Response of an acetolactate synthase (ALS)‐resistant biotype of Amaranthus rudis to selected ALS‐inhibiting and alternative herbicides

Summary An Amaranthus rudis Saner (common water‐hemp) biotype from a field treated for two consecutive years with a mixture of chlorimuron and metribuzin was tested in greenhouse and laboratory studies to assess resistance and cross‐resistance to four acetolactale synthase (ALS)‐inhibiting herbicides. The biotype demonstrated >1920‐fold resistance at the whole plant level, and >850‐fold resistance at the ALS enzyme to chlorimuron, compared with a susceptible biotype. This chlorimuron‐resistant biotype was also cross‐resistant to primisuifu‐ron, haiosulfuron and imazethapyr. In greenhouse studies, atrazine alone or in combination with ALS‐inhibiting herbicides provided excellent control of the resistant biotype of A. rudis. Combinations of dicamba and ALS‐inhibitors also provided adequate control. Additionally, premixtures of flumetsulam and metolachlor and of dicamba and atrazine furnished excellent control of this chlorimuron‐resistant A. rudis biotype.