Use of a Fenclorim Seed Treatment to Safen Rice to a Delayed Preemergence Application of S-metolachlor on a Clay Soil.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is a highly problematic weed in flooded rice (Oryza sativa L.) that can result in significant yield losses when left unchecked. New methods of barnyardgrass control will be needed to preserve high yields in Arkansas rice production. S-metolachlor is a chloroacetamide herbicide that provides residual control of grasses and small-seeded broadleaf weeds. An experiment was conducted in Keiser, Ark. to evaluate the efficacy of S-metolachlor in a rice system in conjunction with a fenclorim seed treatment to mitigate the risk of crop injury. Three rates of S-metolachlor (0.5, 1.0, and 1.5 lb ai/ac) were applied delayed-preemergence to 'Diamond' rice that was treated with fenclorim at 0 or 2.5 lb of ai/1000 lb-seed. Visual injury to rice and visual control of barnyardgrass were rated in comparison with the nontreated control and were evaluated throughout the season. Rough rice yield was evaluated after harvest. While visual rice injury was higher on average in the first two weeks after treatment without fenclorim, by 28 days after treatment (DAT) the low rate of S-metolachlor combined with a fenclorim seed treatment caused less than 17% injury. S-metolachlor provided effective visual control of barnyardgrass at all three rates up to 28 DAT that exceeded 90% control. Overall, the presence of fenclorim reduced injury to rice at each rate of S-metolachlor, while not having an impact on weed control. At a rate of 0.5 lb ai/ac of S-metolachlor with a fenclorim seed treatment, rice yield was comparable with the nontreated control. However, increasing rates of metolachlor reduced yield. If S-metolachlor becomes labeled for use in rice, this will provide an alternative site of action for weed control without requiring a herbicide resistance trait. [ABSTRACT FROM AUTHOR]