Evaluation of Acetochlor and other Very-Long-Chain Fatty Acid-Inhibiting Herbicides in Arkansas Rice

The prevalence of herbicide resistance and lack of effective management options for controlling problematic weeds such as barnyardgrass and weedy rice in Arkansas rice production has led to exploration of alternative herbicide sites of action (SOA). Very-long-chain fatty acid (VLCFA)-inhibiting herbicides are not currently labeled for use in U.S. rice production but have been used with success in other row crops and in rice production in Asia. Based on preliminary research, rice tolerance and weed control were evaluated following various application timings and rates of acetochlor and pethoxamid, in addition to several other VLCFA-inhibiting herbicides. Rice tolerance to acetochlor was maximized when applied in a microencapsulated (ME) formulation at the 1-leaf growth stage. Rice also demonstrated adequate tolerance to ME acetochlor applied delayed-preemergence (DPRE); however, when activating rainfall was received soon after application, unacceptable rice injury was observed and is therefore not recommended. When properly activated, barnyardgrass control and rough rice yield was comparable between acetochlor-based herbicide programs and clomazone-based programs in Clearfield and Provisia rice systems. However, it should be noted that early-season barnyardgrass control and rough rice yields were generally higher following clomazone-based herbicide programs due to minimal rice injury and excellent barnyardgrass control in all environments. Both ME acetochlor and pethoxamid provided early-season control of weedy rice and other grass species when applied soon after planting. Although DPRE applications were the most effective for weed control, they pose extreme risk for rice injury and should be avoided. In contrast, weed control was slightly reduced by delaying applications to 1-leaf rice but risk for rice injury was also decreased. Winter-applied VLCFA-inhibiting herbicides caused tolerable injury to rice planted the following spring. Microencapsulated acetochlor and pyroxasulfone provided considerable suppression of weedy rice for as long as seven weeks after planting, suggesting an alternative method for controlling weedy rice. Should ME acetochlor and pethoxamid be registered for use in U.S. rice production, they have potential to provide growers with an alternative SOA to combat herbicide resistance and control problematic weed species.