Introduction
Wheat is one of the most important grain crops globally, ranking second in overall production (Shahbandeh Reference Shahbandeh2023). In 2022, global wheat production was approximately 781 million metric tons, with China accounting for 137 million metric tons. Since 1990, wheat yields have increased by over 131% but have become more stagnant in the last 5 yr. The United States cranks fifth in global wheat production, with Arkansas accounting for over 60,000 ha of harvested area (USDA NASS 2022). One of the major limiting factors of wheat production is yield loss from weed infestations. Although wheat can tolerate weed competition better than corn (Zea mays L.) or soybean [Glycine max (L.) Merr.], an estimated 25% of yield loss in the United States is associated with weed competition (Flessner et al. Reference Flessner, Burke, Dille, Everman, VanGessel, Tidemann, Manuchehri, Soltani and Sikkema2021; Soltani et al. Reference Soltani, Dille, Burke, Everman, VanGessel, Davis and Sikkema2016, Reference Soltani, Dille, Burke, Everman, VanGessel, Davis and Sikkema2017).
Italian ryegrass is a highly problematic weed in wheat. One way that Italian ryegrass is detrimental to winter wheat yield is through direct interference, where yield reductions have been seen as high as 4.2% per 10 Italian ryegrass plants in a square meter (Liebl and Worsham Reference Liebl and Worsham1987). Wheat yield reductions have ranged from 33% to 44% at higher densities of Italian ryegrass, particularly when high nitrogen (N) levels are present in the field (Appleby et al. Reference Appleby, Olson and Colbert1976; Hashem et al. Reference Hashem, Radosevich and Roush1998). Controlling Italian ryegrass has become more problematic as herbicide resistance has continued evolving in populations nationwide. In Arkansas alone, Italian ryegrass is resistant to three unique sites of action, particularly Herbicide Resistance Action Committee (HRAC)/Weed Science Society of America (WSSA) Groups 1, 2, and 9 (Heap Reference Heap2024). Because of this resistance, Italian ryegrass has become more troublesome in Arkansas, both in wheat production and for spring burndown applications (T. Butts 2023, personal communication).
One of the best methods for controlling Italian ryegrass is using residual herbicides to reduce weed emergence. Researchers in Mississippi found that pyroxasulfone and S-metolachlor could provide up to 93% control of glyphosate-resistant Italian ryegrass up to 6 mo after application as a fall residual application (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014). Pyroxasulfone is labeled for use in several row crops, including corn, cotton (Gossypium hirsutum L.), soybean, and wheat. Studies have shown that pyroxasulfone is much more persistent in the soil than S-metolachlor, so although they have demonstrated comparable control in previous research, pyroxasulfone is typically available in the soil for a longer period (Westra et al. Reference Westra, Shaner, Westra and Chapman2014). Whereas application timings differ across cropping systems, the most common application timing for pyroxasulfone is preemergence (PRE) to reduce early-season weed interference. In wheat, delayed preemergence (DPRE) to early postemergence is the optimal time of application to reduce the chance of injury to wheat; however, this delay in application timing can result in reduced weed control.
S-metolachlor is a HRAC/WSSA Group 15 chloroacetamide herbicide that provides residual control of grasses and small-seeded broadleaf weeds. It is currently labeled in numerous cropping systems, including corn, cotton, and soybean. Wheat is typically sensitive to chloroacetamide herbicides, with significant injury occurring in the root zone and symptomology such as chlorosis and necrosis of the leaves (Qu et al. Reference Qu, Li, Zhang, Cui, Zhao, Liu, Lu and Qian2021). Stunting of wheat plants is one of the major forms of injury that results from applications of S-metolachlor. Still, previous work has indicated that recovery is possible later in the growing season (Ritter and Menbere Reference Ritter and Menbere2002). This study also showed that S-metolachlor could effectively control Italian ryegrass through wheat harvest.
One option for utilizing herbicides that are traditionally unsafe for grass crops is using herbicide safeners, specifically as a seed treatment. The use of a herbicide safening seed treatment is commercially available in grain sorghum (Sorghum bicolor (L.) Moench), where fluxofenim allows the application of S-metolachlor preemergence in conjunction with a foliar-applied safener, benoxacor (Jablonkai Reference Jablonkai2013) (Concep III; Syngenta Crop Protection, LLC, Greensboro, NC). Fluxofenim has also been evaluated under greenhouse conditions as a herbicide safening seed treatment in wheat using dimethenamid, pyroxasulfone, and S-metolachlor (Raiyemo et al. Reference Raiyemo, Price, Rauch, Campbell, Xiao, Ma, Gross and Prather2021). In some instances, there was increased shoot biomass for wheat sprayed with S-metolachlor or dimethenamid when a fluxofenim seed treatment was used compared to that of the nontreated seed.
Fenclorim is a herbicide safener previously commercially available in a premixed product known as Sofit (Quadranti and Ebner Reference Quadranti and Ebner1983). Althoughit was originally used in Asian water-seeded rice (Oryza sativa L.), work has been done in the United States to evaluate the efficacy of a fenclorim seed treatment in rice to applications of a microencapsulated (ME) formulation of acetochlor (Avent et al. Reference Avent, Norsworthy, Butts, Roberts and Bateman2022a, Reference Avent, Norsworthy, Butts, Roberts and Bateman2022b). Several studies were conducted to determine if acetochlor could be safely applied in the presence of fenclorim-treated rice, and data showed that fenclorim could provide adequate crop safety when applications were made at a DPRE timing.
One aspect that can significantly influence both the efficacy and potential injury to crops from a herbicide is the formulation. Although several options are available, a slow-release formulation has shown the potential for increased crop safety when dealing with traditionally unsafe herbicides for crops such as rice and wheat (Fogleman et al. Reference Fogleman, Norsworthy, Barber and Gbur2019). Slow-release formulations allow for a herbicide to be released slowly over time, which can provide both prolonged weed control and reduced phytotoxicity to the crop. A capsule suspension (CS) is a slow-release formulation commonly used in residual herbicides such as Prowl H2O (BASF Corp., Research Triangle Park, NC, 27709). S-metolachlor has traditionally been utilized in emulsifiable concentrate (EC) formulated products such as Dual Magnum (Syngenta, Greensboro, NC 27419). However, based on previous research investigating the effects of formulation on an herbicide in the same family, acetochlor, there is a belief that using a CS formulation may mitigate the risk of injury in wheat.
Based on the findings of both the use of a fenclorim seed treatment in rice and the reduced injury from a slow-release formulation of acetochlor, the potential for an effective CS formulation of S-metolachlor was evaluated for potential use in wheat. A series of experiments were conducted to answer two main questions: (i). Can a CS formulation of S-metolachlor be used in conjunction with a fenclorim seed treatment to provide adequate crop safety in wheat?(ii)Can the herbicide effectively control problematic weeds in wheat, specifically Italian ryegrass?
Materials and Methods
Field Experimental Sites
Two field experiments were initiated in 2021 and 2022 in Fayetteville, AR, at the Milo J. Shult Research and Extension Center (36.097475 N; 94.099431 W) to evaluate the efficacy of a CS formulation of S-metolachlor in winter wheat. Two varieties were used over the six total sites, four with ‘Smith’s Gold’ (Oklahoma State University Extension, Stillwater, OK 74078) and two with ‘Croplan 9606’ (Winfield United, Arden Hills, MN 55126). In 2023, an additional two site-years were conducted (one PRE and one DPRE), and ‘Croplan 9606’ was planted because of insufficient ‘Smith’s Gold’ seed. Wheat was drilled at 3.4 million seeds ha–1 at a 1.9-cm depth with 19-cm row spacing. Two sites, one with a preemergence application timing (PRE) and one with a delayed-preemergence (DPRE) timing, were initiated in 2021 by planting on October 8 after establishing a weed-free seedbed with tillage. Wheat emerged on October 18. In 2022, two additional sets of experiments were initiated similarly, with one set being planted on October 13 and emerging on October 20. The final set was planted on November 1 and emerged on November 11. All wheat trials were managed according to the University of Arkansas Cooperative Extension Service’s fertility recommendations with a preplant application of poultry litter. The four sites containing ‘Smith’s Gold’ were planted on a Pembroke silt loam composed of 11% sand, 68% silt, and 21% clay with a pH of 5.7 and 1.6% organic matter. The final two sites with ‘Croplan 9606’ were planted on a Captina silt loam composed of 14% sand, 68% silt, and 21% clay with a pH of 6.2 and 1.8% organic matter.
Common Methodology
Plot dimensions were 1.8 by 5.2 m each with four replications for each site. Both the PRE and DPRE experiments were designed as a two-factor factorial within a randomized complete block. One factor was herbicide treatment, which included a CS formulation of S-metolachlor at 0.37, 0.74, and 1.12 kg ai ha–1, along with ME-formulated acetochlor at 1.05 kg ai ha–1 and a nontreated control. Each herbicide rate was tested against three rates of a fenclorim seed treatment (0, 0.5, 2.0 g ai kg–1 of seed). Three total sites were used to evaluate the PRE application timing of the herbicides, and a similar number of sites were conducted on the DPRE application timing. All applications were made with a CO2-pressurized backpack sprayer calibrated to deliver 140 Lh preemergence–1 at 4.82 km h–1 with three AIXR 110015 nozzles at 276 kPa.
Preemergence Application Experiments
Three site-years were utilized to evaluate CS S-metolachlor and ME acetochlor in a wheat production system. Because most residual herbicide applications in wheat occur from the PRE to one-leaf application timing, a PRE application timing was chosen to determine the efficacy of chloroacetamide herbicides in wheat. PRE applications were made on the day of planting, and crop emergence occurred 7 to 10 d after planting.
Delayed-Preemergence Application Experiments
Three site-years were utilized to evaluate CS S-metolachlor and ME acetochlor at a DPRE application timing, which occurs after seed germination but before crop emergence. A common practice for wheat production involves delaying application timing from PRE to DPRE or one-leaf to reduce the chance of severe injury. This experiment was designed to determine if a fenclorim seed treatment could provide adequate crop safety when applications are made DPRE. This trial was initiated at the same time as the PRE experiments, but applications were made 2 to 7 d after planting following sprouting of the wheat.
Data Collection and Statistical Analysis
Visible Italian ryegrass control was evaluated relative to the nontreated control on a scale of 0 to 100, with 0 being no control and 100 being complete control. Visible wheat injury was also evaluated, with 0 being no injury and 100 meaning complete plant mortality. Italian ryegrass panicle counts were taken in two randomly selected 0.25 m2 quadrants in each plot to assess end-of-season weed pressure. To quantify the reduction in panicles from the nontreated control, the panicle amount from each plot was divided by the nontreated treatment in each repetition, respectively. Wheat grain yield was collected at harvest using a small-plot combine that harvested the center eight rows of each plot. Weights were adjusted to 12.5% moisture. Rainfall data were recorded daily for the first3 wk following planting using amounts approximately 0.5 km from the test site based on the National Weather Service at https://www.weather.gov/wrh/Climate?wfo=tsa
The PRE and DPRE experiments were analyzed separately, with “site-year” and “block” considered random effects within the model. All distributions were tested in the JMP distribution checker (JMP PRO v17.2; SAS Institute Inc., Cary, NC) 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 non-normally distributed, the Akaike Information Criterion was used to select the most appropriate data distribution according to the JMP software. Crop injury was found to be non-normally distributed and was analyzed using a gamma distribution in JMP PRO v17.2, whereas wheat yield was found to be normally distributed and analyzed as such. To determine the efficacy of fenclorim, crop injury and yield were analyzed using planned orthogonal contrasts. Contrasts were performed on each herbicide rate independently to compare each seed treatment to the nontreated seed. Italian ryegrass control was found to follow a gamma distribution and was analyzed by herbicide. Weed control and relative panicle counts were subjected to ANOVA), and means were separated using Tukey’s highly significant differences (HSD) with (α = 0.05) using a generalized linear mixed model in JMP Pro 17.2.
Greenhouse Experiment
To make assumptions about the effect of application timing, a greenhouse experiment was conducted with two runs to evaluate the application timing of CS S-metolachlor in conjunction with a fenclorim seed treatment. The experiment was designed with three factors: application timing, S-metolachlor rate, and seed treatment. A split-plot design with two whole-plot factors, application timing and S-metolachlor rate, and a split-plot of seed treatment was utilized for this experiment. The application timings evaluated were PRE and DPRE, with three rates (0.37, 0.74, and 1.12 kg ai ha–1) of CS S-metolachlor. The greenhouse was 28 C during the day with a 14-h photoperiod and 21 C at night.
Each treatment consisted of nontreated wheat and fenclorim-treated wheat evenly spaced in four rows (two per seed treatment) with 10 seeds per row planted into plastic tubs with 12- by 35- by 21-cm dimensions. Each tub had holes in the bottom to facilitate drainage and was filled to 95% capacity with silt loam soil, the same soil used for field experiments. Tubs of soil were watered twice daily to maximize the risk of injury to wheat from the herbicides. Visible injury ratings were made weekly from 7 to 21 d after emergence (DAE) on a scale of 0 to 100%, with 0 meaning no visible injury and 100 being complete plant mortality. Additionally, stand counts were taken 7 DAE and made relative to the nontreated control by dividing the stands in each treatment by the stand taken in the nontreated plots. 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 non-normally distributed, the Akaike Information Criterion was used to select the most appropriate data distribution according to the JMP software. Crop injury ratings and relative stand counts were determined to be a gamma distribution. Data were subjected to ANOVA, and means were separated using Tukey’s HSD (α = 0.05).
Results and Discussion
Rainfall
In 2021, rainfall occurred 6 d after the PRE application and on the same day as the DPRE application, totaling 6.7 and 3.4 cm of rain, respectively (Figure 1). In 2022, two runs of each experiment were conducted. The first run in 2022 was planted with adequate soil moisture and received rainfall 4 d after the PRE application and the day after the DPRE application with 1.1 cm of rain. The second set of experiments in 2022 received rainfall 7 d after the PRE and the day after the DPRE application with 2 cm of rain. While there is some variation among the activation dates and rainfall amounts, each treatment received rainfall within 7 d, with additional rain events occurring in the 2 weeks following the initial activation. Rainfall activation and activation K timing can play a significant role in the potential efficacy of a residual herbicide, and all applications appear to meet the requirements for good activation (Jones et al. Reference Jones, Banks and Radcliffe1990).
Figure 1. Daily rainfall amounts for 3wk following planting at the Milo J. Shult Research and Extension Center. Abbreviations: PRE, preemergence application; DPRE, delayed-preemergence application.
Weed Control
In the PRE-applied experiment, seed treatment did not affect Italian ryegrass control or panicle counts (p-value >0.05); therefore, only the main effect of herbicide treatments is presented (data not shown). A CS formulation of S-metolachlor at 1.12 kg ai ha–1 applied PRE provided more than 90% visible control of Italian ryegrass through 35 d after treatment (DAT) (Table 1). However, by the preharvest assessment, control with the higher rate was less than 80% and was comparable to the two lower rates of S-metolachlor. Previous research has indicated that Italian ryegrass control can range from 92% to 100% before harvest when S-metolachlor is applied PRE in wheat at 1.12 kg ai ha–1, with only a slight reduction in control as the herbicide rate decreased (Ritter and Menbere Reference Ritter and Menbere2002). The only treatment that had fallen below 50% control of Italian ryegrass by preharvest was ME acetochlor at 1.05 kg ai ha–1. For context, pyroxasulfone is one of the most effective herbicides for Italian ryegrass in wheat, providing 90% to 100% control in other research (Hulting et al. Reference Hulting, Dauer, Hinds-Cook, Curtis, Koepke-Hill and Mallory-Smith2012).
Table 1. Influence of herbicide rate at a preemergence application timing on Italian ryegrass control and panicle counts.
a Abbreviations: DAT, d after treatment
b Means within a column not containing the same letter are different according to Tukey’s HSD (α = 0.05)
Relative panicle counts of Italian ryegrass indicated a similar pattern to visible control of the weed, with all three rates of S-metolachlor reducing the number of panicles from 54% to 59% compared to the nontreated control. Acetochlor treatments resulted in a panicle reduction of 38% compared to the nontreated control, which aligns with the reduced visible weed control preharvest. The results from this experiment have indicated that S-metolachlor could effectively control Italian ryegrass throughout the season. At the same time, ME acetochlor as an option was not as effective when applied PRE.
By 28 DAT, S-metolachlor at 0.74 and 1.12 kg ai ha–1 applied DPRE provided greater than 90% Italian ryegrass control, but reduced control occurred at the lowest rate of S-metolachlor and with ME acetochlor (Table 2). Overall, the two highest rates of S-metolachlor applied DPRE controlled Italian ryegrass effectively (>80%) for the entire growing season, which is similar to the level of control provided by pyroxasulfone in wheat in other research (Bond et al. Reference Bond, Eubank, Bond, Golden and Edwards2014).
Table 2. Influence of herbicide rate at a delayed preemergence application timing on Italian ryegrass control.
a Abbreviations: DAT, d after treatment
b Means within a column not containing the same letter are different according to Tukey’s HSD (α=0.05)
Based on the results of these experiments, a CS formulation of S-metolachlor could be an effective herbicide option in wheat. S-metolachlor at 0.74 to 1.12 kg ai ha–1 applied PRE or DPRE could effectively control Italian ryegrass through harvest. In both experiments, an ME formulation of acetochlor at 1.05 kg ai ha–1 performed worse than S-metolachlor at each rate and would likely not be an effective standalone herbicide at either application timing in a wheat production system.
Preemergence Crop Response
Crop injury, in the form of delayed emergence, from the fenclorim seed treatment at 2 g ai kg–1 translated into shorter wheat 21 DAT than the nontreated control, but this injury was gone by the preharvest assessment, with no reflection in yield loss at the end of the season (Table 3). Similar results have been observed in other herbicide safeners when delayed germination occurred (Abu-Qare and Duncan Reference Abu-Qare and Duncan2002). One possible explanation for the lack of differences between the nontreated control with no herbicide applied was a substantial density of Italian ryegrass that resulted in heavy competition for nutrients and lodging. Differences in injury among seed treatments could not be detected preharvest, and grain yield was not affected at any level of seed treatment, respectively for the contrasts with the low and high rate of fenclorim. Wheat yield from the three rates of seed treatments applied with S-metolachlor at a rate of 0.37 kg ai ha–1 averaged 2,970 kg ha–1.
Table 3. Significance of contrast between fenclorim seed treatment rates by herbicide rate at a preemergence application timing for wheat injury and yield.
a Abbreviations: DAT, d after treatment.
b Seed treatment, fenclorim in g ai kg–1 of seed.
c P value less than 0.05 indicates statistical significance.
d Bold P values are significant.
As the S-metolachlor rate increased to 0.74 kg ai ha–1, crop injury averaged 25% at 21 DAT and 17% by preharvest (Table 3). At preharvest, the high rate of fenclorim provided increased crop safety compared to the nontreated seed. Yield differences were not significant at this rate.
S-metolachlor is labeled for use in other crops, such as soybean and corn, at a rate of 1.06 to 2.13 kg ai ha–1; therefore, a rate of 1.12 kg ai ha–1 was evaluated in wheat. Both rates of the fenclorim seed treatment provided significant crop safening to S-metolachlor at 21 DAT when applied at the highest rate (Table 3). The increased safety from fenclorim was still noticeable preharvest, with a 16-percentage point reduction in crop injury when the high rate of fenclorim was used instead of nontreated seed. Similar work with a different herbicide safening seed treatment, fluxofenim, showed enhanced crop safety in over half of the wheat varieties treated with S-metolachlor at 1.01 kg ai ha–1 (Raiyemo et al. Reference Raiyemo, Price, Rauch, Campbell, Xiao, Ma, Gross and Prather2021). In terms of yield, wheat treated with S-metolachlor at a rate of 1.12 kg ai ha–1 with a high rate of a fenclorim seed treatment yielded 2,510 kg ha–1 compared to only 1,500 kg ha–1 from the nontreated seed, resulting in a 67% yield increase. A ME formulation of acetochlor resulted in less than 10% injury to wheat at all three seed treatment levels at preharvest, with no significant differences observed in crop yield. Although enhanced crop safety was observed with fenclorim, the injury was not deemed acceptable at the highest rate of S-metolachlor.
Delayed-Preemergence Crop Response
Visible crop injury evaluations of DPRE applied S-metolachlor and acetochlor followed a different trend from PRE applications. The greatest level of injury observed throughout this set of experiments was 17% (Table 4). The effect of fenclorim at each herbicide rate was analyzed, but no safening was observed at 28 DAT or preharvest. Although the presence of fenclorim did not increase crop safety, there were several instances of the high rate of fenclorim causing increased injury at 28 DAT in the form of stunting to wheat where injury was not observed from the herbicide alone. This injury did not reflect yield loss at the end of the season, as there were no differences in yield due to the presence or absence of fenclorim-treated seed.
Table 4. Significance of contrasts between fenclorim seed treatment rates by herbicide rate at a delayed preemergence application timing for wheat injury and yield.
a Abbreviations: DAT, d after treatment.
b Fenclorim seed treatment in g ai kg-1 of seed.
c P value less than 0.05 indicates statistical significance.
d Bold P values are significant.
Although field studies were not conducted to observe the differences between preemergence and delayed-preemergence application timings directly, injury ranged from 8% to 34% for preemergence applications and 8% to 17% for delayed preemergence applications (Tables 3 and 4). Because comparisons could not be made in the field, a greenhouse experiment was conducted to make assumptions about the effect of application timing and the impact on crop injury from S-metolachlor in wheat.
Greenhouse Experiment
To directly compare application timing, herbicide rate, and seed treatment, a greenhouse experiment was conducted in 2023 with two separate runs. At a PRE application timing, wheat injury ranged from 41% to 87% without fenclorim and 13% to 33% with fenclorim 7 DAE (Table 5). Additionally, injury to wheat was reduced at each rate of S-metolachlor applied PRE when fenclorim was present. DPRE applications, on average, caused less injury regardless of herbicide rate or seed treatment, ranging from 8% to 20% injury 7 DAE. The 0.37 and 1.12 kg ai ha–1 rates of S-metolachlor, when fenclorim was present, were comparable to all treatments applied DPRE 7 DAE.
Table 5. Visible wheat injury and crop stand relative to the nontreated control in the greenhouse.
a Abbreviations: DAE, d after emergence; APPL, application timing; HR, herbicide rate; SDTR, seed treatment; PRE, preemergence; DPRE, delayed preemergence.
b S-metolachlor rate in kilograms of active ingredient per hectare.
c Fenclorim seed treatment in g ai kg-1 of seed.
d P value less than 0.05 indicates statistical significance.
e Means within a column not containing the same letter are different according to Tukey’s HSD. (α = 0.05).
By 21 DAE, the interaction between application timing, herbicide rate, and seed treatment was insignificant (Table 5). When averaged over herbicide rate, application timing and seed treatment had a significant interaction with a reduction in injury from 69% to 23% at the PRE application timing when fenclorim was present. For the DPRE application, the effect of fenclorim was insignificant, indicating no additional crop safety, possibly because of the low levels of injury in the experiment. Whereas interactions containing herbicide rates were negligible, differences were observed between S-metolachlor at 0.37 and 0.74 kg ha ha–1, whereas the two highest rates of S-metolachlor did not separate statistically.
Stand loss is typically associated with injury caused by HRAC/WSSA Group 15 herbicides. The relative stand was evaluated at 7 DAE and reflected a similar pattern to crop injury (Table 5). The relative stand was lowest for PRE applications of S-metolachlor when averaged over seed treatment, ranging from a 24- to 40-percentage-point reduction in relative stand. However, this reduction was less than 8 percentage points for all treatments applied DPRE, with no significant differences among the three rates of S-metolachlor. When evaluating the herbicide rate by seed treatment interaction, the only significant difference between the no-fenclorim (68%) and fenclorim-treated (90%) wheat occurred with S-metolachlor at 0.74 kg ai ha–1.
Practical Implications
At both the PRE and DPRE application timings, weed control was effective through harvest, with only slight variation based on the rate of S-metolachlor used. An ME acetochlor was least effective on Italian ryegrass at both application timings. S-metolachlor could be used in a winter wheat production system without factoring in the crop safety component for control of glyphosate-resistant and ACCase-resistant Italian ryegrass, especially when integrated with other weed control tactics.
Crop safety is the most critical factor when looking at a potential new product in a crop not currently labeled for use. Although S-metolachlor was found to effectively control Italian ryegrass, the biggest question was how a fenclorim seed treatment could potentially reduce the injury typically seen in wheat when the herbicide is applied. Based on experiments conducted at the PRE application timing, fenclorim provided substantial safening over the nontreated seed at the two highest rates of S-metolachlor. Additionally, wheat yield was improved in the presence of fenclorim at both rates. However, when S-metolachlor was applied DPRE, there was little to no effect of fenclorim on wheat injury or yield, regardless of the rate applied.
When both application timings were conducted in the same experiment in the greenhouse, similar results were seen based on the findings in the field (Table 5). Fenclorim provided increased crop safety in the presence of S-metolachlor applied PRE, with more substantial differences seen at the higher herbicide rates. However, fenclorim had little to no effect at the DPRE timing, signifying that fenclorim was unnecessary when applications were at this timing. The need for additional data, including commercially standard products and varietal tolerance screenings, could provide a better understanding of how fenclorim can be used with winter wheat.
Acknowledgments
The assistance with plot establishment and maintenance by Weed Science 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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