Growth and fecundity of Palmer amaranth escaping glufosinate in soybean with and without grass competition

Field experiments were conducted at Clayton and Rocky Mount, North Carolina, during the summer of 2020 to determine the growth and fecundity of Palmer amaranth plants surviving glufosinate with and without grass competition in soybean. Glufosinate (590 g ai ha-1) was applied at early postemergence (5 cm Palmer amaranth height), mid-postemergence (7-10 cm), and late postemergence (>10 cm) and at orthogonal combinations of those timings. Non-treated Palmer amaranth was grown in weedy (i.e., intraspecific and grass competition), weed-free in-crop (WFIC), and weed-free fallow (WFNC) conditions for comparisons. No Palmer amaranth plants survived the sequential glufosinate applications and control decreased as the plants were treated at a larger size for both experiments. The apical and circumference growth rate of Palmer amaranth surviving glufosinate was reduced by more than 44% when compared to the WFNC Palmer amaranth. The biomass of Palmer amaranth plants surviving glufosinate was reduced by more than 87% when compared to the WFNC Palmer amaranth. The fecundity of Palmer amaranth surviving glufosinate was reduced by more than 70% when compared to WFNC Palmer amaranth. Palmer amaranth surviving glufosinate were as fecund as the WFIC Palmer amaranth in both experiments for soybean. The results prove that despite the significant vegetative growth rate decrease of Palmer amaranth surviving glufosinate, plants can be fecund as non-treated plants in soybean. The trends of growth and fecundity of Palmer amaranth surviving glufosinate with and without grass competition were similar. These results suggest that glufosinate-treated grass weeds may not reduce the growth or fecundity of Palmer amaranth surviving glufosinate.