Introduction
In most instances, residual herbicides are commonly applied at planting or before weed or crop emergence, primarily to control weeds before they emerge from the soil (Radosevich et al. Reference Radosevich, Holt and Ghersa2007). Residual herbicides provide prolonged weed control due to their persistence and availability in the soil, which also reduces selection for resistance to POST herbicides (Cahoon et al. Reference Cahoon, York, Jordan, Everman, Seagroves, Braswell and Jennings2015; Helling Reference Helling and Van Acker2005). A PRE followed by a POST application to some crops often provides better weed control than a single application (Khaliq et al. Reference Khaliq, Matioob, Shafiq, Cheema and Wahid2011). Agronomic crop producers commonly use overlapping residual herbicides, especially in the mid-southern United States, to reduce the number of weeds that need to be controlled by a POST herbicide (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012).
A survey conducted in 2020 showed that barnyardgrass was the most important weed in Arkansas flood- and furrow-irrigated rice fields (Butts et al. Reference Butts, Kouame, Norsworthy and Barber2022). Barnyardgrass can reduce rice yields by 79% when it is uncontrolled for an entire growing season, resulting in producers losing profit and barnyardgrass seed being deposited in the soil seedbank (Smith Reference Smith1968). In addition, in Arkansas, barnyardgrass was reported to be resistant to six sites of action, making the weed difficult to control in rice production systems (Barber et al. Reference Barber, Butts, Boyd, Smith, Cunningham, Selden, Norsworthy, Burgos and Bertucci2023; Heap Reference Heap2024). Herbicide-resistant barnyardgrass plants are difficult to control due to the limited number of effective herbicides labeled for use on rice. Thus, additional herbicide sites of action are needed in dry-seeded rice production systems to control barnyardgrass.
The Roxy® Rice Production System (RRPS) is a new herbicide-resistant technology that allows oxyfluorfen to be applied in-crop both PRE and POST (McKenzie et al. Reference McKenzie, Andaya, Andaya and Leon2021). The Herbicide Resistance Action Committee (HRAC)/Weed Science Society of America (WSSA) categorizes oxyfluorfen as a Group 14 herbicide. The non-genetically modified organism trait used in the RRPS confers resistance to oxyfluorfen, allowing producers to use a Group 14 herbicide in rice production. No barnyardgrass populations have been reported to be resistant to HRAC/WSSA Group 14 herbicides (Heap Reference Heap2024). Using a Group 14 herbicide would provide a new site of action for barnyardgrass control in rice if it is proven to be effective.
The RRPS was developed by researchers at the California Rice Research Station following discovery of a trait in rice that confers resistance to oxyfluorfen (McKenzie et al. Reference McKenzie, Andaya, Andaya and Leon2021). The resistance trait was obtained through mutagenesis. Albaugh LLC is working with the California Rice Research Station to develop the RRPS and complementary herbicide chemistry. Due to limited research, the use of oxyfluorfen on rice crops still needs to be evaluated in a dry-seeded rice production system, which is common in the mid-southern United States, as opposed to the water-seeded production system that are common in California.
Oxyfluorfen inhibits protoporphyrinogen IX oxidase (PPO), and is used to control grass and broadleaf weed species (Price et al. Reference Price, Koger, Wilcut, Miller and Santen2008). Oxyfluorfen acts as a contact herbicide and can be applied PRE or POST to various crops, including broccoli (Brassica oleracea var. italica), cabbage (B. oleracea L.), cauliflower (B. oleracea var. botrytis), cotton (Gossypium hirsutum L.) (postdirected), and conifer and deciduous trees (Anonymous 2014). With commercialization of the RRPS, producers may choose to mix oxyfluorfen with other commonly applied PRE herbicides, such as clomazone or quinclorac, which are used to control barnyardgrass (Norsworthy et al. Reference Norsworthy, Bond and Scott2013).
Clomazone (a Group 13 herbicide) is labeled for barnyardgrass control on rice, soybean [Glycine max (L.) Merr.], and other crops (Anonymous 2021; Mills and Witt Reference Mills and Witt1989; Webster et al. Reference Webster, Baldwin and Dillion1999). Clomazone applied alone PRE on a silt loam soil effectively controls barnyardgrass (Talbert and Burgos Reference Talbert and Burgos2007; Westberg et al. Reference Westberg, Oliver and Frans1989; Zhang et al. Reference Zhang, Webster and Blouin2005). However, barnyardgrass populations in Arkansas have been confirmed to be resistant to clomazone, indicating that additional herbicide sites of action are needed to effectively control the weed (Barber et al. Reference Barber, Butts, Boyd, Smith, Cunningham, Selden, Norsworthy, Burgos and Bertucci2023; Heap Reference Heap2024).
Quinclorac is categorized as both a Group 4 (broadleaves) and Group 29 (grasses) herbicide that can be applied PRE or POST to control grass weeds (Kiebling and Pfenning Reference Kiebling, Pfenning, Grayson, Green and Copping1990). Quinclorac can be used on rice, rangelands, grain sorghum [Sorghum bicolor (L.) Moench], and preplant on wheat (Triticum aestivum L.) (Anonymous 2016; Moyer et al. Reference Moyer, Esau and Boswall1999). Quinclorac can control barnyardgrass, large crabgrass [Digitaria sanguinalis (L.) Scop], and broadleaf signalgrass [Urochloa platyphylla (Munro ex. C. Wright) R.D. Webster], all of which are common weeds in rice production fields (Anonymous 2016; Street and Mueller Reference Street and Mueller1993).
Barnyardgrass has evolved resistance to herbicides in HRAC/WSSA groups 1, 2, 4, 7, 13, and 29 in Arkansas, indicating that new herbicide sites of action will be needed to sustain chemical control practices in rice production (Barber et al. Reference Barber, Butts, Boyd, Smith, Cunningham, Selden, Norsworthy, Burgos and Bertucci2023; Heap Reference Heap2024). Therefore, experiments were conducted to evaluate the control of barnyardgrass when oxyfluorfen was used in sequential applications with clomazone or oxyfluorfen in an RRPS. An additional experiment was conducted to determine the efficacy of oxyfluorfen on barnyardgrass when following application of standard PRE herbicides (clomazone and quinclorac) in a dry-seeded, flood-irrigated RRPS. Furthermore, the tolerance of oxyfluorfen-resistant rice to oxyfluorfen was assessed with each unique combination of herbicides and herbicide rates.
Materials and Methods
Common Methodology
Field experiments were conducted in the 2021 and 2022 growing seasons at the Rice Research and Extension Center (RREC) near Stuttgart, AR, and the University of Arkansas Pine Bluff Small Farm Research Center (UAPB) near Lonoke, AR. Experiments conducted at the RREC were on a Dewitt silt loam soil (Fine, smectitic, thermic Typic Albaqualfs) (2023) with 27% sand, 54% silt, 19% clay, and 1.75% organic matter, pH 5.6. The experiments conducted at UAPB were on an Immanuel silt loam soil (Fine-silty, mixed, thermic Oxyaquic Glossudalfs) (2023) consisting of 14% sand, 72% silt, 14% clay, and 1.25% organic matter, pH 5.6. Both soil types were determined by the Arkansas Agricultural Diagnostic Laboratory, in Fayetteville, AR. Conventional tillage was used to prepare the seedbed for planting at RREC and UAPB. An oxyfluorfen-resistant long-grain rice cultivar (‘Roxy’; Albaugh LLC, St. Joseph, MO) was planted at 72 seeds per meter of row. Plots at the RREC were 2 m wide and 5 m long, and planted with a nine-row small plot drill, while plots at UAPB were 3 m wide and 8 m long, and planted with a seven-row small plot drill. All rice was planted with a spacing of 19 cm between each row.
A four-nozzle CO2-pressurized backpack sprayer calibrated to deliver 140 L ha−1 at 4.8 kph with AIXR 110015 nozzles (Teejet Technologies, Springfield, IL) was used for all herbicide applications at RREC. At UAPB, herbicides were applied using a multiboom, tractor-mounted sprayer calibrated to deliver 94 L ha−1 at 4.8 kph with AIXR 110015 nozzles. All treatments were replicated four times at each site. The PRE herbicides were applied to the soil within 1 d of planting, and the POST herbicides were applied to 2-leaf and 4-leaf rice at RREC and UAPB, respectively. Methylated seed oil was included at 1% v/v in all POST applications. At the POST application, barnyardgrass had one to two leaves at RREC and mostly two to three leaves at UAPB. Fertilizer was applied according to published recommendations (Roberts et al. Reference Roberts, Slaton, Wilson and Norman2021).
Visible injury and weed control ratings were evaluated on a scale of 0 to 100, with 0 representing no injury or weed control and 100 being crop death or complete weed control compared with nontreated plots. Barnyardgrass seed production was estimated 2 to 3 wk before harvest by collecting 20 panicles from the trial area. The average number of seeds per panicle was determined by threshing the 20 samples, weighing 500 seeds, and adjusting to seeds per panicle. This average seed production per panicle was then used to assess seed production per plot by multiplying the total number of panicles in each square meter. Rough rice grain was harvested with a small-plot combine, adjusted to 12% moisture, and reported in kilograms per hectare.
Clomazone and Oxyfluorfen in Sequential Applications
Field experiments were conducted during the 2021 and 2022 growing seasons at RREC and UAPB. The experiments at RREC and UAPB were initiated at planting (Table 1), and were aimed at comparing ratios of clomazone and oxyfluorfen when used in sequential applications for barnyardgrass control. The experiments were designed as randomized complete blocks, with seven treatments, including a nontreated control used for comparison. Clomazone (280 or 336 g ai ha−1) and oxyfluorfen (673 or 840 g ha−1) were applied separately and in combination (280 plus 840 g ha−1 and 336 plus 673 g ha−1) PRE. Following the PRE applications, the same herbicide rates and combinations were subsequently applied at the 2- or 4-leaf rice growth stages at RREC and UAPB, respectively. Visible injury and weed control ratings were taken 7 and 14 d after emergence, and 7, 21, and 35 d after the POST application at RREC. Assessments at UAPB were made 14 d after emergence, and again 7 and 14 d after the POST application. Barnyardgrass plants were counted and removed weekly until flood establishment in two 0.25-m2 quadrants established 7 d after rice emergence in each plot at RREC. At UAPB, barnyardgrass plants were counted 7 d after POST. Barnyardgrass panicles were counted in two 0.25-m2 quadrants at RREC and UAPB before harvest.
Table 1. Barnyardgrass control following a postemergence application of oxyfluorfen when used sequentially on a silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after rice emergence; DAPOST, days after postemergence application; fb, followed by; oxy, oxyfluorfen.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
Oxyfluorfen Following Oxyfluorfen or Commercial Standard Herbicides
Field experiments conducted at RREC were initiated at planting to evaluate oxyfluorfen applied POST following a PRE application of clomazone, oxyfluorfen, or quinclorac (Table 1). The experiment was designed as a randomized complete block with 11 treatments, including a nontreated control. Two treatments containing clomazone (336 g ha−1) or quinclorac (400 g ha−1) alone were applied PRE, and oxyfluorfen applied alone at 840 or 1,120 g ha−1. Additional PRE-applied treatments containing clomazone or quinclorac plus oxyfluorfen (840 or 1,120 g ha−1) were included in the experiment. At the 2-leaf rice growth stage, oxyfluorfen was applied at 560 or 840 g ha−1 to all treated plots; therefore, the annual sum of oxyfluorfen was ≤1,680 g ha−1 (C. Shelton, personal communication). Rice shoot counts were collected 14 d after the PRE but before the POST application. Visible injury and weed control ratings were recorded at 14 d after emergence, and 7, 21, and 35 d after POST. Barnyardgrass densities were counted at flooding, and panicles were counted before harvest in two 0.25-m2 quadrants per plot.
Data Analysis
All data were analyzed using JMP Pro software (v.17.0; SAS Institute Inc, Cary, NC). The experiments were analyzed as continuous distributions using the Fit Model function in JMP, and the Akaike information criterion was used to select the best fit for the data. All data were beta distributed; therefore, all data were analyzed using the GLIMMIX procedure with SAS software (Gbur et al. Reference Gbur, Stroup, McCarter, Durham, Young, Christman, West and Kramer2012). All data were subjected to an analysis of variance and separated using Tukey’s honestly significant difference test with an alpha value of 0.05. Because of a significant site-year by treatment interaction for each location, all data were analyzed by site-year.
The experiments evaluating clomazone and oxyfluorfen alone and in combination included a preplanned orthogonal contrast comparing sequential applications that contained one or multiple herbicides. The experiments comparing oxyfluorfen to clomazone and quinclorac were analyzed using preplanned orthogonal contrasts. Multiple rates of oxyfluorfen were compared with those of clomazone and quinclorac alone to determine whether superior weed control was provided by oxyfluorfen. Combinations of clomazone or quinclorac plus oxyfluorfen were compared to each other and to oxyfluorfen alone to assess whether adding either herbicide to oxyfluorfen would increase barnyardgrass control.
Results and Discussion
Clomazone and Oxyfluorfen in Sequential Applications
Weed Control
In one of four site-years, there was more barnyardgrass control with clomazone plus oxyfluorfen PRE combinations at 14 d after emergence than either herbicide alone (Tables 1 and 2). Following the POST application, all treatments provided a similar level of barnyardgrass control at all observations (Tables 1 and 2). At the RREC, barnyardgrass control never fell below 97% following any POST application. This indicates that the sequential use of clomazone, oxyfluorfen, and a combination of the two was highly effective, especially when overlaying these residual herbicides. The sequential applications at UAPB were generally less effective, which may be due to a later planting date during a time when there is typically less moisture in the soil, and a delay in applying the POST applications until the 4-leaf stage of rice. Conversely, POST applications at the RREC were timely for 2-leaf rice.
Table 2. Barnyardgrass control, barnyardgrass density, and seed production following a postemergence application of oxyfluorfen when used sequentially on a silt loam soil in 2021 and 2022 at the University of Arkansas at Pine Bluff Small Farm Research Center near Lonoke, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after rice emergence; DAPOST, days after postemergence application; fb, followed by; oxy, oxyfluorfen.
b Postemergence applications were applied at the 4-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
The number of barnyardgrass plants in all site-years was comparable across all herbicide treatments at 7 d after POST, and similarly, contrast revealed that single or multiple herbicides did not provide a difference in barnyardgrass density (Tables 2 and 3). All combinations of clomazone and oxyfluorfen resulted in comparable barnyardgrass seed production at both rates of clomazone and oxyfluorfen alone. Contrasts showed that sequential applications containing multiple herbicides resulted in fewer barnyardgrass seeds than applications containing only one herbicide.
Table 3. Cumulative barnyardgrass density, barnyardgrass seed production before harvest, and rough rice yields following a postemergence application of oxyfluorfen when used sequentially on a silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after rice emergence; DAPOST, days after postemergence application; fb, followed by; oxy, oxyfluorfen.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
Overlapping herbicides with effective POST and residual activity should prevent weeds from emerging and control any weeds that emerged since the previous herbicide application (Norsworthy et al. Reference Norsworthy, Bond and Scott2013; Vishwakarma et al. Reference Vishwakarma, Meena, Das, Jha, Biswas, Bharati, Hati, Chaudhary, Shirale, Lakaria, Gurav and Patra2023). Clomazone applied at 280 g ha−1 has shown the potential to control barnyardgrass when applied PRE (Westberg et al. Reference Westberg, Oliver and Frans1989). Barnyardgrass has shown susceptibility to oxyfluorfen applied at 800 g ha−1 POST; therefore, additional control would be expected when combined with a residual herbicide (Lee et al. Reference Lee, Matsumoto, Pyon and Ishizuka1991).
Broadleaf signalgrass was absent at UAPB; therefore, the weed was evaluated only at RREC. Broadleaf signalgrass control was comparable across all herbicide treatments in both site-years, except for less control with clomazone applied alone at 280 g ha−1 in 2022 at 14 d after emergence (Table 4). At 7 d after POST, there were no differences in broadleaf signalgrass control for both site-years, indicating that treatments containing oxyfluorfen were comparable to that of clomazone alone. All treatments provided similar levels of broadleaf signalgrass control at 35 d after POST in 2021, as control never fell below 99%. Clomazone at 336 g ha−1 applied both PRE and POST provided comparable barnyardgrass control to that of all other treatments at 35 d after POST in 2022, except for clomazone at 280 g ha−1. In 2022, broadleaf signalgrass control ranged from 59% to 95% at 35 d after treatment. The combinations of clomazone plus oxyfluorfen can improve broadleaf signalgrass control in most instances over the herbicides alone at 35 d after treatment. The findings from this research differ from those of previous studies, which have shown that low levels of broadleaf signalgrass control with clomazone alone are not common (O’Barr et al. Reference O’Barr, McCauley, Bovey, Senseman and Chandler2007; Willingham et al. Reference Willingham, Falkenberg, McCauley and Chandler2008). In 2021 and 2022, there were 72 and 509 plants m−2, respectively, at the time of the POST application. The differing broadleaf signalgrass control observed in 2022 could be due to a dense weed population.
Table 4. Broadleaf signalgrass control following a postemergence application of oxyfluorfen when used sequentially on silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after rice emergence; DAPOST, days after postemergence application; fb, followed by; oxy, oxyfluorfen.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
Tolerance
In one of the two years at RREC, higher injury levels were observed with oxyfluorfen than clomazone alone applied at 14 d after emergence; however, rice in all treatments exhibited comparable injury levels in 2021 (Table 5). In 2022, a contrast revealed that using multiple herbicides in sequential applications leads to greater injury than applications containing only one herbicide. The significance of this contrast appears to be mainly driven by low levels of injury incurred with sequential applications of clomazone alone versus the higher injury levels caused by oxyfluorfen. In general, sequential applications of oxyfluorfen plus clomazone caused injury to rice that was similar to that of sequential oxyfluorfen applied alone. The most noticeable visible injury symptom following clomazone applied PRE was the bleaching of rice, whereas oxyfluorfen caused reduced plant vigor and necrosis, which is consistent with a protoporphyrinogen oxidase inhibitor. In one of two site years at RREC, rice injury was greatest with oxyfluorfen treatments compared to clomazone alone 7, 21, and 35 d after POST. The injury observed following the POST application was due to the rice being stunted following the PRE application, which would lead to an increased risk for injury from the POST application (C. Arnold, unpublished data). Observations at 35 d after POST indicated that oxyfluorfen-resistant rice may not always recover from sequential applications of clomazone plus oxyfluorfen by this evaluation.
Table 5. Oxyfluorfen-resistant rice injury following a postemergence application of oxyfluorfen when used sequentially on silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after rice emergence; DAPOST, days after postemergence application; fb, followed by; oxy, oxyfluorfen.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α=0.05.
At UAPB, there was no difference in rice injury among treatments at 14 d after emergence in one of two site-years (Table 6). In 2022 at 14 d after emergence, the injury to rice from clomazone plus oxyfluorfen was similar to all other treatments. At 7 d after POST, there was no difference in rice injury between herbicide treatments, likely due to the size of plants at the time of application. After the POST application, a contrast revealed no difference in injury when single or multiple herbicides were used in sequential applications. When the herbicide was applied POST, the rice was at the 4-leaf growth stage in both site years. Differences in plant size at the time of the POST application could influence the amount of injury and weed control observed with oxyfluorfen. As rice plants increase in size, the amount of injury caused by oxyfluorfen decreases (C. Arnold, unpublished data).
Table 6. Oxyfluorfen-resistant rice injury and rough rice yields for the sequential application of clomazone and oxyfluorfen experiment on a silt loam soil in 2021 and 2022 at the University of Arkansas at Pine Bluff Small Farm Research Center near Lonoke, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after rice emergence; DAPOST, days after postemergence application; fb, followed by; oxy, oxyfluorfen.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
There were no differences between herbicide treatments in rough rice yield at RREC in both site years (Table 3). The injury observed in the experiment did not appear to affect the amount of rough rice collected from the experiment at harvest. However, the number of barnyardgrass plants that produced seeds following each treatment likely influenced the amount of rough rice harvested. Barnyardgrass that competes with rice will reduce rough rice yields to varying degrees based on the length of competition (Smith Reference Smith1968). It is worth noting that rough rice yields at both locations were generally low compared with the Arkansas state average for rice (Hardke Reference Hardke2021), which may partly indicate that the oxyfluorfen-resistant trait is not in a high-yielding, adapted cultivar for the region.
Applications of oxyfluorfen alone resulted in rough rice yields that were comparable to those of all other herbicide treatments at UAPB in 2022 (Table 6). A contrast revealed that applications of multiple herbicides resulted in greater rough rice yields than applications that contained only one herbicide. It is noted that rice yields were numerically lowest following sequential applications of clomazone at 280 g ha−1, which is less than the labeled rate for the soil texture; however, removing the yield data for the lowest rate of clomazone from the contrasts comparing single and multiple herbicides still results in a significant improvement in rice yield of 710 kg ha−1. There was minimal injury in the trial at UAPB; therefore, injury from the herbicide likely did not influence rough rice yields. The number of barnyardgrass plants in each plot that produced seed prior to harvest likely influenced rough rice yields; however, the cultivar is neither high-yielding nor adapted to the mid-southern United States.
Oxyfluorfen Applied Following Oxyfluorfen or Standard Preemergence Herbicides
Weed Control
Based on contrasts, oxyfluorfen applied PRE at 840 and 1,120 g ha−1 provided a comparable level of barnyardgrass control to that of clomazone or quinclorac alone in both site years at all observation timings (Table 7). Hence, oxyfluorfen could serve as an alternative site of action for barnyardgrass control in oxyfluorfen-resistant rice. An alternative site of action would help producers diversify their herbicide programs and control barnyardgrass that has become resistant to other herbicides (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012).
Table 7. Barnyardgrass control following standard preemergence herbicides on a silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after emergence; DAPOST, days after postemergence; oxy low, oxyfluorfen at 840 g ha−1 preemergence; oxy high, oxyfluorfen at 1,120 g ha−1 preemergence; PRE, preemergence; Quin, quinclorac; std, clomazone and quinclorac.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
Contrasts revealed no difference in barnyardgrass control between applications of clomazone plus oxyfluorfen, quinclorac plus oxyfluorfen, or oxyfluorfen alone 14 d after emergence (Table 7). However, contrasts showed greater barnyardgrass control with quinclorac plus oxyfluorfen than oxyfluorfen alone at 7 d after POST in 2021. Although there were differences in barnyardgrass control observed at 7 d after POST, there were little to no biological differences.
Before harvest, all contrasts revealed a similar number of barnyardgrass panicles in 2021 (Table 8). The only difference observed in either site year occurred after the 840 g ha−1 rate of oxyfluorfen was applied PRE, which produced more barnyardgrass panicles than clomazone- or quinclorac-alone treatments. The findings from this research indicate that oxyfluorfen applied alone at 840 g ha−1 or in sequential applications allows for greater production of barnyardgrass panicles than clomazone or quinclorac alone, followed by oxyfluorfen at 840 g ha−1. The number of barnyardgrass panicles that were produced appeared to have been influenced by barnyardgrass control at 35 d after POST. The results support the need to use multiple sites of action in a weed control program.
Table 8. Barnyardgrass counts at flooding, panicle counts before harvest, and stand counts following standard PRE herbicides on silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after emergence; DAPOST, days after postemergence; oxy low, oxyfluorfen at 840 g ha−1 preemergence; oxy high, oxyfluorfen at 1,120 g ha−1 preemergence; PRE, preemergence; Quin, quinclorac; std, clomazone and quinclorac.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
Oxyfluorfen can provide broadleaf signalgrass control that is comparable to that of herbicides commonly used PRE in mid-southern U.S. rice production. At 14 d after emergence, broadleaf signalgrass control was greater with oxyfluorfen applied at 1,120 g ha−1 than clomazone or quinclorac applied alone PRE in one of two site years (Table 9). However, little to no biological variation was observed in broadleaf signalgrass control. Oxyfluorfen applied alone at 840 g ha−1 did not improve broadleaf signalgrass control compared to clomazone or quinclorac applied alone PRE at all observation timings in both years.
Table 9. Broadleaf signalgrass control following standard preemergence herbicides on a silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after emergence; DAPOST, days after postemergence; oxy low, oxyfluorfen at 840 g ha−1 preemergence; oxy high, oxyfluorfen at 1,120 g ha−1 preemergence; PRE, preemergence; Quin, quinclorac; std, clomazone and quinclorac.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α = 0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
Broadleaf signalgrass control was improved with quinclorac plus oxyfluorfen compared to clomazone plus oxyfluorfen or to oxyfluorfen applied alone at 14 d after emergence and 21 d after POST in one of two site years (Table 9). By 35 d after POST, broadleaf signalgrass control in both site years was comparable for quinclorac plus oxyfluorfen, clomazone plus oxyfluorfen, and oxyfluorfen alone. Oxyfluorfen alone provided broadleaf signalgrass control similar to that of quinclorac and clomazone.
Tolerance
The 840 and 1,120 g ha−1 rates of oxyfluorfen applied PRE caused greater injury in the form of stand reduction than clomazone or quinclorac applied alone 14 d after emergence in one of two site-years (Tables 9 and 10). Contrasts revealed that oxyfluorfen applied at 1,120 g ha−1 followed by oxyfluorfen at 560 g ha−1 caused greater injury to oxyfluorfen-resistant rice than clomazone and quinclorac followed by oxyfluorfen in one site-year at 35 d after POST (Table 10). Preemergence applications of clomazone resulted in bleaching, quinclorac caused leaf malformation, and oxyfluorfen reduced the rice stand. Rice plants can compensate for a reduction in stand by producing more tillers; however, there is a point at which the plants cannot compensate for the stand loss (Bond et al. Reference Bond, Walker, Bollich, Koger and Gerard2005; Harrell and Blanche Reference Harrell and Blanche2010). The injury observed with oxyfluorfen applied PRE could explain why the herbicide is not labeled for use in rice production (Anonymous 2014).
Table 10. Oxyfluorfen-resistant rice injury following standard preemergence herbicides on silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR. a,b,c
a Abbreviations: clom, clomazone; DAE, days after emergence; DAPOST, days after postemergence; oxy low, oxyfluorfen at 840 g ha−1 preemergence; oxy high, oxyfluorfen at 1,120 g ha−1 preemergence; PRE, preemergence; Quin, quinclorac; std, clomazone and quinclorac.
b Postemergence applications were applied at the 2-leaf growth stage of rice.
c Means within the same column followed by the same letter are not different according to Tukey’s honestly significant difference test (α.=.0.05); the absence of letters indicates no treatment difference was present. An asterisk indicates a significant contrast at α = 0.05.
Although oxyfluorfen mixed with quinclorac was compatible with oxyfluorfen, the interaction of the herbicides in-crop was damaging to oxyfluorfen-resistant rice. Combining quinclorac and oxyfluorfen and applying them PRE increased injury to oxyfluorfen-resistant rice over oxyfluorfen alone in one of two site-years at all observation timings (Table 10). The increased injury could be due to the use of multiple herbicides that act upon oxyfluorfen-resistant rice. However, clomazone plus oxyfluorfen applied PRE caused comparable injury at all observation timings in both site years. Carfentrazone or saflufenacil, an alternative protoporphyrinogen oxidase-inhibiting herbicide, applied PRE, has shown the ability to cause >20% injury to rice (Montgomery et al. Reference Montgomery, Bond, Golden, Gore, Edwards, Eubank and Walker2014).
Practical Implications
Barnyardgrass resistance to HRAC/WSSA Group 13 herbicides has been documented (Heap Reference Heap2024), indicating that alternative herbicides should be used for residual control of this weed. Producers should combine clomazone and oxyfluorfen for barnyardgrass and broadleaf signalgrass control in a RRPS to diversify the herbicides used. Using oxyfluorfen in a weed control program may potentially provide control of herbicide-resistant barnyardgrass, because barnyardgrass has no documented resistance to Group 14 herbicides in the mid-southern United States. The recommended use rate of clomazone on silt loam soil is 336 g ha−1; therefore, clomazone and oxyfluorfen applied PRE would likely be recommended at 336 and 673 g ha−1, respectively. The potential annual use limit of oxyfluorfen is 1,680 g ha−1; therefore, sequential applications of clomazone and oxyfluorfen at 336 and 673 g ha−1 would be allowable to oxyfluorfen-resistant rice (C. Shelton, personal communication). While this combination provides control of barnyardgrass, further research is needed to determine the full spectrum of weeds that would be controlled.
If oxyfluorfen becomes labeled for use on oxyfluorfen-resistant rice, the herbicide could provide rice producers with a new site of action for barnyardgrass and broadleaf signalgrass control. Based on this research, producers could use oxyfluorfen alone or with standard PRE herbicides to control barnyardgrass on silt loam soil. The rice injury varied among site-years, and the crop sometimes struggled to recover from the early season injury. Tolerance trials without weeds would be needed to determine whether early season injury from oxyfluorfen translates into grain yield loss. Oxyfluorfen mixed with quinclorac or clomazone would likely improve barnyardgrass control in some instances, reduce the selection for target-site resistance, and broaden the spectrum for control based on the weeds listed on current oxyfluorfen product labels (Anonymous 2014; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012).
Acknowledgments
We thank staff members at the Rice Research and Extension Center and the University of Arkansas Pine Bluff Small Farm Research Center for their assistance with plot establishment and maintenance.
Funding
The Arkansas Rice Promotion Board and Albaugh LLC provided partial funding for this research.
Competing Interests
Chad Shelton is an employee of Albaugh LLC.
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