Annual Ryegrass Control in Corn With Glyphosate plus Residual Herbicides Applied Preplant in the Spring

Glyphosate needs to be partnered with other herbicides that have residual biological activity on ryegrass ( Lolium multiflorum Lam.) to improve the efficacy and consistency of annual ryegrass control in corn. Five field experiments were conducted from 2019 to 2021 near Exeter, Ontario to evaluate various glyphosate tank mixes applied preplant (PP) in the spring in corn for the control of annual ryegrass seeded as a cover crop in the fall of the previous year (2018 to 2020). At 1 week after application (WAA), all glyphosate tank mixes evaluated provided minimal annual ryegrass control (14-28%). At 2 WAA, the addition of dimethenamid-p/saflufenacil to glyphosate improved annual ryegrass control from 55% to 68%; there was no improvement in annual ryegrass control with the other 14 tank mixes evaluated. At 3 WAA, the addition of dimethenamid-p/saflufenacil and mesotrione + rimsulfuron to glyphosate controlled annual ryegrass 91 and 90%, respectively. At 4 WAA, the addition of dimethenamid-p, dimethenamid-p/saflufenacil, mesotrione + rimsulfuron, S -metholachlor, or bicyclopyrone/mesotrione/ S -metolachlor to glyphosate improved annual ryegrass control 7, 7, 10, 8, and 6%, respectively. At 6 WAA, the addition of pyroxasulfone, pyroxasulfone + atrazine, dimethenamid-p, dimethenamid-p/saflufenacil, mesotrione + rimsulfuron, S -metholachlor, atrazine/ S -metolachlor, or bicyclopyrone/mesotrione/ S -metolachlor to glyphosate improved annual ryegrass control 10, 9, 12, 10, 17, 13, 10, and 12%, respectively but the addition of all other herbicides to glyphosate did not improve annual ryegrass control. Density and biomass reductions of annual ryegrass with glyphosate tank mix evaluated generally followed a similar trend as the visible control. Annual ryegrass interference reduced corn yield by up to 83% compared to the non-treated control. The addition of a residual herbicide to glyphosate did not result in an improvement in the seed yield of corn. Among the glyphosate tank mixes evaluated glyphosate + mesotrione + rimsulfuron provided the most consistent control of annual ryegrass in corn.


Introduction
Corn (Zea mays L.) is one of the most valuable agricultural crops grown in Canada (Molenhuis, 2018). Canada is the 11 th top corn-producing country in the world and Ontario farmers grow nearly two-thirds of all corn produced in Canada (Molenhuis, 2018). Corn growers in Ontario seeded 890,000 hectares and produced nearly 9 million tonnes of corn with a farm gate value of approximately $1.8 billion in 2020 (OMAFRA, 2021). Nearly 60% of the grain corn produced is used for feed and the remaining 40% is used for various industrial uses (OMAFRA, 2021). Corn producers need to modify their crop production practices to improve soil health and reduce the ecological downturn associated with conventional corn production practices while maintaining their competitiveness in the global marketplace (Lu et al., 2000). Inclusion of fall-sown cover crops in cropping systems has been shown to enhance soil health by improving organic matter, water holding capacity, nutrient sequestration, nutrient availability, and soil structure and reducing nutrient leaching and soil erosion (Blanco-Canqui et al., 2015;Moore et al., 1994;Snapp et al., 2005;Teasdale et al., 2007;Thilakarathna et al., 2015). Cover crops have also been shown to suppress weeds and reduce weed seeds in the soil seedbank by providing a natural habitat for beneficial organisms that feed on weed seeds (Cholette et al., 2018;Clark, 2012;Moore et al., 1994;Teasdale, 1996;Teasdale et al., 2007). Additionally, some cover crops have been shown to exude allelopathic chemicals into the soil that reduced weed growth (Li et al., 2008;Weston, 1996). Leguminous cover crops can also fix nitrogen and decrease the need for synthetic nitrogen application in subsequent crops (Lu et al., 2000).
Annual ryegrass (Lolium multiflorum Lam.), also known as Italian ryegrass, is a fast-growing cover crop that is suitable for cooler regions of the USA and Canada (Nandula et al., 2007). Annual ryegrass has very dense roots which allow it to sequester nitrogen and be very effective in breaking up compacted soils (Grant, 2018). Although annual ryegrass has many beneficial attributes for inclusion as a fall-sown cover crop in cropping systems in the USA and Canada, it has not been adopted by many growers mainly due to the difficulty of controlling it in the spring prior to seeding corn (Nandula, 2014). Effective control of annual ryegrass before seeding corn is crucial to eliminate early-season interference and subsequent corn yield losses. Earlier studies have reported as much as 49% corn seed yield losses when annual ryegrass was not controlled (Kobayashi et al., 1987;Nandula, 2014).
Earlier research has shown that glyphosate can be an effective herbicide for the control of annual ryegrass in Ontario; however, glyphosate alone does not provide consistent control of ryegrass prior to seeding corn (Soltani et al., 2020(Soltani et al., , 2021. Glyphosate needs to be partnered with other herbicides that have residual biological activity on ryegrass to improve the efficacy and consistency of annual ryegrass control in corn (Soltani et al., 2020(Soltani et al., , 2021. Currently registered preplant residual herbicides in Ontario that can be tank mixed with glyphosate for the control of annual ryegrass include atrazine/dicamba, atrazine + pendimethalin, pendimethalin, atrazine + isoxaflutole, atrazine, atrazine/mesotrione/S-metolachlor, atrazine/bicyclopyrone/mesotrione/S-metolachlor, pyroxasulfone + atrazine, pyroxasulfone, atrazine/S-metolachlor, dimethenamid-P/saflufenacil, dimethenamid-P, bicyclopyrone/mesotrione/S-metolachlor, S-metolachlor, and mesotrione + rimsulfuron (OMAFRA, 2022). Earlier studies have shown that the efficacy of glyphosate to control some weeds can be decreased when tank mixed with other herbicides (Flint & Barrett, 1989;Jordan et al., 2001;Wicks & Hanson, 1995). To our knowledge, no study has cumulatively compared the efficacy of glyphosate plus these residual herbicides for the control of annual ryegrass in corn under Ontario environmental conditions. The purpose of this research was to evaluate the effect of various glyphosate tank mixes applied pre-plant (PP) in the spring for the control of annual ryegrass seeded as a cover crop in the fall of the previous year.

Materials and Methods
Five field experiments were established from 2019 to 2021 at Huron Research Station, University of Guelph [43.316305, -81.504763] near Exeter, Ontario. There was one trial in 2019 and two trials (separated in time and space each year) in 2020 and 2021. Trials were started in the fall of 2018, 2019, and 2020 when annual ryegrass was seeded after harvesting the previous crop and finished after corn harvest in the following year (2019, 2020, and 2021). Each experiment was designed as a randomized complete block design with four replications. Treatments evaluated were non-treated control (weedy), glyphosate, glyphosate + atrazine/dicamba, glyphosate + atrazine + pendimethalin, glyphosate + pendimethalin, glyphosate + atrazine + isoxaflutole, glyphosate + atrazine, glyphosate + atrazine/mesotrione/S-metolachlor, glyphosate + atrazine/bicyclopyrone/mesotrione/S-metolachlor, glyphosate + pyroxasulfone + atrazine, glyphosate + pyroxasulfone, glyphosate + atrazine/S-metolachlor, glyphosate + dimethenamid-P/saflufenacil, glyphosate + dimethenamid-P, glyphosate + bicyclopyrone/mesotrione/S-metolachlor, glyphosate + S-metolachlor, and glyphosate + mesotrione + rimsulfuron applied PP at the manufacturers' recommended rates. The rates and adjuvants used with each treatment are presented in Tables 3 and 4. Note. a Specimen labels for each product and manufacturer contact information can be found at https://pr-rp.hc-sc.gc.ca/ls-re/index-eng.php Experimental plots were 3 m wide and 10 m long. Annual ryegrass (Speare Seeds Limited, Harriston, Ontario) was seeded in rows spaced 18 cm apart (seeding rate of 22 kg ha -1 ) after the previous crop was harvested. Herbicide treatments were applied prior to seeding corn; treatment dates are presented in Table 2. Glyphosate/glufosinate-resistant corn ('DKC 42-60RIB') was seeded approximately 5 cm deep from April to June of each year at the rate of approximately 86,000 seeds ha -1 in rows that were spaced 75 cm apart. Herbicides were applied preplant (when annual ryegrass was 20 cm or less) using a CO 2 -pressurized backpack sprayer (delivery rate of 200 L ha -1 at a pressure of 240 kPa). The spray boom was 1.5 m long and had four ultra low drift (ULD 120-02, Pentair-Hypro, New Brighton, Minnesota) nozzles spaced 0.5 m apart, producing a spray width of 2.0 m.
Annual ryegrass control was assessed visually 1, 2, 3, 4, and 6 weeks after treatment application (WAA) using a scale of 0 to 100, with 0 representing no control and 100 representing total control. Annual ryegrass density and biomass were determined 4 WAA from two 0.25 m 2 quadrats placed between the corn rows. Annual ryegrass plants were counted (density) and clipped at the soil surface, placed in paper bags, dried at 60 o C, and then weighed (biomass). Grain corn yield (adjusted to 15.5% moisture) was determined at maturity by combining the middle two rows of each plot using a small plot combine.

Statistical Analysis
Data were analyzed using the GLIMMIX procedure via SAS Studio v9.4, OnDemand for Academics (SAS Institute, Cary, NC). Variance was partitioned into the fixed effect of herbicide treatment and the random effects jas.ccsenet.org Vol. 14, No. 11;2022 of environment (location-year combinations), block nested within environment, and the environment-by-treatment interaction. Annual ryegrass control at 1, 2, 3, 4, and 6 WAA were analyzed using the beta distribution and complementary log-log link. The weedy control was assigned a value of 0% and was therefore excluded from the analysis due to zero variance. Annual ryegrass density and biomass were analyzed using the lognormal distribution with identity link; treatment means were analyzed and compared on the log scale. Corn yield was analyzed using a normal distribution with identity link. The weedy control was included for the analysis of density, biomass, and corn yield data. The Pearson chi-square/degrees of freedom ratio and Shapiro-wilk statistic were used to determine model fitness and avoid potential overdispersion for all parameters. Studentized residual plots were used to confirm homogeneity of variance and normal probability plots were used to verify the assumptions of normality. Density and biomass data were back transformed using the omega method (M. Edwards, Ontario Agricultural College Statistician, University of Guelph, personal communication). Mean estimates were separated using Tukey-Kramer Least Significant Difference (LSD) at an alpha level of 0.05. Density and biomass treatment means were compared to the weed-free control using P-values generated via the Least Squares Means output.
At 3 WAA, glyphosate controlled annual ryegrass 81%; the addition of dimethenamid-p/saflufenacil or mesotrione + rimsulfuron to glyphosate improved annual ryegrass control to 91 and 90%, respectively. The addition of all other residual herbicides to glyphosate did improve annual ryegrass control (Table 3).