Control of herbicide-resistant common waterhemp (Amaranthus tuberculatus var. rudis) with pre- and post-emergence herbicides in soybean

Common waterhemp was first identified in Ontario in 2002. Eight trials were conducted over a 3-yr period (2003-2005) at Cottam and Petrolia, Ontario to determine the efficacy of various preemergence and postemergence herbicides for the control of herbicide-resistant waterhemp in soybean [Glycine max (L.) Merr.]. Waterhemp was resistant to the acetolactate synthase-inhibiting herbicides at both locations and to the photosystem II-inhibiting herbicides at Petrolia; therefore, results varied by location. Results at Petrolia also varied across years. Waterhemp interference resulted in soybean yield losses of up to 73% in weedy versus weed-free checks. Preemergence-applied metolachlor, dimethenamid, and linuron provided greater than 80% control and metolachlor plus metribuzin 94% or greater control of waterhemp 70 d after emergence at both locations. Acifluorfen, fomesafen, imazamox plus fomesafen, and glyphosate applied postemergence resulted in 87% or greater waterhemp control 70 d after application at Cottam in 2003 and 2004 and at Petrolia in 2004; however, waterhemp control with these herbicides was only 60 to 66% at Petrolia in 2005 due to prolonged weed emergence. Multiple herbicide applications per season may be required for effective waterhemp control in soybean.