Many problematic weeds have evolved resistance to herbicides in mid-southern U.S. rice fields. With the lack of new effective herbicides, rice producers seek alternatives that are currently not labeled for rice production. Very-long chain fatty acid elongase (VLCFA) inhibitors are currently not labeled for U.S. rice but are labeled for use in other U.S. row cropping systems and rice production in Asia. Previous research has demonstrated the utility of VLCFA inhibitors for weed control in rice; however, these herbicides induce variable amounts of injury to the crop when applied early in the growing season. Experiments were initiated in 2020 and 2021 at the Rice Research and Extension Center near Stuttgart, AR, to evaluate rice tolerance and weed control with acetochlor and a herbicide safener, fenclorim, seed treatment. Three rates of a microencapsulated formulation of acetochlor (630, 1,260, and 1,890 g ai ha-1), four application timings [preemergence (PRE), delayed-preemergence (DPRE), spiking, and 1-leaf], and without or with the fenclorim seed treatment (2.5 g kg-1 of seed) were used to evaluate rice tolerance, weedy rice control, and barnyardgrass control. Acetochlor applied DPRE at 1,260 g ai ha-1 provided better weedy rice and barnyardgrass control than 1-leaf applications at the same rate. Acetochlor rates of 1,260 and 1,890 g ai ha-1 reduced barnyardgrass and weedy rice densities greater than 630 g ai ha-1. The fenclorim seed treatment did not influence weedy rice or barnyardgrass control but did reduce injury for DPRE acetochlor applications. Based on these results, acetochlor can be safely applied to rice DPRE (≤ 19% injury) at 1,260 g ai ha-1 when the seed is treated with fenclorim, leading to ≥ 88% barnyardgrass and ≥ 45% weedy rice control 28 days after treatment.

Rice producers in the U.S. need effective herbicides to control problematic weeds. Previous research has demonstrated that acetochlor can provide in-season weed control in rice; however, undesirable injury is common. Thus, trials were initiated in 2020 and 2021 to evaluate 1) rice cultivar tolerance to microencapsulated (ME) acetochlor with the use of a fenclorim seed treatment at 2.5 g ai kg-1 of seed, 2) a dose-response of a fenclorim seed treatment with ME acetochlor, and 3) rice tolerance to fenclorim and ME acetochlor under cool, wet conditions. For all trials, acetochlor was applied delayed-preemergence (4 to 7 days after planting). In the dose-response trials and in the presence of acetochlor, the fenclorim seed treatment rate of 2.5 g ai kg-1 reduced rice injury and increased rice plant heights and shoot numbers relative to acetochlor without fenclorim and was comparable to the nontreated control in all evaluations. In the cultivar screening, 14 of 16 cultivars exhibited < 20% injury with acetochlor at 1,260 g ai ha-1 and fenclorim at 2.5 g ai kg-1 2 weeks after emergence (WAE) at the Pine Tree Research Station (PTRS). At the Rice Research and Extension Center (RREC) 2 and 4 WAE and PTRS 4 WAE, all cultivars exhibited < 20% injury with acetochlor and fenclorim. The fenclorim seed treatment in the presence of acetochlor provided comparable rice plant height, shoot numbers, groundcover, and rough rice yield to the nontreated control. Under cool, wet conditions, rice injury without fenclorim ranged from 15 to 60% with acetochlor at 1,050 g ai ha-1, while injury from acetochlor with the fenclorim seed treatment ranged from 0 to 20%. Based on the results of these experiments, the fenclorim seed treatment appears to safen an assortment of rice cultivars from injury caused by ME acetochlor.

Applications of very long-chained fatty acid elongase (VLCFA)-inhibiting herbicides are currently not labeled within U.S. rice (Oryza sativa L.) production but have been used in Asian rice production to suppress weedy rice and control other annual grasses and small-seeded broadleaves. Therefore, separate experiments were initiated in the fall of 2019 and 2020 to evaluate rice tolerance and weedy rice (O. sativa L.) control with acetochlor and pyroxasulfone in the following spring. A three-factor experiment within a randomized complete block design was implemented with the factors being (1) herbicide choice (acetochlor or pyroxasulfone), (2) a low or high herbicide rate (2100 and 4200 g ai ha −1 , and at 237 and 475 g ai ha −1 acetochlor and pyroxasulfone, respectively), and (3) without or with a fenclorim seed treatment (2.5 g ai kg −1 of seed). Throughout the experiments, rice exhibited <10% injury with acetochlor.The fenclorim seed treatment reduced injury (averaged over herbicide choice and rate) from 28% to 21% 28 days after emergence of rice for both trial types. Fenclorim also improved rough-rice yield (averaged over herbicide choice and rate) by 12 and 18 percentage points for the two different studies. However, regardless of fenclorim, rice did not demonstrate <20% injury with pyroxasulfone. The fenclorim seed treatment or any interaction containing fenclorim did not influence weedy rice control, and weedy rice control with acetochlor ranged from 0% to 48%. Findings from this experiment demonstrate the effects of a fenclorim seed treatment for fall-applied acetochlor and pyroxasulfone and the efficacy of acetochlor and pyroxasulfone for controlling weedy rice. INTRODUCTIONWeedy rice (Oryza sativa L.) is one of the most troublesome weeds for flooded-rice (O. sativa L.