Spray Interval, Application Order, and Plant Height Influences Control of Dicamba-Resistant Palmer amaranth

Palmer amaranth, which is resistant to glyphosate and protoporphyrinogen oxidase inhibitors, remains a threat to cotton and soybean production in Tennessee. This is partly due to the recent evolution of dicamba-resistant Palmer amaranth in West Tennessee, which further complicates weed management. Experiments were conducted in 2021 and 2022 to determine the best timing between sequential applications and in what order 2,4-D or dicamba should be used with glufosinate to control resistant Palmer amaranth. Palmer amaranth control increased when the interval between postemergence herbicide applications decreased from 21 to 7 days. At the 7-day interval in a dicamba-based system, the order of herbicides did not affect Palmer amaranth control. However, in a 2,4-D-based system, the greatest control was achieved when 2,4-D was applied first, followed by either 2,4-D or glufosinate. While weed height at the time of application had a significant effect on Palmer amaranth control with auxin herbicides, control was still unacceptable in the field at the labeled rates of dicamba or 2,4-D when applied to <10 cm tall weeds (48% and 53%, respectively). Neither dicamba nor 2,4-D provided acceptable control of the Palmer amaranth populations evaluated. Sequential applications separated by 7 days provided better weed control than those separated by 21 days. Given that the better 7-day sequential treatments provided less than 90% control and resulted in more than 64,000 surviving Palmer amaranth ha-1 suggests that relying solely on these herbicides for Palmer amaranth control is not a sustainable weed management strategy.