenning 1990). The gap between the labeling of these newer herbicides often caused growers to over-rely on propanil, many times making more than one application per year, until barnyardgrass was eventually confirmed resistant to propanil in Arkansas in 1990 (Baltazar and Smith 1994; Carey et al. 1995). Since then, barnyardgrass has developed resistance to several POST-applied herbicide SOAs, including synthetic auxins, acetyl-CoA carboxylase

(ACCase)-inhibitors, and acetolactate synthase (ALS)- inhibitors in the US alone (Fischer et al.

2000; Heap 2018; Lovelace et al. 2007; Wilson 2009). Although the occurrences of four-way multiple-resistance are known to be isolated, populations of barnyardgrass with resistance to two

SOA are common, putting high levels of selection pressure on the herbicides that are still effective (Heap 2018). Therefore, it is now more important than ever to investigate alternative

SOAs for use in rice cropping systems to ease the selection pressure of the herbicides that still provide high levels of control.

Herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) exhibit encouraging characteristics, which may indicate their potential for use in rice cropping systems.

Even though only one HPPD-inhibiting herbicide (benzobicyclon) is labeled for use in the US,

HPPD inhibitors have been used since the 1970s in Asian rice-producing countries (Ahrens et al.

2013; Anonymous 2017). The drawback of these early HPPD-inhibiting herbicides was that high use rates (3 to 4 kg ai ha-1) were essential for sufficient levels of weed control, making them uneconomical for use in large-scale rice farming operations such as those in the US (Ahrens et al. 2013). However, modern HPPD inhibitors are considerably more active on a per weight basis when compared to their predecessors, requiring approximately 94 to 99.4% less active ingredient

(Ahrens et al. 2013; Anonymous 2014a).

The HPPD SOA has three distinct classes: pyrazolones, isoxazoles, and triketones

(Grossman and Ehrhardt 2007). In the US, herbicides from all three classes are labeled for use in corn (Zea mays L.), including topramezone (pyrazolone), isoxaflutole (isoxazole), mesotrione

(triketone), and tembotrione (triketone). Historically, the early HPPD inhibitors used in Asian rice were from the pyrazolone family. Therefore, topramezone is the only pyrazolone HPPD inhibitor labeled in the US. The low use rate of topramezone (24 g ae ha-1) and high level of control of weeds commonly found in rice production, including barnyardgrass, makes a compelling argument that it could be an excellent chemical tool for rice farmers (Soltani et al.

2011).

Even though the HPPD inhibitor, benzobicyclon, is already labeled for use in California rice production, it has some disadvantages when compared to topramezone. Control of weeds with benzobicyclon is attainable only by applying the herbicide after permanent flooding occurs

(Davis et al. 2014; Sekino 2002). This required delayed timing is due to the active ingredient in benzobicyclon being formed via hydrolysis; only then is it made available to be taken up by foliage and root systems (Komatsubara et al. 2009). In contrast, topramezone is readily available

for uptake even with the absence of permanent flooding (Grossman and Ehrhardt 2007).

Consequently, topramezone application in rice is less costly than benzobicyclon application since topramezone is able to be applied via ground unlike benzobicyclon, which is applied aerially.

Weeds present prior to flooding are generally less mature than those following flood establishment, causing them to be more susceptible to herbicide applications.

Therefore, the objective of the following research was to evaluate the efficacy and compatibility of topramezone applied alone or in mixtures with commonly used rice herbicides on barnyardgrass in field situations.

Materials and Methods

Field experiments were conducted during 2016 and 2017 at the University of Arkansas—

Pine Bluff Research Farm near Lonoke, Arkansas (UAPB) and in 2017 at the University of

Arkansas Rice Research and Extension Center (RREC) near Stuttgart, Arkansas. The soil texture for both years consisted of Immanuel silt loam (fine-silty, mixed, active, thermic Oxyaquic

Glossudalfs) at UAPB and DeWitt silt loam (fine, smectitic, thermic Typic Albaqualfs) at RREC.

Plots measured 1.8 by 4.3 m in 2016 and 1.3 by 4.6 m in 2017. ClearfieldÒ (BASF Corporation,

Florham Park, NJ) rice cultivars ‘CL151’ (2016) and ‘CL172’ (2017) were seeded in 18-cm- wide rows using a tractor-mounted drill adjusted to deliver 82 seeds m-1 at a 1.8-cm depth.

Herbicide applications were made via a CO2-pressurized backpack sprayer with a handheld boom fitted with 110015 AIXR flat-fan nozzles (Teejet Technologies, Springfield, IL) calibrated to deliver 140 L ha-1 output volume at 276 kPa.

Trials were conducted as a two-factor factorial [topramezone rates (0, 12.5, or 24 g ae ha-

1) by mixture partner (quinclorac, fenoxaprop, cyhalofop, halosulfuron, propanil, imazethapyr, imazamox, saflufenacil, clomazone, florpyruaxifen-benzyl, or triclopyr)], randomized complete

block design with four replications. Treatments consisted of topramezone (ArmezonÒ, BASF corporation, Florham Park, NJ) applied alone or in combination with 11 rice herbicides, at recommended or predicted labeled rates, when the rice reached the 2- to 3-leaf stage to resemble a typical early postemergence (EPOST) application timing used by producers (Table 1). All applications, except those containing propanil, were mixed with crop oil concentrate at 1% v/v.

Herbicide treatments were applied June 3, 2016, and June 9, 2017, at UAPB, and June 8, 2017, at

RREC. Following application, visual estimates of barnyardgrass control were recorded 2 and 4 weeks after treatment on a 0 to 100% scale, where 0% represented no control and 100% represented complete weed control within the plot. Weather data for these experiments are contained in Figures 1, 2, and 3. However, environmental conditions following herbicide application did not affect herbicide activity on barnyardgrass control.

All data were subjected to two-way analysis of variance (ANOVA), and significant means were separated via Fisher’s protected LSD (P = 0.05) with block considered as a random effect. Data were combined over site years due to no significant treatment by year interaction

(P>0.05). Herbicide mixtures with topramezone were then determined to be either antagonistic, synergistic, or additive when combined with topramezone by utilizing Colby’s method as described by the equation

()+) ( = ) + + − 100 where X and Y are percent control of barnyardgrass with topramezone and each mixture partner applied alone, and E is the expected response when topramezone and the mixture partners are combined (Colby 1967). Mixtures were deemed antagonistic, synergistic, or additive by comparing expected and observed responses using t-tests (P = 0.05). Antagonistic mixtures significantly lower the amount of control of at least one of the herbicides applied alone whereas

synergistic mixtures significantly raise the level of control compared to both herbicides applied alone, and additive effects neither significantly raise or lower the control of either herbicides.

Results and Discussion

Topramezone applied alone provided moderate levels of barnyardgrass control with 77% visible control at 12 g ae ha-1 and 78% control at 24 g ae ha-1 at 2 WAT (Table 1). Although not exceptional, the level of control that topramezone provided when applied alone was on par with herbicides currently recommended for barnyardgrass management at 2 WAT such as quinclorac

(75%), fenoxaprop (82%), cyhalofop (71%), and imazethapyr (84%) (Scott et al. 2018) (Table

1). The sub-optimal level of control observed by herbicides typically more effective at controlling barnyardgrass may be attributed to the excessive density of barnyardgrass present at the time of application, due to the fields being left fallow once every other year and letting the weeds go to seed: UAPB 2016—103 plants m-2; UAPB 2017—202 plants m-2; RREC—100 plants m-2. The high plant density likely limited the amount of spray solution reaching the target surface due to the barnyardgrass and rice overlapping and physically blocking smaller plants, thus reducing the amount of herbicide being received by the plant and reducing efficacy (Hogar et al. 2004; Winkle et al. 1981). Furthermore, the barnyardgrass present in the field trials conducted were often times 4- to 5-leaf stage at the time of application, larger than recommended by the labels from the herbicides stated above at the EPOST timing (Anonymous 2011, 2014b,

2016a, 2016b). However, when mixed with either rate of topramezone, control observed with quinclorac, clomazone, or florpyrauxifen-benzyl was greater than that of topramezone or the individual herbicide applied alone according to two-way ANOVA (Table 1). Although halosulfuron and triclopyr had little effect on barnyardgrass control (27% and 30% at 2 WAT),

when either herbicide was mixed with topramezone at 24 g ae ha-1 both combinations provided greater control (>90%) than when any of the three herbicides were applied alone (Table 1).

Studies conducted on control of common bermudagrass (Cynodon dactylon L.) in tall fescue

(Festuca arundinacea Shchreb.) resulted in similar findings, which indicated that the combination of topramezone and halosulfuron or triclopyr controlled bermudagrass greater than either applied alone (Brosnan and Breeden 2012). Furthermore, Young et al. (2018) reported that, in some cases, halosulfuron mixed with benzobicyclon, another HPPD-inhibiting herbicide, improved barnyardgrass control over benzobicyclon alone.

Although several herbicides exhibited greater control when mixed with topramezone than applied alone, according to Colby’s method, no mixtures were synergistic. All but two herbicides were additive (Table 1). The two that were antagonistic rather than additive when mixed with topramezone were saflufenacil and propanil, the only contact herbicides in the experiment. The antagonism observed following application of these mixtures is likely due to the rapid destruction of leaf tissue caused by saflufenacil and propanil, which can prevent the uptake and translocation of topramezone, an interaction often observed between systemic and contact herbicides (Green 1989). In addition, careful consideration of the weather data presented in

Figures 1, 2 and 3 indicate that environmental conditions had little impact on the visible control data. Though the conditions were different in each run of this study, barnyardgrass was controlled similarly each time, indicating that barnyardgrass is highly susceptible to topramezone.

Conclusions

The results obtained from these field trials indicate that many of the herbicides commonly used in rice production are compatible in mixtures containing topramezone. However, treatments

containing saflufenacil and propanil should be avoided due to the antagonism of topramezone when barnyardgrass is the target weed. Furthermore, although not considered synergistic by

Colby’s method, growers can increase barnyardgrass control with mixtures containing quinclorac, fenoxaprop, cyhalofop, halosulfuron, imazamox, clomazone, florpyrauxifen-benzyl, or triclopyr by adding topramezone to the spray tank. This research signifies that while topramezone has never been used in rice production before, it can be used in controlling barnyardgrass, a key weed in rice production systems. More field research should be conducted on other weeds such as weedy rice to determine the effectiveness of topramezone on other species of weeds and rice tolerance.

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Tables and Figures Table 1. Observed and expected barnyardgrass control 2 and 4 WAT (weeks after treatment) following early postemergence applications of common rice herbicides applied alone or with 12 or 24 g ai ha-1 of topramezone; average of means at the University of Arkansas—Pine Bluff Research Farm in 2016 and 2017 and the University of Arkansas—Rice Research and Extension Center in 2017a. Barnyardgrass controlbc Application rate 2 WAT 4 WAT Herbicide Herbicide Topramezone Observed Expected Observed Expected g ai or ae ha-1 ------% ------Topramezone - 0 ------

12 77 defg - 72 efghi -

24 78 defg - 73 efghi - Quinclorac 420 0 75 efgh - 68 ghi -

12 90 abc 91 86 abcde 89

24 91 abc 92 90 abc 90 25 Fenoxaprop 120 0 82 cdefg - 76 defghi -

12 88 abcd 92 87 abcd 88

24 92 abc 94 92 abc 92 Cyhalofop 315 0 71 ghij - 63 hij -

12 88 abcd 90 84 abcdef 85

24 93 abc 91 88 abcd 89 Halosulfuron 30 0 27 m - 20 n -

12 74 efghi 81 52 jkl 77

24 91 abc 83 87 abcd 83 Propanil 4490 0 44 kl - 17 n -

12 60 j 84 26 mn 75

24 62 ij 85 39 lm 78 Imazethapyr 105 0 84 bcdef - 79 cdefg -

12 88 abcd 95 80 cdefg 88 24 91 abc 96 89 abcd 93

Table 1. Cont. Barnyardgrass controlbc Application rate 2 WAT 4 WAT Herbicide Herbicide Topramezone Observed Expected Observed Expected g ai or ae ha-1 ------% ------Imazamox 45 0 63 ij - 60 ijk -

12 88 abcd 91 78 cdefg 85

24 89 abcd 92 81 bcdefg 87 Saflufenacil 25 0 40 kl - 16 n -

12 63 ij 85 41 l 77

24 71 fghij 85 49 kl 78 Clomazone 340 0 63 ij - 39 lm -

12 92 abc 88 88 abcd 79

26 24 94 ab 90 96 a 82 Florpyruaxifen-benzyl 30 0 65 hij - 71 fghi -

12 92 abc 90 87 abcd 89

24 97 a 90 95 ab 91 Triclopyr 280 0 30 lm - 18 n -

12 83 bcdef 83 79 cdefg 75 24 91 abc 83 91 abc 82 a All treatments contained 1% v/v crop oil concentrate except those containing propanil. b Means followed by different letters in the “observed” column indicate significant differences in barnyardgrass control. c Expected columns are derived from Colby’s method—E = X + Y — (XY)/100. Bolded values indicate significant differences obtained via two-sided t-test between the observed and expected figures for each treatment.

40 8.0 Precipitation Low Temp High Temp 7.5 35 7.0 6.5 30 6.0 5.5 25 5.0 4.5 20 4.0 3.5 15 3.0 Temperature (C) Temperature Precipitation (cm) Precipitation 2.5 10 2.0

27 1.5

5 1.0 0.5 0 0.0 6/1/16 6/3/16 6/5/16 6/7/16 6/9/16 7/1/16 7/3/16 7/5/16 7/7/16 7/9/16 5/18/16 5/20/16 5/22/16 5/24/16 5/26/16 5/28/16 5/30/16 6/11/16 6/13/16 6/15/16 6/17/16 6/19/16 6/21/16 6/23/16 6/25/16 6/27/16 6/29/16 7/11/16 7/13/16 7/15/16 7/17/16 Date

Figure 1. Environmental conditions at the University of Arkansas—Pine Bluff research farm near Lonoke, AR, in 2017 beginning at planting. Treatments for this herbicide mixtures efficacy trial were applied on June 3, 2016. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx)

40 8.0 Precipitation Low Temp High Temp 7.5 35 7.0 6.5 30 6.0 5.5 25 5.0 4.5 20 4.0 3.5 15 3.0 Temperature (C) Temperature Precipitation (cm) Precipitation 2.5 10 2.0 28 1.5

5 1.0 0.5 0 0.0 6/2/17 6/4/17 6/6/17 6/8/17 7/2/17 7/4/17 7/6/17 7/8/17 5/17/17 5/19/17 5/21/17 5/23/17 5/25/17 5/27/17 5/29/17 5/31/17 6/10/17 6/12/17 6/14/17 6/16/17 6/18/17 6/20/17 6/22/17 6/24/17 6/26/17 6/28/17 6/30/17 7/10/17 7/12/17 7/14/17 7/16/17 Date

Figure 2. Environmental conditions at the University of Arkansas—Pine Bluff research farm near Lonoke, AR, in 2017 beginning at planting. Treatments for this herbicide mixtures efficacy trial were applied on June 9, 2017. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx)

40 8.0 Precipitation Low Temp High Temp 7.5 35 7.0 6.5 30 6.0 5.5 25 5.0 4.5 20 4.0 3.5 15 3.0 Temperature (ºC) Temperature Precipitation (cm) Precipitation 2.5 10 2.0 29 1.5

5 1.0 0.5 0 0.0 6/1/17 6/3/17 6/5/17 6/7/17 6/9/17 7/1/17 7/3/17 7/5/17 7/7/17 7/9/17 5/18/17 5/20/17 5/22/17 5/24/17 5/26/17 5/28/17 5/30/17 6/11/17 6/13/17 6/15/17 6/17/17 6/19/17 6/21/17 6/23/17 6/25/17 6/27/17 6/29/17 7/11/17 7/13/17 Date

Figure 3. Environmental conditions at the University of Arkansas—Rice Research and Extension Center near Stuttgart, AR in 2017 beginning at planting. Treatments for this herbicide mixtures efficacy trial were applied on June 8, 2017. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx)

Chapter 3

Efficacy of Topramezone on Weedy Rice and Barnyardgrass Populations in Arkansas

Abstract

Two of the most troubling weeds in rice production, weedy rice and barnyardgrass, are rapidly developing resistance to herbicide sites of action (SOA) that were once effective.

Therefore, new SOA are needed to combat further development of herbicide resistance in these weeds. In 2017 and 2018, greenhouse experiments were conducted to evaluate the efficacy of topramezone applied at 24 g ae ha-1, a 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide that is not currently used in rice production, on 101 weedy rice and 81 barnyardgrass accessions collected throughout Arkansas. Results varied in the weedy rice accession experiment conducted in a greenhouse. For 31 of these accessions, mortality of weedy rice differed by at least 50% between blocks. Although all plants were treated in the same fashion at the same time and were placed in the same greenhouse, the position of each accession is suspected to have caused this wide variation in mortality due to external buildings blocking natural sunlight in some areas causing different temperatures and photosynthetically active radiation (PAR) throughout the greenhouse. Similar results were observed in a separate field experiment, which studied injury following applications of topramezone on 10 rice cultivars among site years. This implies that minute changes in environmental conditions can have a significant impact on topramezone efficacy regardless of whether it is utilized for weedy rice or cultivated rice. In the barnyardgrass experiment, the efficacy of topramezone was compared to propanil, quinclorac, fenoxaprop, and imazethapyr. Only imazethapyr was more effective at controlling barnyardgrass than topramezone. Propanil was the least effective, with 63 accessions exhibiting less than 75% control of barnyardgrass followed by quinclorac (43), fenoxaprop (14), topramezone (7), and

imazethapyr (5). Results from these experiments suggest that topramezone may be highly effective in controlling barnyardgrass, but rice producers cannot confidently rely on it for controlling weedy rice. Further research should be conducted on weedy rice to determine the effect of environmental conditions on topramezone efficacy.

Nomenclature: topramezone; weedy rice, Oryza sativa L.; barnyardgrass, Echinochloa crus- galli L. Beauv.; rice, Oryza sativa L.

Key words: topramezone, HPPD, rice, barnyardgrass, weedy rice

Introduction

Weedy rice and barnyardgrass are two of the toughest weeds to control in rice cropping systems on every rice cultivating continent (Catala et al. 2002; Gealy 2005; Holm et al. 1977;

Olajumoke et al. 2016). Depending on the density of each weed, both may cause a 5 to 100% reduction in yield loss in cultivated fields (Ottis et al. 2005; Ottis and Talbert 2007; Smith 1988).

If not mitigated through herbicide programs or crop rotation, a commercial rice farm that has even a small area infested with either weedy rice or barnyardgrass has the potential to infest the entire field from which it originated as well as other fields. This may be due to both weeds producing a large number of seeds, mechanical harvesters casting the seeds large distances during the harvesting process, movement of seed from field to field on unclean machines, and movement through water passages (Walsh and Powles 2007). One of the principal reasons that weedy rice and barnyardgrass are especially hard to control is the narrow range of effective herbicide sites of action (SOA) that do not cause injury to the rice crop.

Weedy rice is classified in the same genus and species as cultivated rice; therefore, there were no herbicide SOAs that were able to control the weed postemergence until the introduction of imidazolinone-resistant cultivars in the United States in 2002 (Craigmiles 1978; Tan et al.

2004). Before then, cultural controls such as crop rotation into soybean (Glycine max L. Merr.) or other crops where applications of herbicides that can control weedy rice may be applied were used to curb the weedy rice population (Huey and Baldwin 1978). In the years following the release of imidazolinone-resistant rice cultivars until 2012, the number of total hectares of these cultivars rapidly increased to over 65% of the rice in the southern United States (Linscombe

2015). Although this technology has significantly helped control weedy rice populations in commercial rice programs, instances of weedy rice that are resistant to imidazolinone herbicides

are on the rise (Burgos et al. 2008; Goulart et al. 2013). The increase in these resistant populations is due to both the outcrossing of weedy rice with imidazolinone-resistant cultivars and a lack of SOA diversity in weed control programs (Burgos et al. 2014; Scott et al. 2018).

Unlike in weedy rice, there have been several herbicide SOAs within the past 60 years that have been effective in controlling barnyardgrass. However, large gaps between the discovery and commercial release of these herbicides caused growers to over-apply single SOAs; this inadvertently selected for resistant barnyardgrass populations (Talbert and Burgos 2007).

Currently, populations of barnyardgrass are known to be resistant to acetolactate synthase inhibitors (ALS), synthetic auxins, cellulose inhibitors, microtubule inhibitors, photosystem II inhibitors (PSII), long chain fatty acid inhibitors, lipid inhibitors, acetyl-CoA carboxylase

(ACCase) inhibitors, and 1-deoxy-D-xylulose 5-phosphate synthase (DOXP) inhibitors (Heap

2018). Furthermore, some of these populations are resistant to multiple SOA (up to four SOA;

Mississippi, US) (Heap 2018). The loss of a SOA causes immense selection pressure on the remaining effective herbicides, which will eventually lead to resistance to these herbicides, leaving no herbicide options for controlling barnyardgrass.

The most effective way to mitigate herbicide resistance in both weedy rice and barnyardgrass is to implement best management practices (BMPs), which combine cultural, mechanical, and chemical strategies to control weeds (Norsworthy et al. 2012). One of the main pillars in the chemical portion of practicing BMPs is rotation of herbicide SOAs; however, this may be difficult or impossible to accomplish if there are only a few herbicides still effective on the target weeds (Norsworthy et al. 2012). The increasing occurrences of weeds developing resistance to commercially available herbicides along with the decrease in the discovery of new

herbicide SOAs makes finding alternative means of controlling resistant weeds increasingly important (Peters and Strek 2018).

One solution to lessening the selection on effective herbicides is to investigate the adoption of herbicides with different SOA not labeled for use in crops where herbicide-resistant

(HR) weeds are becoming an issue. In the cases of weedy rice and barnyardgrass in commercial rice fields, there has been an increasing interest in using 4-hydoxyphenolpyruvate dioxygenase

(HPPD)-inhibiting herbicides, which are commonly used in corn (Zea mays L.). The first HPPD- inhibiting herbicide labeled for use in the United States (California only) was benzobicyclon in

2017 (Anonymous 2017). The success of benzobicyclon has brought attention to other herbicides in the HPPD-inhibiting SOA, more specifically topramezone. Unlike benzobicyclon, which may be effectively applied only post-flood, topramezone could be used in pre-flood situations.

Therefore, instead of needing an aerial applicator to apply the herbicide, the growers may apply the herbicide themselves via ground application thus reducing the cost of application.

Topramezone has also been shown to control weeds commonly found in rice growing systems including broadleaves and annual grasses (barnyardgrass); however, the effect of topramezone on weedy rice is currently unknown (Soltani et al. 2011). If labeled for use in rice, topramezone would contribute an alternative SOA that may be used in rotation with other effective SOA for controlling HR barnyardgrass and potentially, weedy rice.

The objective of this research was to evaluate the efficacy of topramezone on barnyardgrass and weedy rice accessions collected throughout Arkansas.

Materials and Methods

Weedy Rice.

In the fall of 2016, panicles of 101 weedy rice accessions in 19 Arkansas counties were collected from rice fields. At the time of collection, coordinates were recorded via the mobile application My GPS Coordinates Pro (TappiAppsÓ) utilizing the global positioning system contained in an iPhone 7 (Apple Inc., Cupertino, CA). Sample locations were determined by searching for the presence of weedy rice within commercial rice fields or were contributed by

University of Arkansas Division of Agriculture county agents and crop consultants in the state.

The presence of herbicide resistance was unknown throughout this research. Following collection, accessions were threshed, taxonomically identified (presence of awn; hull color), and stored in a refrigerated room (4.5 C) for approximately 6 months before being taken out and stored at room temperature for ~2 months so that dormancy would not hinder germination (Table

1).

In April of the following year, a greenhouse experiment was initiated at the University of

Arkansas Division of Agriculture—Lonoke County Extension Office in Lonoke, Arkansas. Prior to planting, rectangular polypropylene trays (23 by 16.5 cm) were filled with potting mix (Sun

Gro Horticulture, Seba Beach, Canada) and misted with water to prepare a seedbed. The experiment was arranged in a randomized complete block with two replications. Twenty-five seeds from a single accession were planted in each tray and watered every day until the conclusion of the experiment. Temperatures in the greenhouse ranged from 32 C during the day to 22 C at night. At the end of the first week, plants were thinned so that every tray contained 20 weedy rice plants. When plants reached the 2- to 3-leaf stage, trays were placed side-by-side and

-1 sprayed with topramezone (24 g ae ha ) and 1% v/v crop oil concentrate (COC) utilizing a CO2-

pressurized backpack sprayer outfitted with a handheld boom fitted with 110015 AIXR flat-fan nozzles (Teejet Technologies, Springfield, IL) calibrated to deliver 140 L ha-1 output volume at

276 kPa. Trays were then placed back into their corresponding positions inside the greenhouse.

Following application, live plants were counted 4 weeks after treatment (WAT). Data were then analyzed using three one-way analysis of variance (ANOVA) in JMP Pro 12.1 (SAS

Institute Inc. Cary, NC 27513) where the treatments represented sample number, hull color, or presence of awns. The response was percent mortality. A frequency table as well as a distribution map were then created to display differential tolerances of accessions to topramezone and to visualize the spatial distribution of percent mortality (Table 2 and Figure 1). Due to drastic variations in mortality among blocks and runs, accessions could not be statistically evaluated accurately; therefore, means and standard error were reported for each individual accession

(Table 1).

Barnyardgrass.

In the fall of 2017, 81 barnyardgrass samples were collected and threshed in the same manner as stated in the previous section (Table 3). Following threshing, accessions were stored at room temperature for approximately 2 months before planting. On October 7th of the same year, the greenhouse experiment was initiated at the University of Arkansas – Altheimer laboratory in Fayetteville, AR and conducted again on November 6th. The experiment was arranged as a randomized complete block with four replications. Prior to herbicide treatments, trays were prepared in a similar fashion as the previous experiment; however, instead of planting

25 seeds, an unknown amount of seed was planted in each tray. When seedling barnyardgrass started to emerge, three plants from each accession were transplanted to six individual 12-cm- diameter pots for each replication. The plants were then given one week to recover from

transplanting. On October 16th as the plants reached the 2- to 3-leaf stage, each accession was treated with either topramezone (24 g ae ha-1), propanil (4480 g ai ha-1), fenoxaprop (120 g ai ha-

1), quinclorac (420 g ai ha-1), or imazethapyr (105 g ai ha-1). All herbicide treatments contained

1% v/v crop oil concentrate (COC), with the exception of propanil. A nontreated was also included for comparison while rating visible control. Treatments were applied using a spray chamber outfitted with a two-nozzle boom with flat-fan 800067 nozzles (TeeJet Technologies,

Springfield, IL) calibrated to deliver 187 L ha-1 at 276 kPa.

Visual estimates of control of each accession relative to each nontreated as well as the number of live plants were recorded 2 and 3 WAT (0% = no injury, 100% = complete control;

0% = all plants alive, 100% = complete mortality). Accessions that averaged less than 75% control were considered likely resistant to the corresponding herbicide and were recorded via a frequency table, distribution map, and Venn diagram (Table 4; Figure 2; Figure 3). Prior to preparing the frequency table and distribution map, accessions were categorized into groups based on their average percent control (0 to 50%, 51 to 75%, 76 to 99%, and 100%). A multinomial logit regression analysis utilizing the GLIMMIX procedure in SAS 9.4 (SAS

Institute Inc., Cary, NC 27513) was also performed with groupings as a response, herbicide as the treatment, and sample number as a random effect.

Results and Discussion

Weedy Rice.

In 31 accessions treated with topramezone, mortality differed by 50% or greater between the blocks (data not shown). Although the data from the frequency table and distribution map suggests that 30 accessions were controlled by 75% or more, only 12 were consistently controlled by at least 75% and 30 were consistently controlled by 74% or less when taking

standard error in consideration (Tables 1 and 2, Figure 1). For example, weedy rice population

WR5 expressed 75% mortality, but with a standard error of 25, indicating a tremendous amount of variability in response of this accession to topramezone (Table 1). However, in that same table population WR92 from Poinsett County clearly expressed a high mortality rate (93%) with a SE of only 3.

Environmental conditions following applications of HPPD-inhibitors are known to affect herbicide uptake and translocations within plants (Grossman and Ehrhardt 2007; Johnson and

Young 2002; Shulte and Köcher 2009). Although most conditions were stable throughout the entirety of the experiment due to the positioning of the greenhouse, natural sunlight was obstructed by another building for several hours of the day. Even though high-pressure sodium lamps were used for supplemental lighting, natural sunlight typically emits between 2- to 3-times the amount of photosynthetically active radiation (Donnelly and Fisher 2002). A study conducted by Grossman and Ehrardt (2007) on corn (Zea mays L.), giant foxtail (Setaria faberi L.), and

European black nightshade (Solanum nigrum L.) indicated that as light intensity decreased, the uptake and translocation of topramezone throughout the plants in the hours following application likewise decreased. This could have adverse effects on efficacy if the plant does not receive a lethal dose of herbicide because of reduced uptake or if the plant metabolizes the herbicide faster than a lethal dose can translocate throughout the plant. Similar results occurred in a field study conducted in 2016 and 2017 at the University of Arkansas—Pine Bluff Research Farm and in

2017 at the University of Arkansas—Rice Research and Extension Center where large differences in injury were observed between site years when topramezone was applied at 24 g ae ha-1 to ten commercially available rice cultivars: the wide variation among site years in this study was attributed to varying application timings throughout the day causing some site years to

receive differing levels of sunlight and temperature in the following hours after application

(Moore et al. 2018). Results from this study and the experiments conducted herein lead to the suggestion that rice and weedy rice, regardless of phenotypic characteristics or population, is somewhat tolerant to topramezone. Therefore, any reduction in uptake and translocation may have a detrimental effect on control.

Barnyardgrass.

Out of the 81 barnyardgrass accessions evaluated, only 13 were deemed susceptible to all herbicides tested by the baseline of 75% or greater visible control (Figure 2). As expected, due to its longevity as a labeled herbicide for use in Arkansas rice cropping systems, accessions were most widely resistant to propanil (63) followed by quinclorac (43), fenoxaprop (14), topramezone (7), and imazethapyr (5) with 44 samples resistant to two or more of these herbicides (Table 4; Figure 2). Instances of herbicide resistance in this study were somewhat higher than seen in a study conducted by Rouse et al. (2017) from 2006 to 2016; however, collection was conducted differently between the two studies. The level of control for each herbicide was directly related to the number of resistant populations, with imazethapyr providing the greatest level of control followed by topramezone, fenoxaprop, quinclorac, and propanil

Table 4; Figure 3) Of the 81 accessions applied with topramezone, 29 were controlled 74% or less in terms of mortality (Table 3). However, only 7 were 75% or less in visual estimations of control (Table 4). Therefore, while some accessions were not completely eliminated, most were severely injured to the point at which they would likely not cause interference in an in-field situation (Table 3). In addition, across the 81 populations evaluated topramezone visually controlled barnyardgrass populations more completely than all other herbicides except imazethapyr, which controlled 76 of 81 populations 100% (Table 4).

Conclusions

The results obtained in these trials indicate that while there is potential for topramezone use as an early postemergence herbicide for barnyardgrass, the efficacy of topramezone on weedy rice is erratic. Therefore, if labeled for use in commercial rice cropping systems, growers may expect to have a high level of control for barnyardgrass but should use an alternative SOA or practice a form of cultural control such as crop rotation for curbing weedy rice populations. In addition, further research needs to be conducted to assess the effect of environmental conditions

(including light and temperature intensity) on weedy rice when treated with topramezone.

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Tables and Figures

Table 1. Weedy rice accessions as characterized by county, coordinates, hull color, and presence of awns as well as % mortality and standard errors (SE) 4 weeks after treatment following applications of topramezone (24 g ae ha-1) at the 2- to 3-leaf stagea Hull Awns Mortalityb Accession County Latitude Longitude color Yes/No % SE WR1 Desha 33.89222 -91.42690 Straw Yes 70 10 WR2 Clay 36.39195 -90.49305 Straw No 75 20 WR3 Cross 35.31990 -90.52490 Black Yes 83 18 WR4 Clay 36.34972 -90.52500 Straw No 85 5 WR5 Greene 35.99862 -90.44167 Straw No 75 25 WR6 Phillips 34.50612 -90.77445 Black Yes 65 5 WR7 Cross 35.41945 -90.99862 Mixed Yes 63 33 WR8 Clay 36.39250 -90.48055 Straw No 63 38 WR9 Crittenden 35.40945 -90.30472 Black Yes 45 45 WR10 Greene 36.23612 -90.53500 Straw No 73 3 WR11 Greene 36.23112 -90.44055 Straw No 60 25 WR12 Poinsett 35.55028 -90.48805 Black Yes 78 23 WR13 Cross 35.40028 -90.61833 Mixed Yes 60 20 WR14 Craighead 35.93960 -90.43670 Black Yes 88 8 WR15 Poinsett 35.47778 -90.46722 Straw No 60 30 WR16 Phillips 34.47417 -90.83250 Straw No 80 20 WR17 Cross 35.39722 -90.99362 Mixed Yes 15 15 WR18 Mississippi 35.91612 -90.22917 Black Yes 58 8 WR19 Poinsett 35.55000 -90.84362 Straw Yes 80 10 WR20 Chicot 33.40167 -91.31528 Straw No 80 10 WR21 Arkansas 34.08112 -91.37278 Black Yes 73 8 WR22 Poinsett 35.57815 -90.41953 Straw No 63 38 WR23 Poinsett 35.45695 -90.55167 Straw No 50 20 WR24 Drew 33.49333 -91.50138 Straw No 23 8 WR25 Poinsett 35.49055 -90.48388 Straw No 43 33 WR26 Jefferson 34.19528 -91.90528 Straw No 83 18 WR27 Chicot 33.48805 -91.34862 Straw Yes 43 8 WR28 Drew 33.50500 -91.51888 Straw No 83 3 WR29 Arkansas 34.03555 -91.36972 Black Yes 65 5 WR30 Drew 33.50000 -91.51222 Straw No 45 15 WR31 Lincoln 34.09222 -91.72445 Straw No 45 15 WR32 Drew 33.49333 -91.50555 Straw No 48 18 WR33 Drew 33.75888 -91.46305 Straw No 50 30 WR34 Chicot 33.46833 -91.34055 Straw Yes 58 43

Table 1. Cont. Hull Awns Mortalityb Accession County Latitude Longitude color Yes/No % SE

WR35 Phillips 34.49612 -90.68000 Black Yes 55 20 WR36 Poinsett 35.47388 -90.44888 Straw No 50 40 WR37 Phillips 34.47805 -90.78112 Straw No 60 10 WR38 Crittenden 35.41000 -90.29362 Black Yes 43 33 WR39 Crittenden 35.42472 -90.34695 Black Yes 45 10 WR40 Chicot 33.34917 -91.34750 Straw No 50 20 WR41 Poinsett 35.48528 -90.45778 Straw No 70 15 WR42 Drew 33.52833 -91.53278 Straw No 88 13 WR43 Mississippi 35.64770 -90.18135 Straw No 68 33 WR44 Crittenden 35.40972 -90.31417 Black Yes 30 15 WR45 Arkansas 34.32138 -91.15528 Black Yes 50 20 WR46 Desha 33.92555 -91.40112 Black Yes 55 20 WR47 Greene 35.97417 -90.44195 Straw No 58 18 WR48 St. Francis 35.05722 -90.40222 Mixed Yes 90 10 WR49 Phillips 34.33668 -91.00708 Straw No 43 23 WR50 Phillips 34.32653 -90.95475 Black Yes 53 23 WR51 Phillips 34.40243 -90.69038 Straw No 68 18 WR52 Phillips 34.31403 -90.86337 Black Yes 75 25 WR53 Phillips 34.47098 -90.79922 Straw No 70 15 WR54 Lee 34.70305 -91.00917 Straw No 55 25 WR55 Lee 34.70917 -91.04972 Straw No 85 0 WR56 Lee 34.76388 -90.86528 Straw No 45 0 WR57 Lee 34.77778 -91.04972 Black Yes 53 48 WR58 Lee 34.85722 -90.98778 Black Yes 83 3 WR59 Crittenden 35.40968 -90.24877 Straw No 85 5 WR60 Mississippi 35.72552 -90.00216 Black Yes 45 45 WR61 Mississippi 35.71455 -90.03293 Straw No 70 20 WR62 Crittenden 35.41138 -90.49780 Straw No 88 3 WR63 Mississippi 35.71455 -90.02856 Straw No 85 5 WR64 Mississippi 35.72373 -90.01976 Straw No 90 10 WR65 Mississippi 35.43957 -90.23127 Straw No 65 5 WR66 Mississippi 35.79520 -90.07360 Mixed Yes 83 13 WR67 Cross 35.26528 -90.60733 Mixed Yes 23 13 WR68 Mississippi 35.98707 -90.00100 Straw No 48 18 WR69 Mississippi 35.80368 -90.17384 Straw No 83 8 WR70 Mississippi 35.81373 -90.07384 Mixed Yes 68 3 Table 1. Cont.

Hull Awns Mortalityb Accession County Latitude Longitude color Yes/No % SE

WR71 Mississippi 35.98693 -90.00090 Straw No 63 3 WR72 Greene 36.14785 -90.72072 Black Yes 55 45 WR73 Greene 36.19860 -90.74378 Straw No 83 18 WR74 Greene 36.14270 -90.71914 Straw Yes 58 33 WR75 Greene 36.05475 -90.72709 Straw No 53 28 WR76 Greene 36.19467 -90.74408 Black Yes 50 40 WR77 Greene 36.09428 -90.37802 Straw No 50 10 WR78 Greene 36.11836 -90.76234 Black Yes 70 15 WR79 Greene 36.19592 -90.74748 Straw No 43 28 WR80 Greene 36.02309 -90.81116 Straw No 70 25 WR81 Greene 36.05161 -90.71337 Straw No 68 28 WR82 Greene 36.16052 -90.66238 Black Yes 53 28 WR83 Greene 36.15320 -90.66260 Straw No 60 30 WR84 Greene 36.05183 -90.43446 Straw No 50 20 WR85 Conway 35.18139 -92.81111 Straw No 83 8 WR86 Conway 35.18139 -92.81917 Straw No 78 13 WR87 Conway 35.15583 -92.82444 Straw No 55 35 WR88 Poinsett 35.62152 -90.75030 Black Yes 55 40 WR89 Poinsett 35.64257 -90.71317 Straw No 20 5 WR90 Poinsett 35.49196 -90.91373 Straw No 43 3 WR91 Poinsett 35.64738 -90.77502 Straw No 88 13 WR92 Poinsett 35.55401 -90.48740 Straw No 93 3 WR93 Clay 36.39144 -90.42190 Straw No 88 3 WR94 Clay 36.39105 -90.39951 Straw No 53 18 WR95 Clay 36.39222 -90.49349 Straw No 83 8 WR96 Clay 36.43271 -90.35404 Straw No 55 35 WR97 Clay 36.40966 -90.88354 Straw No 45 35 WR98 Lonoke 34.51200 -91.87570 Black Yes 43 33 WR99 Lafayette 33.07912 -93.77061 Straw No 45 35 WR100 Lafayette 33.06460 -93.78014 Straw No 50 10 WR101 Lafayette 33.06505 -93.78678 Straw No 35 20 a Applications contained 1% v/v of crop oil concentrate. b Mortality and standard errors calculated as percent of plants alive at 4 WAT relative to plants alive (20) at treatment application.

Table 2. Percentage mortality of 101 weedy rice accessions to topramezone at 24 g ae ha-1 grouped by hull color and presence of awnsa Awn Number of Mortalityb Hull color presence accessions 0-25% 26-50% 51-75% 76-100% Straw Yes 5 0 1 3 2 No 64 2 12 29 21 Both 69 2 13 32 2 3 Black Yes 25 0 6 14 5

Mixed Yes 7 2 0 3 2 All Yes 37 2 7 20 8 a Topramezone applications contained 1% v/v of crop oil concentrate. b Mortality was calculated as percent of plants alive at 4 WAT relative to plants alive (20) at treatment application.

Table 3. Responses of 81 barnyardgrass accessions 3 weeks after treatment following applications of topramezone (24 g ae ha-1)a at the 2- to 3-leaf stage. Mortalityb Controlc Accession County Latitude Longitude % SE % SE B1 Lonoke 34.511373 -91.948450 83 10 95 3 B2 Jefferson 34.304721 -91.871205 75 8 89 4 B3 Ashley 33.269082 -91.654495 100 - 100 - B4 Chicot 33.271934 -91.442626 100 - 100 - B5 Chicot 33.311309 -91.356700 100 - 100 - B6 Chicot 33.347288 -91.280438 92 8 96 4 B7 Chicot 33.398624 -91.312171 100 - 100 - B8 Chicot 33.468517 -91.339649 75 8 89 4 B9 Desha 33.605948 -91.275142 83 10 91 5 B10 Arkansas 34.070087 -91.372892 75 8 85 5 B11 Arkansas 34.197967 -91.307556 100 - 100 - B12 Jefferson 34.394893 -91.903546 83 10 90 6 B13 Arkansas 34.462468 -91.403329 100 - 100 - B14 Arkansas 34.537310 -91.584142 100 - 100 - B15 Lonoke 34.845236 -91.881794 100 - 100 - B16 Prairie 34.998801 -91.672891 100 - 100 - B17 Prairie 34.978065 -91.571995 83 10 93 5 B18 Prairie 35.006256 -91.446012 92 8 95 5 B19 Woodruff 35.005666 -91.271690 100 - 100 - B20 Monroe 34.823669 -91.254854 100 - 100 - B21 Monroe 34.605314 -91.195766 100 - 100 - B22 Monroe 34.691562 -91.179575 83 10 95 4 B23 St. Francis 35.135497 -90.847487 100 - 100 - B24 St. Francis 35.119092 -90.927258 100 - 100 - B25 St. Francis 35.140500 -91.006888 92 8 96 4 B26 Cross 35.192894 -90.945996 67 14 84 6 B27 Cross 35.439459 -90.751872 100 - 100 - B28 Greene 35.986746 -90.441945 100 - 100 - B29 Greene 36.052485 -90.457757 100 - 100 - B30 Greene 36.115176 -90.439335 83 10 96 2 B31 Greene 36.221892 -90.355938 100 - 100 - B32 Clay 36.367320 -90.197826 92 8 95 5 B33 Clay 36.367774 -90.197869 75 8 91 3 B34 Clay 36.434875 -90.442106 100 - 100 - B35 Craighead 35.843722 -90.838339 67 - 79 7

Table 3. Cont Mortalityb Controlc Accession County Latitude Longitude % SE % SE B36 Craighead 35.733420 -90.984712 83 10 89 7 B37 Jackson 35.651631 -91.097788 92 8 98 3 B38 Jackson 35.564451 -91.357869 100 - 100 - B39 Poinsett 35.558977 -90.570494 83 10 96 2 B40 Poinsett 35.472027 -90.343608 83 10 93 6 B41 Crittenden 35.281280 -90.228436 75 8 88 6 B42 Mississippi 35.535883 -90.121338 92 8 98 3 B43 Mississippi 35.807423 -90.074660 100 - 100 - B44 Poinsett 35.618208 -90.369647 92 8 95 5 B45 Mississippi 35.588400 -89.999388 100 - 100 - B46 Mississippi 35.582680 -89.991272 100 - 100 - B47 Poinsett 35.499138 -90.336138 83 10 90 6 B48 Crittenden 35.232905 -90.357738 100 - 100 - B49 Crittenden 35.188920 -90.391500 83 10 98 1 B50 Crittenden 35.360241 -90.273586 100 - 100 - B51 Crittenden 35.356847 -90.323519 92 8 99 1 B52 Crittenden 35.145141 -90.255445 92 8 95 5 B53 Mississippi 35.430000 -90.226000 92 8 93 8 B54 Mississippi 35.423000 -90.221000 92 8 94 6 B55 Poinsett 35.554546 -90.607593 8 8 43 3 B56 Craighead 35.913000 -90.819000 100 - 100 - B57 Craighead 35.787000 -90.916000 92 8 98 3 B58 Phillips 34.512600 -90.671300 92 8 95 5 B59 Phillips 34.405050 -90.729000 100 - 100 - B60 Phillips 34.428400 -90.683683 83 10 85 9 B61 Poinsett 35.529826 -90.834031 33 24 63 18 B62 Poinsett 35.565019 -90.844549 92 8 96 4 B63 Poinsett 35.656250 -90.713388 83 10 84 9 B64 Poinsett 35.571511 -90.749765 83 10 93 5 B65 Poinsett 35.693230 -90.729890 100 - 100 - B66 Monroe 34.633700 -91.189900 42 8 71 11 B67 Monroe 34.916400 -91.221200 75 8 86 6 B68 Monroe 34.802500 -91.196300 100 - 100 - B69 Monroe 34.915000 -91.171500 8 8 13 9 B70 Monroe 34.721100 -91.179800 67 - 81 1 B71 Crittenden 35.340000 -90.390000 50 17 68 11 B72 Poinsett 35.580711 -90.559853 33 14 55 9

Table 3. Cont. Mortalityb Controlc Accession County Latitude Longitude % SE % SE B73 Poinsett 35.553336 -90.545889 100 - 100 - B74 Mississippi 35.994172 -89.987894 100 - 100 - B75 Poinsett 35.485572 -90.618636 100 - 100 - B76 Mississippi 35.675536 -90.213722 75 8 86 6 B77 Mississippi 35.596386 -90.258789 92 8 99 1 B78 Poinsett 35.531503 -90.343617 83 10 91 5 B79 Poinsett 35.535175 -90.379708 92 8 94 6 B80 Lawrence 35.906135 -91.228412 33 14 59 10 B81 Lawrence 36.065953 -90.852307 92 8 95 5 a Applications contained 1% v/v of crop oil concentrate. b Mortality and standard errors were calculated as percent of plants alive at 3 WAT relative to plants alive at treatment application (3). c Control and standard errors were calculated as percent control relative to the nontreated for each accession where 0% is no control and 100% is complete plant death.

Table 4. Barnyardgrass accession distribution of visible control with respect to corresponding herbicide application at 3 weeks after treatment. Herbicidea Rate Controlb g ae or ai ha-1 0-50% 51-75% 76-99% 100%

Propanil 4480 49 14 14 4

Quinclorac 420 39 4 14 24

Fenoxaprop 120 9 5 26 41

Imazethapyr 105 2 2 1 76

Topramezone 24 2 5 42 32 a Herbicide applications included 1% v/v of crop oil concentrate with exception of propanil. b Visible control relative to nontreated of each accession where 0% is no control and 100% is complete plant mortality.

Legend 0 - 25 % 26 - 50% 51 - 75% 76 - 100%

Figure 4. County map of Arkansas visualizing the percent mortality of 101 weedy rice accessions following treatments of topramezone at 24 g ha-1 applied at the 2- to 3-leaf stage. Applications contained 1% v/v of crop oil concentrate. [Graph Builder; US Counties; JMP Pro 12.1 (SAS Institute Inc. Cary, NC 27513)]

Figure 5. Number of barnyardgrass accessions with resistance to topramezone and other commonly applied rice herbicides in Arkansas. Individual ovals signify a single herbicide. Consequently, overlapping ovals indicate that accessions within overlapping areas are resistant to each corresponding herbicide.

Legend 0 - 50% 51 - 75% 76 - 99% 100%

Figure 6. Distribution map of location and control of 81 barnyardgrass accessions following applications of topramezone at 24 g ae ha-1. Applications contained 1% v/v of crop oil concentrate and were applied at the 2- to 3-lf stage. [Graph Builder; US Counties; JMP Pro 12.1 (SAS Institute Inc. Cary, NC 27513)]

Chapter 4

Influence of Mixture, Rate, Timing, and Cultivar on Rice Tolerance to Topramezone

Abstract

The introduction of benzobicyclon for US rice production has led to the study of other 4- hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides for potential in-crop use.

Topramezone, in particular, would provide producers with the benefit of low use rates and high efficacy on weeds, including broadleaves as well as annual grasses, if found safe for use in rice.

Topramezone could be especially valuable to producers who are dealing with barnyardgrass resistance to multiple sites of action (SOA). Three field experiments were conducted to evaluate rice tolerance to 1) common rice herbicides applied alone or in combination with topramezone,

2) topramezone applied at different timings and rates, and 3) topramezone compared to two other

HPPD-inhibiting herbicides applied to ten commonly planted rice cultivars. Results from these studies varied substantially by year in the amount of injury seen from topramezone in each trial conducted. For example, in the topramezone mixtures trial in 2016, <4% injury was observed with all herbicide mixtures applied at the 2- to 3-leaf rice stage. However, in 2017 at the same location, there were large differences between the 2 years where florpyrauxifen-benzyl + topramezone (24 g ae ha-1) caused 35% injury at 2 weeks after treatment (WAT). For the rate and timing trials, injury 2 WAT in 2017 at the University of Arkansas Rice Research and Extension

Center (RREC) following topramezone at 12 and 24 g ae ha-1 was 97% and 98%, respectively, when applied at 1- to 2-leaf rice, 15% and 46% when applied at 4- to 5-leaf rice, and 31% and

33% when applied at 1 week postflood. Injury from the same treatments, topramezone at 12 and

24 g ae ha-1 at the three growth stages, at the University of Arkansas Pine Bluff Research Farm

(UAPB) were 0 and 13%, 1 and 3%, and 18 and 28%, respectively. In the cultivar tolerance trial

in 2016, visible injury to all cultivars was 1 to 4% from topramezone applied at 2 WAT. In the trial in 2017 at the RREC, applications of topramezone resulted in seven of the ten cultivars exhibiting 24 to 45% injury 2 WAT. In 2017 at the UAPB, injury was >45% for all cultivars.

Similar variation in injury levels occurred in the topramezone mixture and rate and timing trials.

Differences in injury at each site year following treatments containing topramezone may be due to varying environmental conditions in the subsequent hours after application; however, it cannot be confirmed through the trials conducted here. These studies indicate that although topramezone may sometimes be applied without crop injury, severe injury can occur, which brings into question the future of topramezone in rice.

Nomenclature: topramezone; corn, Zea mays L.; rice, Oryza sativa L.

Key words: topramezone, HPPD, rice, mixtures, rate, timing, cultivar, tolerance

Introduction

The increasing number of weeds resistant to commonly used rice herbicides has resulted in research focused on alternative sites of actions (SOAs) that are typically used in other crops.

One SOA that has shown promise in rice cropping systems is the 4-hydroxyphenolpyruvate dioxygenase (HPPD)-inhibiting herbicides. HPPD inhibitors have been used in Asian rice production as early as the 1970s; however, these early herbicides required extremely high application rates (3 to 4 kg ai ha-1) for adequate weed control (Ahrens et al. 2013). In the US, benzobicyclon is the only HPPD-inhibiting herbicide labeled for use and is labeled only in

Californian rice production (Anonymous 2017). Benzobicyclon controls many broadleaves, sedges, and some annual grass weeds, but has been shown to have marginal control of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] in rice cropping systems (Davis et al. 2013;

Young et al. 2016). The efficacy of benzobicyclon is also largely dependent on flood depth, making pre-flood applications not possible for weed control (Davis et al. 2013; Sekino 2002).

The success of benzobicyclon has garnered interest in researching other HPPD-inhibiting herbicides in rice. Of these herbicides, one that has shown promising results in early testing is topramezone (Moore et al. 2017; Scott et al. 2016). Topramezone is an HPPD-inhibiting herbicide currently labeled for use in US corn and some turfgrass species. Much like other

HPPD-inhibitors, topramezone has been used for control of broadleaves and annual grasses, including a high level of control of barnyardgrass (Soltani et al. 2011). In contrast to benzobicyclon, activity of topramezone is not dependent on flooded field conditions. Although research has been thoroughly documented on rice tolerance to some HPPD-inhibiting herbicides, little has been done with topramezone. Often, herbicides within the same SOA can produce significantly different crop tolerance outcomes. For example, dicamba and 2,4-D are safe and

labeled for use on corn, but severe injury occurs when quinclorac is applied even though the three herbicides are all synthetic auxin herbicides (Sunohara et al. 1999).

Environmental conditions during and after application have been documented to influence the efficacy of HPPD-inhibiting herbicides as well as crop tolerance. Foliar uptake of

HPPD-inhibitors is extremely rapid. A study conducted by Grossman and Ehrardt (2007) on corn, giant foxtail (Setaria faberi Herrm.), and European black nightshade (Solanum nigrum L.) found that temperature, relative humidity, and amount of photosynthetically active radiation

(PAR) received in the following hours after application (HAA) were associated with the level of foliar uptake and translocation of the herbicide within the plant. Results from their study indicated that as the temperature, relative humidity, and amount of PAR received by the plant increased, the more topramezone was absorbed and transferred throughout the plant (Grossman and Ehrardt 2007). Although selectivity of topramezone is not typically attributed to decreased foliar uptake and translocation, plants with moderate tolerance to the herbicide can be affected by environmental impacts that reduce adsorption and movement throughout the organism

(Coupland 1987; Hammerton 1967).

Rapid metabolism and lower enzyme target site sensitivity of plants are most associated with increased tolerance to HPPD-inhibiting herbicides (Ahrens et al. 2013; Godar et al. 2015;

Grossman and Ehrardt 2007; Mitchell et al. 2001). Environmental factors affecting metabolism of HPPD-inhibitors differ, as a general trend, between C3 and C4 plants. C4 plants such as corn,

Palmer amaranth (Amaranthus palmeri (S.) Wats.), common waterhemp (Amaranthus rudis

Saur.), and large crabgrass (Digitaria sanguinalis L.) exhibit lower herbicide metabolism as temperatures decreases, resulting in elevated injury when applied with mesotrione, another

HPPD-inhibitor (Godar et al. 2015; Johnson and Young 2002). In contrast, C3 plants including

velvetleaf (Abutilon theophrasti Medik.) and common cocklebur (Xanthium strumarium L.) express higher levels of injury as temperature increases (Johnson and Young 2002). Differences in injury between photosynthetic pathways were attributed to C4 plants having a greater increase in growth rate and metabolism as temperature increased in comparison to C3 plants.

Concurrently, injury at higher temperatures in C3 plants was associated with higher foliar uptake of mesotrione. Target-site sensitivity to HPPD-inhibitors vary among plant species, with monocots typically being less sensitive than dicots (Ndikuryayo et al. 2017; Siehl et al. 2014).

If found safe for application on rice, topramezone could provide a new SOA for use in controlling key weeds including barnyardgrass. Therefore, the objectives of this research were to evaluate the tolerance of rice to 1) topramezone compared to two other HPPD-inhibiting herbicides applied to ten commonly planted rice cultivars 2)common rice herbicides applied alone or in combination with topramezone, and 3) topramezone applied at different timings and rates.

Materials and Methods

In 2016 and 2017, field experiments were conducted to evaluate the tolerance of rice to topramezone. All trials were conducted as two-factor factorials in a randomized complete block design. Treatments were applied using a CO2-pressurized backpack sprayer fitted with a handheld boom with 110015 AIXR flat-fan nozzles (Teejet Technologies, Springfield, IL) calibrated to deliver 140 L ha-1 at 276 kPa. Furthermore, all experiments contained herein were fertilized in accordance with the Arkansas Rice Production Handbook (Hardke 2012)

Visible crop injury estimations were taken every 2 weeks for 4 weeks following each treatment in the cultivar tolerance and topramezone mixture trials and once a week for 4 weeks following each treatment for the application timing trial. Ratings were taken on a 0 to 100%

scale, where 0% indicates no injury and 100% represents complete crop death. Canopy heights

(cm) were measured in all studies when the rice reached the flowering stage by laying a corrugated fiberboard across three areas within each plot and measuring where the fiberboard rested on top of the canopy using a standard meter stick. Heights from the three measurements were then averaged within each plot. At maturity, yield data were collected via small plot combine, weighed, and grain moisture was adjusted to 13%.

Because of variation in environmental conditions (Figures 1-5), all years and/or sites for each trial were analyzed separately. Cultivar and herbicide (Tables 1-5), companion herbicide and topramezone rate (Tables 6-8), or timing and topramezone rate (Tables 9-11) were analyzed as fixed effects using block as a random effect. Nontreated plots as well as treatments where no injury was detected in all blocks were excluded from the analysis. All data were then subjected to two-way analysis of variance (ANOVA) with significant means separated using Fisher’s protected LSD (P = 0.05) in SAS 9.4 (SAS Institute Inc., Cary, NC) utilizing the GLIMMIX procedure. For treatments that did not produce a significant two-way interaction, main factors were reported when significant. Injury for the cultivar tolerance study as well as heights and yields were fitted with normal distribution and no transformations. Heights and yields in the cultivar tolerance trial were analyzed within each respective cultivar due to varietal differences between these two factors (Table 2). In the mixture, application timing, and rate tolerance trials, injury data were transformed using log(& − 1) (Tables 6-11). Data were then fitted using a beta ' distribution. After analysis, injury data were back-transformed for representation in Tables 7, 8,

10, and 11.

Varietal Tolerance to HPPD-inhibiting Herbicides.

Field experiments were conducted in 2016 and 2017 at the University of Arkansas—Pine

Bluff research farm (UAPB) and in 2017 at the Rice Research and Extension Center (RREC).

Soil texture was an Immanuel silt loam (fine-silty, mixed, active, thermic Oxyaquic Glossudalfs) at UAPB and a DeWitt silt loam (fine, smectitic, thermic Type Albaqualfs) at RREC. Plots in

2016 at UAPB were maintained weed free with a preemergence (PRE) application of quinclorac

(FacetÒ L, BASF Corporation, Florham Park, NJ) at 420 g ai ha-1 followed by (fb) a POST application of propanil + thiobencarb (RicebeauxÒ, RiceCo, Memphis, TN) at 3360 + 3360 g ai ha-1 fb a postflood (POSTFLD) application of cyhalofop (ClincherÒ SF, DOW AgroSciences,

Indianapolis, IN) at 313 g ai ha-1. In 2017, at the same location, plots were maintained weed free using a PRE application of clomazone (CommandÒ 3ME, FMC Corporation, Philadelphia, PA) + quinclorac (FacetÒ L, BASF corporation, Florham Park, NJ) at 263 + 420 g ai ha-1 fb a POST

Ò application of pendimethalin (Prowl H2O, BASF corporation, Florham Park, NJ) + thiobencarb

(BoleroÒ 8 EC, Valent U.S.A. Corporation, Walnut Creek, CA) at 1066 + 2805 g ai ha-1). At the

RREC location, weeds were managed with PRE applications of clomazone + quinclorac at 263 +

420 g ai ha-1.

Each trial was cone planted with 10 rice cultivars grown in Arkansas rice production including: Roy J, Diamond, LaKast, Jupiter, Titan, Rondo CL151, CL172, CLXL745, and

XL753. Based on previous studies conducted by Kwon et al. (2012) and Young et al. (2017) of the cultivars studied, Rondo was the only pureline cultivar thought to have a high level of indica background, which is known to be highly susceptible to benzobicyclon, whereas all others were either japonica or japonica X indica crosses. Treatments were applied at the EPOST application timing and included: mesotrione at 210 g ai ha-1, tembotrione at 185 g ai ha-1, or topramezone at

24 g ae ha-1. A nontreated for each variety was included for comparison. In all site years, pureline cultivars were planted at 72 seed m-1 row and hybrid cultivars at 23 seed m-1 row with

18 cm between each row at a 1.8-cm depth. Herbicide treatments were applied on June 3, 2016

(UAPB), June 7, 2017 (UAPB), and June 8, 2017 (RREC).

Tolerance to Mixtures.

Field experiments were conducted during 2016 and 2017 at UAPB. In 2016, plots measured 1.8 by 4.3 m, plots in 2017 were 1.3 by 4.6m. ClearfieldÒ (BASF corporation,

Florham Park, NJ) rice cultivars ‘CL151’ (2016) and ‘CL172’ (2017) were seeded in 18-cm- wide rows using a tractor-mounted drill adjusted to deliver 82 seeds m-1 at a 1.8-cm depth. Plots were maintained in the same fashion as the previous trial at the UAPB except the addition of imazethapyr (Newpath, BASF corporation, Florham Park, NJ) at 70 g ai ha-1to the last treatment of pendimethalin and thiobencarb in 2017.

Treatments consisted of topramezone (ArmezonÒ, BASF corporation, Florham Park, NJ) applied at 0, 12, or 24 g ae ha-1 alone and in combination with 11 rice herbicides currently or soon to be labeled when the rice reached the 2- to 3-leaf stage, which resembles a typical early-

POST (EPOST) application timing used by producers (Table 7). Herbicide treatments were applied on June 3, 2016 and June 9, 2017, respectively.

Application Timing and Rate.

Field studies were conducted in 2016 and 2017 at UAPB and in 2017 at the Rice

Research and Extension Center (RREC) near Stuttgart, AR. Experiments conducted were identical in soil series, cultivar, and size of those conducted in the previous studies at each corresponding location. CL172 rice was seeded on 18-cm-wide rows with a tractor-mounted drill at a rate of 86 seed m-1 into plots measuring 2.1 by 4.6 m-1. Plots near Stuttgart were maintained weed free with clomazone (CommandÒ 3ME, FMC Corporation, Philadelphia, PA) + quinclorac

(FacetÒ L, BASF corporation, Florham Park, NJ) at 263 + 420 g ai ha-1 applied PRE and

followed by a POSTFLD application of cyhalofop (ClincherÒ SF, DOW AgroSciences,

Indianapolis, IN) at 313 g ai ha-1 whereas plots at UAPB were maintained in identical fashion to the herbicide mixtures study.

Topramezone at 12 and 24 g ae ha-1 was applied PRE, 5 days after planting (DPRE), at the 2- to 3-leaf stage (EPOST), 5- to 6-leaf stage—preflood (PREFLD), or 1 week after flood

(POSTFLD) timings. Topramezone treatments were applied on the following days: PRE on May

18, 2016, May 17, 2017 (UAPB), and June 9, 2017 (RREC); DPRE on May 23, 2016, May 24,

2017 (UAPB), and June 14, 2017 (RREC); EPOST on June 3, 2016, May 30, 2017 (UAPB), and

June 19, 2017 (RREC); PREFLD on June 13, 2016, June 22, 2017 (UAPB), and June 29, 2017

(RREC); POSTFLD on July 13, 2016, July 6, 2017 (UAPB), and July 6, 2017 (RREC).

Nontreated plots were also included in each trial for comparison.

Results

Varietal Tolerance to HPPD-inhibiting Herbicides.

A significant herbicide by rice cultivar interaction was observed for all site years at both

2 and 4 WAT (Table 1). However, there were significant differences with regard to the extent of injury between site years. UAPB (2016) had the least injury followed by RREC in 2017, which had moderate levels of injury and the highest injury was observed at UAPB in 2017 (Tables 3-5).

The rice cultivar Rondo was the most sensitive cultivar at both RREC and UAPB in 2017; regardless of which herbicide was applied, where injury was at least 90% at 4 WAT (Tables 4 and 5). In 2016, the indica cultivar, Rondo, exhibited differing levels of tolerance to each herbicide, with mesotrione causing the greatest injury (97%) followed by tembotrione (76%) and then topramezone (4%) at 2 WAT (Table 3). With the exception of Rondo, all other inbred rice cultivars treated with HPPD-inhibiting herbicides had less than 10% injury at both assessments

in 2016. The hybrid cultivars acted similarly to Rondo, with treatments containing topramezone resulting in < 4% injury. Hybrid cultivars treated with tembotrione had 11 to 18% injury from topramezone at both assessments, and applications containing mesotrione were greater than tembotrione (20 to 33%) at both 2 and 4 WAT (Table 3).

A significant herbicide main effect was observed for height on Roy J, Diamond, LaKast,

Jupiter, and Rondo (Table 1). For cultivars Roy J, Diamond, and LaKast, treatments containing tembotrione caused the greatest reduction in height by 8, 9, and 7 cm, respectively (Table 3).

Topramezone applied to LaKast resulted in height reduction, albeit to a lesser extent than tembotrione. For the cultivars Jupiter and Rondo, the greatest decrease in height occurred following applications of mesotrione (4 and 15 cm decrease); Rondo rice was also reduced with applications of topramezone (6 cm decrease). Other treatments did not result in height reductions relative to the corresponding nontreated (Table 2). Rondo was the only cultivar to exhibit yield losses when applied with any HPPD-inhibiting herbicide, with topramezone and tembotrione applications being less damaging than mesotrione when compared to the nontreated (1730, 2380, and 6030 kg ha-1 reductions) (Tables 2 and 3).

In 2017 at UAPB, visible crop injury was considerably different from at UAPB in 2016

(Tables 9 and 10). The cultivar Rondo was most injured by HPPD-inhibiting herbicides with over 90% injury for all treatments at either injury assessment date (Table 4). However, in addition to treatments applied to Rondo, tembotrione also resulted in the greatest injury when applied to CL172 or XL753. As a general trend, treatments containing tembotrione were most injurious to each cultivar with over 80% injury detected when applied to any cultivar 2 WAT and at least 75% injury at 4 WAT. Following tembotrione, mesotrione was most injurious with at least 80% injury at 2 WAT when applied to Jupiter, Rondo, CL172, CLXL745, or XL753 and at

least 75% observed at 4 WAT on Rondo, CLXL745, or XL753. With exception to Rondo, topramezone applications on non-imidazolinone-resistant cultivars resulted in the least amount of injury at either rating. Jupiter and Titan were most tolerant to topramezone with 45% and 49% injury assessed 2 WAT and 14% and 33% injury observed 4 WAT. Rice treated with topramezone also recovered more quickly than treatments containing tembotrione or mesotrione when severe visible injury was observed, e.g., XL753 recovered by 37% from 2 to 4 WAT as opposed to tembotrione (4%) or mesotrione (7%).

Heights and yields were reduced by at least one HPPD inhibitor for all cultivars in 2017 at UAPB (Table 4). Canopy height for Roy J, Diamond, LaKast, Jupiter, Titan, or CL151 was not reduced when treated with topramezone relative to the nontreated. However, these cultivars along with Rondo, CL172, CLXL745 and XL753 resulted in less reduction in height following applications of topramezone than with mesotrione or tembotrione. Treatments containing topramezone did not result in the greatest height reduction when applied to any cultivar; tembotrione resulted in the greatest reduction on Titan, CL151, and CL172 and mesotrione applications caused the most reduction of Jupiter, Rondo, and CL172. Similar to height, grain yield was also reduced by at least one HPPD-inhibiting herbicide in all cultivars (Table 8). Roy

J, Diamond, Titan, CL151, and CL172 yields were reduced the most by tembotrione, whereas

Jupiter, CLXL745, and XL753 were most affected by applications of mesotrione (Table 4).

Maximum yield reductions on LaKast and Rondo were comparable following applications of mesotrione and tembotrione. Out of the three herbicides evaluated, topramezone was the least harmful to rice when all parameters were taken into consideration; however, reductions in yield were still seen in all cultivars except Jupiter, CL151, and XL753 following topramezone application.

In 2017 at RREC, injury levels were less severe than at UAPB the same year (Tables 3-

5). Much like the other site years, Rondo was injured the greatest following applications of any

HPPD-inhibiting herbicide at either injury rating. However, in contrast to trials conducted at

UAPB, topramezone was the most injurious of the three herbicides when applied to Roy J,

Diamond, LaKast, Titan, CLXL745, or XL753 at 2 WAT (Table 5). By 4 WAT, injury resulting from topramezone applications in the conventional cultivars (with exception of Rondo) had declined close to or below the level of injury seen by tembotrione and mesotrione. Inbred imidazolinone-resistant cultivars ranged in injury from 9 to 31% whereas applications of mesotrione injured CL151 the least, and mesotrione injured CL172 the most 2 WAT. At 4 WAT, ratings from CL151 ranged from 14 to 19% with no differences between treatments, however on

CL172, injury from treatments containing tembotrione or topramezone were lower than mesotrione (11%, 8%, and 35%). Of the hybrid cultivars, CLXL745 was injured the most with over 80% injury for all treatments at 4 WAT, whereas XL753 exhibited a maximum of 41% injury when treated with topramezone at the same rating.

Height was reduced by at least one treatment within each cultivar with exception of

LaKast, Jupiter, and Titan compared to the nontreated (Tables 2 and 5). Treatments containing topramezone and tembotrione reduced height the most when applied to Roy J, Diamond, and

XL753; topramezone and mesotrione resulted in maximum height reduction on CL172; tembotrione and mesotrione resulted in maximum height reduction on CL151 and all three reduced height the greatest on CLXL753. Rondo exhibited the most height reduction of all cultivars with mesotrione causing crop death, topramezone a 64 cm decrease, and tembotrione a

27 cm decrease. Rice yield reductions occurred following at least one of the herbicide treatments for the cultivars: Diamond, LaKast, Rondo, CL172, CLXL745, and XL753 (Tables 8 and 11).

All three treatments resulted in similar yield losses in CL172 and CLXL745 with respect to nontreated (Table 5). Diamond was affected only by applications of topramezone. Tembotrione was the most yield reductive on LaKast and mesotrione was most injurious on Rondo and

XL753. This high level of variability would obviously be unacceptable in production rice fields.

Tolerance to Mixtures.

Visible crop injury as influenced by herbicide and rate of topramezone varied across site years. In 2016, no two-way interaction between herbicide and rate of topramezone was seen for injury (2 and 4 WAT), height, and grain yield (Table 6). However, rice injury in 2017 was greater than injury in 2016 (data not shown). At 2 WAT, the addition of topramezone at both rates increased injury more than that of the mixture partner herbicide alone, except in the case of propanil, saflufenacil, and clomazone which are known to cause rice injury (Camargo 2011;

Smith 1965; Webster 1999) (Table 7). Maximum injury at 2 WAT was greater with florpyruaxafen-benzyl + topramezone at 24 g ae ha-1 (35%) than with florpyruaxafen-benzyl + topramezone at 12 g ae ha-1 (9%) or either herbicide alone (0%). Similar results were observed with mixtures containing fenoxaprop or halosulfuron, although injury from mixtures of those herbicides applied at 12 g ae ha-1 did not differ from topramezone at 12 g ae ha-1 applied alone.

Treatments containing quinclorac, cyhalofop, propanil, imazethapyr, imazamox, saflufenacil, clomazone, or triclopyr did not differ for injury at 2 WAT when the rate of topramezone was increased. Visible injury decreased among all treatments at the 4 WAT rating (data not shown).

Although a two-way interaction for injury was not observed 4 WAT, the main-effect of topramezone rate was significant (p<0.0001) (Table 6). Rice treated with topramezone at 24 g ae ha-1 had greater injury than 12 g ae ha-1, averaged over all mixture partners, and the least injury was from treatments that did not contain topramezone (Table 8).

As with injury 4 WAT, only the main-effect of topramezone rate was significant for height and rough rice yield (Table 6). When topramezone was added at either rate to labeled herbicides, an average reduction in height of 2 cm occurred compared to when topramezone was not present (Table 8). Grain yield averaged across herbicides other than topramezone was reduced 430 kg ha-1 by the highest rate of 24 g ae ha-1. Mixtures containing the low rate of topramezone were not different from those treatments that did not contain topramezone.

Application Timing and Rate.

There was no rate by timing interaction for injury at UAPB, but application timing was significant for all parameters assessed (Table 9). Rice injury was generally greatest following the

POSTFLD application for all weeks assessed (Table 10). EPOST and PREFLD applications also injured rice (6 and 2%) 2 WAT, but at 3 and 4 WAT no injury was observed following these applications. No injury was apparent from PRE or DPRE applications (Table 10). Reduction in height was similar to injury assessments where the most severe reduction was observed on treatments applied at the POSTFLD timing (72 cm) when compared to the nontreated (79 cm).

Grain yield was reduced the most following EPOST (7740 kg ha-1) and POSTFLD (7470 kg ha-1) applications of topramezone, regardless of rate (Table 10).

A significant rate by timing interaction was observed for rice injury at 2 and 3 WAT, height, and yield at RREC in 2017. Although no interaction was observed 4 WAT, both main factors were significant (Table 6). In contradiction to the trial conducted at UAPB, injury following both rates of topramezone applied EPOST was over 95% both2 and 3 WAT assessments (Table 11). Topramezone applied PRFLD at 24 g ae ha-1 resulted in the second most injury (46%) at 2 WAT. Similar to the study conducted at UAPB, treatments applied POSTFLD injured rice 31 and 33% (12 and 24 g ae ha-1), but PRE and DPRE applications caused no injury

(Table 11). At 4 WAT, EPOST topramezone treatments were most injurious when compared to the other timings (Table 11). Later in the season, EPOST applications of topramezone reduced rice height and grain yield, with height reduced at least 12 cm and yield reduced by at least 3,200 kg ha-1. Grain yield was reduced 530 kg ha-1 from PRFLD applications of topramezone at 24 g ae ha-1 when compared to the nontreated.

Discussion

Overall, large differences in rice injury were observed from trial to trial depending on site year. Studies conducted on the triketone sub-family of HPPD-inhibiting herbicides suggest that indica and indica X japonica rice cultivars are more susceptible to crop injury than japonica cultivars (Kim et al. 2010; Kwon et al. 2012; Young et al. 2017). This tolerance is often associated with expression of the hydroxyphenylpyruvate dioxygenase inhibitor sensitive gene

No. 1 (HIS1) where rice cultivars containing more homologous HIS1 genes exhibited more tolerance compared to cultivars having a deletion or insertion within the gene (Kato et al. 2015).

However, Kim et al. (2016) revealed differential tolerance within japonica cultivars even though the HIS1 gene was present, indicating that the gene was expressed at different levels within each cultivar.

In addition, this injury was expressed more as the average temperature increased from 24

C to 27 C. From trials conducted by Song et al. (2016) in 2009 and 2010 on rice treated with tefurlytrione, another triketone HPPD inhibitor, it was concluded that the rate of tefurlytrione required to reduce rice growth 50% was roughly half in 2010 of that in 2009. The elevated injury was attributed to increased air temperatures following application in 2010 compared to 2009

(26.8 C and 25.4 C). Results from these studies indicate that slight increases in temperature following application of HPPD inhibitors can result in crop injury on seemingly tolerant

cultivars, regardless of whether the HPPD herbicide is in the triketone sub-family (tembotrione and mesotrione) or in the pyrazolone sub-family (topramezone).

In contrast to benzobicyclon, where only a small portion (~3%) of the herbicide is absorbed by rice and is slowly released through hydrolysis, the HPPD inhibitors studied in these trials are adsorbed quickly and in greater amounts through the crop foliage (Grossman and

Ehrhardt 2007; Komatsubara et al. 2009; Mitchell et al. 2001; Schulte and Köcher 2009). The rate at which these herbicides are taken up and translocated throughout the plant have been correlated to both temperature and light intensity received soon after application (Grossman and

Ehrhardt 2007; Johnson and Young 2002; Schulte and Köcher 2009). Furthermore, Johnson and

Young (2002) hypothesized that the efficacy of mesotrione on certain C3 weed species was largely dependent on herbicide uptake and metabolic activity of weeds. With rice being a C3 plant, changes in environmental conditions following application of HPPD-inhibiting herbicides have the potential to produce differential levels of injury following applications of these products.

The study by Grossman and Ehrhardt (2007) demonstrated that the majority of the applied topramezone was taken up within 8 hours after application (corn=60 to 65%; giant foxtail=48 to 50%; European black nightshade=55 to 60%) with maximum uptake (80%) achieved by 24 HAA when the temperature was 25 C, the relative humidity was 75%, and the photosynthetically active radiation (PAR) was 400 µmol m-2 s-1. However, when plants were exposed to lower PAR, the giant foxtail and European black nightshade exhibited reduced foliar uptake by 50% whereas corn displayed a 25% decrease. In the same study, humidity and temperature in the hours following also influenced uptake of topramezone. Relative humidity below 75% resulted in 5 to 10% reduction of herbicide taken up by plants. Furthermore, higher

temperatures following application produced higher rates of uptake and translocation throughout each species. When temperature was reduced from 22 C to 15 C a 15% decrease in foliar adsorption as well as a 10% reduction in translocation outside of the treated leaves occurred.

Temperatures below 15 C resulted in an even more prolific decrease in herbicide uptake with

50% or more reduction in topramezone found within the plants in comparison to applications made at 22 C (Grossman and Ehrhardt 2007).

All treatments causing severe phytotoxicity within the trials conducted in Arkansas strongly correspond to the environmental conditions following application and the time of herbicide application. Cultivar tolerance trials were applied at approximately 17:00 in 2016 with temperature and PAR similar to those seen in the mixture trial conducted the same year. All cultivars were minimally injured as a result with exception of Rondo, which is known to be highly sensitive to benzobicyclon (Table 9) (Young et al. 2017). In contrast, in 2017 at UAPB, herbicide applications for the cultivar tolerance trial were applied on June 7 earlier in the morning (9:00) than in 2016 where in the following hours, plants were exposed to greater PAR and an average temperature of 26.5 C in the hours preceding nightfall (Tables 6-8). Injury resulting from treatments of HPPD-inhibiting herbicides on this date were greatest overall among the three cultivar tolerance tests (Tables 3-5). Treatments in the cultivar tolerance test at RREC were applied one day later at 14:00. The average temperature following application was lower than the previous studies at 21.8 C; however, PAR above 500 µmol m-2 s-1 was received for 4 hours after application which is between the amount received in the hours before PAR reached 0

(2016 UAPB—45 minutes; 2017 UAPB—9 hrs) (Figures 1, 3, and 4). Injury observed at RREC was intermediate between the trials at UAPB in 2016 and 2017.

In 2016, the mixture trial was applied to plots at 17:30 in the afternoon and in the

subsequent hours, light intensity and temperature rapidly declined from 1,000 to 0 µmol m-2 s-1 and 27.2 to 21.7 C as night fell. Additionally, a rainfall event occurred approximately 2 hours after application. Although the commercially available form of topramezone, ArmezonÒ, states that the product should be applied one hour before rainfall, uptake of topramezone occurs up to

24 hours after application (Anonymous 2014; Grossman and Ehrhardt 2007).In the experiments conducted by Grossman and Ehrhardt (2007), a methylated seed oil (MSO) adjuvant was added to the spray mixture whereas a crop oil concentrate (COC) was used in all studies reported herein, except those containing propanil. Mixtures containing MSO are taken up by a plant more quickly and in greater quantity than mixtures containing COC; consequently, the rainfall event after application may have washed off some of the herbicide preventing maximum uptake.

(Hartzler 2018). Furthermore, the following day was cloudy with an average temperature of 24.4

C (Figures 1 and 2). The next year, herbicide applications were made at 13:00 with temperatures averaging 25.2 C, and greater PAR was received during the remaining hours of daylight (Figures

1-4). As a result, injury was minimal in 2016 where lower temperature, lower PAR, and rainfall were observed than in 2017 (injury data not shown for 2016; Tables 6-8).

Perhaps the greatest example of the impact of temperature on crop injury following treatments of topramezone is seen with the timing and rate trials, especially at the 1- to 2-leaf application timings where rice is beginning rapid vegetative growth. The trial conducted at

UAPB received treatments of topramezone at the 1- to 2-leaf stage on June 2, 2017. With an average temperature of 22.2 C in the following 24 hours after application. Substantial precipitation after planting at RREC necessitated a replant of the rate and timing trial on June 8,

2017 (Figure 4). This led to delayed herbicide applications later in the summer relative to UAPB

(Figures 3 and 4). Treatments of topramezone were applied to rice at the 1- to 2-lf stage on June

16, with an average temperature of 28.9 C in the following 24 hours after application (Figure 4).

Consequently, injury from topramezone applied at 12 and 24 g ae ha-1 at RREC was 97 and 85%,

97 and 99%, and 91 and 95%, respectively, greater than injury at UAPB 2, 3, and 4 WAT.

Subsequent treatments also resulted in greater visible crop injury, although to a lesser extent than treatments applied at earlier growth stages, suggesting that crop tolerance to topramezone increases as the rice matures. Height and yield loss also seemed to respond to varying temperatures. Height and yield reductions after applications at the 1- to 2-leaf stage averaged 11 cm and 2930 kg ha-1 greater at RREC than at UAPB (Tables 10 and 11). In contrast, height and yield reductions following the POSTFLD timing were 7 cm and 550 kg ha-1 greater at UAPB than at RREC with temperatures after application averaging 29.7 C at UAPB and 26.2 C at

RREC (Tables 10 and 11; Figures 4 and 5).

Conclusions

The results obtained through trials conducted in 2016 and 2017 emphasize alarming uncertainty about rice tolerance to topramezone as well as other HPPD-inhibiting herbicides currently used in U.S. corn production. Despite the fact that some trials imply that topramezone may be safely used in rice without impacts on visible injury, crop height, or grain yield, others, often with the same treatments, indicate that applications can result in severe injury that is detrimental to the crop, which has potential to cause complete crop failure. Furthermore, though the rice cultivars Diamond, LaKast, Jupiter, Titan, and XL753 exhibited greater tolerance to topramezone when high levels of injury were observed at UAPB in 2017, the wide range of injury seen from site year to site year insinuates that there is potential for applications to cause even greater injury under certain conditions. Solar radiation data collected from the following hours after application via cultivar tolerance and herbicide mixture trials on June 3rd, 2016 and

June 7th, 8th, and 9th in 2017 at UAPB appear to be an indicator of the level of injury observed in rice (Figures 1 and 2). More rice injury was observed when solar radiation was accumulated largely within the first 8 hours of application (Tables 3, 4, and 7; Figure 2). Although the weather data collected from these trials lead us to strongly suggest that environmental conditions following applications of topramezone have a direct impact on the tolerance of rice, this hypothesis cannot be confirmed through the research conducted within these studies. As a result, pursuit of a label for topramezone on rice cannot be recommended due to the potential for extensive crop injury, reductions in height, and yield losses.

It is possible that future research into safeners, other classes of HPPD-inhibiting herbicides, environmental conditions or tolerant varieties could make this technology safe for use on rice. Data from other research does indicate that this chemistry could be a valuable tool for rice farmers in the fight against key weed pests in rice and the buildup of resistant barnyardgrass.

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Tables and Figures

Table 1. P values for rice injury on 10 commonly planted rice cultivars from HPPD-inhibiting herbicides conducted at UAPB and RREC in 2016 and 2017.a Injuryb

2 WAT 4 WAT

UAPB RREC UAPB RREC Factor 2016 2017 2017 2016 2017 2017

------P Values from ANOVA------Cultivar <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

Herbicide <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

Cultivar ☓ <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 Herbicide aAbbreviations: UAPB, University of Arkansas Pine Bluff Research Farm; RREC, University of 78 Arkansas Rice Research and Extension Center; WAT, weeks after treatment bBolded values indicate significance (P=0.05).

Table 2. P values for rice height and yield from HPPD-inhibiting herbicides (topramezone, tembotrione, or mesotrione) applied on rice cultivars conducted at UAPB and RREC in 2016 and 2017.a Herbicide effectb Height Yield UAPB RREC UAPB RREC Cultivar 2016 2017 2017 2016 2017 2017 ------P Values from ANOVA------Roy J <0.0001 0.0077 0.0086 0.8206 <0.0001 0.4507

Diamond <0.0001 0.0124 0.0141 0.8220 <0.0001 0.0133

LaKast <0.0001 0.0063 0.6445 0.4258 <0.0001 0.0048

Jupiter 0.0018 0.0074 0.1546 0.2636 0.0083 0.2560

Titan 0.8747 0.0309 0.3969 0.0543 <0.0001 0.5600

Rondo <0.0001 0.0157 <0.0001 <0.0001 <0.0001 <0.0001

CL151 0.1235 0.0146 0.0015 0.1867 0.0004 0.6610

CL172 0.1752 0.0752 0.0024 0.6163 <0.0001 0.0015

CLXL745 0.5826 <0.0001 <0.0001 0.5587 <0.0001 <0.0001

XL753 0.0514 <0.0001 0.0991 0.5182 <0.0001 0.0056 aAbbreviations: UAPB, University of Arkansas Pine Bluff Research Farm; RREC, University of Arkansas Rice Research and Extension Center bHeight and yields were analyzed by herbicide within each cultivar. cBolded values indicate significance; for values that did not produce a two-way interaction, significant main effects are bolded.

Table 3. Rice injury (2 and 4 weeks after treatment), height (at flowering), and grain yield as influenced by a cultivar and HPPD -inhibiting herbicide at UAPB in 2016. b Injury c Cultivar Herbicide 2 WAT 4 WAT Height Grain yield -1 ------%------cm kg ha Roy J None - - 91 a 7560 a Topramezone 2 g 1 fg 88 ab 7480 a Tembotrione 2 g 2 fg 83 b 7280 a Mesotrione 4 fg <1 g 89 a 7100 a

Diamond None - - 94 a 7920 a Topramezone 3 g 3 fg 92 a 7740 a Tembotrione 4 fg 1 fg 85 b 7730 a Mesotrione 5 fg 4 fg 94 a 7510 a

LaKast None - - 94 a 8300 a Topramezone 2 g 0 g 90 b 7900 a 80

Tembotrione 3 g 2 fg 87 c 8220 a Mesotrione 6 efg 5 f 92 ab 8190 a

Jupiter None - - 85 a 7760 a Topramezone 2 g 0 g 85 a 7630 a Tembotrione 4 fg 1 fg 85 a 8070 a Mesotrione 4 fg <1 g 81 b 7610 a

Titan None - - 84 a 8420 a Topramezone 1 g 1 fg 84 a 7910 a Tembotrione 3 g 1 fg 84 a 8140 a Mesotrione 8 ef 3 fg 84 a 7810 a

Rondo None - - 91 a 8370 a Topramezone 4 fg 3 fg 85 b 6640 b Tembotrione 76 b 83 b 88 ab 5990 b Mesotrione 97 a 97 a 76 c 2340 c

Table 3. Cont. Injury Cultivar Herbicide 2 WAT 4 WAT Height Grain yield -1 ------%------cm kg ha CL151 None - - 94 a 7430 a Topramezone 2 g 0 g 94 a 7040 a Tembotrione 2 g 0 g 92 a 7080 a Mesotrione 3 g 0 g 94 a 7160 a

CL172 None - - 86 a 7100 a Topramezone 2 g 0 g 84 a 7220 a Tembotrione 5 fg 3 fg 84 a 7270 a Mesotrione 7 efg 4 fg 85 a 7050 a

CLXL745 None - - 103 a 9220 a Topramezone 2 g 0 g 103 a 9340 a 81 Tembotrione 11 e 4 fg 102 a 9150 a Mesotrione 23 d 20 d 101 a 9030 a

XL753 None - - 98 a 9420 a Topramezone 3 g 0 g 98 a 9490 a Tembotrione 18 d 14 e 97 a 9240 a Mesotrione 33 c 29 c 96 a 9250 a aAbbreviations: UAPB, University of Arkansas Pine Bluff Research Farm; WAT, weeks after treatment bMeans for injury in each column followed by the same letter are not significantly different at P=0.05 according to Fisher’s protected LSD. cMeans in each column followed by the same letter for height and grain yield are not significantly different within the same cultivar at P=0.05 according to Fisher’s protected LSD.

Table 4. Rice injury (2 and 4 weeks after treatment), height (at flowering), and grain yield as influenced by a cultivar and HPPD -inhibiting herbicide at UAPB in 2017. b Injury c Cultivar Herbicide 2 WAT 4 WAT Height Grain yield -1 ------%------cm kg ha Roy J None - - 88 a 8020 a Topramezone 73 efg 58 kl 84 ab 6310 b Tembotrione 83 cd 86 bcd 80 b 3010 c Mesotrione 73 efgh 65 hijk 80 b 6230 b

Diamond None - - 85 a 8380 a Topramezone 65 ghi 53 lm 84 ab 6900 b Tembotrione 82 cd 81 cde 76 b 1690 d

Mesotrione 74 defg 64 ijk 80 ab 5120 c

LaKast None - - 84 a 7500 a 82 Topramezone 56 ij 44 mn 80 a 5990 b Tembotrione 81 cde 76 defg 72 b 3070 c Mesotrione 74 defg 66 ghijk 70 b 4080 c

Jupiter None - - 81 a 8420 a Topramezone 45 k 14 p 77 ab 8290 a Tembotrione 86 bc 75 defgh 75 bc 7880 a Mesotrione 85 c 70 fghj 71 c 6670 b

Titan None - - 74 a 7730 a Topramezone 49 jk 33 o 69 a 6210 b Tembotrione 80 cdef 75 defgh 61 b 3660 c Mesotrione 46 k 40 no 68 ab 6710 b

Rondo None - - 98 a 8350 a Topramezone 97 a 90 abc 83 b 2610 b Tembotrione 99 a 99 a 50 c 930 c Mesotrione 99 a 96 ab 35 d 1540 c

Table 4. Cont. Injury Cultivar Herbicide 2 WAT 4 WAT Height Grain yield -1 ------%------cm kg ha CL151 None - - 80 a 8610 a Topramezone 64 hi 44 mn 78 a 8290 a Tembotrione 86 bc 79 def 71 b 6080 b Mesotrione 54 jk 31 o 82 a 8300 a

CL172 None - - 76 a 8050 a Topramezone 84 c 66 ghik 71 b 5710 b Tembotrione 95 ab 91 abc 69 b 1510 c Mesotrione 82 cde 74 efghi 64 c 5640 b

CLXL745 None - - 96 a 8600 a Topramezone 80 cdef 63 jkl 88 b 7180 b 83 Tembotrione 85 c 80 cdef 67 c 3370 c Mesotrione 86 bc 81 cde 84 b 2350 d

XL753 None - - 94 a 8780 a Topramezone 71 fgh 34 no 84 b 8730 a Tembotrione 96 ab 94 ab 73 c 2830 b Mesotrione 88 bc 81 cde 68 c 1960 c aAbbreviations: UAPB, University of Arkansas Pine Bluff Research Farm; WAT, weeks after treatment bMeans within each column for injury followed by the same letter are not significantly different at P=0.05 according to Fisher’s protected LSD. cMeans within each column followed by the same letter for height and grain yield are not significantly different within the same cultivar at ⍺=0.05 according to Fisher’s protected LSD.

Table 5. Rice injury (2 and 4 weeks after treatment), height (at flowering), and grain yield as influenced by cultivar and HPPD-inhibiting herbicide at RREC in 2017.a

b Injury c Cultivar Herbicide 2 WAT 4 WAT Height Grain yield

------%------cm kg ha-1 Roy J None - - 95 a 8570 a Topramezone 44 def 28 ef 85 bc 8680 a Tembotrione 23 kl 15 hij 83 c 8920 a Mesotrione 26 jkl 18 ghi 90 ab 8660 a

Diamond None - - 92 a 8650 a Topramezone 43 defg 28 ef 80 b 7630 b Tembotrione 29 hijk 16 ghij 86 ab 8580 a Mesotrione 30 Ijk 21 fgh 91 a 8600 a

LaKast None - - 93 a 7810 a

84 Topramezone 39 efgh 25 fg 92 a 7610 bc Tembotrione 24 kl 14 hij 92 a 7200 c Mesotrione 24 kl 22 fgh 93 a 8210 ab

Jupiter None - - 90 ab 8480 a Topramezone 31 hijk 15 hij 87 b 8340 a Tembotrione 26 jkl 18 ghi 93 a 8580 a Mesotrione 36 fghi 20 fgh 90 ab 8030 a

Titan None - - 89 a 8370 a Topramezone 45 def 28 ef 88 a 8430 a Tembotrione 34 ghij 23 fgh 87 a 8620 a Mesotrione 26 jkl 23 fgh 87 a 8940 a

Rondo None - - 88 a 7310 a Topramezone 88 a 95 ab 24 c 1710 c Tembotrione 83 ab 92 abc 61 b 2630 b Mesotrione 91 a 98 a 0 d 0 d

Table 5. Cont.

Injury

Cultivar Herbicide 2 WAT 4 WAT Height Grain yield -1 ------%------cm kg ha CL151 - - 85 a 7610 a Topramezone 25 jkl 16 ghij 84 a 7400 a Tembotrione 29 ijk 19 fghi 70 b 7720 a

Mesotrione 9 m 14 hij 74 b 7500 a

CL172 None - - 90 a 8480 a

Topramezone 24 kl 11 ghij 85 b 7770 b

Tembotrione 18 lm 8 fghi 90 a 7480 b

Mesotrione 31 hijk 35 de 85 b 7530 b

CLXL745 None - - 88 a 7900 a Topramezone 86 a 88 bc 64 b 3120 b 85 Tembotrione 83 ab 83 c 66 b 3180 b

Mesotrione 75 b 90 abc 65 b 2810 b

XL753 None - - 98 a 8990 a

Topramezone 64 c 41 d 91 b 8410 bc

Tembotrione 46 de 28 ef 91 b 8810 ab

Mesotrione 49 d 38 d 93 ab 8010 c

aAbbreviations: RREC, University of Arkansas Rice Research and Extension Center; WAT, weeks after treatment

bMeans within each column for injury followed by the same letter are not significantly different at P=0.05

according to Fisher’s protected LSD.

cMeans within each column followed by the same letter for height and grain yield are not significantly different

within the same cultivar at P=0.05 according to Fisher’s protected LSD.

Table 6. P values for rice injury, height, and yield from topramezone mixture trials conducted at UAPB in 2016 and 2017.a Injurybc 2 WAT 4 WAT Height Yield Factor 2016 2017 2016 2017 2016 2017 2016 2017 ------P values from ANOVA------Herbicide 0.3419 0.0136 0.4804 0.5903 0.4523 0.0621 0.2693 0.9999

Topramezone rate 0.7705 <0.0001 0.0978 <0.0001 0.9285 0.0039 0.6374 0.0072

Herbicide ☓ 0.8912 0.0264 0.9637 0.7933 0.3606 0.6082 0.2157 0.9818 Topramezone rate aAbbreviations: UAPB, University of Arkansas Pine Bluff Research Farm; WAT, weeks after treatment bANOVA for rice injury was calculated using beta distribution; normal distribution was used for height and yield. 86 cBolded values indicate significance (P=0.05).

Table 7. Rice injury (2 weeks after treatment) caused by herbicides applied alone or with topramezone at 12 or 24 g ae ha-1 at UAPB in 2017.a Rate Injury Herbicide Herbicide Topramezone 2 WAT g ai or ae ha-1 % None - - - 12 b7 i 24 21 bcd Quinclorac 420 - 0 12 10 ghi 24 15 cdefgh Fenoxaprop 120 - 0 12 15 cdefgh 24 28 ab Cyhalofop 315 - 0 12 11 efghi 24 19 bcde Halosulfuron 30 - 0 12 9 ghi 24 24 bc Propanil 4480 - 11 efghi 12 15 cdefgh 24 18 bcdef Imazethapyr 105 - 0 12 9 ghi 24 11 efghi Imazamox 45 - 0 12 9 ghi 24 14 defghi Saflufenacil 25 - 15 cdefgh 12 15 cdefgh 24 16 cdeg Clomazone 340 - 8 hi 12 11 efghi 24 13 defghi Florpyrauxifen-benzyl 30 - 0 12 9 ghi 24 35 a Triclopyr 280 - 0 12 10 ghi 24 14 defghi aAbbreviations: UAPB, University of Arkansas Pine Bluff Research Farm; WAT, weeks after treatment bMeans followed by the same letter are not significantly different according to Fisher’s protected LSD (P=0.05).

Table 8. Main effects from mixture tolerance tests conducted at UAPB in 2017 for responses that did not produce a herbicide by topramezone rate interaction at P=0.05.a

Effect Treatment Injury 4 WAT Height Grain yield

% cm kg ha-1

Herbicide Noneb 4 90 7720

Quinclorac 2 90 7830

Fenoxaprop 5 89 7750

Cyhalofop 3 90 7840

Halosulfuron 5 91 7860

Propanil 6 89 7730

Imazethapyr 3 90 7810

Imazamox 0 93 7890

Saflufenacil 6 89 7870

Clomazone 4 91 7690

Florpyrauxifen-benzyl 8 88 7750

Triclopyr 5 89 7810

Topramezone rate 0 g ae ha-1 1 cc 91 a 7970 a

12 g ae ha-1 3 b 89 b 7890 ab

24 g ae ha-1 10 a 88 b 7540 b aAbbreviations: UAPB, University of Arkansas Pine Bluff Research Farm; WAT, weeks after treatment; NS, not significant b “None” refers to no starting herbicide. Therefore, applications within the “None” treatment contained topramezone at 0, 12, or 24 g ae ha-1 with no other herbicide in the mixture. cMeans followed by different letters indicate significant differences in injury when mixed with topramezone than that of the starting herbicide alone.

Table 9. P values for rice injury, height, and yield from topramezone rate and timing tolerance trials conducted at UAPB a and RREC in 2017. bc Injury 2 WAT 3 WAT 4 WAT Height Yield

Factor UAPB RREC UAPB RREC UAPB RREC UAPB RREC UAPB RREC

------P values from ANOVA------Timing 0.0003 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.0019 <0.0001

Rate 0.1211 0.0282 0.5499 0.0162 0.7773 0.0045 0.1227 0.0041 0.4399 0.4154

Timing☓Rate 0.8645 0.0034 0.2483 0.0031 0.8638 0.2588 0.3667 0.0014 0.4119 0.0333 aAbbreviations: UAPB, University of Arkansas Pine Bluff Research Farm; RREC, University of Arkansas Rice Research and Extension Center; WAT, weeks after treatment bANOVA for rice injury was calculated using beta distribution; normal distribution was used for height and yield. cBolded values indicate significance (P=0.05).

Table 10. Rice injury (2, 3, and 4 weeks after treatment), height (at flowering), and grain yield after applications of topramezone at PREa, DPRE, EPOST, PRFLD, and POSTFLD timings at UAPB in 2017. Injury

Timing Topramezone rate 2 WAT 3 WAT 4 WAT Height Grain yield g ae ha-1 ------%------cm kg ha-1 Nontreated - - - - 79 8330 PRE 12 0 0 0 80 8190 24 0 0 0 80 8070 DPRE 12 0 0 0 79 7910 24 0 0 0 79 8350 EPOST 12 0 0 0 78 7790 24 13 0 0 77 7690 PRFLD 12 1 0 0 79 7950 24 3 0 0 77 8210

90 POSTFLD 12 18 15 13 74 7490

24 28 25 20 69 7450 Main effect Timing PRE 0 0 0 80 a 8130 a DPRE 0 0 0 79 a 8130 a EPOST 6 b 0 0 77 a 7740 bc PRFLD 2 b 0 0 78 a 8080 ab POSTFLD 22 a 20 18 72 b 7470 c Rate 12 g ae ha-1 4 3 3 78 7870 24 g ae ha-1 8 5 5 76 7950 aAbbreviations: PRE, preemergence; DPRE, delayed-preemergence; EPOST, 2- to 3-leaf; PRFLD, 1 week prior to flood; POSTFLD, 1 week post-flood; UAPB, University of Arkansas—Pine Bluff Research Farm; WAT, weeks after treatment bMeans followed by the same letter within a column are not significantly different according to Fisher’s protected LSD (P=0.05).

Table 11. Rice injury (2, 3, and 4 weeks after treatment), height (taken at flowering), and grain yield after applications of topramezone at PRE, DPRE, EPOST, PRFLD, and POSTFLD timings at RREC in 2017.a Injury Timing Topramezone rate 2 WAT 3 WAT 4 WAT Height Grain yield g ae ha-1 ------%------cm kg ha-1 Nontreated - - - - 81 ab 8080 a PRE 12 0 0 0 82 a 7890 ab 24 0 0 0 82 a 7710 ab DPRE 12 0 0 0 83 a 7900 ab 24 0 0 0 82 a 7610 ab EPOST 12 97 a 97 a 91 69 b 4850 c 24 98 a 99 a 95 66 b 4280 d PRFLD 12 15 d 11 c 9 83 a 7980 ab 24 46 b 31 b 21 81 a 7550 b POSTFLD 12 31 c 26 b 16 81 a 7740 ab 91

24 33 c 28 b 21 83 a 7800 ab Main effect

Timing PRE 0 0 0 82 7800

DPRE 0 0 0 82 7750

EPOST 98 98 93 a 68 4560

PRFLD 31 21 15 b 82 7810 POSTFLD 32 27 19 b 82 7770

Rate -1 12 g ae ha 29 27 39 b 77 7180 24 g ae ha-1 35 31 46 a 76 7100 aAbbreviations: PRE, preemergence; DPRE, delayed-preemergence; EPOST, 2- to 3-leaf; PRFLD, 1 week prior to flood; POSTFLD, 1 week post-flood; RREC, University of Arkansas Rice Research and Extension Center; WAT, weeks after treatment bMeans followed by the same letter within a column are not significantly different according to Fisher’s protected LSD (P=0.05). If no two-way interactions were observed, statistics were reported for main effects.

3000

2500 1 -

s 2000 2 -

1500

1000 PAR light µmol m light PAR 92

500

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Hours after application

6/3/16 6/7/17 6/8/17 6/9/17

120000

100000

80000 2 -

60000

Cumulative W m W Cumulative 40000 93

20000

0 6/3/16 6/7/17 6/8/17 6/9/17 Date of application

8 hours after application 24 Hours after application Figure 2. Total W m-2 accumulated 8 and 24 hours after application at Lonoke following treatments applied June 3, 2016; June 7, 2017; June 8, 2017; June 9, 2017 for herbicide mixture and cultivar tolerance trials conducted in 2016 and 2017. Data collected via the Arkansas State Plant Board Weather Web in W m-2 (http://170.94.200.136/weather/Default.aspx).

40 8.0 Precipitation Low Temp High Temp 7.5 35 7.0 6.5 30 6.0 5.5 25 5.0 4.5 20 4.0 3.5 15 3.0 Temperature (C) Temperature Precipitation (cm) Precipitation 2.5 10 2.0 94

1.5

40 8.0 Precipitation Low Temp High Temp 7.5 35 7.0 6.5 30 6.0 5.5 25 5.0 4.5 20 4.0 3.5 15 3.0 Temperature (C) Temperature Precipitation (cm) Precipitation 2.5 10 2.0 95

1.5

Figure 4. Environmental conditions at the University of Arkansas—Pine Bluff research farm near Lonoke, AR in 2017 beginning at planting. Application Timings—Cultivar tolerance—June 7, 2017; Herbicide mixtures tolerance—June 9, 2017; Timing and rate tolerance: PRE—May 17, 2017, DPRE—May 22, 2017, 1- to 2-lf—June 2, 2017, 4- to 5-lf PRFLD—June 22, 2017, 1 WK PSTFLD—July 6, 2017. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx).

40 8.0 Precipitation Low Temp High Temp 7.5 35 7.0 6.5 30 6.0 5.5 25 5.0 4.5 20 4.0 3.5 15 3.0 Temperature (ºC) Temperature Precipitation (cm) Precipitation 2.5 10 2.0 96

1.5

Figure 5. Environmental conditions at the University of Arkansas—Rice Research and Extension Center near Stuttgart, AR in 2017 beginning at cultivar tolerance planting; timing and rate experiment planted on June 8, 2017. Application Timings—Cultivar tolerance: June 8, 2017; Timing and rate: PRE—June 8, 2017, DPRE—June 12, 2017, 1- to 2-leaf—June 16, 2017, 4- to 5-leaf PRFLD—June 29, 2017, 1 WK PSTFLD—July 6, 2017. Data collected via the Arkansas State Plant Board Weather Web (http://170.94.200.136/weather/Default.aspx)

Chapter 5

Soybean Sensitivity to Low Rates of Topramezone Applied Early-Postemergence

Abstract

Off-target movement of herbicides onto crops without tolerance to the herbicide can cause severe injury and, at times, reduce yield. When herbicides are being studied for potential use in a crop, it is important to study the effects of low-dose applications on crops that are commonly planted adjacent to the target crop. Topramezone, a 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide, is currently being examined for potential use in rice production. Field studies were conducted in 2017 to determine the effects of the HPPD inhibitors topramezone and benzobicyclon and the acetolactate synthase-inhibiting rice herbicide bispyribac on soybean, which is often planted in close proximity to rice. Topramezone, benzobicyclon, and bispyribac were applied to soybean at 1/10, 1/20, 1/40, 1/80, 1/160, 1/320, or

1/640 rates of the 1X rate for each herbicide (24 g ae ha-1, 369 g ai ha-1, and 28 g ai ha-1, respectively) when the plants had reached the V3 growth stage. Maximum visible injury to soybeans for each herbicide was observed 2 weeks after treatment (WAT) with the 1/10X rate.

Soybean were injured by bispyribac (72%) followed by topramezone (49%) and benzobicyclon

(31%). Soybean terminal height was reduced 25% with bispyribac, 18% with topramezone, and

5% with benzobicyclon at the 1/10X rate. All herbicides caused similar soybean yield losses regardless of rate, with the largest loss observed at the 1/10X rate where a 33% loss occurred for bispyribac, a 29% loss occurred for topramezone, and a 27% loss occurred for benzobicyclon.

With each parameter measured, as the rate decreased with each herbicide, injury, height reduction, and yield were reduced. Applications of these herbicides to rice must be used with caution when the crop is directly adjacent to soybeans.

Nomenclature: benzobicyclon, bispyribac; topramezone; rice, Oryza sativa L.;

soybean, Glycine max L. Merr.

Key words: HPPD, tolerance, off-target movement 98

Introduction

Since the discovery of herbicide-resistant (HR) weeds, most notably glyphosate-resistant

Palmer amaranth (Amaranthus palmeri S. Wats.) in 2005, management of problematic weeds in

all crops has changed significantly (Culpepper et al. 2006). Prior to this development, a typical

weed management program in glyphosate-resistant cotton (Gossypium hirsutum L.) and

soybeans consisted of glyphosate followed by glyphosate with no other sites of action (SOA)

contained in the program (Culpepper and York 1998; Shaner 2000). The overuse of glyphosate

led to widespread resistance of Palmer amaranth, tall waterhemp (Amaranthus tuberculatus Moq. 99

Sauer), johnsongrass (Sorghum halepense L. Pers.), and several other problematic weeds

(Culpepper et al. 2006; Heap 2018; Rosenbaum and Bradley 2013; Vila-Aiub et al. 2007). As a

result of these herbicide resistant (HR) weeds, it is now more important than ever to recommend

the use of an integrated weed management (IWM) system, including the use of cultural,

mechanical, and chemical methods of weed control. Multiple SOAs, including combinations of

residual and postemergence herbicides, should be overlapped throughout the growing season to

combat the further development of HR weeds (Norsworthy et al. 2012; Owen et al. 2014).

The evolution of glyphosate-resistant weeds is arguably the most well-known instance of

weed resistance; however, it is not the first. In rice production, several weeds have already

become resistant to multiple herbicide SOAs. Introduced in 1956, propanil, a photosystem II

(PSII) inhibitor, was one of the first commercially available rice herbicide (Smith 1961). The

herbicide was widely used for its capability to control many problematic weeds such as

barnyardgrass (Echinochloa crus-galli L. Beauv.), sedges, broadleaves, and others. Similar to

glyphosate with Palmer amaranth, the overuse of propanil resulted in barnyardgrass becoming

resistant (Baltazar and Smith 1994; Carey et al. 1995). Currently, barnyardgrass biotypes have

been documented to be resistant to five herbicide sites of action used in rice (Norsworthy et al.

2012; Wilson et al. 2014). Therefore, it is critical that new sites of action be tested for potential

use in rice production.

Topramezone, a 4-hydroxyphenolpyruvate dioxygenase (HPPD) inhibitor, is being

considered for early-postemergence (EPOST) weed control in rice production. Currently for use

in corn (Zea mays L.), topramezone provides POST control of key annual grasses found in rice,

including barnyardgrass (Grossman and Ehrhardt 2007; Senseman 2007; Soltani et al. 2011).

Plants sensitive to topramezone display severe bleaching of newly developed growth as is seen 100

with other HPPD-inhibiting herbicides. This is caused by the inhibition of carotenoid

biosynthesis, which in turn leaves the plant vulnerable to ultraviolet rays used in photosynthesis

and the destruction of chlorophyll and eventual plant death (Grossman and Ehrhardt 2007; van

Almsick 2009). Another HPPD-inhibiting herbicide, benzobicyclon has been successfully used

in rice production; however, as a post-flood herbicide, it must be applied when a permanent

flood is established to achieve optimal efficacy (Davis et al. 2013; Young et al. 2017). If labeled,

topramezone will provide rice producers with an EPOST HPPD-inhibiting herbicide to aid in

reducing the risk of herbicide resistance.

Off-target movement is possible with topramezone and benzobicyclon. In Arkansas, rice

and soybean are the two most widely produced agronomic crops. Often, there are instances

where the two are grown in adjacent fields. Herbicides that are applied on rice can be devastating

to soybean and vice-versa. For example, drift studies published by Bevitori and Talbert (1998)

on bispyribac, a frequently applied rice herbicide, revealed that soybean may be damaged with

exposure as little as 0.22 g ai ha-1 or 1/100X of the labeled rate. The close proximity of rice and

soybean combined with extreme sensitivity to low-rates of off-target herbicide make it vital to

100

document the effects of potential labeled herbicides at various rates. Currently, there has been

limited published research on the effects of low rates of topramezone or benzobicyclon on

soybean. Therefore, the objective of the following research was to evaluate the response of

soybean exposed low rates of the HPPD-inhibiting herbicides, topramezone and benzobicyclon,

relative to bispyribac applied during the V3 growth stage.

Materials and Methods

Field experiments were conducted in 2017 at the University of Arkansas-Agricultural

Research and Extension Center (AAREC) in Fayetteville, AR, and the Newport Extension 101

Center (NEC) near Newport, AR. At each location, a non-sulfonylurea tolerant (STS)

glufosinate-resistant soybean was planted. P48T67 (Pioneer, DuPont, Johnston, IA) was planted

on May 10, 2017, in Fayetteville and DG4967L (Delta Grow, Delta Grow Seed Co Inc, England,

AR) was planted on June 8, 2017, near Newport. In Fayetteville, plots were 3.7 m wide by 6.1 m

in length spaced 91 cm apart with a seeding rate of 285,000 seeds ha-1 at a 2.5-cm depth. In

Newport, plot sizes were 2.3 m in width by 6.1 m in length spaced 76 cm apart at a seeding rate

of 365,000 seeds ha-1. At both sites, seeds were planted in four-row plots. Both sites were

irrigated three to five times using overhead irrigation systems and kept weed free by means of

labeled herbicides as recommended by the University of Arkansas Division of Agriculture and

hand removal (Scott et al. 2018). The soil series at the AAREC was a mixture between Captina

silt loam (fine-silty, siliceous, active, mesic Typic Fragiudults) and a Leaf Silt loam (fine, mixed,

active, thermic Typic Albaqualts) and a Dexter silt loam (fine-silty, mixed, active, thermic Ultic

Hapludalfs) at the NEC. Studies were fertilized in accordance with the recommendations

provided in the Arkansas Soybean Production Handbook (Slaton et al. 2013) Precipitation and

101

high/low temperatures were documented each day following treatment application for both sites

until the final injury rating was taken at 4 WAT (Figures 1 and 2).

Experiments were arranged as randomized complete block factorials with four

replications and two factors: application of topramezone, benzobicyclon, or bispyribac and

1/10X, 1/20X, 1/40X, 1/80X, 1/160X, 1/320X, or 1/640X rate of the 1X rate of topramezone at

24 g ae ha-1, benzobicyclon at 369 g ai ha-1, or bispyribac at 28 g ai ha-1. A nontreated control

was also included at both locations. All herbicide treatments were applied to the V3 growth

stage (three nodes on the main stem with a fully developed trifoliate) with a CO2-pressurized

102 -1

backpack sprayer calibrated to deliver 140 L ha at 276 kPa.

Visible crop injury was assessed 2 and 4 weeks after application (WAT). These ratings

were based on a scale of 0 to 100% injury, with 0 indicating no crop injury and 100 indicating

crop death (Frans and Talbert 1977). Soybean terminal heights were taken 4 WAT and at

maturity from three representative plants contained in each plot. Soybean yield data were

collected at maturity from the center two rows of each plot via small-plot machine harvester with

grain moisture adjusted to 14%. Visible injury data were subject to nonlinear regression using a

three-parameter Gompertz model as described by:

y=a*exp{-exp[-b*(x-c)]}

where y is visible injury expressed as a percentage relative to a nontreated, a is the asymptote, b

is the growth rate, c is the inflection point, and x is the herbicide dose. Height and yield data

were analyzed using a three-parameter exponential model as expressed by:

y=a*exp('*x)

where y is height or yield response, a is the asymptote, b is the growth rate, and x is the herbicide

dose. The three-parameter Gompertz and exponential models were used due to their wide use in

102

herbicide dose-response studies (Miller and Norsworthy 2018; Salas et al. 2016; Schwartz et al.

2017). Regression curves were created using SigmaPlot 14.0 (Systat Software Inc., San Jose,

CA) and differentiated using rate estimates based on confidence intervals (P=0.05) for each

herbicide in each analysis created using SAS 9.4 (SAS Institute Inc., Cary, NC) utilizing the

GLIMMIX procedure with site-years analyzed together and block included as a random effect.

Results and Discussion

The greatest level of injury for all herbicides was observed at 2 WAT at the 1/10X rate 103

with bispyribac exhibiting an average of 72% visible injury, topramezone exhibiting 49%, and

benzobicyclon exhibiting 31% (Table 1). It is also notable that at the 1/10X rate benzobicyclon

recovered more rapidly from 2 to 4 WAT than the other two herbicides from 31% to 5% visible

injury where treatments of topramezone at the same rate recovered from 49% to 19% and

bispyribac recovered from 72% to 38% (Table 1; Figures 1 and 2). Injury levels following

applications of topramezone were similar to a previous study during which low-rates of

mesotrione, another HPPD-inhibitor, were applied to soybeans (Young et al. 2003). However,

results were slightly different from a study conducted by Brown et al. (2008). Young et al.

(2003) found injury levels following applications of mesotrione from 25 to 78% injury at 2 WAT

from rates between 1/6X and 1/2X rates (1X rate—210 g ai ha-1). However, Brown et al. (2008)

observed 1 to 16% injury at the 1/200X rate and 23 to 59% injury at the 1/10X rate 2 WAT

depending on the site year. Topramezone also acted similarly to mesotrione as time progressed,

with maximum injury observed at 2 WAT and less injury observed at 4 WAT (Young et al.

2003; Brown et al. 2008).

103

Bispyribac was most injurious at both 2 and 4 WAT. Confidence intervals indicate the

rate at which bispyribac differentiated itself from topramezone was at the 0.009X rate and at

0.056X for benzobicyclon 2 WAT (Table 2; Figure 1). While at a slightly lower rate with 0.081X

for topramezone and 0.02X for benzobicyclon at 4 WAT (Table 2; Figure 2). Between the two

HPPD-inhibitors, treatments containing topramezone exhibited more visible injury starting at the

0.012X rate at 2 WAT and 0.015X at 4 WAT (Table 2; Figures 1 and 2).

Bispyribac and topramezone caused the greatest height reduction at 25% and 18%

with respect to the nontreated (Table 1). Terminal height following treatment of topramezone 104

and benzobicyclon is also dependent on the rate which was applied. The 1/10X and 1/20X rate of

topramezone resulted in the reduction of canopy height whereas lower rates starting at 1/40X

were similar to the nontreated (Table 1; Figure 3). Although bispyribac differentiated from

topramezone and benzobicyclon based on visible injury, topramezone and bispyribac were not

different in terms of soybean height at any of the estimated rates. However, bispyribac and

topramezone were different than benzobicyclon in height at 0.067X and 0.077X rates (Table 2;

Figure 3).

Soybean yields in response to all herbicides tested did not differ according to the

confidence intervals provided by the regression (data not shown). Nonetheless, there were large

differences in yield between the largest and smallest rates for bispyribac, benzobicyclon, and

topramezone where the low dose for each herbicide resulted in 95, 99, and 91% relative to the

nontreated and the high dose resulted in 67, 73, and 71% (Table 1). Low rates of topramezone

resulted in a greater reduction for canopy height and yield than the mesotrione study conducted

by Young et al. (2003) but acted in similar fashion to the study conducted by Brown et al.

(2008). Young et al. (2003) reported a 5% reduction in canopy height and 11% reduction in yield

104

at the 1/6X rate whereas Brown et al. (2008) reported a 26% reduction in canopy height and a

16% reduction in yield at the 1/20X rate.

Conclusions

Results from this study suggest exposure to non-target applications of bispyribac,

benzobicyclon, or topramezone has the potential to reduce soybean yield and may do so even if

the plants have visibly recovered in the weeks following the drift occurrence. These findings are

similar to the results from studies conducted by Young et al. (2003) and Brown et al. (2008)

where visible injury caused by drift rates of the HPPD-inhibitor mesotrione were greatest at 2 105

WAT and had almost disappeared at 4 WAT but still caused canopy height and yield loss.

Furthermore, the height reduction following applications of low-doses to bispyribac and

topramezone indicate that canopy development of soybean is impeded which can lead to issues

in weed control. Therefore, growers applying bispyribac, benzobicyclon, or topramezone to a

rice crop should take precautions before applying these herbicides near soybean.

105 Literature Cited

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Bevitori R, Talbert R (1998) Response of soybean, cotton, grain sorghum, and tomato to simulated drift of bispyribac. BR Wells Rice Research Studies AAES Research Series 468:32-34

Brown LR, Robinson DE, Chandler K, Swanton CJ, Nurse RE, Sikkema PH (2008) Simulated mesotrione drift followed be glyphosate, imazethapyr, bentazon or glyphosate plus chlorimuron in soybean. Can J Plant Sci 89: 265-272

Carey VF, Hoagland RE, Talbert RE (1995) Verification and distribution of propanil-resistant barnyardgrass (Echinochloa crus-galli) in Arkansas. Weed Technol 9:366-372

Culpepper AS, York AC (1998) Weed management in glyphosate-tolerant cotton. J Cotton Sci 2:174-185

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

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108 Tables and Figures

Topramezone 1/10X 49 5 19 1 82 5 71 3

1/20X 35 3 8 2 87 3 78 4

1/40X 30 3 5 1 97 3 84 4

1/80X 17 2 4 1 98 2 84 2

1/160X 12 2 4 2 97 3 84 3

1/320X 12 3 3 1 100 2 90 2 1/640X 10 1 3 1 99 4 91 4

Benzobicyclon 1/10X 31 1 5 2 95 3 73 3

1/20X 19 1 2 1 94 4 81 3

1/40X 14 1 2 1 97 2 85 2

1/80X 14 2 3 1 96 2 88 2

1/160X 9 1 1 1 101 4 94 4

1/320X 9 1 1 1 100 3 99 4 1/640X 6 1 0 - 99 2 99 2

Bispyribac-sodium 1/10X 72 2 38 3 75 5 67 3

1/20X 42 4 9 1 89 3 81 5

1/40X 32 3 6 1 93 3 72 5

1/80X 19 2 4 2 101 4 86 3

1/160X 16 2 1 1 96 3 94 5

1/320X 12 2 2 1 102 2 90 2 1/640X 9 2 1 1 97 3 95 4 a Injury, height, and yields were calculated relative to the nontreated where 0% injury is no injury and 100% injury is complete crop death and 0% height and yield indicate no crop was present and 100% indicates treatments were equal to height and yield calculated from the nontreated (Newport—94 cm and 3750 kg ha-2; Fayetteville—88 cm and 3450 kg ha-2). b WAT = weeks after treatment; SE = standard error c 1X rates were based on 24 g ae ha-1, 369 g ai ha-1, and 28 g ai ha-1 for topramezone, benzobicyclon, and bispyribac, respectively.

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Table 2. Rates at which injury (2 and 4 WAT) or height (4 WAT) intersect and diverge following applications of low-rates of topramezone, benzobicyclon, or bispyribac on soybean at AREC and NEC in 2017 at the 95% confidence interval.abcd

Rate at Herbicide 1 Herbicide 2 Parameter analyzed Herbicide 1 Herbicide 2 differentiation confidence intervals confidence intervals Injury at 2 WAT Bispyribac Benzobicyclon 0.009X 14.35-20.45 10.27-14.35 Topramezone Benzobicyclon 0.012X 14.60-20.74 10.41-14.60 Bispyribac Topramezone 0.056X 43.94-53.61 35.75-43.94

Injury at 4 WAT Bispyribac Benzobicyclon 0.020X 3.12-6.19 0.91-3.12 Topramezone Benzobicyclon 0.015X 2.58-5.90 0.78-2.58 Bispyribac Topramezone 0.081X 17.97-28.84 10.75-17.97

110 % Height reduction Bispyribac Benzobicyclon 0.067X 10.95-21.86 2.13-10.95 at 4 WAT

Topramezone Benzobicyclon 0.077X 10.46-20.86 2.21-10.46 aAbbreviations: WAT—weeks after treatment, AREC—University of Arkansas—Agricultural Research and Extension Center, NEC—University of Arkansas-Newport Extension Center bNo significant differences on yield between topramezone, benzobicyclon, or bispyribac (p=0.05). c95% confidence intervals based on nonlinear 3-parameter Gompertz model for crop injury and 3-parameter exponential model for height. d The 1X rates were based on 24 g ae ha-1, 369 g ai ha-1, and 28 g ai ha-1 for topramezone, benzobicyclon, and bispyribac, respectively.

y = a*exp(-exp(-b(x-c)))

Injury (%) WAT at 2 111

Proportion of typical rate

Bispyribac

Figure 1. Soybean injury 2 weeks after treatment following applications of low-doses of bispyribac, benzobicyclon or topramezone at the vegetative third trifoliate growth stage. Rates are proportional to a typical rate of each herbicide (bispyribac at 28 g ai ha-1 benzobicyclon at 369 g ai ha-1, or topramezone at 24 g ae ha-1). Three parameter Gompertz model y = a*exp(- exp(-b(x-c))) where y is the response, a is the asymptote, b is the growth rate, and c is the inflection point. Parameter estimates and standard errors: Bispyribac—a=86.19(11.17), b=22.68(4.39), c=0.03(0.01); benzobicyclon—a=44.39(20.77), b=15.22(10.39), c=0.04(0.03); topramezone—a=48.43(3.57), b=37.66(7.8), c=0.01(0.002)

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y = a*exp(-exp(-b(x-c)))

4 WAT

Injury (%) WAT at 4

112

Proportion of typical rate

Bispyribac

Figure 2. Soybean injury 4 weeks after treatment following applications of low-doses of bispyribac, benzobicyclon or topramezone at the vegetative third trifoliate growth stage. Rates are proportional to a typical rate of each herbicide (bispyribac at 28 g ai ha-1 benzobicyclon at 369 g ai ha-1, or topramezone at 24 g ae ha-1). Three parameter Gompertz model y = a*exp(- exp(-b(x-c))) where y is the response, a is the asymptote, b is the growth rate, and c is the inflection point. Parameter estimates and standard errors: Bispyribac—a=69.40(30.78), b=15.74(5.06), c=0.09(0.03); benzobicyclon—a=190.82(7080.53), b=3.61(31.54), c=0.46(5.93); topramezone—a=27.51(7.78), b=19.66(6.92), c=0.05(0.02)

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y = a*exp(b*x)

Log rate Fractional rate Topramezone rate Benzobicyclon rate Bispyribac rate ln(rate) (1x) (g ae ha-1) (g ai ha-1) (g ai ha-1) -6.46 1/640 0.0375 0.577 0.044 -5.77 1/320 0.075 1.153 0.088 -5.08 1/160 0.15 2.306 0.175 -4.38 1/80 0.3 4.613 0.35 -3.69 1/40 0.6 9.23 0.7 -3.00 1/20 1.2 18.45 1.4 -2.30 1/10 2.4 36.9 2.8

Figure 3. Soybean canopy height reduction at 4 weeks after treatment following applications of low-doses of bispyribac, benzobicyclon, or topramezone at the vegetative third trifoliate growth stage. Three parameter exponential model y = a*exp(b-x) where y is the response, a is the asymptote, and b is the growth rate. Parameter estimates and standard errors: Bispyribac—a=- 2.94.19(3.33), b=141.48(99.06), c=0.74(0.31); benzobicyclon—a=-71.95(1471.44), b=83.68(1460.1), c=0.03(0.58); topramezone—a=-6.68(6.14), b=71.93(36.48), c=0.49(0.29)

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Figure 4. Environmental conditions at the University of Arkansas—Agricultural Research and Extension Center in Fayetteville, AR in 2017 beginning at herbicide application (June 6).

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Figure 5. Environmental conditions at the University of Arkansas—Newport Extension Center near Newport, AR in 2017 beginning at herbicide application (July 6).

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General Conclusions

The studies conducted throughout the course of this research demonstrate that topramezone can effectively control barnyardgrass and in some cases, weedy rice in cultivated rice cropping systems. Soybean yield loss caused by exposure to low rates of topramezone can occur and is similar to that of benzobicyclon and bispyribac. Growers applying topramezone near soybean fields should use extra precaution before application. However, because of frequent observations of severe crop injury, regardless of mixture with other rice herbicides, timing, rate, or cultivar, the future of topramezone for postemergence use in rice is in doubt. Despite these concerns, the effectiveness of topramezone for controlling barnyardgrass and the history of

HPPD-inhibiting herbicide use in Asian rice cropping systems exhibit the potential for other

HPPD-inhibitors to benefit growers in the United States. Further research should be conducted to determine the causes of these instances of crop injury and potential use of other HPPD-inhibiting herbicides in commercial rice fields.

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