Introduction
Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is one of the most problematic weeds in mid-southern U.S. rice production due to extreme crop competition (up to 82% yield loss), widespread occurrence of herbicide resistance, and prolific seed production (Bagavathiannan et al. Reference Bagavathiannan, Norsworthy, Smith and Neve2012; Butts et al. Reference Butts, Kouame, Norsworthy and Barber2022; Rouse et al. Reference Rouse, Roma-Burgos, Norsworthy, Tseng, Starkey and Scott2018; Smith Reference Smith1968). Barnyardgrass has been reported resistant to six herbicide sites of action, including Herbicide Resistance Action Committee (HRAC)/Weed Science Society of America (WSSA) Group 1, Group 2, Group 4, Group 5, Group 13, and Group 29 (Heap Reference Heap2023). Resistance to herbicides like clomazone limits residual herbicide options for barnyardgrass control, leading to greater reliance on POST herbicides. Because of the lack of options for early-season residual herbicides, more producers have relied on cultivars with herbicide resistance traits, which can lead to additional failures without proper stewardship (Burgos et al. Reference Burgos, Norsworthy, Scott and Smith2008; Shivrain et al. Reference Shivrain, Burgos, Sales, Mauromoustakos, Gealy, Smith, Black and Jia2009). Arkansas rice producers need additional residual herbicide options, and HRAC/WSSA Group 15 very-long-chain fatty-acid (VLCFA) elongase inhibitors may offer a solution.
The VLCFA elongase inhibitors, such as acetochlor, dimethenamid-p, pyroxasulfone, and S-metolachlor, are some commonly applied residual herbicides in the United States (Jhala et al. Reference Jhala, Singh, Shergill, Singh, Jugulam, Riechers, Ganie, Selby, Werle and Norsworthy2024). Specifically, the chloroacetamide herbicides acetochlor and S-metolachlor are heavily used in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. No chloroacetamide herbicides are available in rice due to the risk of severe injury. However, experiments have been conducted to evaluate the use of acetochlor and S-metolachlor in rice at various application times (Godwin et al. Reference Godwin, Norsworthy and Scott2018). Godwin et al. found that rice injury was greatest with S-metolachlor at the delayed preemergence (DPRE) application timing but that injury was reduced as the application was delayed. Similar findings were observed when evaluating acetochlor use in rice, where an early POST application reduced rice injury by 74% compared to a preemergence application 2 wk after treatment (Fogleman et al. Reference Fogleman, Norsworthy, Barber and Gbur2019).
A method that has been effectively used to allow safe applications of S-metolachlor in both corn and grain sorghum [Sorghum bicolor (L.) Moench] has been the utilization of herbicide safeners. Fluxofenim has been used as an herbicide safening seed treatment in grain sorghum, allowing S-metolachlor to be applied PRE with minimal injury (Concep® III, Syngenta Crop Protection, Wilmington, DE, USA). Although rice has traditionally been sensitive to applications of chloroacetamide herbicides, an herbicide safener, fenclorim, has been utilized in Asian water-seeded rice in a premixture with the herbicide pretilachlor (Quadranti and Ebner Reference Quadranti and Ebner1983).
One reason for the success of fenclorim as an herbicide safener is based on the ability to increase the activity of glutathione S-transferase in rice due to an increased expression of metabolic enzymes that allow rice to metabolize herbicides much faster and increase crop safety (Han and Hatzios Reference Han and Hatzios1991; Hu et al. Reference Hu, Yao, Cai, Pan, Liu and Bai2020). Specifically, studies have shown that fenclorim could reduce pretilachlor persistence in rice shoots, whereas HRAC/WSSA Group 15 herbicides typically inhibit plant growth and overall health (Scarponi et al. Reference Scarponi, Buono and Vischetti2003). Fenclorim allowed for the use of pretilachlor in rice; therefore increased safety from traditionally detrimental herbicides in rice like acetochlor and S-metolachlor may be possible.
Fenclorim has been commercially used only with pretilachlor; however, research has been conducted to determine if other HRAC/WSSA Group 15 chloroacetamide herbicides could be used in rice with the help of an herbicide safener (Avent et al. Reference Avent, Norsworthy, Butts, Roberts and Bateman2022a, Reference Avent, Norsworthy, Butts, Roberts and Bateman2022b). Experiments were conducted evaluating the use of fenclorim as a seed treatment in conjunction with the herbicide acetochlor. Specifically, a microencapsulated (ME) acetochlor was utilized to mitigate additional injury to rice due to a slow-release formulation that generally reduces herbicide phytotoxicity (Beestman and Kowite Reference Beestman and Kowite1994). Previous work found that rice injury could be reduced using a ME formulation of acetochlor over an emulsifiable concentrate (EC) (Fogleman et al. Reference Fogleman, Norsworthy, Barber and Gbur2019). A difference in phytotoxicity of the same rate of the herbicide acetochlor was attributed to the quick absorption of an EC formulation that allows for immediate availability compared to a ME, which has a slower release over time. Avent et al. (Reference Avent, Norsworthy, Butts, Roberts and Bateman2022a) found that when fenclorim was applied to rice seed at a rate of 2.5 g ai kg−1 of seed, a ME acetochlor could be safely applied to rice at a DPRE application timing. Additionally, the research showed that rice injury was subsequently reduced when the application timing of acetochlor was delayed from PRE to DPRE or 1-leaf application timings.
Avent et al. (Reference Avent, Norsworthy, Butts, Roberts and Bateman2022a, Reference Avent, Norsworthy, Butts, Roberts and Bateman2022b) found that using a ME acetochlor allowed for a slow release suitable for use in rice. A capsule suspension (CS) formulation has a similar structure to microencapsulation, with a polymeric membrane encompassing the herbicide molecule, allowing for encapsulation (Hazra and Purkait Reference Hazra and Purkait2019; Knowles 2008). A CS formulation involves a controlled release of the herbicide over an extended period and can be impacted by the capsule wall thickness, with benefits like reduced phytotoxicity of the crop and prolonged residual control of weeds (Beestman and Kowite Reference Beestman and Kowite1994; Seaman Reference Seaman1990; Sopeña et al. Reference Sopeña, Maqueda and Morillo2009).
S-metolachlor is an herbicide that has shown excellent control of grasses and small-seeded broadleaf weeds in numerous row crops (Daramola et al. Reference Daramola, Iboyi, MacDonald, Kanissery, Tillman, Singh and Devkota2024). Additionally, research has shown that S-metolachlor can control problematic rice weeds, such as weedy rice (Oryza sativa L.), at levels greater than 90% (Zemolin et al. Reference Zemolin, Avila, Agostinetto, Cassol, Bastiani and Pestana2014). Although severe injury to rice is possible, adding a fenclorim seed treatment and CS formulation may allow for applications in rice. To determine if S-metolachlor could be viable in a delayed-flood rice production system, herbicide rate, application timing, and fenclorim safening efficacy must be evaluated. An experiment was conducted to determine if fenclorim could adequately safen rice to DPRE applications of S-metolachlor, and an additional experiment was conducted to assess a 1- to 2- leaf (EPOST) application in rice.
Materials and Methods
Experimental Sites
An experiment was conducted at the Pine Tree Research Station near Colt, AR, in 2022 and 2023 to evaluate the efficacy of a fenclorim seed treatment on two commonly used mid-southern U.S. rice cultivars. A CS formulation of S-metolachlor (UPL, King of Prussia, PA, USA) was applied DPRE to both ‘Diamond’ and ‘DG263L’ rice to determine if fenclorim could provide adequate crop safety. Additionally, another experiment was conducted in both years to evaluate the tolerance of fenclorim-treated rice to a 1- to 2-leaf (EPOST) application of an EC and CS S-metolachlor. All experiments were conducted adjacently in 2022 and 2023 on a Calhoun silt loam soil with 17% sand, 68% silt, 15% clay, and 1.4% organic matter.
Common Methodology
For all experiments, plot dimensions were 1.5 × 5.2 m, and experiments were conducted under a drill-seeded, delayed-flood rice production system. Rice was planted on May 13, 2022, with emergence 6 d later, and on April 12, 2023, with emergence on May 1. Rice was drilled at 72 seeds per m−1 of row with a 19-cm spacing at a 1.9-cm depth. All trials were fertilized according to the University of Arkansas Cooperative Extension Service’s rice fertility recommendations (Roberts et al. Reference Roberts, Slaton, Wilson and Norman2016). Trials in 2022 and 2023 were flooded on June 12 and June 2, respectively. Herbicide applications were made when necessary to maintain a weed-free environment in both trials to understand the effect of S-metolachlor on rough rice yield. All applications were made with a CO2-pressurized backpack sprayer with a four-nozzle boom calibrated to deliver 140 L ha−1 at 4.8 kph with AIXR 110015 nozzles at 276 kPa. Rough rice yields were collected at harvest using a small-plot combine (Kincaid, Haven, KS, USA) and adjusted to 12% moisture.
DPRE Experiment
This experiment was designed as a two-factor factorial within a randomized complete block with four replicates. Two site-years used ‘Diamond’ rice, while the other used ‘DG263L’ rice. Though two rice cultivars were utilized in this experiment, the cultivars were not tested against each other due to the experimental design. One of the factors evaluated was the herbicide rate (0, 0.42, 0.84, and 1.68 kg ai ha−1) of the CS formulation of S-metolachlor, and the other was the presence or absence of a fenclorim seed treatment at 2.5 g ai kg−1 of seed. Applications for these experiments were made DPRE, occurring after rice germination but before crop emergence. These experiments aimed to determine if S-metolachlor and a fenclorim seed treatment would be viable for crop safety in a delayed-flooded rice system. Visible rice injury was evaluated, with 0% being no injury and 100% being complete plant mortality at 14 and 42 d after emergence (DAE).
All distributions were tested in the JMP (SAS Institute, Cary, NC, USA) distribution checker to determine if the data were normally distributed. Additionally, the Shapiro–Wilkes test was utilized to further test the normality of the data. If data were found to be nonnormally distributed, the Akaike information criterion (AIC) was used to select the most appropriate data distribution according to the JMP software. Crop injury was determined to be nonnormally distributed and was analyzed using a generalized linear mixed model with a gamma distribution (Gbur et al. Reference Gbur, Stroup, McCarter, Durham, Young, Christman, West and Kramer2020). Rough rice grain yield was examined relative to the nontreated, no fenclorim control, and was found to be normally distributed. Data were analyzed with year as a fixed effect and block as a random effect and were subjected to analysis of variance (ANOVA) with means separated using Tukey’s honestly significant difference (HSD) (α = 0.05). Student’s t-tests were conducted in instances when fewer than four treatments were analyzed.
EPOST Experiment
To determine the potential use of POST-applied CS S-metolachlor in a rice production system, an experiment was conducted to evaluate rice tolerance when the herbicide was applied at the crop’s 1- to 2-leaf stage. The experiment was a randomized complete-block design with four replicates. The rice cultivar evaluated was ‘Diamond’. S-metolachlor formulations were CS and EC applied at 0.56 and 1.12 kg ai ha−1. In addition to these treatments, propanil at 4.48 kg ai ha−1 was evaluated alone and in conjunction with the previous treatments to determine if the mixture would increase the risk of injury. A standard treatment of clomazone at 0.336 kg ai ha−1 and pendimethalin at 1.12 kg ai ha−1 applied DPRE was included for injury and yield reference. Visible crop injury was evaluated at 7, 21, and 35 d after the EPOST treatment on a scale similar to the cultivar evaluation. Rough rice grain yield was analyzed relative to the commercial standard treatment of clomazone and pendimethalin.
The data were subjected to a repeated-measures analysis. Herbicide treatment and evaluation timing were evaluated, as was the interaction between both factors, with analysis taking place by year and the blocks nested within each site-year as random. All distributions were tested in the JMP distribution checker to determine if the data were normally distributed. Additionally, the Shapiro–Wilkes test was utilized to further test the normality of the data. If data were found to be nonnormally distributed, the AIC was used to select the most appropriate data distribution according to the JMP software. Crop injury was determined to be nonnormally distributed and was analyzed using a generalized linear mixed model with a gamma distribution, while relative rice yield was normally distributed (Gbur et al. Reference Gbur, Stroup, McCarter, Durham, Young, Christman, West and Kramer2020). Data were subjected to an ANOVA, and means were separated using Tukey’s HSD (α = 0.05).
Results and Discussion
Rainfall
Rainfall amounts were recorded daily for the first 3 wk following planting and are shown in Figure 1. In 2022, rainfall occurred 6 d after the DPRE application and 3 d after crop emergence, totaling 5.0 cm of rain. The average daily temperature ranged from 20 to 30 C for the first 3 wk. Environmental conditions were different in 2023, primarily due to planting occurring approximately 1 mo earlier. Planting occurred on April 12, and the DPRE application was made 5 d later. The first rainfall after the DPRE occurred 5 d later, on April 19, totaling 6.8 cm. Rice later emerged on May 1. The earlier planting in 2023 resulted in cooler conditions over the 3 subsequent wk, with daily average temperatures ranging from 13 to 23 C. Because of the differences in site-year and the subsequent effect on the results of the experiments, data were analyzed by year for both sets of experiments. Each treatment received an activating rainfall within 7 d, and all applications appeared to meet the requirements for good activation (Jones et al. Reference Jones, Banks and Radcliffe1990; Parker et al. Reference Parker, Simmons and LM2005).
Figure 1. Rainfall amount and average daily temperatures for 3 wk after planting at the Pine Tree Research Station near Colt, AR, in 2022 and 2023. Abbreviation: DPRE, delayed preemergence.
DPRE Experiment
The DPRE experiment did not compare the two rice cultivars (‘Diamond’ and ‘DG263L’) but aimed to determine if adding a fenclorim seed treatment would improve the tolerance of each rice cultivar to DPRE applications of CS S-metolachlor. Rice injury from HRAC/WSSA Group 15 chloroacetamide herbicides typically results in stunting or reduced stand. Injury is more prominent immediately after emergence, but rice can sometimes recover from early-season injury without a yield reduction (Godwin et al. Reference Godwin, Norsworthy and Scott2018). Godwin et al. found that some treatments of S-metolachlor resulted in up to 40% visible injury but maintained comparable yields to the nontreated control. In 2022, the interaction between S-metolachlor rate and fenclorim was significant at 14 DAE for ‘DG263L’, with improved crop safety occurring when fenclorim-treated rice was treated with S-metolachlor at 1.68 kg ai ha−1 (Table 1). The greatest injury to ‘DG263L’ rice at 14 DAE was 39%, caused by S-metolachlor at 1.68 kg ai ha−1 in the absence of fenclorim. At the two lowest rates of S-metolachlor, there were no differences in injury to ‘DG263L’ between fenclorim-treated and nontreated rice. As the S-metolachlor rate increased, injury to ‘DG263L’ generally increased, regardless of the use of fenclorim, except at the highest herbicide rate.
Table 1. Visible rice injury and yield relative to the nontreated control to delayed preemergence applications of S-metolachlor with and without a fenclorim seed treatment for two commercial cultivarsa,b,c,d .
a Abbreviations: DAE, days after emergence; HR, herbicide rate; SDTR, seed treatment.
b ‘DG263L’ rough rice grain yield averaged 10,000 and 12,100 kg ha−1 in nontreated plots in 2022 and 2023, respectively. ‘Diamond’ rough rice grain yield averaged 9,620 and 12,500 kg ha−1 in 2022 and 2023, respectively.
c P < 0.05 indicates statistical significance.
d Means within a column not containing the same letter are different according to Tukey’s HSD (α = 0.05).
e S-metolachlor rate in kg ae ha−1.
f Fenclorim seed treatment in g ai kg−1 of seed.
With the earlier planting in 2023, injury to ‘DG263L’ ranged from 31% to 98% (Table 1). Although the interaction between S-metolachlor rate and fenclorim was not significant 14 DAE in 2023, the main effects were significant. The high rate, 1.68 kg ai ha−1, of S-metolachlor resulted in 83% injury to ‘DG263L’ when averaged over fenclorim seed treatment, with reduced injury occurring as the S-metolachlor rate decreased. The fenclorim seed treatment, averaged over S-metolachlor rates, reduced rice injury by 32 percentage points, from 79% to 47%, at 14 DAE, indicating increased crop safety when fenclorim was present. Environmental factors can affect rice when S-metolachlor is applied DPRE, resulting in differing injury levels year to year. A strong environmental effect on S-metolachlor in rice was observed in other research evaluating the herbicide without seed treatments (Godwin et al. Reference Godwin, Norsworthy and Scott2018).
‘Diamond’ rice tended to be injured to a lesser extent in 2022 than in 2023 (Table 1). In 2022, injury to ‘Diamond’ ranged from 9% to 54% at 14 DAE, whereas injury ratings of 25% to 98% occurred in 2023. The main effects of fenclorim seed treatment and S-metolachlor rates were significant in both years at 14 DAE. Injury to ‘Diamond’ rice increased with each increase in S-metolachlor rate, averaged over seed treatment, in 2022. Similarly, the two highest rates of S-metolachlor caused comparable levels of injury to ‘Diamond’ in 2023. In both years at 14 DAE, the fenclorim seed treatment provided significant safening of ‘Diamond’ to S-metolachlor; however, the level of injury was still not commercially acceptable.
At 42 DAE, there was no indication that rice was recovering from injury, at least that caused by the higher rates. For example, injury to ‘DG263L’ was 86% to 91% at the highest rate of S-metolachlor in the absence of fenclorim. Similarly, ‘Diamond’ without fenclorim was injured 90% to 91% by the highest herbicide rate. For both ‘Diamond’ and ‘DG263L’ at 42 DAE, there were still main effects (P = 0.0005 to <0.0001) for S-metolachlor rate and fenclorim use in both years. As the S-metolachlor rate increased, there was greater injury, and in the absence of fenclorim, both cultivars exhibited greater injury.
‘DG263L’ relative rice yield had a significant interaction of herbicide rate and fenclorim in 2022 and 2023 (Table 1). In 2022, relative rice yields ranged from 31% to 88% of the nontreated control. Only the highest rate of S-metolachlor showed an effect of fenclorim with fenclorim-treated rice, yielding 65% of the nontreated compared to 31% in the absence of fenclorim. Relative rice yield ranged from 77% to 108% of the nontreated in 2023, with yield loss occurring only at the high rate of S-metolachlor without fenclorim.
‘Diamond’ rice had a similar response with relative yield as ‘DG263L’, with significant interactions between herbicide rate and fenclorim in 2022 and 2023 (Table 1). Relative rice yields ranged from 12% to 105% in 2022 and from 50% to 100% in 2023. Additionally, in both 2022 and 2023, when S-metolachlor was applied at 1.68 kg ai ha−1, fenclorim increased yields over the absence of the seed treatment. However, while there was an apparent effect of the fenclorim seed treatment on rice yield, in 2022, there was still yield loss following the highest rate of S-metolachlor with fenclorim, indicating that the seed treatment did not provide adequate crop safety at the highest herbicide rate.
One possible explanation for differences between the 2022 and 2023 site-years is the environmental conditions in which rice was planted and, subsequently, the herbicide applied. Cool, wet conditions can sometimes exacerbate the extent of herbicide injury to the crop. When combined with the delayed germination of rice in 2023, site-year differences were likely.
EPOST Experiment
The first injury evaluation was taken 7 d after treatment (DAT) to capture any immediate injury following application, especially when mixed with an EC formulation of propanil. In 2022, visible rice injury 7 DAT caused by the high rate (1.12 kg ai ha−1) of CS S-metolachlor plus propanil at 4.48 kg ai ha−1 was not different from the standard residual treatment (Table 2). Similarly, within the first 7 d in 2023, all treatments were comparable to the commercial standard treatment (clomazone + pendimethalin DPRE) with no statistical separation, regardless of the rate of S-metolachlor applied.
Table 2. Visible rice injury and relative rice yield to a commercial standard residual herbicide applied to 1-leaf rice a,b,c,d .
a Abbreviations: CS, capsule suspension; DAT, days after treatment; EC, emulsifiable concentrate; prop, propanil.
b Rice was flooded on June 12, 2022, and on June 2, 2023.
c Means within a year not containing the same letter are different according to Tukey’s HSD (α = 0.05).
d P < 0.05 indicates statistical significance.
e Average rough rice grain yield in 2022 and 2023 was 10,600 and 11,600 kg ha−1, respectively.
f Clomazone at 0.336 kg ha−1 + pendimethalin at 1.12 kg ha−1 applied delayed preemergence.
g 0.56 kg ai ha−1 of S-metolachlor.
h 1.12 kg ai ha−1 of S-metolachlor.
Rice injury from the high rate of S-metolachlor, regardless of formulation or the addition of propanil, was more severe (40% to 48%) than the commercial standard treatment in 2022 by 21 DAT (Table 2). Rice injury from the three treatments with S-metolachlor at 1.12 kg ai ha−1 more than doubled by 21 DAT, initially observed at 7 DAT. The rice injury is most likely associated with introducing the flood to the rice before the 21 DAT evaluation. Treatments containing the lowest S-metolachlor rate were comparable to the standard residual treatment 21 DAT. Although a statistical separation did not occur for the three treatments containing the high rate of S-metolachlor from 7 to 21 DAT in 2023, rice injury had nearly doubled in this timeframe, similarly to the previous year. Though this analysis did not allow for comparisons across years, the response of rice to flooded conditions appeared to exacerbate the injury.
By 35 DAT in 2022, the three treatments containing S-metolachlor at 1.12 kg ai ha−1 still caused greater than 40% injury to rice (Table 2). This injury level indicated that no recovery occurred between 21 and 35 DAT. Treatments containing the low rate of S-metolachlor caused a similar level of injury as the standard residual treatment, which was less than 20% injury by 35 DAT. Rice injury 35 DAT in 2023 indicated greater differences between the EC and CS formulations of S-metolachlor. Whereas the CS formulation caused 51% injury to rice 35 DAT, rice treated with the EC formulation of S-metolachlor had completely recovered. One potential explanation for this drastic difference in injury caused by the EC and CS formulations of S-metolachlor is persistence in the soil. Whereas a slow-release formulation has proven beneficial in mitigating rice injury from PRE or DPRE applications, the presence of S-metolachlor later in the season can result in injury that increases over time due to the herbicide’s ability to be highly mobile in water. Godwin et al. (Reference Godwin, Norsworthy and Scott2018) observed a similar phenomenon where treatments with a higher rate of S-metolachlor applied POST in rice showed increased injury after flood establishment. Another result that differed from 2022 to 2023 was rice response to treatments containing a low rate of S-metolachlor, which caused 24% and 25% injury and were not comparable to the standard residual.
The differences between 2022 and 2023 illustrate the possible variability in crop response with S-metolachlor in rice. Rice injury has been highly variable yearly, similar to previous work done with S-metolachlor in rice (Godwin et al. Reference Godwin, Norsworthy and Scott2018). Rice yield response was evaluated relative to the standard residual treatment. In 2022, rice in all treatments, except EC S-metolachlor at 1.12 kg ai ha−1 and CS S-metolachlor at 1.12 kg ai ha−1 with propanil, had yields comparable to the standard treatment (Table 2). However, in 2023, only treatments containing CS S-metolachlor at 1.12 kg ai ha−1 resulted in reduced yield in comparison to the nontreated control.
Practical Implications
A CS formulation of S-metolachlor in rice can be safened using a fenclorim seed treatment and a DPRE application timing; however, the extent of injury, at least at the highest rate tested, is likely more than what growers will tolerate. Furthermore, there was sometimes yield loss at the highest rate, even when using the safener, indicating that the formulation of S-metolachlor tested is unlikely to be a commercially viable option. Across all evaluation timings with both ‘Diamond’ and ‘DG263L’ rice, fenclorim enhanced crop safety when averaged over the three rates of S-metolachlor; however, this reduction in injury did not always translate into rice yields comparable to the nontreated control. Although ‘Diamond’ and ‘DG263L’ could not be directly compared, the cultivars had similar trends regarding rice injury and yield relative to the nontreated controls. Additionally, both cultivars showed increased yield with fenclorim-treated rice over the nontreated seed when averaged over the herbicide rate.
The yield response at the highest rate of S-metolachlor in 2022 indicated that even with fenclorim, yield loss occurred compared to the nontreated control. With the addition of S-metolachlor, rice producers could control more problematic weed species that may be resistant to more traditional rice herbicides; however, environmental conditions can increase the risk of rice injury and subsequent yield loss. Additionally, reduced weed control is likely if herbicide rates are lowered to levels where rice is tolerant.
S-metolachlor is available for use in various row crops at multiple application times. In rice, the use of overlapping residual herbicides is recommended to reduce weed density throughout the growing season, and the addition of S-metolachlor to this system would allow for a more diverse selection of herbicides and reduce the selection from resistance placed on other rice herbicides (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). By introducing a new herbicide to rice, S-metolachlor can improve barnyardgrass and weedy rice control, especially at an EPOST application timing when the PRE or DPRE residuals have typically dissipated. Generally, some degree of rice injury is tolerable early in the season of a typical rice production system; however, rice injury generally remained stagnant or worsened with the use of CS S-metolachlor at 1.12 kg ai ha−1 at an EPOST application, particularly after flooded conditions were introduced. If S-metolachlor becomes labeled for an EPOST application timing, the desired rate would likely be less than 1.12 kg ai ha−1, as this rate can cause increased injury to rice under anaerobic conditions. At rates lower than 1.12 kg ai ha−1, weed control would likely be diminished beyond a level that would be viable in a rice production system.
Acknowledgments
The assistance with plot establishment and maintenance by graduate students at the University of Arkansas is appreciated.
Funding
UPL provided funding for this research.
Competing interests
The authors declare no conflicts of interest.
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