Herbicide-Resistant Palmer amaranth (Amaranthus palmeri S. Wats.) in the United States — Mechanisms of Resistance, Impact, and Management

Palmer amaranth, a dioecious summer annual species, is one of the most trouble‐ some weeds in the agronomic crop production systems in the United States. In the last two decades, continuous reliance on herbicide(s) with the same mode of action as the sole weed management strategy has resulted in the evolution of herbicideresistant (HR) weeds, including Palmer amaranth. By 2015, Palmer amaranth bio‐ types had been confirmed resistant to acetolactate synthase (ALS)-inhibitors, dinitroanilines, glyphosate, hydroxyphenylpyruvate dioxygenase (HPPD)-inhibi‐ tors, and triazine herbicides in some parts of the United States along with multiple HR biotypes. Mechanisms of herbicide-resistance in Palmer amaranth are discussed in this chapter. Preplant herbicide options including glufosinate, 2,4-D, and dicam‐ ba provide excellent Palmer amaranth control; however, their application is limited before planting crops, which is often not possible due to unfavorable weather con‐ ditions. Agricultural biotechnology companies are developing new multiple HR crops that will allow the post-emergence application of respective herbicides for management of HR weeds, including Palmer amaranth. For the effective in-crop management of Palmer amaranth, and to reduce the potential for the evolution of other HR weeds, growers should apply herbicides with different modes of action in tank-mixture and should also incorporate cultural practices including inversion till‐ age and cover crops along with herbicide programs.