Weed Control in Corn and Soybean with Group 15 ( VLCFA Inhibitor ) Herbicides Applied Preemergence

Limited information exists on the efficacy of pethoxamid for annual grass and broadleaf control in corn and soybean in Ontario. A total of 10 field experiments (5 with corn and 5 with soybean) were conducted during 2015 to 2017 in Ontario, Canada, to compare the weed control efficacy of dimethenamid-P at 544 g·ai·ha, pethoxamid at 840 g·ai·ha, pyroxasulfone at 100 g·ai·ha, and Smetolachlor at 1050 g·ai·ha applied preemergence (PRE). Reduced weed interference with pyroxasulfone and dimethenamid-P resulted in corn yield that was similar to the weed-free control; however, weed interference with pethoxamid and S-metolachlor reduced corn yield 28 and 33%, respectively. Reduced weed interference with pyroxasulfone resulted in soybean yield that was similar to the weed-free control; however, weed interference with pethoxamid, dimethenamid-P, and S-metolachlor reduced soybean yield 27, 27, and 30%, respectively. At 4 and 8weeks after application (WAA), all VLCFA inhibitor herbicides (Group 15) provided excellent redroot pigweed control (90 to 99%) in corn. 'ere were no differences in common ragweed control, density, and dry weight among the VLCFA inhibitor herbicide evaluated; pyroxasulfone provided highest numeric common ragweed control and lowest numeric density and dry weight. At 4 and 8 WAA, pyroxasulfone provided the best common lambsquarters and wild mustard control and lowest numeric density and dry weight in corn and soybean. At 8 WAA, the VLCFA inhibitor herbicides controlled green foxtail 91 to 96% in corn; dimethenamid-P provided better control of green foxtail than pethoxamid in soybean. 'ere were no differences in barnyard grass control among the VLCFA inhibitor herbicides evaluated.


Introduction
Soybean and corn are two major agricultural crops produced in Canada [1,2].Currently, nearly 50% of soybean and >60% of corn produced in Canada is grown in Ontario [2].Soybean and corn are ranked as the 1st and 2nd most important field crops grown in Ontario, respectively [1].In 2016, Ontario soybean and corn growers harvested 1.09 and 0.81 million ha and produced 3.38 and 8.05 million tonnes with a total farm gate value of $1.59 and $1.54 billion, respectively [1].Effective weed management is critical for profitable soybean and corn production.Weed management has been identified as the single most important aspect of crop production for protecting the full yield potential of the crop [3].Growers need new weed management options to control problematic weeds in corn and soybean.
Pethoxamid is a new very long chain fatty acid (VLCFA) inhibitor herbicide from the chloroacetamide chemical family that is under consideration for registration in corn and soybean in North America.In sensitive plants, pethoxamid inhibits very long chain fatty acid (VLCFA) elongases, which inhibits fatty acid and subsequent lipid production [4][5][6].Pethoxamid controls key grasses such as Setaria spp., Digitaria spp., Echinochloa spp., and broadleaf weeds such as Amaranthus spp., Chenopodium spp., and Polygonum spp.[7].Pethoxamid also has activity against Group 2, 5, and 9 herbicide-resistant weeds including Amaranthus palmeri (S.Watson), Amaranthus rudis (L)., and other important annual grass and broadleaf weeds [8].Pethoxamid primarily interferes with weed seedling development in sensitive plants [8].In addition to corn and soybean, pethoxamid is currently under consideration for registration in canola, sunflower, cotton, and rice in other regions of North America [8].
Herbicide diversity is crucial for long-term sustainable crop production.Evolution of glyphosate-resistant and multiple-resistant weeds especially weeds resistant to Group 2, 5, 9, 14, and 27 herbicides in recent years is problematic for many growers in North America [9].ese modes-of-action are among the most valuable tools for weed management in corn and soybean [9].VLCFA inhibitor herbicides provide an alternative mode-of-action to commonly used herbicides and are an effective herbicide option for weed control in corn and soybean.Currently, other VLCFA inhibitor herbicides registered preemergence (PRE) in corn and soybean in Ontario include dimethenamid-P, pyroxasulfone, and S-metolachlor.However, pethoxamid is not registered for use in corn and soybean in Ontario.Registration of this herbicide may provide corn and soybean growers with a new option to control problematic grass and broadleaf weeds including herbicideresistant biotypes.

Materials and Methods
Treatments were arranged in a randomized complete block design.Herbicide treatments (based on manufacturer's recommendation) were dimethenamid-P at 544 g•ai•ha −1 , pethoxamid at 840 g•ai•ha −1 , pyroxasulfone at 100 g•ai•ha −1 , and S-metolachlor at 1050 g•ai•ha −1 .Each experiment included a weedy and a weed-free control.Plots included four rows of corn ("DKC 53-56" or "DKC 42-42 RIB") or soybean ("30-61 RY" or "DKB 27-60 RY") spaced 0.75 m apart in rows that were 10 m long at Exeter and 8 m long at Ridgetown.Corn and soybean were seeded 4 cm deep at a rate of approximately 80,000 and 380,000 seeds•ha −1 , respectively.All herbicides were applied 1-2 days after planting using a CO 2 pressurized backpack sprayer calibrated to apply 200 L•ha −1 at 240 kPa equipped with a 1.5 m wide boom with four ULD 120-02 nozzles spaced 0.5 m apart.e weed-free control was kept weed-free in corn with S-metolachlor/atrazine (2880 g•ai•ha −1 ) plus mesotrione (140 g•ai•ha −1 ), applied PRE, and in soybean with imazethapyr (100 g•ai•ha −1 ) plus metribuzin (400 g•ai•ha −1 ), applied PRE, followed by hoeing and hand weeding as needed during the growing season.
Weed control was visually estimated at 4 and 8 weeks after herbicide application (WAA) on a scale of 0 to 100, with 0 representing no apparent control and 100 representing complete control.Weed density and the aboveground shoot dry weight (biomass) at 8 WAA were determined from two 0.5 m −2 quadrats in each plot.Weeds were separated by weed species and counted before being cut at soil level.Plants were then placed in a paper bag and dried in an oven at 60 °C for 2 weeks until constant moisture before recording the dry weights.Weeds selected for analysis needed to be present in at least 2 environments.At corn and soybean maturity, the two middle rows of each plot were harvested with a small plot combine for yield and grain moisture determinations.Seed yields were adjusted to 15.5% seed moisture content for corn and 13% seed moisture content for soybean.
e GLIMMIX procedure in SAS (SAS Institute Inc. 2016 Base SAS ® 9.4 Procedures Guide: Statistical Pro- cedures, Fifth Edition, SAS Institute Inc, Cary, NC.) was used with the Laplace estimation method to perform data analysis.Model fixed effect was herbicide treatment, and random effects were environment (location-year combinations), environment by treatment interaction, and replicate within environment.In order to broaden the inference space and for results to be applicable to new location-years, environment and its associated interactions were chosen to be random effects [10,11].e significance of fixed and random effects was tested using the F-test and likelihood ratio tests, respectively.For each parameter, different distributions were assessed on the model scale and compared using the AICC, Pearson chi-squared/df, examination of the residual plots, and Shapiro-Wilk statistic.Once the best distribution was chosen, the least square means (LSMEANS) were calculated on the data scale using the inverse link function.Tukey's adjustment was applied to pairwise comparisons to determine differences among treatment means at a significance level of 0.05.Percent visible control 4 and 8 WAA for all weed species and common ragweed density and dry weight per square meter were best described using a Gaussian distribution and identity link.
In corn, a lognormal distribution (identity link) was used for green pigweed, common lambsquarters, green foxtail density and dry weight, and ragweed and wild mustard density.In soybean, a lognormal distribution was used for common lambsquarters, wild mustard and green foxtail density and dry weight, and barnyardgrass dry weight.e gamma distribution and log link was used for barnyardgrass density.In corn and soybean analysis, for parameters where values of zero were among the values observed (other than 2 International Journal of Agronomy for percent control in the weedy control, or density and dry weight in the weed-free control), a value of one was added to all data points.Each density and dry weight data point had a value of one added prior to analysis to accommodate for observed zero values, and the final LSMEANS were adjusted by subtracting one.Treatment means calculated using the lognormal distribution were backtransformed for presentation using a correction for log bias [12].

Results and Discussion
Weeds selected for analysis needed to be present in at least 2 environments.For corn experiments, weed species analyzed included redroot pigweed, common ragweed, common lambsquarters, wild mustard, and green foxtail.For soybean experiments, weed species analyzed included common ragweed, common lambsquarters, wild mustard, green foxtail, and barnyardgrass.

Corn and Soybean Yield.
In corn, weeds interference caused a 51% yield reduction (Table 1).Reduced weed interference with pyroxasulfone and dimethenamid-P resulted in corn yield that was similar to the weed-free control (Table 1).However, weed interference with pethoxamid and S-metolachlor reduced corn yield 28 and 33%, respectively.
Stephenson et al. [9] in Louisiana found no yield loss with pyroxasulfone (PRE) at 125 g•ai•ha −1 in corn.Knezevic et al. [13] in Nebraska reported that pyroxasulfone has to be applied PRE at approximately 195 g•ai•ha −1 to maintain corn yield comparable to 95% of the weed-free control.
In soybean, weeds interference caused 39% yield reduction.Reduced weed interference with the application of pyroxasulfone resulted in soybean yield that was similar to the weed-free control.However, weed interference with dimethenamid-P, pethoxamid, and Smetolachlor reduced soybean yield 27, 27, and 30%, respectively (Table 1).In other studies, pyroxasulfone applied PRE at 89 g•ai•ha −1 decreased soybean yield 18% due to weed interference [14].

Wild Mustard.
e VLCFA inhibitor herbicides evaluated did not provide adequate wild mustard control in corn and soybean.At 4 WAA, dimethenamid-P (PRE) at 544 g•ai•ha −1 , pethoxamid (PRE) at 840 g•ai•ha −1 , pyroxasulfone (PRE) at 100 g•ai•ha −1 , and S-metolachlor (PRE) at 1050 g•ai•ha −1 controlled wild mustard 34, 24, 54, and 32% in corn and 62, 48, 70, and 51% in soybean, respectively (Table 5).Wild mustard density and aboveground dry weight were generally similar to the weedy plots for both crops, except in soybean in which density was reduced 83% with pyroxasulfone (Table 5).Results are consistent with other studies that have reported  Means followed by the same alphabetical letter within a column for each crop are not significantly different according to the Tukey-Kramer multiple range test at P < 0.05.

Conclusions
Corn grain yield was similar to weed-free control with pyroxasulfone and dimethenamid-P.ere was excellent redroot pigweed control with all VLCFA inhibitor herbicides in corn.ere were no differences in common ragweed control, but pyroxasulfone provided the highest numeric control with the lowest density and aboveground dry weight among the VLCFA inhibitor herbicides evaluated in corn.Pyroxasulfone provided the best control of common lambsquarters and wild mustard among the VLCFA inhibitor herbicides evaluated in corn.
Soybean yield was similar to weed-free with pyroxasulfone.ere were no differences in common ragweed and common lambsquarters control, but pyroxasulfone provided the highest numeric control with the lowest weed density and aboveground dry weight among the VLCFA inhibitor herbicides in soybean.Pyroxasulfone provided the best control of wild mustard in soybean.Pethoxamid provided the poorest control of green foxtail and barnyardgrass among the VLCFA inhibitor herbicides evaluated in soybean.
Based on this study, pethoxamid at the rate evaluated in this study does not provide improved annual grass and broadleaf weed control compared to currently registered VLCFA inhibitor herbicides in corn and soybean.Further research is needed to evaluate the impact of higher rates of pethoxamid and possible tankmix combinations to determine if it has a fit for weed management in corn and soybean in Ontario, Canada.

Table 1 :
Yield of corn and soybean treated with various VLCFA inhibitor herbicides, applied preemergence (PRE), near Exeter and Ridgetown in 2015-2017 a .
a Means followed by the same alphabetical letter within a column for each crop are not significantly different according to the Tukey-Kramer multiple range test at P < 0.05.

Table 2 :
Control, density, and dry weight of redroot pigweed in corn treated with VLCFA inhibitor herbicides, applied preemergence (PRE), near Exeter in 2016 and 2017 and Ridgetown in 2017 a .Means followed by the same alphabetical letter within a column for each crop are not significantly different according to the Tukey-Kramer multiple range test at P < 0.05.

Table 3 :
Control, density, and dry weight of common ragweed in corn and soybean treated with VLCFA inhibitor herbicides, applied preemergence (PRE), near Exeter and Ridgetown (2015-2017) a .

Table 4 :
Control, density, and dry weight of common lambsquarters in corn and soybean treated with VLCFA inhibitor herbicides, applied preemergence (PRE), near Exeter in 2015-2017 and Ridgetown in 2017 a .Means followed by the same alphabetical letter within a column for each crop are not significantly different according to the Tukey-Kramer multiple range test at P < 0.05.

Table 5 :
Control, density, and dry weight of wild mustard in corn and soybean treated with VLCFA inhibitor herbicides, applied preemergence (PRE), near Exeter in 2015 and 2017 a .

Table 6 :
Control, density, and dry weight of green foxtail in corn and soybean treated with VLCFA inhibitor herbicides, applied preemergence (PRE), near Exeter in 2015-2017 and Ridgetown in 2017 a .Means followed by the same alphabetical letter within a column for each crop are not significantly different according to the Tukey-Kramer multiple range test at P < 0.05.

Table 7 :
Control, density, and dry weight of barnyardgrass treated with VLCFA inhibitor herbicides, applied preemergence (PRE), in soybean near Exeter (2017) and Ridgetown (2017) a .Means followed by the same alphabetical letter within a column for each crop are not significantly different according to the Tukey-Kramer multiple range test at P < 0.05.
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