The evolution of herbicide resistance is making it extremely difficult for US rice producers to use chemical control on weed species such as barnyardgrass and red rice. To combat herbicide resistance, it is imperative that alternative herbicide sites of action (SOAs) be incorporated into rice whenever possible. There are currently no very-long-chain fatty acid–inhibiting herbicides (WSSA Group 15) labeled for use in US rice; however, pethoxamid is one such herbicide currently under development. If appropriate rice tolerance and weed control can be established, pethoxamid would represent a unique herbicide SOA for use in US rice. We conducted field trials near Stuttgart, AR, in 2015 and near Colt and Lonoke, AR, in 2016 to assess selectivity of pethoxamid and weed control alone and in combination with other herbicides as a delayed preemergence (DPRE) application in drill-seeded rice. Pethoxamid was applied at 0, 420, or 560 g ai ha–1 alone and in combination with clomazone, imazethapyr, pendimethalin, and quinclorac. Minimal rice injury occurred with any treatment assessed. A reduction in rice shoot density and plant height compared to the nontreated control followed the use of pethoxamid; however, no decrease in yield resulted. The highest levels of barnyardgrass control followed the use of imazethapyr at 91% and quinclorac at 89% regardless of the presence of pethoxamid near Lonoke; however, pethoxamid applied at both rates in combination with clomazone and quinclorac increased barnyardgrass control compared to clomazone and quinclorac applied alone. Near Colt, barnyardgrass control of 92% and 96% resulted from pethoxamid alone, averaged over the high and low rates. Based on these data, rice can tolerate pethoxamid when applied DPRE, and adequate levels of barnyardgrass control can be achieved at the rates evaluated within a program; hence, pethoxamid appears to be a viable option for use in rice to allow for increased rotation of herbicide SOAs to combat herbicide-resistant and difficult-to-control weeds.

Field studies were conducted in 1996 and 1997 to determine the effects of RP-201772 and RP-201772 tank mixtures applied to emerged Zea mays. RP-201772 at 105 g ha−1 and tank-mixed metolachlor/benoxacor or atrazine were applied preemergence and to spike, 2-leaf, and 4-leaf Z. mays. Herbicide treatments over the four application timings provided greater than 90% Chenopodium album, Ambrosia artemisiifolia, Amaranthus retroflexus, and Abutilon theophrasti control. However, Setaria faberi control varied among years and application timings. Severe Z. mays injury, 70% and 40%, was observed when RP-201772 tank-mixed with metolachlor/benoxacor was applied to 2-leaf and 4-leaf Z. mays, respectively. Greenhouse studies confirmed Z. mays sensitivity from delayed applications of RP-201772 tank-mixed with metolachlor/benoxacor. Similarly, increased Z. mays injury was observed from postemergence applications of RP-201772 tank-mixed with acetochlor/MON-13900. Herbicide absorption, translocation, metabolism, and retention studies were conducted to determine the physiological basis for the observed Z. mays injury from delayed applications of the RP-201772 tank mixture with metolachlor/benoxacor. Metolachlor/benoxacor increased radiolabeled RP-201772 absorption when applied to spike, 2-leaf, and 4-leaf Z. mays. RP-201772 translocation and metabolism did not explain enhanced Z. mays injury. However, RP-201772 retention increased fivefold when metolachlor/benoxacor was present in the spray solution and applied to 2-leaf and 4-leaf Z. mays. Increased RP-201772 absorption and retention appeared to be the basis for Z. mays injury when tank-mixed with metolachlor/benoxacor and applied to emerged Z. mays.