Acetolactate synthase (ALS)-inhibiting herbicides are often used to control Italian ryegrass in winter wheat in Texas. An Italian ryegrass biotype near Waco, TX was evaluated for resistance to mesosulfuron in field and greenhouse experiments. Control of the biotype in the field was less than 10 with the label rate of mesosulfuron (15 g ai/ha). Greenhouse studies confirmed that the biotype was resistant to mesosulfuron; control of the biotype was less than 35 at 120 g ai/ha mesosulfuron. The herbicide dose required to reduce plant biomass of a susceptible and the Waco biotype by 50 (GR(50)) was 1.3 and 31 g ai/ha, respectively, indicating a resistance level of 24-fold in the Waco biotype. However, the Waco biotype was controlled with the acetyl-CoA carboxylase inhibitors diclofop anti pinoxaden.
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