Control of glyphosate-resistant waterhemp (Amaranthus tuberculatus var rudis) with dicamba and dimethenamid-P in Ontario

Glyphosate-resistant (GR) waterhemp is the fifth confirmed GR weed in Canada and was first confirmed in 2014 in the province of Ontario. In 2015 and 2016, two field experiments were conducted across four site–years each to determine the effectiveness of dicamba at two rates (300 and 600 g a.i. ha⁻¹) in addition to dimethenamid-P (693 g a.i. ha⁻¹) in a single and sequential management program. Due to traited soybean seed availability, all treatments were applied in corn. Waterhemp density, biomass, visual control estimates, and corn yield were collected. Waterhemp density and dryweight were found to be lower in treatments including a sequential application of dica-maba and those that included dimethenamid-P. The sequential management strategy provided >95% control of GR waterhemp at 12 weeks after application. Dicamba applied after emergence at 300 and 600 g a.i. ha⁻¹ controlled GR waterhemp 80% and 91%, respectively; in contrast, dicamba applied before emergence at 300 and 600 g a.i. ha⁻¹ provided only 9% and 30% control of GR waterhemp, respectively. Dimethenamid-P controlled waterhemp better than dicamba when applied before emergence. Corn yield was also impacted by herbicide timing as corn yield was reduced by 17%–23% comparing preemergence vs postemergence applications in the experiments presented.