Journal of Agricultural Science; Vol. 15, No. 5; 2023 ISSN 1916-9752 E-ISSN 1916-9760 Published by Canadian Center of Science and Education 17 Control of Multiple-Herbicide-Resistant Canada Fleabane With Fall, Spring, and Sequential Herbicide Applications in Winter Wheat Nader Soltani 1 , Christy Shropshire 1 & Peter H. Sikkema 1 1 University of Guelph Ridgetown Campus, Ridgetown, ON, Canada Correspondence: Nader Soltani, University of Guelph Ridgetown Campus, 120 Main St. East, Ridgetown, ON, N0P 2C0, Canada. E-mail: soltanin@uoguelph.ca Received: February 27, 2023 Accepted: March 24, 2023 Online Published: April 15, 2023 doi:10.5539/jas.v15n5p17 URL: https://doi.org/10.5539/jas.v15n5p17 Abstract Limited information exists on the efficacy of pyrasulfotole/bromoxynil, fluroxypyr/halauxifen-methyl + MCPA EHE, and clopyralid applied in the fall, spring, or sequentially [fall followed by (fb) spring] for the control of multiple-herbicide-resistant (MHR) Canada fleabane in winter wheat under Ontario environmental conditions. Three field experiments were initiated in the autumn of 2020 and 2021 for a total of 6 site-years to evaluate fall- and spring-applied herbicides and their sequential applications for the control of MHR Canada fleabane in winter wheat in Ontario. Pyrasulfotole/bromoxynil applied in the fall, spring, or sequentially controlled MHR Canada fleabane 83, 99, and 100%, respectively at 8 weeks after the spring application (WAB); the spring and sequential applications provided better control than the fall application. Fluroxypyr/halauxifen + MCPA and clopyralid applied in the fall, spring, or sequentially controlled MHR Canada fleabane 97 to 100% and 99 to 100%, respectively at 8 WAB. Based on orthogonal contrasts the spring and sequential herbicide applications provided greater control than the fall application (8 WAB). MHR Canada fleabane interference reduced winter wheat yield up to 27% in this study. Based on orthogonal contrasts reduced MHR Canada fleabane interference with the fall application resulted in 17% higher winter wheat yield than when herbicide application was delayed to the spring. Although MHR Canada fleabane was controlled very effectively with clopyralid winter wheat yield was lower, presumably due to crop injury; this observation will have to be explored further in future research. Results from this study indicate that pyrasulfotole/bromoxynil and fluroxypyr/halauxifen + MCPA applied in the fall can be used to effectively control MHR Canada fleabane and minimize winter wheat yield loss due to weed interference. Keywords: glyphosate-resistant, winter wheat injury, sequential herbicide application, weed control, orthogonal contrasts, yield 1. Introduction Winter wheat (Triticum aestivum L.) is one of the most important crops grown globally. In Ontario, winter wheat is commonly grown in rotation with soybean, corn, and dry bean. In 2021, nearly 2.7 million tonnes of winter wheat valued at approximately $980 million was grown on approximately 440,000 hectares in Ontario (OMAFRA 2022). One of the major impediments to maximizing winter wheat production is yield loss from weed interference (Klein, 2019). A meta-analysis published by the Weed Science Society of America (WSSA) estimated that winter wheat yield would be reduced by an average of 23% in North America if weeds are not controlled and the monetary loss was estimated to be US$2.19 billion (Flessner et al., 2021). The spread of glyphosate-resistant (GR) and multiple-herbicide-resistant (MHR) weeds in recent years in Ontario fields has further complicated weed control in winter wheat. MHR Canada fleabane (Conyza canadensis (L.) Cronq.) has become a troublesome weed in winter wheat in recent years. MHR Canada fleabane was first confirmed in Canada from seed collected in 2010 in Essex County, Ontario (Byker et al., 2013). Since then, MHR Canada fleabane has been confirmed across southern Ontario and is estimated to be present on 5% of the field crop hectares in Ontario (Budd et al., 2017; Soltani et al., 2022). The MHR Canada fleabane found in Ontario carries resistance to both glyphosate (Group 9) and cloransulam (Group 2), while resistance to paraquat (Group 22) is slightly less frequent (Corteva Agriscience Canada, 2022). Potential yield loss in winter wheat from MHR Canada fleabane has been estimated to be 8% with an economic loss of approximately $1.7 million (Soltani et al., 2022). There are limited herbicide options to control MHR Canada fleabane in winter wheat in