Increased Absorption and Translocation Contribute to Improved Efficacy of Dicamba to Control Early Growth Stage Palmer Amaranth (Amaranthus palmeri)

Rapid growth of Palmer amaranth (Amaranthus palmeri S. Watson) poses a challenge for timely management of this weed. Dose–response studies were conducted in 2017 and 2018 under field and greenhouse conditions near Garden City and Manhattan, KS, respectively, to evaluate the efficacy of dicamba to control ≤10-, 15-cm, and 30-cm-tall A. palmeri, which mimics three herbicide application timings: on-time application (Day 0) and 1- (Day 1) and 4-d (Day 4) delays. Visual injury rating and reduction in shoot biomass (% of nontreated), and mortality were assessed at 4 wk after treatment using a three- and four-parameter log-logistic model in R. Increasing dicamba doses increased A. palmeri control regardless of plant height in both the field and greenhouse studies. The results suggest that delaying application 1 (15 cm) and 4 d (30 cm) resulted in 2- and 27-fold increases in the effective dose of dicamba on A. palmeri, respectively, under field conditions. However, in the greenhouse, for the same level of A. palmeri control, more than 1- and 2-fold increases in dicamba dose, respectively, were required. Similarly, the effective dose of dicamba required for 50% reduction in A. palmeri shoot biomass (GR₅₀) increased more than 4- and 8-fold or more than 1- and 2-fold when dicamba application was delayed by 1 (15 cm) and 4 d (30 cm), in the field or in the greenhouse, respectively. To understand the basis of increased efficacy of dicamba in controlling early growth stages of A. palmeri, dicamba absorption and translocation studies were conducted. Results indicate a significant reduction in dicamba absorption (7%) and translocation (15%) with increase in A. palmeri height. Therefore, increased absorption and translocation of dicamba results in increased efficacy in improving A. palmeri control at early growth stages.