The lack of POST herbicides to control grasses in grain sorghum prompted researchers to develop acetolactate synthase (ALS)–resistant grain sorghum. Field experiments were conducted to evaluate the differential response of ALS-resistant grain sorghum to POST application of nicosulfuron rimsulfuron applied at three growth stages. ALS-resistant grain sorghum was treated with 0, 13 7, 26 13, 39 20, 52 26, 65 33, 78 39, and 91 46 g ai ha⁻¹ of nicosulfuron rimsulfuron when plants were at the three- to five-leaf, seven- to nine-leaf, or 11- to 13-leaf stage. In general, as nicosulfuron rimsulfuron rates increased, visible injury increased at the three- to five-leaf and seven- to nine-leaf stages. Injury was greatest 1 wk after treatment for the three- to five-leaf and seven- to nine-leaf stages across all ratings, and plants then began to recover. No injury was observed at any rating time for the 11- to 13-leaf stage. Plant height and sorghum grain yield were reduced as nicosulfuron rimsulfuron rates increased when applied at the three- to five-leaf stage. However, nicosulfuron rimsulfuron applied at the seven- to nine-leaf and 11- to 13-leaf stages did not decrease sorghum yield. This research indicated that nicosulfuron rimsulfuron application at the three- to five-leaf stage injured ALS-resistant grain sorghum; however, application at the seven- to nine-leaf or 11- to 13-leaf stages did not result in grain yield reduction.

Nomenclature: Nicosulfuron; rimsulfuron; grain sorghum, Sorghum bicolor (L.) Moench.