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

• Published in: Crop, forage & turfgrass management Vol. 10; no. 1
• Main Authors: Avent, Tristen H., Norsworthy, Jason K.
• Format: Journal Article
• Language: English
• Published: 01-06-2024
• ISSN: 2374-3832; 2374-3832
• DOI: 10.1002/cft2.20274

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.
Plain Language Summary
Rice producers in the Southern United States need new effective herbicides to control problematic weeds. Research has shown that acetochlor (Warrant herbicide), while not labeled, could be used to control barnyardgrass in rice, but crop tolerance is a concern. Fenclorim has been used as an herbicide safener seed treatment to reduce rice injury to microencapsulated acetochlor, but this research was conducted on medium textured soils (silt loam). Rice tolerance has not been demonstrated on clay soils and heavier soils tend to bind more herbicide, which would mean higher rates are needed for adequate control. We observed good crop tolerance with acetochlor at 1.7 lb ai/acre (4.5 pt/A warrant) and a fenclorim seed treatment. This rate also demonstrated up to 82% barnyardgrass and Palmer amaranth control 28 days after the rice emerged. If a microencapsulated acetochlor were to become labeled in rice, producers could use it as an alternative residual herbicide option within a program.