Sequential Applications of Synthetic Auxins and Glufosinate for Escaped Palmer Amaranth Control

Field and greenhouse studies were conducted to investigate the influence of sequence and timing of synthetic auxins and glufosinate on large Palmer amaranth (Amaranthus palmeri) control. Field studies were performed in Henry County, AL where treatments were applied to Palmer amaranth with average heights of 37 and 59 cm in 2018 and 2019, respectively. Sequential applications of 2,4-D/dicamba + glyphosate followed by (fb) glufosinate at labeled rates 3 or 7 days after initial treatment (DAIT) were used in addition to the reverse sequence with a 7-day interval. Time intervals of 3 or 7 days between applications did not influence Palmer amaranth control. Palmer amaranth was controlled 100% by dicamba + glyphosate fb glufosinate and 2,4-D + glufosinate fb glufosinate 7 DAIT in 2018. However, herbicide performance was reduced due to delayed application and taller plants in 2019 with up to 23% less visual injury. To further investigate Palmer amaranth response to dicamba and glufosinate applied sequentially, a greenhouse study was conducted in 2019 where physiological measurements were recorded over a 35-day period. Treatments were applied to Palmer amaranth averaging 38 cm tall and included dicamba + glyphosate fb glufosinate 7 DAIT, the reverse sequence, and a single application of dicamba + glufosinate + glyphosate. Glufosinate severely inhibited mid-day photosynthesis compared to dicamba with up to 90% reductions in CO2 assimilation 1 DAIT. In general, Palmer amaranth respiration and stomatal conductance were not affected by herbicides in this study. Applications of dicamba + glyphosate fb glufosinate 7 DAIT was the only treatment hindered Palmer amaranth regrowth with 52% reduction in leaf biomass compared to nontreated control. These data suggest Palmer amaranth infested fields are more likely to be rescued with sequential applications of synthetic auxins and glufosinate, but consistent control of large Palmer is not probable.