International Journal of Agronomy
Volume 2019 (2019), Article ID 8159671, 7 pages
https://doi.org/10.1155/2019/8159671

Research Article

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

Nader Soltani, Lynette R. Brown, and Peter H. Sikkema

University of Guelph Ridgetown Campus, Ridgetown, ON N0P 2C0, Canada

Correspondence should be addressed to Nader Soltani; soltanin@uoguelph.ca

Received 13 November 2018; Accepted 28 February 2019; Published 7 April 2019

Academic Editor: Maria Serrano

Copyright © 2019 Nader Soltani et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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⁻¹, pethoxamid at 840 g·ai·ha⁻¹, pyroxasulfone at 100 g·ai·ha⁻¹, and S-metolachlor at 1050 g·ai·ha⁻¹ 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 8 weeks after application (WAA), all VLCFA inhibitor herbicides (Group 15) provided excellent redroot pigweed control (90 to 99%) in corn. There 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. There were no differences in barnyard grass control among the VLCFA inhibitor herbicides evaluated.

1. 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 [4–6]. 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]. These 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 herbicide-resistant biotypes.

To our knowledge, there is no published research that has collectively compared the weed control efficacy of VLCFA inhibitor herbicides including dimethenamid-P, pethoxamid, pyroxasulfone, and S-metolachlor in corn and soybean in Ontario. Accordingly, the objective of this research was to compare the efficacy of four VLCFA inhibitor herbicides including 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 applied PRE for the control of problematic weeds in corn and soybean in Ontario.

2. Materials and Methods

Field studies were conducted in 2015, 2016, and 2017 near Exeter (41°68′N, 83°11′W) and Ridgetown (42.7392°N, 81.8871°W), Ontario. There were 5 field experiments with corn (1 in 2015 (Exeter); 2 in 2016 (Ridgetown and Exeter); and 2 in 2017 (Ridgetown and Exeter)) and 5 experiments with soybean (1 in 2015 (Exeter); 2 in 2016 (Ridgetown and Exeter); and 2 in 2017 (Ridgetown and Exeter)). The soil at Exeter was a Brookston clay loam (Orthic Humic Gleysol, mixed, mesic, and poorly drained) with 29–37% sand, 37–44% silt, 26-27% clay, 3.2–3.6% organic matter, and a pH of 7.7. The soil at Ridgetown was a Watford (Gray-Brown Brunisolic, mixed, mesic, sandy, and imperfectly drained)-Brady (Gleyed Brunisolic Gray-Brown Luvisol, mixed, mesic, sandy, and imperfectly drained) with 50-51% sand, 24-25% silt, 25% clay, 3.2–3.9% organic matter, and a pH of 7.1–7.2.

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 CO₂ 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. The 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.

The GLIMMIX procedure in SAS (SAS Institute Inc. 2016 Base SAS® 9.4 Procedures Guide: Statistical Procedures, 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]. The 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. The 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 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].

3. 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.

3.1. 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.

Table 1: Yield of corn and soybean treated with various VLCFA inhibitor herbicides, applied preemergence (PRE), near Exeter and Ridgetown in 2015–2017a.

Treatment
Rate (g·ai·ha⁻¹)
Yield (T·ha⁻¹)

Corn

Weed-free control

11.9a

Weedy control

5.8c

Dimethenamid-P
544
9.0ab

Pethoxamid
840
8.6bc

Pyroxasulfone
100
9.6ab

S-metolachlor
1050
8.0bc

Soybean

Weed-free control

3.24a

Weedy control

1.99b

Dimethenamid-P
544
2.37b

Pethoxamid
840
2.36b

Pyroxasulfone
100
2.68ab

S-metolachlor
1050
2.28b

aMeans 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 .

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 S-metolachlor 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].

3.2. Redroot Pigweed

There was excellent control of redroot pigweed with all VLCFA inhibitor herbicides in corn. 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 controlled redroot pigweed as much as 99, 98, 99, and 98%; reduced redroot pigweed density 98, 96, 93, and 92%; and reduced redroot pigweed aboveground dry weight 99, 94, 93, and 91%, respectively (Table 2).

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 2017a.

Treatment
Rate (g·ai·ha⁻¹)
Control (%)
Density (no. m⁻²)
Dry weight (g·m⁻²)

4 WAA
8 WAA

Corn

Weed-free control

100
100
0a
0.0a

Weedy control

0b
0b
10.1b
8.9b

Dimethenamid-p
544
99a
99a
0.2a
0.1a

Pethoxamid
840
98a
90a
0.4a
0.5a

Pyroxasulfone
100
99a
99a
0.7a
0.6a

S-metolachlor
1050
98a
96a
0.8a
0.8a

aMeans 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 .

Results are similar to that of Mahoney et al.’s [14], who found 100% control of redroot pigweed with pyroxasulfone (PRE) at 89 g·ai·ha⁻¹ in soybean. Li et al. [15] found 99 to 100% of redroot pigweed with dimethenamid-P (PRE) at 544 g·ai·ha⁻¹ and S-metolachlor (PRE) at 1050 g·ai·ha⁻¹ in dry bean. Taziar et al. [16] reported 96 to 98% redroot and green pigweed control with dimethenamid-P (PRE) at 544 g·ai·ha⁻¹, pyroxasulfone (PRE) at 100 g·ai·ha⁻¹, and S-metolachlor (PRE) at 1050 g·ai·ha⁻¹ in dry bean. Additionally, pyroxasulfone (PRE) at 150 g·ai·ha⁻¹ controlled other Amaranthus species such as Palmer amaranth 93 to 96% in corn [9]. Geier et al. [17] reported 82 to 85% control of Palmer amaranth with pyroxasulfone applied PRE at 125 g·ai·ha⁻¹ in corn. However, S-metolachlor (PRE) at 1070 g·ai·ha⁻¹ controlled Palmer amaranth 57 to 87% in the same study [17]. Another study found as much as 97% control of Palmer amaranth with pyroxasulfone (PRE) at 166 g·ai·ha⁻¹ in corn [18].

3.3. Common Ragweed

The VLCFA inhibitor herbicides evaluated provided poor control of common ragweed in corn and soybean. At 4 WAA, 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 controlled common ragweed 56, 66, 77, and 42% in corn and 13, 19, 28, and 7% in soybean, respectively (Table 3). However, the results were not always significantly different from the weedy control (Table 3). Generally, there was no decrease in common ragweed density and aboveground dry weight relative to the weedy plots in both crops, except in corn where density of common ragweed was reduced 52, 59, and 63% with dimethenamid-P, pethoxamid, and pyroxasulfone, respectively (Table 3).

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.

Treatment
Rate (g·ai·ha⁻¹)
Control (%)
Density (no. m⁻²)
Dry weight (g·m⁻²)

4 WAA
8 WAA

Corn

Weed-free control

100
100
0.0a
0.0a

Weedy control

0b
0a
51.2c
128.6b

Dimethenamid-P
544
56a
13a
24.8b
86.7b

Pethoxamid
840
66a
20a
20.9b
120.0b

Pyroxasulfone
100
77a
30a
18.7b
91.9b

S-metolachlor
1050
42a
12a
36.1bc
141.3b

Soybean

Weed-free control

100
100
0a
0.0a

Weedy control

0b
0b
26b
29.1b

Dimethenamid-P
544
13ab
12ab
26b
37.7b

Pethoxamid
840
17a
19a
23b
31.2b

Pyroxasulfone
100
28a
26a
19b
27.2b

S-metolachlor
1050
7ab
6ab
26b
34.7b

aMeans 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 .

Results are consistent with other research studies that have shown poor control of common ragweed (7 to 28%) with dimethenamid-P (PRE) at 693 g·ai·ha⁻¹, pethoxamid (PRE) at 1200 g·ai·ha⁻¹, pyroxasulfone (PRE) at 150 g·ai·ha⁻¹, and S-metolachlor (PRE) at 1600 g·ai·ha⁻¹ in corn [19]. Mahoney et al. [14] also reported only 45% common ragweed control with pyroxasulfone (PRE) at 89 g·ai·ha⁻¹ in soybean. Belfry et al. [20] also found poor control of common ragweed with pyroxasulfone (PRE) at 100 g·ai·ha⁻¹ in soybean. Additionally, Taziar et al. [16] reported poor common ragweed control (5 to 39%) with dimethenamid-P (PRE) at 544 g·ai·ha⁻¹, pyroxasulfone (PRE) at 100 g·ai·ha⁻¹, and S-metolachlor (PRE) at 1050 g·ai·ha⁻¹ in dry bean.

3.4. Common Lambsquarters

There were less than adequate common lambsquarters control with all VLCFA inhibitor herbicides in corn and soybean. Pyroxasulfone (PRE) at 100 g·ai·ha⁻¹ was the best treatment for the control of common lambsquarters in corn (Table 4). At 4 weeks after herbicide treatment, 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 common lambsquarters 68, 70, 84, and 60% in corn and 43, 33, 56, and 32% in soybean, respectively (Table 4). However, results were not always significantly different from the weedy control (Table 4). Common lambsquarters density was reduced by 74, 78, 92, and 66% in corn with dimethenamid-P, pethoxamid, pyroxasulfone, and S-metolachlor, respectively (Table 4). Dimethenamid-P, pethoxamid, and pyroxasulfone also reduced common lambsquarters density 73, 67, and 82% in soybean, respectively. Common lambsquarters aboveground dry weight was generally comparable to the weedy plots for both crops, except in corn in which dry weight was reduced 87% with pethoxamid and 93% with pyroxasulfone.

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 2017a.

Treatment
Rate (g·ai·ha⁻¹)
Control (%)
Density (no. m⁻²)
Dry weight (g·m⁻²)

4 WAA
8 WAA

Corn

Weed-free control

100
100
0.0a
0.0a

Weedy control

0c
0b
15.8d
11.9c

Dimethenamid-P
544
68ab
63a
4.1bc
4.1bc

Pethoxamid
840
70ab
77a
3.4bc
1.6b

Pyroxasulfone
100
84a
84a
1.3b
0.8a

S-metolachlor
1050
60b
65a
5.4c
7.0bc

Soybean

Weed-free control

100
100
0a
0.0a

Weedy control

0b
0b
33c
19.4b

Dimethenamid-P
544
43a
32ab
9b
8.9b

Pethoxamid
840
33a
30ab
11b
9.9b

Pyroxasulfone
100
56a
43a
6b
7.6b

S-metolachlor
1050
32a
30ab
15bc
15.8b

aMeans 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 .

In other studies, Jha et al. [21] found that common lambsquarters was controlled 15 to 66% with pyroxasulfone (PRE) at 149 g·ai·ha⁻¹, 33 to 77% with dimethenamid-P (PRE) at 840 g·ai·ha⁻¹, and 12 to 65% with acetochlor (PRE) at 1960 g·ai·ha⁻¹ in corn. Mahoney et al. [14] found 76% common lambsquarters control with pyroxasulfone applied PRE at 89 g·ai·ha⁻¹ in soybean. In contrast, Belfry et al. [20] reported minimal control (8 to 9%) of common lambsquarters with 100 g·ai·ha⁻¹ of pyroxasulfone. Other studies have also shown poor common lambsquarters control (8 to 54%) with dimethenamid-P (PRE) at 544 g·ai·ha⁻¹, pyroxasulfone (PRE) at 100 g·ai·ha⁻¹, and S-metolachlor (PRE) at 1050 g·ai·ha⁻¹ in dry bean [15, 16].

3.5. Wild Mustard

The 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 14 to 48, 2 to 33, and 63 to 95% control of wild mustard with dimethenamid-P at 544 g·ai·ha⁻¹, pyroxasulfone at 100 g·ai·ha⁻¹, and S-metolachlor at 1050 g·ai·ha⁻¹ applied PRE in dry bean, respectively [16].

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 2017a.

Treatment
Rate (g·ai·ha⁻¹)
Control (%)
Density (no. m⁻²)
Dry weight (g·m⁻²)

4 WAA
8 WAA

Corn

Weed-free control

100
100
0.0a
0.0a

Weedy control

0b
0b
118.2b
195.4b

Dimethenamid-P
544
34ab
25a
56.3b
109.7b

Pethoxamid
840
24ab
31a
53.7b
143.3b

Pyroxasulfone
100
54a
57a
9.5b
37.0ab

S-metolachlor
1050
32ab
19ab
71.0b
160.3b

Soybean

Weed-free control

100
100
0a
0.0a

Weedy control

0b
0c
23c
76.9b

Dimethenamid-P
544
62a
35ab
9abc
20.2b

Pethoxamid
840
48a
23bc
20c
58.2b

Pyroxasulfone
100
70a
51a
4ab
8.1ab

S-metolachlor
1050
51a
16bc
14bc
35.6b

aMeans 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 .

3.6. Green Foxtail

The VLCFA inhibitor herbicides controlled green foxtail 55–96% in corn and soybean (Table 6). At 8 WAA, 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 controlled green foxtail 96, 91, 96, and 96% in corn and 89, 60, 74, and 78% in soybean, respectively (Table 6). Dimethenamid-P, pethoxamid, pyroxasulfone, and S-metolachlor reduced green foxtail density 99, 88, 95, and 95% in corn and 92, 62, 90, and 88% in soybean, respectively (Table 6). Similarly, dimethenamid-P, pethoxamid, pyroxasulfone, and S-metolachlor reduced green foxtail aboveground dry weight 99, 91, 93, and 97% in corn and 93, 63, 93, and 93% in soybean, respectively.

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 2017a.

Treatment
Rate (g·ai·ha⁻¹)
Control (%)
Density (no. m⁻²)
Dry weight (g·m⁻²)

4 WAA
8 WAA

Corn

Weed-free control

100
100
0.0a
0.0a

Weedy control

0b
0b
29.9d
11.3b

Dimethenamid-P
544
84a
96a
0.4ab
0.1a

Pethoxamid
840
71a
91a
3.6c
1.0a

Pyroxasulfone
100
81a
96a
1.6bc
0.8a

S-metolachlor
1050
94a
96a
1.5bc
0.3a

Soybean

Weed-free control

100
100
0a
0.0a

Weedy control

0c
0c
50c
22.8d

Dimethenamid-P
544
88a
89a
4b
1.5b

Pethoxamid
840
55b
60b
19c
8.4c

Pyroxasulfone
100
70ab
74ab
5b
1.7b

S-metolachlor
1050
78a
78ab
6b
1.5b

aMeans 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 .

In other studies, Geier et al. [17] reported 95% green foxtail control with pyroxasulfone applied PRE at 125 g·ai·ha⁻¹ in corn. However, S-metolachlor (PRE) at 1070 g·ai·ha⁻¹ controlled green foxtail 88% in the same study [17]. Mahoney et al. [14] reported 86% control of green foxtail with pyroxasulfone (PRE) at 89 g·ai·ha⁻¹ in soybean. Belfry et al. [20] found as much as 61% control of green foxtail with pyroxasulfone (PRE) at 100 g·ai·ha⁻¹ in soybean. Taziar et al. [16] also found 99, 99, and 98% green foxtail control with dimethenamid-P (PRE) at 544 g·ai·ha⁻¹, pyroxasulfone (PRE) at 100 g·ai·ha⁻¹, and S-metolachlor (PRE) at 1050 g·ai·ha⁻¹ in dry bean, respectively. Additionally, Li et al. [15] found 92 to 99% control of green foxtail with dimethenamid-P (PRE) at 544 g·ai·ha⁻¹ and S-metolachlor (PRE) at 1050 g·ai·ha⁻¹ in dry bean.

3.7. Barnyardgrass

VLCFA inhibitor herbicides evaluated did not consistently control barnyardgrass in soybean. At 4 and 8 WAA, 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 controlled barnyardgrass up to 80, 48, 56, and 61% in soybean, respectively (Table 7). However, results were not always significantly different from the weedy control (Table 7). Density and aboveground dry weight of barnyardgrass were comparable to the weedy plots for all VLCFA inhibitor herbicides except for S-metolachlor which reduced which decreased density 86% and aboveground dry weight 97% (Table 7). In other studies, pyroxasulfone (PRE) at 150 g·ai·ha⁻¹ controlled barnyardgrass 93 to 96% in corn [9]. Yamaji et al. [22] also found 100% barnyardgrass control with pyroxasulfone (PRE) at 125 to 250 g·ai·ha⁻¹ in corn.

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.

Treatment
Rate (g·ai·ha⁻¹)
Control (%)
Density (no. m⁻²)
Dry weight (g·m⁻²)

4 WAA
8 WAA

Soybean

Weed-free control

100
100
0a
0.0a

Weedy control

0b
0b
36c
39.8c

Dimethenamid-P
544
80a
80a
11bc
3.5abc

Pethoxamid
840
48ab
35ab
14bc
12.9bc

Pyroxasulfone
100
56ab
53ab
13bc
4.0abc

S-metolachlor
1050
60a
61ab
5ab
1.2ab

aMeans 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 .

4. Conclusions

Corn grain yield was similar to weed-free control with pyroxasulfone and dimethenamid-P. There was excellent redroot pigweed control with all VLCFA inhibitor herbicides in corn. There 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. There 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.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This project was funded in part by Grain Farmers of Canada (GFO) and the Growing Forward II Program of the Agricultural Adaptation Council.

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