Efficacy of Early-Season Applications of Acetochlor and Pethoxamid in Rice

Limited options for controlling herbicide-resistant weedy rice and barnyard grass in Arkansas rice has led to the exploration of alternative herbicide sites of action (SOA). Very long-chain fatty acid (VLCFA)-inhibiting herbicides have been used successfully in US row crops and Asian rice production for control of annual grasses and smallseeded broadleaves but are not labeled for use in US rice. Preliminary experiments have indicated adequate rice tolerance to acetochlor and pethoxamid; however, limited weed control information in rice systems is available. Field experiments were conducted in 2016 and 2017 to evaluate weed control with early-season applications of acetochlor and pethoxamid on weedy rice and annual grasses in rice. In separate experiments, microencapsulated acetochlor at 1050 and 1470 g ai ha-1 or pethoxamid at 420 and 840 g ai ha-1 was applied alone delayed preemergence (DPRE), at spiking, 1-2 leaf, and 1-2 leaf rice. In both years, injury less than 10 and 20% was observed for all acetochlor and pethoxamid treatments, respectively, 2 weeks after treatment (WAT). Both herbicides controlled barnyard grass >92% and suppressed weedy rice 33 to 63% 2 WAT. Regardless of application timing or rate, acetochlor and pethoxamid reduced weedy rice density relative to the non-treated 4 WAT. Control of weedy rice, barnyard grass, broadleaf signal-grass, and large crabgrass was maximized when either herbicide was applied DPRE or to spiking rice and generally decreased as application timing was delayed. Furthermore, control of weed species early in the season influenced rough rice yield, as the highest yields were harvested when acetochlor or pethoxamid was applied DPRE or at spiking. Residual control of annual grasses and suppression of weedy rice from early-season applications of acetochlor and pethoxamid indicate they could be valuable in a season-long rice herbicide program while providing an alternative SOA to combat herbicide-resistant weeds.