Glufosinate controls glyphosate-resistant Palmer amaranth, but growers struggle to make timely applications. XtendFlexTM cotton, resistant to dicamba, glufosinate, and glyphosate, may provide growers an option to control larger weeds. Palmer amaranth control and cotton growth, yield, and fiber quality were evaluated in a rescue situation created by delaying the first POST herbicide application. Treatments consisted of two POST applications of dicamba plus glufosinate, separated by 14 d, with the first application timely (0-d delay) or delayed 7, 14, 21, or 28 d. All treatments included a layby application of diuron plus MSMA. Palmer amaranth, 14 d after first POST, was controlled 99, 96, 89, 75, and 73% with 0-, 7-, 14-, 21-, or 28-d delays, respectively. Control increased following the second application, and the weed was controlled at least 94% following layby. Cotton yield decreased linearly as first POST application was delayed, with yield reductions ranging from 8 to 42% with 7- to 28-d delays. Delays in first POST application delayed cotton maturity but did not affect fiber quality.

Nomenclature: Dicamba; glufosinate; glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats; cotton, Gossypium hirsutum L.