Smooth pigweed (Amaranthus hybridus) and livid amaranth (A-lividus) response to several imidazolinone and sulfonylurea herbicides

The effects of chlorimuron, imazaquin, imazethapyr, nicosulfuron, primisulfuron, and thifensulfuron were evaluated on a population of smooth pigweed in Painter, VA with no history of treatment with acetolactate synthase (ALS)-inhibitor herbicides. Imazethapyr and nicosulfuron gave the greatest smooth pigweed control, and subsequently were used in field and greenhouse studies to investigate susceptibility of smooth pigweed and livid amaranth populations to ALS-inhibitor herbicides. Approximately 5 million smooth pigweed plants from Painter were treated with imazethapyr or nicosulfuron from 1992 to 1994 and no ALS-inhibitor-resistant plants were identified. In the greenhouse, the response of smooth pigweed from Painter,VA, Marion, MD, and Oak Hall, VA and livid amaranth from Warren County, NJ to imazaquin or imazethapyr and nicosulfuron was investigated. Smooth pigweed from Marion and Oak Hall and livid amaranth from NJ had histories of treatment with ALS-inhibitors. Painter smooth pigweed control was 81 to 97% by imazethapyr and nicosulfuron while control of the Marion and Oak Hall populations was 3 and 18% by imazaquin at 560 and 1120 g ai/ha, respectively, and control by nicosulfuron at 35 g ai/ha was 50 to 73%. Control of livid amaranth from Warren County, NJ was 8 to 15% by imazethapyr at 560 g ai/ha, and was 30 to 58% by nicosulfuron at 35 g/ha.