Selectivity of Very‐Long‐Chain Fatty Acid‐Inhibiting Herbicides in Rice as Influenced by Application Timing and Soil Texture

Abstract

Core Ideas Alternative herbicide sites of action are desperately needed in US rice. Rice exhibited tolerance to the currently non‐labeled herbicides acetochlor and pethoxamid when applied at the one‐ to two‐leaf growth stage on a silt loam and clay soil. Delaying application of very‐long‐chain fatty acid‐inhibiting herbicides until drill‐seeded rice has completely emerged enhanced crop tolerance. Very‐long‐chain fatty acid (VLCFA)‐inhibiting herbicides include pyroxasulfone, S‐metolachlor, acetochlor, and pethoxamid. Currently, no VLCFA‐inhibiting herbicides are labeled for use in US rice production; however, if rice tolerance can be established they would provide an alternative herbicide site of action (SOA). In 2015 and 2016, pyroxasulfone (Zidua) at 0.14 lb a.i. ac−1, S‐metolachlor (Dual Magnum) at 0.95 lb a.i. ac−1, acetochlor (Warrant) at 0.95 lb a.i. ac−1, and pethoxamid at 0.75 lb a.i. ac−1 were applied delayed preemergence (DPRE) and to spiking and one‐ to two‐leaf rice on a Dewitt silt loam and a Sharkey clay soil. Trials were conducted at separate locations to determine the effect differing soil properties may have on the tolerance of rice to VLCFA‐inhibiting herbicides. Substantial rice injury was observed on the Dewitt silt loam soil following all pyroxasulfone and S‐metolachlor treatments, ranging from 20 to 100% injury 3 weeks after rice flooding. On the Dewitt silt loam, treatments containing acetochlor and pethoxamid applied to one‐ to two‐leaf rice resulted in minimal injury and the highest yields observed among VLCFA‐inhibiting herbicides. In general, rice injury ratings were less on the Sharkey clay soil. Pethoxamid and acetochlor applied to rice at any application timing resulted in ≤ 1% injury 2 weeks after treatment. Hence, contingent on a label, pethoxamid and acetochlor can be applied under similar conditions as early as DPRE on the Sharkey clay soil and not until the one‐ to two‐leaf rice stage on the Dewitt silt loam at the rates assessed.