Optimization of Metamitron Rate Applied PRE in Sugar Beet for Management of Palmer amaranth ( <i>Amaranthus palmeri</i> S. Watson)

Abstract The prevalence of glyphosate- and ALS-inhibitor-resistant Palmer amaranth has substantially impacted sugar beet yield across CO and NE since 2020. While metamitron applied PRE has been previously reported to effectively control glyphosate-resistant (GR) Palmer amaranth, the rates evaluated in the literature are likely higher than what is practical for growers. Consequently, identifying an optimal rate will reduce potential herbicide use while still effectively controlling Palmer amaranth. Lower rates of metamitron may be sufficient to allow control of Palmer amaranth through the two true-leaf stage (2 TL) of growth, where very-long chain fatty acid (VLCFA) inhibiting herbicides can be used to layer additional residual herbicides and prevent Palmer amaranth emergence. Multiple dose-response analysis studies were established in Nebraska in 2020 and 2023, and in Colorado in 2023, to determine the necessary rate of metamitron, with and without a tank-mix partner of ethofumesate, to control Palmer amaranth through the 2 TL sugar beet stage. Metamitron rates ranged from 0 – 6.40 kg ai ha -1 , while ethofumesate was included at a fixed rate of 1.68 and 1.86 kg ai ha -1 in 2020 and 2023, respectively. At all locations, maximum Palmer amaranth control was obtained near 4 kg ai ha -1 when applied alone. Adding ethofumesate as a tank-mix partner reduced metamitron use rate to about 2.9 kg ai ha -1 . Trial results supported the issuance of a Section 18 label for metamitron at 3.27 kg ai ha -1 in Colorado and Nebraska in 2024, which subsequently extended to include Idaho, Wyoming, and Oregon in 2025.