Introduction
Corn (Zea mays L.) is a major crop in Canada; it is primarily grown in Ontario, where more than 60% of Canada’s corn is grown on 1.5 million ha (Statistics Canada 2023), equating to Can$2.35 billion in total farm income in 2023 (OMAFRA 2024). In the United States, corn was grown on 38.1 million ha with a production value of US$91.7 billion (USDA-NASS 2022b, 2023). Weed interference in field corn causes an average yield loss of 50% when no weed management tactics are used (Soltani et al. Reference Soltani, Dille, Burke, Everman, VanGessel, Davis and Sikkema2016). Herbicides are the most common weed management tool, with 96% of corn hectares having received at least one application (USDA-NASS 2022a).
A new premixture herbicide containing pyroxasulfone (a Group 15 herbicide as categorized by the Herbicide-Resistance Action Network [HRAC] and Weed Science Society of America [WSSA]) plus encapsulated saflufenacil (a Group 14 herbicide) could combat challenging weed problems that reduce corn yields. This premixture herbicide combines two herbicide sites of action, a Group 15 very-long-chain fatty acid elongases inhibitor and a Group 14 protoporphyrinogen oxidase inhibitor (Shaner Reference Shaner2014). Pyroxasulfone has been available in Ontario since the 2016 growing season (Health Canada 2016). Pyroxasulfone (200 to 300 g ai ha−1) provides 4 to 6 wk of residual weed control when applied before weed emergence; however, the herbicide can be applied early postemergence with a tank-mix partner to attain activity on emerged weeds. Pyroxasulfone is predominately a rate-dependant grass herbicide with activity on a few broadleaf weeds and sedges (BASF 2022b; OMAFRA 2021; Shaner Reference Shaner2014). Grasses controlled by pyroxasulfone include barnyardgrass, foxtail species, and large crabgrass, while broadleaf weeds include redroot pigweed, velvetleaf, and waterhemp (Nurse et al. Reference Nurse, Sikkema and Robinson2011; OMAFRA 2021; Yamaji et al. Reference Yamaji, Honda, Kobayashi, Hanai and Inoue2014). The suspension concentrate formulation of saflufenacil has been sold in Canada since the 2010 growing season, applied preplant, preplant incorporated, or preemergence for the control of a wide range of broadleaf weeds (Health Canada 2024; OMAFRA 2021). The suspension concentrate formulation of saflufenacil applied before crop emergence controls common lambsquarters, redroot pigweed, common ragweed, Canada fleabane, velvetleaf, wild buckwheat, wild mustard, and stinkweed (Boydston et al. Reference Boydston, Felix and Al-Khatib2012; Geier et al. Reference Geier, Stahlman and Charvat2009; OMAFRA 2021).
In corn-producing regions of North America, growers demand strong weed control with little crop injury. Pyroxasulfone and encapsulated saflufenacil is a new herbicide formulation, but limited research has been conducted to determine the necessary doses to control problematic weed species in southwestern Ontario. By conducting a biologically effective dose study, a better understanding of the rate required to control a particular weed species with this new encapsulated formulation. Thus the objective of this study was to determine the biologically effective dose of pyroxasulfone + encapsulated saflufenacil applied preemergence for control of common lambsquarters, redroot pigweed, and foxtail species as well as assess corn injury.
Materials and Methods
This study consisted of 6 site-years conducted over 2 yr (2022 and 2023) in southwestern Ontario: four in Ridgetown, on the University of Guelph, Ridgetown campus, and two located near Belmont, on the BASF London Research Farm. Trials were set up as a randomized complete block design with nine treatments and four replications; plot size of 2 by 8 m. Field preparation consisted of conventional tillage (chisel ploughed in the fall followed by cultivation in the spring) and fertilization according to Ontario Ministry of Agriculture, Food, and Rural Affairs recommendation based on soil tests. Corn was then planted at approximately 80,000 seeds ha−1 in rows spaced 75 cm apart to a depth of 5 cm. Refer to Table 1 for additional soil and crop information. Following planting, the herbicide treatments were applied preemergence with a CO2-pressurized backpack sprayer calibrated to deliver 200 L ha−1. To establish the dose-response, nine doses of pyroxasulfone + encapsulated saflufenacil (0, 18.25, 36.5, 73, 146, 195, 245, 490, and 980 g ai ha−1) were applied prior to corn and weed emergence. Doses were established based on a proposed label rate of 146 to 245 g ai ha−1. Refer to Table 2 for the treatment list and rate structure for individual active ingredients (pyroxasulfone and encapsulated saflufenacil) in this premixture.
Table 1. Field trial information.a
a Abbreviations: CEC, cation exchange capacity; OM, organic matter.
Table 2. Herbicide treatments and rates.
a The premixture contains pyroxasulfone + encapsulated saflufenacil.
Corn injury was rated 1, 2, and 3 wk after emergence (WAE) on a 0% to 100% scale, where 0% indicates no visible symptoms and 100% represents complete plant death. Weed control was assessed at 4 and 8 WAE on the same 0% to 100% scale. Weed density and biomass were collected at 8 WAE by marking a 0.25-m2 quadrat at two random locations (between corn rows and at least 1 m from the plot edge) in each plot and counting the weeds by species and clipping them at the soil surface and collecting each weed species within the quadrats separately. The collected samples were then dried in a kiln until a consistent moisture was achieved. The dried plant material was then weighed and the aboveground biomass for each plot was recorded. The weed spectrum evaluated across all locations consisted of common lambsquarters, redroot pigweed, and foxtail species. Weed pressure was consistent across replications and sites to ensure accurate evaluations. Yield data were collected at harvest maturity using a mechanical plot harvester to obtain weight and moisture content for each plot, which were used to adjust the yield to 15.5% moisture.
Data from all 6 site-years were combined for analysis and interpretation across multiple environments following the analysis of random effects. Two environments were removed for foxtail species and one for redroot pigweed before analysis due to low weed pressure. The NLIN procedure in SAS software (v.9.4; SAS Institute Inc., Cary, NC), was used to regress weed control, weed density, weed biomass, and corn yield evaluations over herbicide dose. Regression curves that best fit the data from this study were chosen by assessing modeling efficiency and root mean squared error (Archontoulis and Miguez Reference Archontoulis and Miguez2015). Weed control and yield data were regressed over an exponential to a maximum curve (Equation 1):
$${y = a - b (e^{(-c^{*}dose)})}$$
where y is the response parameter, a is the upper asymptote, b is the magnitude constant, and c is the slope. Weed density and biomass was regressed over an inverse exponential curve (Equation 2):
$${y = a + b (e^{(-c^{*}dose)})}$$
where y is the response parameter, a is the lower asymptote, b is the reduction in y from intercept to asymptote, and c is the slope from intercept to a. The effective dose (ED) needed for 50% control, density, and biomass was calculated using Equations 1 and 2. Since density and biomass are not calculated on a 0% to 100% scale, 50% of the untreated control was calculated to establish the ED50. The ED90 was not determined because it would be inappropriate when the dose required exceeded the range used in this study.
Results and Discussion
Weed Control 4 and 8 WAE
A lower rate of pyroxasulfone + encapsulated saflufenacil was needed to control redroot pigweed than common lambsquarters and foxtail species at 4 WAE (Figure 1; Table 3). At 8 WAE redroot pigweed required the lowest rate for control, followed by common lambsquarters and foxtail species (Figure 2; Table 3). The required dose of pyroxasulfone + encapsulated saflufenacil to obtain 50% control of redroot pigweed, foxtail species, and common lambsquarters was 145, 204, and 214 g ai ha−1, respectively, at 4 WAE (Figure 1; Table 3). The required dose of pyroxasulfone + encapsulated saflufenacil to obtain 50% control of redroot pigweed, common lambsquarters, and foxtail species was 170, 219, and 240 g ai ha−1, respectively, at 8 WAE (Figure 1; Table 3). The ED90 for visible weed control was not achieved for any weed species evaluated or evaluation timing (4 and 8 WAE) with the rates applied in this study.
Figure 1. Visible weed control (%) of common lambsquarters, redroot pigweed, and foxtail species with pyroxasulfone + encapsulated saflufenacil 4 wk after emergence (WAE). Vertical bars represent ±SE of means. Dose-response curves were fit to an exponential to a maximum model using nonlinear regression (Equation 1).
Table 3. Nonlinear regression parameters and predicted pyroxasulfone + encapsulated saflufenacil dose required to obtain 50% visual control of weeds at 4 and 8 wk after emergence.a
a Abbreviation: WAE, weeks after emergence.
b Regression parameters were calculated using Equation 1.
c ED50 is the effective dose required to achieve 50% visible control.
Figure 2. Visible weed control (%) of common lambsquarters, redroot pigweed, and foxtail species with pyroxasulfone + encapsulated saflufenacil 8 wk after emergence (WAE). Vertical bars represent ±SE of means. Dose-response curves were fit to an exponential to a maximum model using nonlinear regression (Equation 1).
Weed Density and Biomass
The required dose of pyroxasulfone + encapsulated saflufenacil to obtain a 50% reduction in common lambsquarters, redroot pigweed, and foxtail species density was 34, 86, and 108 g ai ha−1, respectively (Figure 3; Table 4). Higher doses of pyroxasulfone + encapsulated saflufenacil were required to achieve a 50% reduction in biomass. The required dose of pyroxasulfone + encapsulated saflufenacil to obtain a 50% reduction in redroot pigweed, common lambsquarters, and foxtail species biomass was 125, 183, and 528 g ai ha−1, respectively (Figure 4; Table 4). Biomass data were variable, but Figure 4 shows a dramatic decrease in the biomass of foxtail species as the dose increased, while a less dramatic decrease in biomass occurred with common lambsquarters and redroot pigweed. The ED90 for weed density and biomass was not achieved for any weed species evaluated with the rates applied in this study.
Figure 3. Weed density (number of plants per square meter) of common lambsquarters, redroot pigweed, and foxtail species with pyroxasulfone + encapsulated saflufenacil 8 wk after emergence (WAE). Vertical bars represent ±SE of means. Dose-response curves were fit to an inverse exponential model using nonlinear regression (Equation 2).
Table 4. Nonlinear regression parameters and predicted pyroxasulfone + encapsulated saflufenacil dose required for 50% reduction of common lambsquarters, redroot pigweed, and foxtail species density and biomass at 8 wk after emergence.a
a Abbreviation: WAE, weeks after emergence.
b Regression parameters were calculated using Equation 2.
c ED50 is the effective dose required to achieve 50% visible control.
Figure 4. Weed biomass (grams per square meter) of common lambsquarters, redroot pigweed, and foxtail species with pyroxasulfone + encapsulated saflufenacil 8 wk after emergence (WAE). Vertical bars represent ±SE of means. Dose-response curves were fit to an inverse exponential model using nonlinear regression (Equation 2).
A previous field study conducted by Knezevic et al. (Reference Knezevic, Datta, Scott and Charvat2009) determined that the ED90 of pyroxasulfone was 200 to 300 g ai ha−1 for multiple grass and broadleaf weed species. Previous greenhouse research on dose-response of saflufenacil applied preemergence for control of multiple broadleaf weeds demonstrated 90% biomass reduction at 9 g ai ha−1 (Geier et al. Reference Geier, Stahlman and Charvat2009). This contrasts with the results of this study, where the dose required for a 50% reduction in biomass of redroot pigweed, common lambsquarters, and foxtail species was much higher. Higher rates in field trials as opposed to greenhouse studies is to be expected due to increased environmental variance. Mahoney et al. (Reference Mahoney, Shropshire and Sikkema2014) determined through 11 field experiments that the ED50 of a pyroxasulfone/flumioxazin combination for controlling common lambsquarters, pigweed species, and green foxtail at 8 wk after application was 46, 3, and 57 g ai ha−1, respectively. Although a comparison of field trials carried out at different locations can be affected by soil characteristics and environment, the results of this study suggest that weed control efficacy may be reduced with the encapsulated formulation.
Corn Injury and Yield
Pyroxasulfone + encapsulated saflufenacil applied preemergence caused no corn injury in this study, even at extreme rates of 490 and 980 g ai ha−1 (data not shown). This is not surprising given that the suspension concentrate formulation of saflufenacil and pyroxasulfone are both registered for preemergence application to corn (BASF 2022a, 2022b; OMAFRA 2021). Reduced weed interference with increasing doses of pyroxasulfone + encapsulated saflufenacil led to greater corn yields (Figure 5). Corn yield plateaued at 245 g ai ha−1 with no further increase in corn yield when pyroxasulfone + encapsulated saflufenacil was applied at doses above 245 g ai ha−1.
Figure 5. Corn yield (tons per hectare) with pyroxasulfone + encapsulated saflufenacil. Vertical bars represent ±SE of means. Dose-response curves were fit to an exponential to a maximum model using nonlinear regression (Equation 1).
Based on this study, the doses of pyroxasulfone + encapsulated saflufenacil required for 50% control are specific to each weed species. The required dose of pyroxasulfone + encapsulated saflufenacil was lower to control redroot pigweed than it was to control common lambsquarters and foxtail species. Reduced weed interference with increasing doses of pyroxasulfone + encapsulated saflufenacil resulted in an increase in corn yield up to 245 g ai ha−1. In this study the ED90 of all weed species was not achieved with rates up to 980 g ai ha−1 of pyroxasulfone + encapsulated saflufenacil when the proposed label rate is 146 to 245 g ai ha−1. Since the ED90 of pyroxasulfone + encapsulated saflufenacil for controlling redroot pigweed, common lambsquarters, and foxtail species was greater than the doses evaluated in this study, future research should evaluate a complementary pyroxasulfone + encapsulated saflufenacil dose with appropriate tank-mix partners.
Practical Implications
The results of this study provide valuable information for growers and agronomists who use pyroxasulfone + encapsulated saflufenacil as a preemergence herbicide in corn production. No corn injury was observed with pyroxasulfone + encapsulated saflufenacil, supporting the crop safety of this herbicide mixture in corn production systems. The pyroxasulfone + encapsulated saflufenacil herbicide mixture demonstrated species-specific efficacy, requiring lower doses to obtain effective control of redroot pigweed and higher doses for common lambsquarters and foxtail species, particularly extended periods after application (8 wk after emergence). Importantly, higher herbicide rates consistently resulted in reduced weed density, biomass, and weed interference, leading to significant improvements in corn yield, which plateaued at approximately 245 g ai ha−1. Despite these yield benefits, the study found that the doses required to obtain 90% weed control (ED90) were not achieved within the range of doses assessed, especially for more difficult-to-control species like foxtail. Overall, to obtain better weed management and optimal crop yield, this study supports a more tailored approach that accounts for weed species present and potentially integrating higher doses or additional herbicide sites of action. Future product recommendations and weed management programs should incorporate these findings to enhance control efficacy and crop yield while minimizing the risk of herbicide resistance through diversified weed control strategies.
Acknowledgments
We thank Kris McNaughton for her technical assistance.
Funding statement
This project was funded in part by BASF Canada Inc.
Competing Interests
Co-author Chris Budd is Senior Biologist with BASF Canada Inc. The other authors declare they have no conflicts of interest.
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