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.

Nomenclature: Acifluorfen; flumioxazin; fomesafen; glyphosate; S-metolachlor; metribuzin; pyroxasulfone; sulfentrazone; common waterhemp, Amaranthus tuberculatus var. rudis; soybean, Glycine max (L.) Merr.