Glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Wats.) is a major problem in the southeastern U.S. Cooperative Extension personnel are actively promoting resistance management strategies, including integration of herbicides with other modes of action into glyphosate-based programs, to reduce selection pressure. This field experiment, conducted in five environments in North Carolina and Georgia during 2006 and 2007, evaluated residual control of Palmer amaranth by herbicides registered for use in cotton (Gossypium hirsutum L.). Treatments consisted of a factorial arrangement of 13 residual herbicides at 1X rates (manufacturer's suggested use rates) and 1.5X rates. All herbicides were applied preemergence (PRE) to the weeds to evaluate residual effectiveness. Results varied by environment, depending primarily upon time and amount of rainfall. Herbicides at 1.5X rates were an average of 9% more effective 20 d after application compared with 1X rates. Of the herbicides typically applied PRE or preplant, fomesafen, flumioxazin, and pyrithiobac were the most effective. These herbicides controlled Palmer amaranth 74 to 100%, depending upon environment, at 20 d. Fluometuron and diuron were intermediately effective, controlling Palmer amaranth 41 to 91%, and pendimethalin and prometryn were least effective (30 to 82%). Pyrithiobac and S-metolachlor were the most effective of herbicides that could be applied postemergence (POST) to cotton. Pyrithiobac (75 to 97% at 20 d) was more effective than trifloxysulfuron (34 to 88%), and S-metolachlor (57 to 96%) was more effective than metolachlor (32 to 86%). Flumioxazin (82 to 100% at 20 d) was the most effective option for postemergence-directed (POST-DIR) application. Diuron, linuron, linuron plus diuron, and prometryn plus trifloxysulfuron were intermediately effective (48 to 97%), and prometryn was least effective (30 to 79%). Integration of effective residual herbicides into glyphosate-based management systems will help sustain cotton production in areas infested with Palmer amaranth.
