Performance of an S ‐metolachlor capsule suspension for barnyardgrass control and crop tolerance in fenclorim‐treated rice

Abstract Barnyardgrass Echinochloa crus‐galli (L.) P. Beauv. is the most problematic weed of delayed‐flooded rice ( Oryza sativa L.) systems in the mid‐southern United States. Recent findings suggest that acetochlor can achieve effective barnyardgrass control in rice, but rice injury can occur with rainfall soon after application. The use of a herbicide safener such as fenclorim can reduce the risk of rice injury to acetochlor and possibly other chloroacetamide herbicides. An experiment was conducted on clay soil with S ‐metolachlor applied from 0 to 1.68 kg a.i. ha −1 with a fenclorim seed treatment at 0 and 2.5 g a.i. kg −1 seed at a delayed‐preemergence (DPRE) application timing in rice. Another experiment was conducted on a silt loam soil with S ‐metolachlor applied from 0 to 0.84 kg ha −1 DPRE or one‐leaf on fenclorim‐treated rice. In 2022, on the clay soil, S ‐metolachlor at 0.56 kg ha −1 provided greater than 90% barnyardgrass control with less than 20% rice injury at 28 days after treatment. However, in 2023, wet conditions delayed the application, resulting in increased emergence of barnyardgrass before treatments could be applied. On a silt loam soil in 2023, crop safety improved when S ‐metolachlor was applied DPRE and rice was treated with fenclorim compared to the nontreated seed; however, only S ‐metolachlor at 0.28 kg ha −1 resulted in less than 20% injury to rice, comparable to microencapsulated (ME) acetochlor. While 0.28 kg ha −1 of S ‐metolachlor resulted in rice with similar tolerance to ME acetochlor, barnyardgrass control was less than 60%, which is not viable in rice.