Introduction
Barnyardgrass is the most troublesome weed in Mississippi rice production (Webster Reference Webster2012) due to its adaptation to flooded environments, prolific seed production, and rapid growth (Marambe and Amarasinghe Reference Marambe and Amarasinghe2002). Barnyardgrass has evolved resistance to several common herbicide modes of action (Heap Reference Heap2019), including photosystem II (PSII) inhibitors (Carey et al. Reference Carey, Hoagland and Talbert1995; Valverde et al. Reference Valverde, Chaves, Garita, Ramírez, Vargas, Carmiol, Riches and Caseley2001), synthetic auxins (Lopez-Martinez et al. Reference Lopez-Martinez, Marshall and De Prado1997), clomazone (Norsworthy et al. Reference Norsworthy, Scott and Smith2007), and acetyl coA carboxylase (ACCase) inhibitors (Heap Reference Heap2019). Moreover, the intensive use of the acetolactate synthase (ALS)-inhibiting herbicides imazethapyr, penoxsulam, and bispyribac-sodium led to the evolution of ALS-resistant barnyardgrass (Norsworthy et al. Reference Norsworthy, Wilson, Scott and Gbur2014).
Palmer amaranth and hemp sesbania are the most troublesome broadleaf weeds in Mississippi rice production (Webster Reference Webster2012). Palmer amaranth is difficult to control due to its prolific seed production, rapid growth rate (Steckel Reference Steckel2007), and pollen distribution up to 600 m (Sosnoskie et al. Reference Sosnoskie, Webster, MacRae, Grey and Culpepper2012). Hemp sesbania population densities can range from 8,100 to 129,000 plants ha−1 (McWhorter and Anderson Reference McWhorter and Anderson1979) and can reduce rice quality and yield by more than 50% due to shading and competition (Boyette et al. Reference Boyette, Hoagland and Stetina2014).
Flooding is the principal cultural weed control practice for rice (Chauhan and Johnson Reference Chauhan and Johnson2010; Kent and Johnson Reference Kent and Johnson2001; McClung Reference McClung, Smith and Dilday2003; Odero and Rainbolt Reference Odero and Rainbolt2014). Broadleaf weed species do not emerge once a flood is established unless germinated prior to flooding on levees or field areas that are allowed to dry after flooding (Scott et al. Reference Scott, Norsworthy, Barber, Hardke and Hardke2013). Therefore, herbicides that control broadleaf weed species are usually applied shortly before or after flooding (Scott et al. Reference Scott, Norsworthy, Barber, Hardke and Hardke2013). Although clomazone and imazethapyr are among the most commonly used herbicides for barnyardgrass and volunteer rice control in rice, these herbicides provide inadequate control of broadleaf weed species (Camargo et al. Reference Camargo, Senseman, McCauley and Guice2011). Propanil, another commonly used rice herbicide, can control grass and broadleaf weed species when applied POST; however, barnyardgrass has evolved resistance to propanil (Baltazar and Smith Reference Baltazar and Smith1994; Carey et al. Reference Carey, Hoagland and Talbert1995).
Provisia™ rice exhibits enhanced tolerance to herbicides that inhibit ACCase, allowing POST applications of quizalofop, an ACCase-inhibiting herbicide (Burton et al. Reference Burton, Gronwald, Somers, Gengenbach and Wyse1989; Focke and Lichtenthaler Reference Focke and Lichtenthaler1987; Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018). Quizalofop was first registered for use in soybean [Glycine max (L.) Merr.] in the late 1980s, followed by registration for use in cotton (Gossypium hirisutum L.) in the early 1990s (Shaner Reference Shaner2014). A member of the aryloxyphenoxy propionate herbicide family, quizalofop is used to target non-ACCase-resistant red rice [Oryza sativa (L.) Lombardy], volunteer conventional rice, hybrid rice, imidazolinone-resistant rice types, and other common annual and perennial grasses, including barnyardgrass (Anonymous 2017; Konishi and Sasaki Reference Konishi and Sasaki1994; Shaner Reference Shaner2014). However, previous research indicates that efficacy can be compromised when quizalofop is mixed with broadleaf and/or nonselective herbicides, including penoxsulam, penoxsulam plus triclopyr, bispyribac, propanil, and propanil plus thiobencarb (Blackshaw et al. Reference Blackshaw, Walker, Clayton and O’Donovan2006; Chahal and Jhala Reference Chahal and Jhala2015; Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018).
Florpyrauxifen-benzyl is a new active ingredient from the arylpicolinate herbicide family (Epp et al. Reference Epp, Alexander, Balko, Buysse, Brewster, Bryan, Daeuble, Fields, Gast, Green, Irvine, Lo, Lowe, Renga, Richburg, Ruiz, Satchivi, Schmitzer, Siddall, Webster, Weimer, Whiteker and Yerkes2016). A synthetic auxin, florpyrauxifen-benzyl exhibits activity on grass and broadleaf weeds in rice (Miller and Norsworthy Reference Miller and Norsworthy2018). Florpyrauxifen-benzyl can be used on inbred and hybrid rice cultivars, including herbicide-resistant cultivars (Anonymous 2018). Therefore, florpyrauxifen-benzyl represents a new management option for control of ALS-, ACCase-, PSII-, and synthetic auxin-resistant broadleaf and grass weed species (Epp et al. Reference Epp, Alexander, Balko, Buysse, Brewster, Bryan, Daeuble, Fields, Gast, Green, Irvine, Lo, Lowe, Renga, Richburg, Ruiz, Satchivi, Schmitzer, Siddall, Webster, Weimer, Whiteker and Yerkes2016).
Quizalofop is effective at controlling grass weed species but it offers no control of broadleaf weed species; therefore, mixtures of florpyrauxifen-benzyl with quizalofop could be beneficial to increase the spectrum of weed control in ACCase-resistant rice. However, previous research reported negative interactions when quizalofop was mixed with some broadleaf herbicides (Blackshaw et al. Reference Blackshaw, Walker, Clayton and O’Donovan2006; Chahal and Jhala Reference Chahal and Jhala2015). Rustom et al. (Reference Rustom, Webster, Blouin and McKnight2018) reported reductions in barnyardgrass control as great as 54% when quizalofop was mixed with penoxsulam, penoxulam plus triclopyr, or bispyribac. Therefore, this research was conducted to evaluate rice response and control of grass and broadleaf weed species with sequential applications of quizalofop including auxinic herbicides in the first or second treatment.
Materials and Methods
Grass Study
A field study was conducted at the Mississippi State University Delta Research and Extension Center in Stoneville, MS, in 2017 (33.4412°N, 90.9049°W) and 2018 (33.4430°N, 90.9049°W) to evaluate control of barnyardgrass and volunteer rice with sequential applications of quizalofop (Provisia 0.88 EC; BASF Crop Protection, 26 Davis Dr., Research Triangle Park, NC 27709) mixed with auxinic herbicides in the first or second treatment. Soil was a Sharkey clay (very-fine, smectitic, thermic Chromic Eqiaquerts) with a pH of 8.2 and an organic matter content of 2.1%. The experimental site had a rice-fallow rotation with rice seeded every other year. During the fallow year, weeds were allowed to grow and produce seed to maintain the soil seed bank. Glyphosate (Roundup PowerMax 4.5 L, 1,120 g ae ha−1; Monsanto Company, 800 N. Lindburgh Blvd., St. Louis, MO 63167), paraquat (Gramoxone 2.0 SL, 560 g ai ha−1; Syngenta Crop Protection, P.O. Box 18300, Greensboro, NC 27409), or 2,4-D (2,4-D Amine 3.8 SL, 1,120 g ae ha−1; Agri Star, 1525 NE 36th St., Ankeny, IA 50021) were applied in late March to early April each year to control emerged vegetation. Barnyardgrass was surface-seeded prior to rice seeding to ensure uniform infestation.
Rice was drill-seeded May 18, 2017, and May 2, 2018, to a depth of 2 cm using a small-plot grain drill (Great Plains 1520; Great Plains Mfg, Inc., 1525 East North St., Salina, KS 67401) at 356 seed m−2. Plots consisted of eight rows of rice spaced 20 cm apart and 4.6 m in length and were flooded to an approximate depth of 6 to 10 cm when rice reached the one- to two-tiller stage. Rows 3 through 6 in each plot were seeded with ‘Provisia PVL01’ (Horizon Ag, LLC, 8275 Tournament Dr., Memphis, TN 38125). Rows 1 and 8 were seeded with ‘CL151’ (Horizon Ag, LLC), and rows 2 and 7 were seeded with ‘Rex’ (Reg. No. CV-136, PI 661111) to simulate an infestation of volunteer rice. Treated plots were bordered on either end by a 1.5-m alley that contained no rice. Saflufenacil (Sharpen 2.85 SC, 50 g ai ha−1; BASF Crop Protection) was applied at planting followed by halosulfuron (Permit 75 DF, 39 g ai ha−1; Gowan Company, P.O. Box 5569, Yuma, AZ 85364) applied prior to flooding to maintain the experimental site free of broadleaf weeds. Nitrogen fertilizer was applied at 168 kg ha−1 as urea (46-0-0) immediately prior to flood establishment (Norman et al. Reference Norman, Slanton, Roberts and Hardke2013). Rice was managed throughout the growing season utilizing local guidelines to optimize yield (Buehring Reference Buehring2008).
Treatments were arranged as a two-factor factorial within a randomized complete block design and four replications. Factor A was application timing for inclusion of auxinic herbicides and consisted of treatments applied to rice in the two- to three-leaf (EPOST) and four-leaf to one-tiller (LPOST) growth stages. Factor B was auxinic herbicide and consisted of no auxinic herbicide and the herbicide products listed in Table 1. Quizalofop at 119 g ai ha−1 was applied to all plots at the EPOST and LPOST timings and auxinic herbicides were mixed with quizalofop at the designated timings. The no-auxinic-herbicide treatment consisted of quizalofop alone EPOST followed by LPOST. A nontreated control was included for comparison. All treatments contained crop oil concentrate (Herbimax, 83% petroleum oil; Loveland Products, P.O. Box 1286, Greeley, CO 80632) at 1% (v/v) and were applied using a CO2-pressurized backpack sprayer equipped with flat-fan nozzles (Airmix 11002 nozzle; Greenleaf Technologies, 230 E Gibson St., Covington, LA 70433) set to deliver 140 L ha−1 at 206 kPa using water as a carrier.
Table 1. Herbicide common and tradenames, application rates, and herbicide manufacturer information for treatments in the Grass and Broadleaf studies conducted at Stoneville, MS, in 2017 and 2018.
Visible estimates of aboveground rice injury and control of barnyardgrass, ‘CL151’ and ‘Rex’ were recorded 7 d after EPOST (DA-EPOST) and 14 and 28 d after LPOST (DA-LPOST) on a scale of 0% to 100% where 0% indicated no visual effect of herbicides and 100% indicated complete plant death or weed control. Rice plant height was determined 14 DA-LPOST by measuring from the soil surface to the upper most extended leaf and calculating the mean height of five randomly selected plants in each plot. Plant height (cm) and density (no. m−2) for ‘CL151’, ‘Rex’, and barnyardgrass at each application timing are presented in Table 2. Plots were drained approximately 2 wk before harvest maturity. Rice was harvested with a small-plot combine (Wintersteiger Delta; Wintersteiger, Inc., 4705 W. Amelia Earhart Dr., Salt Lake City, UT 84116) at a moisture content of approximately 20% on September 7, 2017, and October 8, 2018. Final rough rice grain yield was adjusted to 12% moisture content.
Table 2. ‘CL151’, ‘Rex’, and barnyardgrass height and density at time of treatment applications in the Grass Study at Stoneville, MS, in 2017 and 2018.a
a Herbicide application timings included rice in the two- to three-leaf (EPOST) and four-leaf to one-tiller (LPOST) growth stages.
The square roots of visible injury and control estimates were arcsine transformed. The transformations did not improve homogeneity of variance based on visual inspection of plotted residuals; therefore, nontransformed data were used in analyses. Data from the nontreated control plots were not included in the analysis of crop injury and weed control estimates to stabilize variance. Data from nontreated control plots were not included in rough rice yield analysis because no data were collected due to severe barnyardgrass infestation. Nontransformed data were subjected to the Mixed Procedure (statistical software Release 9.3; SAS Institute Inc., 100 SAS Campus Drive, Cary, NC 27513-2414) with year and replication (nested within year) as random effect parameters (Blouin et al. Reference Blouin, Webster and Bond2011). Type III statistics were used to test the fixed effects of application timing, auxinic herbicide, and the interaction between these variables. Least square means were calculated and mean separation (P ≤ 0.05) was produced using PDMIX800 in SAS, which is a macro for converting mean separation output to letter groupings (Saxton Reference Saxton1998).
Broadleaf Study
A field study similar to that described for the Grass Sudy was conducted at the Mississippi State University Delta Research and Extension Center in Stoneville, MS, in 2017 (33.4412°N, 90.9049°W) and 2018 (33.4430°N, 90.9049°W). However, this study evaluated control of broadleaf weed species with sequential applications of quizalofop mixed with auxinic herbicides in the first or second treatment. Soil description, site maintenance, plot size, and planting information were the same as for the Grass Study. Hemp sesbania, ivyleaf morningglory, volunteer soybean (Asgrow 4632; Monsanto Company), and Palmer amaranth were surface-seeded prior to rice seeding each year to ensure uniform infestation. Clomazone (Command 3 ME, 560 g ai ha−1; FMC Corporation, 1735 Market St., Philadelphia, PA 19103) was applied PRE followed by bispyribac-sodium (Regiment 80 WP, 28 g ai ha−1; Valent U.S.A. Corporation, P.O. Box 8025, Walnut Creek, CA 94596-8025) as needed to maintain the experimental site free of grass weeds.
The experimental design, treatment structure, and treatment application for the Broadleaf Study was the same as that for the Grass Study. Visible estimates of aboveground rice injury and control of hemp sesbania, ivyleaf morningglory, volunteer soybean, and Palmer amaranth were recorded 7 DA-EPOST and 14 and 28 DA-LPOST on the previously described scale. Rice plant heights were determined 14 DA-LPOST as previously described. Plant height and density of hemp sesbania, ivyleaf morningglory, volunteer soybean, and Palmer amaranth at each application timing are presented in Table 3. Rice was harvested with a small-plot combine on September 7, 2017, and October 8, 2018, and final rough rice grain yield was adjusted to 12% moisture content. Data analyses were performed as previously described for the Grass Study.
Table 3. Height and density of hemp sesbania, ivyleaf morningglory, volunteer soybean, and Palmer amaranth at time of treatment applications in the Broadleaf Study at Stoneville, MS, in 2017 and 2018.a
a Herbicide application timings included rice in the two- to three-leaf (EPOST) and four-leaf to one-tiller (LPOST) growth stages.
Results and Discussions
Grass Study
Main effects of application timing (P = 0.091 to 0.425) and auxinic herbicide treatment (P = 0.075 to 0.542) and the interaction of these variables (P = 0.09 to 0.952) were not significant for rice injury across all evaluations. Rice injury was <10% at all evaluations (data not presented). Additionally, rice plant height 14 DA-LPOST was not influenced by the treatments imposed in this study (P = 0.16 to 0.481).
Control of ‘CL151’ 7 DA-EPOST was influenced by a main effect of auxinic herbicide treatment (P = 0.005). Pooled across application timings, all auxinic herbicide treatments except orthosulfamuron plus quinclorac controlled ‘CL151’ greater than quizalofop alone 7 DA-EPOST (Table 4). Additionally, florpyrauxifen-benzyl controlled more ‘CL151’ than orthosulfamuron plus quinclorac did. Control of ‘CL151’ and ‘Rex’ was 97% to 98% 14 and 28 DA-LPOST and neither were influenced by treatments imposed in this study (P = 0.424 to 0.979; data not presented). In similar research, mixtures of quinclorac and triclopyr with quizalofop at 120 g ha−1 resulted in a neutral response for control of volunteer rice utilizing ‘CLXL-745’ and ‘CL 111’ (Webster et al. Reference Webster, Rustom, McKnight, Blouin and Telo2019).
Table 4. Control of ‘CL151’ 7 d after first application (DA-EPOST) of quizalofop at 119 g ai ha−1 alone and in mixtures with auxinic herbicides in the Grass Study at Stoneville, MS, in 2017 and 2018.a,b
a Data were pooled across two application timings and two experiments. Means followed by the same letter are not different at P ≤ 0.05.
b All treatments included quizalofop at 119 g ai ha−1 in the two- to three-leaf (EPOST) and four-leaf to one-tiller (LPOST) applications.
Main effects of application timing (P = 0.478) and auxinic herbicide treatment (P = 0.530) and the interaction of these variables (P = 0.571) were not significant for control of barnyardgrass 7 DA-EPOST. However, interactions between application timing and auxinic herbicide treatments were detected for barnyardgrass control 14 (P = 0.004) and 28 (P = 0.002) DA-LPOST. Barnyardgrass control at 14 and 28 DA-LPOST with quizalofop applied alone or with auxinic herbicides included in EPOST and LPOST applications was similar, and 96% to 98% for all auxinic herbicide treatments except penoxsulam plus triclopyr in the EPOST treatment, which controlled barnyardgrass 84% at both evaluations (Table 5).
Table 5. Barnyardgrass control 14 and 28 d after final application (DA-LPOST) and rough rice yield following sequential applications of quizalofop at 119 g ai ha−1 alone and in mixtures with auxinic herbicides in the Grass Study at Stoneville, MS, in 2017 and 2018.a,b,c
a Data were pooled across two experiments. Means followed by the same letter for each parameter are not different at P ≤ 0.05.
b Herbicide application timings included application to rice in the two- to three-leaf (EPOST) and four-leaf to one-tiller (LPOST) growth stages.
c All treatments included quizalofop at 119 g ai ha−1 in the EPOST and LPOST applications.
Rough rice yield was also influenced by an interaction between application timing and auxinic herbicide treatment (P = 0.045). Rough rice yield was similar whether auxinic herbicides were included with quizalofop EPOST or LPOST for all auxinic herbicide treatments except penoxsulam plus triclopyr (Table 5). As with barnyardgrass control 14 and 28 DA-LPOST, rough rice yield was reduced when penoxsulam plus triclopyr was included with quizalofop in EPOST compared with LPOST treatments. Furthermore, rough rice yield was 10% less when orthosulfamuron plus quinclorac was mixed with quizalofop LPOST compared with florpyrauxifen-benzyl LPOST or quinclorac EPOST or LPOST.
Acetyl CoA carboxylase-resistant rice allows POST applications of quizalofop to control troublesome grass weed species in rice (Epp et al. Reference Epp, Alexander, Balko, Buysse, Brewster, Bryan, Daeuble, Fields, Gast, Green, Irvine, Lo, Lowe, Renga, Richburg, Ruiz, Satchivi, Schmitzer, Siddall, Webster, Weimer, Whiteker and Yerkes2016; Rustom et al. Reference Rustom, Webster, Blouin and McKnight2018). Lancaster et al. (Reference Lancaster, Norsworthy and Scott2018) reported that quizalofop controlled barnyardgrass >96% at the two- to three-leaf growth stage and >90% at the five- to six-leaf growth stage in ACCase-resistant rice. Webster et al. (Reference Webster, Rustom, McKnight, Blouin and Telo2019) reported that quinclorac and triclopyr antagonized barnyardgrass control 28 d after application when mixed with quizalofop at 120 g ha−1. In the current study, barnyardgrass was controlled ≥96% with quizalofop applied alone or with auxinic herbicide treatments except penoxsulam plus triclopyr EPOST. Therefore, to optimize barnyardgrass control, penoxsulam plus triclopyr should not be mixed with quizalofop for applications to ACCase-resistant rice.
Broadleaf Study
Main effects of application timing (P = 0.091 to 0.170) 7 DA-EPOST and 14 and 28 DA-LPOST, and auxinic herbicide treatment (P = 0.075 to 0.083) 14 and 28 DA-LPOST and the interaction of these variables (P = 0.312 to 0.842) 7 DA-EPOST and 14 and 28 DA-LPOST did not result in significant rice injury. A main effect of auxinic herbicide treatment (P ≤ 0.001) was detected as rice injury 7 DA-EPOST; however, rice injury was <10% at all evaluations (data not presented).
An interaction of application timing and auxinic herbicide treatment (P = 0.025) was significant for rice height 14 DA-LPOST. Rice heights 14 DA-LPOST were similar (≥88 cm for quizalofop alone and in mixture with auxinic herbicide treatments in EPOST treatments); however, rice heights were reduced 12% and 13% with penoxsulam plus triclopyr and quinclorac, respectively, compared to no auxinic herbicide LPOST (Table 6). Rice height was reduced when treatments containing quinclorac were delayed from EPOST to LPOST.
Table 6. Rice plant height 14 d after final application (DA-LPOST) of quizalofop at 119 g ai ha−1 alone and in mixtures with auxinic herbicides in the Broadleaf Study at Stoneville, MS, in 2017 and 2018.a,b,c
a Data were pooled across two experiments. Means followed by the same letter are not different at P ≤ 0.05.
b Herbicide application timings included application to rice in the two- to three-leaf (EPOST) and four-leaf to one-tiller (LPOST) growth stages.
c All treatments included quizalofop at 119 g ai ha−1 in the EPOST and LPOST applications.
Interactions between application timing and auxinic herbicide treatments were detected for control of hemp sesbania 7 DA-EPOST (P ≤ 0.001) and 14 DA-LPOST (P ≤ 0.001), ivyleaf morningglory 7 DA-EPOST (P ≤ 0.001) and 14 DA-LPOST (P ≤ 0.001), and Palmer amaranth 7 DA-EPOST (P ≤ 0.001) and 14 DA-LPOST (P ≤ 0.001). Hemp sesbania control 7 DA-EPOST was greatest (85% with penoxsulam plus triclopyr in EPOST treatments; Table 7). Other auxinic herbicide treatments resulted in ≤77% control of hemp sesbania 7 DA-EPOST. Hemp sesbania was controlled 60% 14 DA-LPOST with triclopyr EPOST compared with ≥96% control with other EPOST auxinic treatments. Mixtures containing florpyrauxifen-benzyl and penoxsulam plus triclopyr LPOST provided the greatest control (≥96% control compared with other auxinic treatments LPOST). Among LPOST treatments, triclopyr controlled hemp sesbania the least.
Table 7. Hemp sesbania and ivyleaf morningglory control 7 d after first application (DA-EPOST) and 14 d after final application (DA-LPOST) following quizalofop at 119 g ai ha−1 alone and in mixtures with auxinic herbicides in the Broadleaf Study at Stoneville, MS, in 2017 and 2018.a,b,c
a Data were pooled across two experiments. Means followed by the same letter for each parameter are not different at P ≤ 0.05.
b Herbicide application timings included application to rice in the two- to three-leaf (EPOST) and four-leaf to one-tiller (LPOST) growth stages.
c All treatments included quizalofop at 119 g ai ha−1 in the EPOST and LPOST applications.
No differences in ivyleaf morningglory control 7 DA-EPOST and 14 DA-LPOST were detected among treatments that included an auxinic herbicide with quizalofop EPOST (Table 7). Control was reduced with no auxinic herbicide treatment because quizalofop does not control broadleaf weed species. Ivyleaf morningglory control 14 DA-LPOST was ≥91% with florpyrauxifen-benzyl, penoxsulam plus triclopyr, and triclopyr LPOST. Control 14 DA-LPOST with quinclorac was greater than with orthosulfamuron plus quinclorac LPOST.
Although Palmer amaranth control 7 DA-EPOST varied with auxinic herbicide treatment, no treatment was able to control Palmer amaranth >64% (Table 8). Palmer amaranth control 14 DA-LPOST was 98% and 88% with florpyrauxifen-benzyl included in EPOST and LPOST treatments, respectively, compared with lower control with other auxinic herbicide treatments. Palmer amaranth was controlled least with mixtures that included quinclorac or orthosulfamuron plus quinclorac.
Table 8. Palmer amaranth control 7 d after first application (DA-EPOST) and 14 d after final application (DA-LPOST) following quizalofop at 119 g ai ha−1 alone and in mixtures with auxinic herbicides at two application timings at Stoneville, MS, in 2017 and 2018.a,b,c
a Data were pooled across two experiments. Means followed by the same letter for each parameter are not different at P ≤ 0.05.
b Herbicide application timings included application to rice in the two- to three leaf (EPOST) and four-leaf to one-tiller (LPOST) growth stages.
c All treatments included quizalofop at 119 g ai ha−1 in the EPOST and LPOST applications.
Control of hemp sesbania 28 DA-LPOST (P ≤ 0.001), ivyleaf morningglory 28 DA-LPOST (P ≤ 0.001), volunteer soybean 14 (P ≤ 0.001) and 28 DA-LPOST (P ≤ 0.001), Palmer amaranth 28 DA-LPOST (P ≤ 0.001), and rough rice yield (P ≤ 0.001) were influenced by a main effect of auxinic herbicide treatment. Hemp sesbania control 28 DA-LPOST was ≥97% with florpyrauxifen-benzyl and penoxsulam plus triclopyr (Table 9). At 28 DA-LPOST, hemp sesbania control with orthosulfamuron plus quinclorac and quinclorac was greater than with triclopyr. Potential causes of poor control with triclopyr EPOST were unclear. Ivyleaf morningglory control 28 DA-LPOST was ≥97% with treatments that included an auxinic herbicide.
Table 9. Control of hemp sesbania, ivyleaf morningglory, volunteer soybean, and Palmer amaranth at different intervals after final application (DA-LPOST) and rough rice yield following quizalofop at 119 g ai ha−1 alone and in mixtures with auxinic herbicides in the Broadleaf Study at Stoneville, MS, in 2017 and 2018.a,b
a Data were pooled across two application timings and two experiments. Means followed by the same letter for each parameter are not different at P ≤ 0.05.
b All treatments included quizalofop at 119 g ai ha−1 in the EPOST and LPOST applications.
Volunteer soybean control 14 and 28 DA-LPOST was ≥94% with florpyrauxifen-benzyl, penoxsulam plus triclopyr, and triclopyr, and control with these treatments was greater than with orthosulfamuron plus quinclorac and quinclorac (Table 9). Less control with orthosulfamuron plus quinclorac was attributed to utilization of a sulfonylurea-tolerant soybean cultivar to simulate volunteer soybean infestation, and orthosulfamuron is not effective for control of sulfonylurea-tolerant soybeans (Edwards et al. Reference Edwards, Peeples, Lawrence, Hydrick, Phillips and Bond2016). For control of broadleaf weeds, quinclorac at the labeled rate of 420 g ha−1 should be applied to weeds that are 0 to 5 cm high (Anonymous 2016). Volunteer soybean height at the time of LPOST application was 8 to 13 cm. Additionally, quinclorac is not recommended for control of volunteer soybean (Bond et al. Reference Bond, Lawrence, Bararpour, Dodds, Golden, Irby, Larson and Reynolds2019). Palmer amaranth control 28 DA-LPOST was 98% and 96% with florpyrauxifen-benzyl and penoxsulam plus triclopyr, respectively. Control was reduced with all other auxinic treatments; however, triclopyr provided greater control than orthosulfamuron plus quinclorac and quinclorac.
Control of all four broadleaf weed species was lowest at each evaluation interval with the no-auxinic-herbicide treatment (Table 9). Quizalofop controls grass weed species but has no activity on broadleaf weed species; therefore, nominal broadleaf weed control observed from the no-auxinic-herbicide treatment (quizalofop alone) at the later evaluations resulted from flooding the experimental area.
Rough rice yield was greater in plots receiving auxinic herbicide treatments compared with plots with no auxinic herbicide (Table 9). Rough rice yield was similar following application of mixtures with florpyrauxifen-benzyl, orthosulfamuron plus quinclorac, penoxsulam plus triclopyr, and quinclorac; however, rough rice yield was lower in plots treated with triclopyr.
A main effect of application timing was detected for control of Palmer amaranth 28 DA-LPOST (P ≤ 0.001). Palmer amaranth control 28 DA-LPOST was 90% with EPOST compared with 84% with LPOST treatments (data not presented). Palmer amaranth can grow 5 to 8 cm per day (Horak and Loughlin Reference Horak and Loughlin2000). Therefore, reduced control with LPOST treatments was likely due to larger Palmer amaranth at application compared with EPOST treatments.
Herbicide timing is a critical component of weed control (Montgomery et al. Reference Montgomery, Bond, Golden, Gore, Edwards, Eubank and Walker2015; Parker et al. Reference Parker, York and Jordan2006). Timely herbicide applications improve weed control and increase crop yield (Parker et al. Reference Parker, York and Jordan2006). Weeds are generally easier to control with POST herbicides when applied to small (< cm) plants that have not reached reproductive stages (Montgomery et al. Reference Montgomery, Bond, Golden, Gore, Edwards, Eubank and Walker2015). Doll (Reference Doll1981) stated that weeds in the two- to three-leaf growth stages are ideal for POST herbicide applications. Previous research reported >96% control of Palmer amaranth with POST applications of florpyrauxifen-benzyl at the three-to four-leaf growth stage with less control from other auxinic herbicides (Miller and Norsworthy Reference Miller and Norsworthy2018). The same study reported 80% and 92% control of hemp sesbania with quinclorac and triclopyr, respectively. In this study, differences in control varied by application timing and auxinic herbicide treatment. Auxinic herbicide treatments applied EPOST controlled the broadleaf weed species evaluated equally or better than LPOST applications. This was true for all auxinic herbicide mixtures with quizalofop except hemp sesbania control 14 DA-LPOST with triclopyr.
Flooding rice reduces infestation of grass weed species (Smith and Shaw Reference Smith and Shaw1966); however, broadleaf weeds can grow under flooded conditions if they emerge before flooding (Kendig et al. Reference Kendig, Williams, Smith, Smith and Dilday2003, Scott et al. Reference Scott, Norsworthy, Barber, Hardke and Hardke2013). In the current study, flooding influenced control of ivyleaf morningglory, volunteer soybean, and Palmer amaranth. Because quizalofop has no activity on broadleaf weeds, control of these weeds with treatments containing no auxinic herbicide was a result of flooding. Flooding had no effect on hemp sesbania; therefore, control with the no auxinic herbicide treatment was 0% at each evaluation.
Although previous research reported negative interactions when quizalofop was mixed with some broadleaf herbicides (Blackshaw et al. Reference Blackshaw, Walker, Clayton and O’Donovan2006; Chahal and Jhala Reference Chahal and Jhala2015), rice injury from quizalofop plus the auxinic herbicides evaluated in the current studies was <10% at all evaluations. Therefore, this research demonstrates that quizalofop mixtures with auxinic herbicides are safe and effective for grass and broadleaf weed control. Additionally, choice of mixture could be adjusted based on weed spectrum.
Acknowledgments
This publication is a contribution of the Mississippi Agricultural and Forestry Experiment Station. Material is based on work supported by the U.S. Department of Agriculture–National Institute of Food and Agriculture Hatch project under accession number 199080. We thank the Mississippi Rice Promotion Board for partially funding this research. We thank personnel at the Mississippi State University Delta Research and Extension Center for their assistance. No conflicts of interest have been declared.
