Journal of Agricultural Science; Vol. 16, No. 3; 2024 ISSN 1916-9752 E-ISSN 1916-9760 Published by Canadian Center of Science and Education 39 What Are the Most Efficacious Herbicides Applied Preplant for Control of Multiple-Herbicide-Resistant Canada Fleabane in Corn? Nader Soltani 1 , Christian A. Willemse 1 & Peter H. Sikkema 1 1 Department of Plant Agriculture, University of Guelph Ridgetown Campus, Ridgetown, ON, Canada Correspondence: Nader Soltani, Department of Plant Agriculture, University of Guelph Ridgetown Campus, 120 Main St. East, Ridgetown, ON, N0P 2C0, Canada. E-mail: soltanin@uoguelph.ca Received: December 21, 2023 Accepted: February 2, 2024 Online Published: February 15, 2024 doi:10.5539/jas.v16n3p39 URL: https://doi.org/10.5539/jas.v16n3p39 Abstract New weed management strategies are needed to effectively control multiple-herbicide-resistant (MHR) Canada fleabane in corn. Five experiments were established in growers’ corn fields with confirmed MHR Canada fleabane to determine the efficacy of various herbicides applied preplant (PP). In 2021 environments, glyphosate + isoxaflutole + atrazine, glyphosate + isoxaflutole/diflufenican + atrazine, glyphosate + S-metolachlor/atrazine/mesotrione/bicyclopyrone, glyphosate + mesotrione + atrazine, glyphosate/dicamba + isoxaflutole/diflufenican, glyphosate/dicamba + isoxaflutole/diflufenican + atrazine, and glyphosate + saflufenacil/dimethenamid-p provided excellent control (90-100%) of MHR Canada fleabane but glyphosate + S-metolachlor/mesotrione/bicyclopyrone and glyphosate + S-metolachlor/atrazine/mesotrione controlled MHR Canada fleabane 77-84% and 87-96%, respectively at 4, 8, and 12 weeks after application (WAA). Herbicide tankmixes evaluated reduced MHR Canada fleabane density and biomass 91-100%. In 2022 environments, all glyphosate tankmixes evaluated provided 97-100% control, 99-100% density reduction, and 100% biomass reduction of MHR Canada fleabane in corn. In 2021 and 2022 environments MHR Canada fleabane interference reduced corn yield 41 and 32%, respectively; reduced MHR Canada fleabane interference with all herbicide treatments resulted in corn yield similar with the weed-free control. Results of this study indicate that glyphosate + isoxaflutole + atrazine, glyphosate + isoxaflutole/diflufenican + atrazine, glyphosate + S-metolachlor/atrazine/mesotrione/bicyclopyrone, glyphosate + mesotrione + atrazine, glyphosate/dicamba + isoxaflutole/diflufenican, glyphosate/dicamba + isoxaflutole/diflufenican + atrazine, and glyphosate + saflufenacil/dimethenamid-p provide excellent and consistent control of MHR Canada fleabane. However, glyphosate + S-metolachlor/atrazine/mesotrione and glyphosate + S-metolachlor/mesotrione/bicyclopyrone do not provide consistent control of MHR Canada fleabane in corn. Keywords: glyphosate-resistant, corn injury, weed density, weed biomass, herbicide tankmix, corn yield 1. Introduction Corn is an important grain crop grown in Ontario. Corn growers in Ontario seeded approximately 920,000 hectares and produced nearly 14 million tonnes of corn with a farm-gate value of over $2.8 billion in 2022 (OMAFRA, 2022). Approximately 60% of the grain corn produced in Ontario is used for feed purposes while the remaining 40% is used for various industrial uses (OMAFRA, 2022). Corn yield loss from weed interference is one of the most important factors preventing growers from obtaining optimum yield (Soltani et al., 2016). Multiple-herbicide-resistant (MHR) weeds and glyphosate-resistant (GR) weeds have become a major issue in corn production in Ontario (Soltani et al., 2022). Currently, one of the most problematic MHR weeds for crop producers in Ontario is Canada fleabane. Canada fleabane is a winter or summer annual weed from the Asteraceae family that can produce as many as a million seeds per plant (Frankton and Mulligan, 1987; Weaver, 2001). Canada fleabane seeds are 1 to 2 mm long and have an attached pappus that aids in wind dispersal; seeds can travel as far as 550 km from the parent plant (Frankton & Mulligan, 1987; Royer & Dickinson, 1999; Shields et al., 2006). Canada fleabane has a long emergence pattern throughout the cropping season (spring, summer, or fall) and therefore is best controlled with a preplant (PP) herbicide application that has residual activity (Loux et al., 2006; Weaver, 2001). The first confirmation of GR Canada fleabane in Canada was from seeds collected in Essex County, ON in 2010 (Byker et al., 2013). GR Canada fleabane has now been confirmed in 30 counties across southern Ontario (Budd