Weed control and rice response to microencapsulated acetochlor and a fenclorim seed treatment on a clay soil

Recent research has demonstrated the ability of a fenclorim seed treatment to reduce rice (Oryza sativa L.) injury to acetochlor. However, all studies were conducted on silt loam soils and have not evaluated rice tolerance or weed control on clay soils. Experiments were initiated in 2021 and 2022 at the Northeast Research and Extension Center near Keiser, AR, to determine rice response and the effectiveness of delayed-preemergence (DPRE)-applied microencapsulated (ME) acetochlor (1.1, 1.7, and 2.3 lb ai acre⁻¹) when applied to a clay soil with and without a fenclorim seed treatment at 0 or 2.5 lb ai 1000-lb⁻¹ of seed. Averaged over the fenclorim seed treatment, acetochlor at 1.1 and 1.7 lb ai acre⁻¹ caused similar injury levels to rice; however, barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] control increased at 1.7 lb ai acre⁻¹, eliciting 19% injury to rice and 82% barnyardgrass control at 28 days after emergence (DAE). Palmer amaranth (Amaranthus palmeri S. Watson) control ranged from 82% to 93%. Additionally, fenclorim did not influence barnyardgrass or Palmer amaranth control, but it did reduce rice injury and increase shoot density, plant height, and rough rice grain yield. At 14 and 28 DAE, fenclorim at 2.5 lb ai 1000-lb⁻¹ of seed (averaged over acetochlor rates) reduced visible rice injury from 61% to 13% and 40% to 8%, respectively. Results from this study indicate ME acetochlor could be successfully applied to rice grown on a clay soil when a fenclorim seed treatment is used, providing producers a new site of action for use in U.S. rice production.