Full-season herbicide programs to control a seven-way-resistant Palmer amaranth accession under soybean technologies

Abstract A seven-way herbicide-resistant Palmer amaranth accession (MSR2) was identified in AR. Herbicide programs providing season-long control of this problematic accession need to be investigated, especially within the current soybean portfolio. Therefore, this study aimed to evaluate the efficacy of different soybean herbicide programs for controlling seven-way-resistant Palmer amaranth accession, MSR2, emphasizing the contribution of residual herbicides to full-season suppression. Field experiments were conducted in 2022 and 2023 in Fayetteville, AR, in an area infested by MSR2. A total of 14 herbicide programs were tested, targeting available soybean technologies that enable glyphosate, glufosinate, dicamba, and 2,4-D. All herbicide programs had one or two postemergence herbicides applied at early postemergence (EPOST) and late postemergence (LPOST). Additionally, eight herbicide programs included residual herbicides at preemergence (PRE; S -metolachlor plus metribuzin) and EPOST ( S -metolachlor). A nontreated control was included for comparison. Visible Palmer amaranth control (%) was assessed at LPOST and 2 weeks after LPOST (2 WA LPOST). Palmer amaranth plants were counted from two 0.25 m 2 quadrats randomly marked at each evaluation, and the density reduction (%) was calculated compared to the nontreated control. Preplanned orthogonal contrasts were conducted to compare herbicide programs with or without residual herbicides. Overall, in both years, the highest MSR2 control at both evaluations was observed in the herbicide programs that included residuals at PRE and EPOST with postemergence treatments of 2,4-D or dicamba (single or mixed). For Palmer amaranth density, herbicide programs that relied on residuals at PRE and EPOST with sequential postemergence applications of 2,4-D plus glufosinate or dicamba plus glyphosate obtained higher reduction levels. Findings reveal that the addition of residual herbicides is crucial in controlling multiple-herbicide-resistant Palmer amaranth biotypes, like MSR2. Herbicide programs based solely on postemergence applications were ineffective in controlling accession MSR2.