Sensitivity of 2021 Palmer amaranth Accessions from Arkansas to Dicamba, 2,4-D, and Glufosinate.

The use of Enlist®, XtendFlex®, and LibertyLink® cropping systems are effective tools to facilitate weed management in cotton (Gossypium hirsutum L.). These technologies enable farmers to use 2,4-D, dicamba, or glufosinate over the top of cotton in season. However, the incidence of Palmer amaranth (Amaranthus palmeri S. Wats.) escapes demonstrates the importance of surveilling the spread of herbicide resistance. Therefore, this study aimed to evaluate herbicide resistance in Palmer amaranth accessions collected after glufosinate and auxin mimic herbicides (2,4-D or dicamba) were applied for weed control. A total of 22 accessions from eastern Arkansas were collected in the 2021 growing season. Herbicide treatments were equivalent to 0.5× or 1× rate of 2,4-D (Enlist One®) at 0.48 and 0.95 lb ae/ac, dicamba (XtendiMax®) at 0.25 and 0.5 lb ae/ac, and glufosinate (Liberty®) at 0.29 and 0.58 lb ai/ac, respectively. All treatments were applied to greenhouse-grown Palmer amaranth plants with 5 to 6 leaves, and 2 runs of 50 plants per accession were sprayed per treatment. Plant mortality (%) was assessed 21 days after treatment. Treatments with 2,4-D, dicamba, and glufosinate at 0.5× resulted in 80% or less mortality to 18, 13, and 6 accessions, respectively. Treatments with a 1× rate provided more than 80% control to all accessions treated with dicamba; meanwhile, only 18 and 21 accessions treated with 2,4-D and glufosinate, respectively, resulted in the same level of mortality. 2 accessions that resulted in less than 60% mortality after treatment with a 1× rate of 2,4-D were further evaluated, and resistance to 2,4-D was confirmed. These findings demonstrate the importance of monitoring weed survival in production fields. Implementing integrated weed management strategies to reduce selective pressure over current herbicide options could help mitigate herbicide resistance cases. [ABSTRACT FROM AUTHOR]