Rice producers in the United States need effective herbicides to control problematic weeds. Previous research has demonstrated that acetochlor can provide in-season weed control in rice; however, undesirable injury is common. Thus, trials were initiated in 2020 and 2021 to evaluate 1) rice cultivar tolerance to microencapsulated (ME) acetochlor with the use of a fenclorim seed treatment at 2.5 g ai kg^–1 of seed; 2) a dose-response of a fenclorim seed treatment with ME acetochlor; and 3) rice tolerance to fenclorim and ME acetochlor under cool, wet conditions. For all trials, acetochlor was applied delayed-preemergence (4 to 7 d after planting). In the dose-response trials and in the presence of acetochlor, the fenclorim seed treatment rate of 2.5 g ai kg^–1 reduced rice injury and increased rice plant heights and shoot numbers relative to acetochlor without fenclorim, and plant heights and shoot numbers were comparable to those of the nontreated control in all evaluations. In the cultivar screening, 14 of 16 cultivars exhibited <20% injury with acetochlor at 1,260 g ai ha^-1 and fenclorim at 2.5 g ai kg^–1 2 wk after emergence (WAE) at the Pine Tree Research Station (PTRS). At the Rice Research and Extension Center (RREC) 2 and 4 WAE and at PTRS 4 WAE, all cultivars exhibited <20% injury with acetochlor and fenclorim. The fenclorim seed treatment in the presence of acetochlor provided comparable rice plant height, shoot numbers, groundcover, and rough rice yield to that of the nontreated control. Under cool, wet conditions, rice injury without fenclorim ranged from 15% to 60% with acetochlor at 1,050 g ai ha^-1, whereas injury from acetochlor with the fenclorim seed treatment ranged from 0% to 20%. Based on the results of these experiments, the fenclorim seed treatment appears to safen an assortment of rice cultivars from injury caused by ME acetochlor.

Nomenclature: Acetochlor; fenclorim; rice, Oryza sativa L.