Glyphosate-resistant (GR) common waterhemp is the fifth GR weed species confirmed in Canada, and the fourth in Ontario. As of 2017, GR common waterhemp has been confirmed in Lambton, Essex, and Chatham-Kent counties in Ontario. Greenhouse and field dose–response experiments revealed that GR common waterhemp in Ontario had a resistance level of 4.5 and 28, respectively, when compared with known glyphosate-susceptible populations. At 12 wk after application, pyroxasulfone/flumioxazin (240 g ai ha−1), pyroxasulfone/sulfentrazone (300 g ai ha−1), and S-metolachlor/metribuzin (1,943 g ai ha−1) controlled GR common waterhemp 97%, 92%, and 87%, respectively. Pyroxasulfone/sulfentrazone or S-metolachlor/metribuzin applied PRE followed by acifluorfen (600 g ai ha−1) or fomesafen (240 g ai ha−1) applied POST controlled GR common waterhemp 98% and performed better than PRE or POST alone. This research is the first to determine the resistance factor of GR common waterhemp in Ontario and identifies control strategies in soybean to mitigate the impact of common waterhemp interference in soybean crop production.

Eleven field experiments were conducted over a 3-yr period (2010, 2011, and 2012) in conventional- and no-till soybean with a flumioxazin and pyroxasulfone premix. PRE and preplant applications were evaluated for soybean injury, weed control, and yield compared to standard herbicides. Early-season soybean injury from flumioxazin/pyroxasulfone ranged from 1 to 19%; however, by harvest, soybean yields were similar across labeled rates (160 and 200 g ai ha−1), standard treatments, and the nontreated control. Flumioxazin/pyroxasulfone provided excellent control (99 to 100%) of velvetleaf, pigweed species (redroot pigweed and smooth pigweed), and common lambsquarters across almost all rates tested (80 to 480 g ai ha−1). Common ragweed, green foxtail, and giant foxtail control increased with flumioxazin/pyroxasulfone rate. The biologically effective rates varied between tillage systems. The flumioxazin/pyroxasulfone rate required to provide 80% control (R80) of pigweed was 3 and 273 g ai ha−1 under conventional- and no-till, respectively. For common ragweed, the R80 was 158 g ai ha−1 under conventional tillage; yet, under no-till, the rate was nonestimable. The results indicate that flumioxazin/pyroxasulfone can provide effective weed control as a setup for subsequent herbicide applications.