Introduction
Rice producers in the United States are challenged with controlling herbicide-resistant weeds. In Arkansas, barnyardgrass is the most problematic weed that rice producers are challenged with controlling, especially in a flooded rice system (Butts et al. Reference Butts, Kouame, Norsworthy and Barber2022; Norsworthy et al. Reference Norsworthy, Bond and Scott2013). Currently, few herbicides are labeled for use in rice that will effectively control barnyardgrass. Barnyardgrass in Arkansas is resistant to the Herbicide Resistance Action Committee (HRAC)/Weed Science Society of America (WSSA) Groups 1, 2, 4, 5, 13, and 29 (Barber et al. 2023; Heap Reference Heap2023). Barnyardgrass resistance is most prevalent to postemergence herbicides, meaning that residual herbicides are the best chemical control option for controlling herbicide-resistant barnyardgrass.
Rice producers in the mid-southern United States typically use residual herbicides on conventional and herbicide-resistant rice, with clomazone being the most employed for grass control (Norsworthy et al. Reference Norsworthy, Bond and Scott2013). For the most effective control with residual herbicides, producers should start with a clean field, apply a residual herbicide, and then make a sequential application 2 to 3 wk later with a residual and postemergence herbicide (Osterholt et al. Reference Osterholt, Webster, Blouin and McKnight2019). Residual herbicides can be mixed with herbicides that have postemergence activity to keep the field weed-free throughout the growing season. Herbicide applications in a rice field containing propanil and a residual herbicide resulted in greater weed control and rice yields than when propanil was applied alone (Crawford and Jordan Reference Crawford and Jordan1995). In other research, thiobencarb and pendimethalin applied with propanil as an early postemergence herbicide provided greater control of propanil-resistant barnyardgrass than propanil alone (Talbert and Burgos Reference Talbert and Burgos2007). Hence, there are scenarios in which using residual herbicides, like clomazone, and a postemergence herbicide will provide better grass control than a postemergence herbicide alone.
Residual herbicides vary in the length of time the parent molecule remains intact in the soil, commonly called herbicide persistence (Colquhoun Reference Colquhoun2006). Residual herbicides can provide varying levels of weed control depending on soil texture, soil pH, and climatic factors (Colquhoun Reference Colquhoun2006; Curran Reference Curran1999; Helling Reference Helling and Van Acker2005; Radosevich et al. Reference Radosevich, Holt and Ghersa2007). Weather patterns in an area where residual herbicides have been applied can influence the herbicide’s persistence. High temperatures allow residual herbicides to be broken down quickly due to chemical and microbial degradation (Curran Reference Curran1999; Radosevich et al. Reference Radosevich, Holt and Ghersa2007). Rainfall or irrigation are common practices for incorporating residual herbicides into the soil; however, excessive rainfall or irrigation may cause herbicide leaching or runoff (Colquhoun Reference Colquhoun2006; Radosevich et al. Reference Radosevich, Holt and Ghersa2007). In rice production, residual herbicides exposed to excessive rainfall or irrigation are prone to runoff, as rice is commonly grown in areas where leaching is not an issue.
Oxyfluorfen is a HRAC/WSSA Group 14 herbicide that inhibits protoporphyrinogen IX oxidase (Anonymous 2013). Oxyfluorfen was introduced to control broadleaf and grass weed species when applied in various crops and fallow areas (Anonymous 2013). Because oxyfluorfen can control grass weed species, combinations of clomazone and oxyfluorfen could allow producers to control problematic grass weed species in rice using multiple sites of action (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). However, oxyfluorfen is not currently labeled in rice and is typically not used in anaerobic conditions.
Oxyfluorfen-resistant rice has been bred at the California Cooperative Rice Research Foundation, potentially allowing for oxyfluorfen use in season for rice producers (McKenzie et al. Reference McKenzie, Andaya, Andaya and De Leon2021). The trait that confers resistance to oxyfluorfen is a mutant allele and was not genetically inserted into the plant (McKenzie et al. Reference McKenzie, Andaya, Andaya and De Leon2021). The oxyfluorfen-resistant trait can be bred into new and existing rice varieties so that producers can use a cultivar that best fits the needs of the farm.
There have been limited studies on oxyfluorfen use in rice, especially regarding weed control; therefore, trials were conducted to evaluate weed control and the level of tolerance of oxyfluorfen-resistant rice to preemergence-applied ratios of clomazone plus oxyfluorfen compared to clomazone alone on silt loam and clay soil.
Materials and Methods
General Methodology
Field experiments were conducted in 2021 and 2022 at the Rice Research and Extension Center (RREC) near Stuttgart (Lat. 34.465164; Long. 91.402792), AR, the University of Arkansas Pine Bluff Small Farm Research Center (UAPB) near Lonoke (Lat. 34.845378; Long. 91.881639 ), AR, and the Northeast Research and Extension Center (NEREC) in Keiser (Lat. 35.666253; Long. 90.081986), AR. Experiments at the RREC were conducted on a Dewitt silt loam soil (Fine, smectitic, thermic Typic Albaqualfs) with 27% sand, 54% silt, 19% clay, 1.75% organic matter, and a pH of 5.6 (USDA-NRCS 2022). Experiments at UAPB were conducted on an Immanuel silt loam soil (Fine-silty, mixed, thermic Oxyaquic Glossudalfs) consisting of 14% sand, 72% silt, 14% clay, and 1.25% organic matter, with a pH of 5.6. Experiments at NEREC were conducted on a Sharkey silty clay with 41% sand, 1% silt, 58% clay, and 2.8% organic matter, with a pH of 5.5. All experiments were planted with an oxyfluorfen-resistant long-grain rice cultivar (Roxy; Albaugh LLC, St Joseph, MO). Rice was planted at 72 seeds m–1 of row on May 14, 2021, and April 29, 2022, at the RREC; April 22, 2021, and May 10, 2022, at the NEREC; and May 24, 2021, and May 11, 2022, at UAPB. The experiments were designed as a randomized complete block, with 9 and 12 treatments for the silt loam and clay soil experiments, respectively. Clomazone (Command® 3ME; FMC Corp., Philadelphia, PA) and oxyfluorfen (ALB2023; Albaugh LLC, St. Joseph, MO) were used in the experiments. Clomazone was applied alone at 224 or 336 g ai ha–1 on silt loam soil and 448, 560, and 672 g ha–1 on clay soil. The amount of oxyfluorfen was determined by using 1:1.5, 1:2, and 1:3 ratios of clomazone to oxyfluorfen. All herbicides were applied preemergence, and a nontreated control was included for comparison. Plots at the RREC and NEREC were 1.8 m wide and 5.2 m long, whereas plots at UAPB were 3.1 m wide and 7.7 m long. The preemergence applications were applied to the soil immediately after planting. Applications at RREC and NEREC were made with a four-nozzle CO2-pressurized backpack sprayer using AIXR 110015 nozzles at 4.8 kph and 140 L ha–1. Applications at UAPB were made using a tractor-mounted, multi-boom sprayer calibrated to deliver 94 L ha–1 at 4.8 kph with AIXR 110015 nozzles. Each treatment at each site was replicated four times. Rice shoot counts were taken in two 1-m sections of a row at either 2 or 3 wk after emergence (WAE). Visible rice injury and barnyardgrass control ratings were recorded 1, 3, and 5 WAE at RREC and UAPB; and 1, 3, 5, and 7 WAE at NEREC. Two 0.25-m2 quadrants were flagged 1 WAE in each plot. Weed species in the quadrants were counted and pulled weekly until the experiments were flooded. Weed counts were only evaluated at UAPB in 2022. Twenty barnyardgrass panicles were collected from each experiment to estimate seed production at RREC and NEREC in 2021 and all locations in 2022. Two applications of azoxystrobin plus difenoconazole (Amistar Top; Syngenta Crop Protection, Greensboro, NC) were applied to each trial each year to protect the crop from diseases. No maintenance herbicide or insecticide applications were made. Rough rice yields were not collected in 2021 at UAPB as a result of severe lodging in the experiment.
Data Analysis
JMP Pro v. 17.0 (JMP) and SAS v 9.4 (SAS) (SAS Institute Inc, Cary, NC) were used to analyze all data for the experiments. Data were subjected to an ANOVA and means separated using Tukey’s honest significant difference test with an alpha value of 0.05 (Gbur et al. Reference Gbur, Stroup, McCarter, Durham, Young, Christman, West and Kramer2012). In addition, all experiments had a significant site-year-by-treatment interaction; therefore, all data were analyzed separately by site-year.
All experiments were analyzed using preplanned orthogonal contrasts. Ratios of clomazone to oxyfluorfen were compared to determine if one ratio was superior to others, and ratios were compared to clomazone alone to determine if the addition of oxyfluorfen improved grass weed control.
Results and Discussion
Silt Loam Soil
Weed Control
A contrast indicated that adding oxyfluorfen to clomazone improved barnyardgrass control in two of four site-years at 1 WAE (Table 1). Adding oxyfluorfen to clomazone improved barnyardgrass control in all site-years by 3 WAE. Barnyardgrass control with clomazone alone ranged from 78% to 100% at 1 WAE for the four site-years evaluated.
Table 1. Barnyardgrass control following a preemergence application of clomazone and oxyfluorfen on silt loam soil in 2021 and 2022.a–c
a Abbreviations: Clom, clomazone; n/a, not applicable; oxy, oxyfluorfen; RREC, Rice Research and Extension Center; UAPB, University of Arkansas Pine Bluff Small Farm Research Center; WAE, wk after emergence.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c * Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen.
A previous study has shown that barnyardgrass is susceptible to oxyfluorfen (Lee et al. Reference Lee, Matsumoto, Pyon and Ishizuka1991). Therefore, adding oxyfluorfen to clomazone could benefit areas where clomazone-resistant barnyardgrass is suspected or confirmed. In addition to improved barnyardgrass control, oxyfluorfen plus clomazone provides two effective sites of action, a strategy known to mitigate the evolution of herbicide resistance (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012).
Contrasts indicated that all ratios of clomazone and oxyfluorfen provided similar levels of barnyardgrass control in three of four site-years at 1 and 3 WAE (Table 1). By 5 WAE, all ratios of oxyfluorfen and clomazone provided similar levels of barnyardgrass control across all site-years. In other research, clomazone at 280 g ha–1 provided 90% to 100% barnyardgrass control (Westberg et al. Reference Westberg, Oliver and Frans1989). Effective control should be expected with all ratios that included clomazone at 336 g ha–1, as both herbicides are labeled for barnyardgrass control in crops on silt loam soil (Anonymous 2013).
All clomazone/oxyfluorfen ratios were comparable in the amount of barnyardgrass seed produced (Table 2). Adding oxyfluorfen to soil-applied clomazone reduced barnyardgrass seed production in one of three site-years. Allowing barnyardgrass plants to interfere with rice and reproduce could substantially affect rough rice grain yield (Smith Reference Smith1968) and lead to greater barnyardgrass infestations due to increases in the soil seedbank. The return of barnyardgrass seeds to the soil seedbank could cause significant ramifications for producers in future growing seasons and lead to the selection of barnyardgrass plants resistant to oxyfluorfen.
Table 2. Cumulative barnyardgrass densities and barnyardgrass seed production following a preemergence application of clomazone and oxyfluorfen on silt loam soil in 2021 and 2022.a–c
a Abbreviations: Clom, clomazone; n/a, not applicable; oxy, oxyfluorfen; RREC, Rice Research and Extension Center; UAPB, University of Arkansas Pine Bluff Small Farm Research Center; WAE, wk after emergence.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c * Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen
Broadleaf signalgrass [Urochloa phatyphylla (Wright) Webster] was present for two site-years compared to four site-years for barnyardgrass. For all observations, contrasts indicated no improved control with adding oxyfluorfen to clomazone in either site-year (Table 3). Broadleaf signalgrass control did not differ among any of the clomazone/oxyfluorfen ratios tested.
Table 3. Broadleaf signalgrass control following a preemergence application of clomazone and oxyfluorfen on silt loam soil in 2021 and 2022 at the Rice Research and Extension Center near Stuttgart, AR.a–c
a Abbreviations: Clom, clomazone; oxy, oxyfluorfen; n/a, not applicable; WAE, wk after emergence.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c *Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen.
Clomazone alone can provide control of broadleaf signalgrass when used at similar rates and is labeled for control of the weed at 336 g ha–1 (Anonymous 2021; O’Barr et al. Reference O’Barr, McCauley, Bovey, Senseman and Chandler2007). Oxyfluorfen at 1,120 g ha–1 has provided 80% control of broadleaf signalgrass 3 WAE (Price et al. Reference Price, Koger, Wilcut, Miller and van Santen2008). Although research has shown that oxyfluorfen provides some control of broadleaf signalgrass, oxyfluorfen is not labeled for control of the weed (Anonymous 2013). Broadleaf signalgrass control will not be improved by adding oxyfluorfen to clomazone.
Tolerance
The visible rice injury following an application of clomazone plus oxyfluorfen was in the form of bleaching caused by clomazone along with stunting, necrosis, and reduced plant vigor caused by oxyfluorfen based on comparison to clomazone alone treatments. At 1 WAE, visible rice injury for all treatments was ≤7% for two of the four site-years, whereas injury was as much as 40% at RREC in 2022 for clomazone at 336 g ha–1 in the 1:3 ratio of clomazone to oxyfluorfen (Table 4). In three of the four site-years, there was less than 5% visible rice injury for all ratios of clomazone and oxyfluorfen at 5 WAE. In general, the rice was able to recover from a preemergence application of clomazone plus oxyfluorfen by 5 WAE; however, in 2022, there was still 23% injury to rice following the 1:3 ratio with clomazone at 336 g ha–1 at RREC. The high injury observed in 2022 at RREC could be attributed to the site receiving ≥1.3 cm of rainfall 7 DAT compared to 15 DAT in 2021.
Table 4. Rice injury following a preemergence application of clomazone and oxyfluorfen on silt loam soil in 2021 and 2022.a–c
a Abbreviations: Clom, clomazone; n/a, not applicable; oxy, oxyfluorfen; RREC, Rice Research and Extension Center; UAPB, University of Arkansas Pine Bluff Small Farm Research Center; WAE, wk after emergence.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c * Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen.
The earlier activating rainfall in 2022 could have made the herbicide available for uptake by germinating seedlings in 2022, leading to greater injury. Oxyfluorfen has demonstrated the ability to cause a greater reduction in biomass when applied to wet soil compared to dry soil (Chon et al. Reference Chon, Guh, Han, Lee and Shin1997). A study conducted in the greenhouse showed that preemergence applications of oxyfluorfen resulted in the most injury when the soil was maintained at 100% saturation (C. Arnold, unpublished data), indicating that increased injury could be observed if the soil in the field was at 100% saturation.
Contrasts revealed that oxyfluorfen addition to clomazone can increase the risk for rice injury over clomazone alone (Table 4). Oxyfluorfen applied preemergence in combination with clomazone caused greater injury in two of the four site-years 1 WAE than clomazone alone, albeit, in one of the significant site-years, the difference was not likely of biological significance. By 5 WAE, visible rice injury was greater in one of four site-years when comparing rice treated with and without oxyfluorfen, potentially a result of the rainfall received before the rice emerged. Excessive water in the experiment could have caused greater injury because of the herbicide being available for uptake by the plant early in the growing season (Paiman, Reference Paiman2019).
Contrasts revealed that a 1:1.5 ratio of clomazone to oxyfluorfen caused less injury to rice than the 1:3 ratio for one of the four site-years at 1 WAE (Table 4). The 1:3 ratio resulted in greater levels of rice injury when contrasted with the 1:2 ratio in two of four site-years. The 1:3 ratio would be expected to have more injury than the 1:1.5 or 1:2 ratio due to the higher rate of oxyfluorfen applied and hence greater herbicide availability for the rice plants. By 3 WAE, contrasts indicated no differences in injury among the ratios of clomazone to oxyfluorfen evaluated, and all ratios resulted in ≤13% injury of oxyfluorfen-resistant rice averaged over clomazone rates.
In some instances, the visible injury was related to a reduced rice stand (Table 5). The number of shoots per meter row of rice was reduced when a 1:3 ratio of clomazone to oxyfluorfen was used, compared to a 1:2 ratio in one of three site-years. The 1:3 ratios decreased the shoot counts compared to the 1:1.5 ratios in two of three site-years. The 1:2 clomazone plus oxyfluorfen ratios had increased rice shoots in one of three site-years. Using oxyfluorfen in oxyfluorfen-resistant rice can cause reduced shoot density by thinning rice via the death of emerging plants. In general, rice shoot densities appear to be consistently reduced when the ratio of clomazone to oxyfluorfen increases.
Table 5. Rice shoot counts and rough rice yield following a preemergence application of clomazone and oxyfluorfen on silt loam soil in 2021 and 2022.a–c
a Abbreviations: Clom, clomazone; n/a, not applicable; oxy, oxyfluorfen; RREC, Rice Research and Extension Center; UAPB, University of Arkansas Pine Bluff Small Farm Research Center; WAE, wk after emergence.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c *Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen.
Rough rice yields were influenced by the ratio of clomazone plus oxyfluorfen used and the addition of oxyfluorfen to clomazone in one of three site-years (Table 5). When oxyfluorfen was applied with clomazone, rough rice yield was increased compared to clomazone alone at UAPB in 2022. A 1:3 ratio of clomazone plus oxyfluorfen resulted in higher rough rice yields than a 1:2 ratio at UAPB in 2022. At RREC in 2022, there was lodging in the trial resulting in extreme yield variability.
Yield at UAPB in 2022 increased as barnyardgrass control numerically increased. However, there did not appear to be a relationship between oxyfluorfen-resistant rice injury and rough rice yield. A severe reduction in rice yields can be observed when barnyardgrass competes with rice for the growing season (Smith Reference Smith1968, Reference Smith1974). There could have been other factors that influenced rice yield; however, these factors would have likely been uniform throughout the trial.
Clay Soil
Weed Control
Contrasts indicated that all ratios of clomazone and oxyfluorfen evaluated on clay soil provided comparable barnyardgrass control through 3 WAE in both site-years (Table 6). By 5 WAE, the 1:3 ratio of clomazone to oxyfluorfen provided greater barnyardgrass control than the 1:1.5 ratio in one of two site-years. Additional control as the oxyfluorfen rate increased equates with the fact that oxyfluorfen alone is labeled for barnyardgrass control in several crops (Anonymous 2013). The same trend was apparent 7 WAE; however, contrasts indicated that barnyardgrass control had decreased to 75% to 86% for all clomazone plus oxyfluorfen ratios. Based on the contrasts conducted, the 1:2 and 1:3 ratios of clomazone to oxyfluorfen would be better suited for barnyardgrass control than the 1:1.5 ratio in drill-seeded rice across varying soil types.
Table 6. Barnyardgrass control following a preemergence application of clomazone and oxyfluorfen on clay soil in 2021 and 2022 at the Northeast Research and Extension Center in Keiser, AR.a–c
a Abbreviations: Clom, clomazone; n/a, not applicable; oxy, oxyfluorfen; WAE, wk after emergence.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c *Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen.
Clomazone is a widely used preemergence herbicide for grass weed control in mid-southern U.S. rice production (Norsworthy et al. Reference Norsworthy, Bond and Scott2013). Contrasts revealed that oxyfluorfen addition to clomazone improved barnyardgrass control in one of two site-years at 1 WAE on clay soil (Table 6). At 3 and 5 WAE, there was improved barnyardgrass control when oxyfluorfen was included with clomazone in both site-years. Adding oxyfluorfen to clomazone reduced the number of barnyardgrass plants that had emerged by 3 and 4 WAE compared to clomazone alone in both site-years (Table 7). The clomazone plus oxyfluorfen ratios evaluated had no influence on the number of emerging barnyardgrass plants up to 3 WAE in both site-years. Using multiple sites of action would allow producers to achieve effective barnyardgrass control and contribute to the sustainability of the Roxy Rice Production System (Green Reference Green2014).
Table 7. Cumulative barnyardgrass densities and barnyardgrass seeds produced following a preemergence application of clomazone and oxyfluorfen on clay soil in 2021 and 2022 at the Northeast Research and Extension Center in Keiser, AR.a–c
a Abbreviations: Clom, clomazone; n/a, not applicable; oxy, oxyfluorfen; WAE, wk after emergence.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c * Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen.
The number of barnyardgrass seeds produced by plants on an area basis was not reduced by adding oxyfluorfen to clomazone (Table 7). However, the 1:3 ratio of clomazone to oxyfluorfen decreased barnyardgrass seed production compared to the 1:1.5 and 1:2 ratios in one of the two site-years. The seed production observed could result from the barnyardgrass control and emergence observed earlier in the growing season. The seed returned to the soil seedbank can potentially cause herbicide control failures in the future or select weeds resistant to oxyfluorfen (Schwartz-Lazaro and Copes Reference Schwartz-Lazaro and Copes2019).
Tolerance
Adding oxyfluorfen to clomazone increased injury levels to oxyfluorfen-resistant rice in both site-years at 1 WAE (Table 8). At 3 WAE, there was no difference in the level of injury observed in both site-years. However, at 5 and 7 WAE, there was greater injury with the addition of oxyfluorfen to clomazone in one of the two site-years.
Table 8. Rice injury following a preemergence application of clomazone and oxyfluorfen on clay soil in 2021 and 2022 at the Northeast Research and Extension Center in Keiser, AR.a–c
a Abbreviations: Clom, clomazone; n/a, not applicable; oxy, oxyfluorfen; WAE, wk after emergence.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c * Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen.
Clomazone alone does not commonly cause severe injury (≥20%) when applied at a similar rate preemergence in rice (Camargo et al. Reference Camargo, Senseman, McCauley and Guice2011; Scherder et al. Reference Scherder, Talbert and Clark2004). However, oxyfluorfen is not labeled for use in rice, possibly because of the potential for crop injury (Anonymous 2013). Using multiple sites of action in-crop will often lead to greater weed control; however, multiple sites of action could potentially lead to greater injury to rice because of the plant being targeted at multiple sites.
The 1:3 ratio of clomazone to oxyfluorfen caused greater injury than the 1:1.5 and 1:2 ratios of clomazone to oxyfluorfen at 1 WAE in 2021 (Table 8). No difference was observed between clomazone and oxyfluorfen ratios at 3 and 5 WAE in both years. Again at 7 WAE, the 1:3 clomazone to oxyfluorfen ratio caused greater injury than the 1:1.5 and 1:2 ratios. The amount of injury observed with an application containing oxyfluorfen in oxyfluorfen-resistant rice cannot be attributed to differing lines. Even though the seed used in 2021 was harvested in Arkansas in 2020 and shipped for cleaning before planting in 2021, and the 2022 seed was shipped directly from California, the line evaluated in trials in both years was the same.
On clay soil, adding oxyfluorfen to clomazone resulted in greater rough rice yields than clomazone alone in 2022 (Table 9). Rough rice yield data comparing oxyfluorfen plus clomazone to clomazone alone followed the same trend as barnyardgrass control 7 WAE in 2022. Barnyardgrass left uncontrolled following the 7 WAE observation could potentially cause a 35% to 43% reduction in rough rice yields (Smith Reference Smith1968). The lower barnyardgrass control observed with clomazone alone would significantly reduce rough rice yields than oxyfluorfen plus clomazone (Smith Reference Smith1968).
Table 9. Rice shoot counts and rough rice yield following a preemergence application of clomazone and oxyfluorfen on clay soil in 2021 and 2022.a–c
a Abbreviations: Clom, clomazone; n/a, not applicable; oxy, oxyfluorfen.
b Means within the same column followed by the same letter are not different according to Tukey’s honest significant difference (α = 0.05); the absence of letters indicates no treatment differences.
c * Indicates significant difference (α = 0.05).
d Ratios indicate clomazone/oxyfluorfen.
Practical Implications
Oxyfluorfen use in oxyfluorfen-resistant rice will provide producers an alternative option for controlling herbicide-resistant barnyardgrass and reducing the risk of target-site herbicide resistance. On silt loam and clay soils, a 1:2 or 1:3 ratio of clomazone to oxyfluorfen should be used to provide the highest levels of barnyardgrass control throughout the growing season. The 1:3 ratio of clomazone to oxyfluorfen will increase the risk of injury to rice compared to the 1:2 ratio on silt loam and clay soils. By applying a 1:2 ratio of clomazone to oxyfluorfen, the labeled recommended rate of clomazone at 336 g ha–1 on a silt loam soil would result in oxyfluorfen being applied at 672 g ha–1 as the herbicides were co-applied as a mixture. The maximum annual use rate of oxyfluorfen in rice is anticipated to be 1,680 g ha–1, which would result in there being an additional 1,008 g ha–1 of the herbicide available for a postemergence application (Chad Shelton, personal communication). If clomazone were applied at 672 g ha–1 on clay soil, the oxyfluorfen rate would be 1,344 g ha–1 at a 1:2 ratio. Under this scenario, it is unlikely that sufficient oxyfluorfen would be available to make a postemergence application as a a result of annual rate limitations on the herbicide. It is also important to note that these recommendations are solely based on barnyardgrass on both soil textures. Other weeds could be present within a field for which oxyfluorfen may or may not effectively control, resulting in different results from those observed here.
Acknowledgments
Assistance with plot establishment and maintenance by the staff at the Rice Research and Extension Center and the University of Arkansas Pine Bluff Small Farm Research Center is appreciated.
Funding
Partial funding for this research was provided by the Arkansas Rice Promotion Board and Albaugh LLC.
Competing interests
Chad Shelton is an employee of Albaugh LLC.
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