Italian Ryegrass Control Using a Capsule Suspension Formulation of <i>S</i>-metolachlor in Fenclorim-treated wheat

Abstract Italian ryegrass control is one of the most significant limitations in wheat production in the United States today. Resistance to Herbicide Resistance Action Committee (HRAC)/Weed Science Society of America (WSSA) Groups 1, 2, and 9 in Arkansas have further complicated postemergence control, whereas residual herbicides still show effective weed control. One problem is the potential of HRAC/WSSA Group 15 herbicides to injure wheat when applied preemergence, indicating the need for a herbicide safener. A series of experiments were conducted in Fayetteville, AR, to evaluate crop tolerance and Italian ryegrass control using a capsule suspension (CS) formulation of S -metolachlor in conjunction with fenclorim-treated wheat. Experiments were conducted as a two-factor factorial with S -metolachlor applied at three rates (0.37, 0.74, and 1.12 kg ai ha – 1 ) and a microencapsulated formulation of acetochlor at 1.05 kg ai ha –1 , and three rates of a fenclorim seed treatment at 0, 0.5, and 2.0 g ai kg –1 of seed. Separate experiments utilized either a preemergence (PRE) or a delayed-preemergence (DPRE) application timing. In both experiments, S -metolachlor at 0.74 and 1.12 kg ai ha – 1 provided 77% to 96% control of Italian ryegrass by preharvest, whereas acetochlor only provided 49% to 72% control. Visible wheat injury from PRE applications ranged from 7% to 49% for all treatments 21 d after treatment (DAT), with a reduction in injury when fenclorim-treated wheat was used for both the 0.74 and 1.12 kg ai ha – 1 rate of S -metolachlor. In the DPRE experiments, wheat injury ranged from 5% to 16% 21 DAT with no noticeable safening from the presence of fenclorim at any herbicide rate. The results of these experiments indicate that a DPRE application using a CS formulation of S -metolachlor would be more favorable for producers to mitigate the potential for injury to wheat while providing Italian ryegrass control. Additionally, at the DPRE application timing, fenclorim is unnecessary for S -metolachlor to be safely applied at the rates evaluated.