Effectiveness of glufosinate, dicamba, and clethodim on glyphosate-resistant and susceptible populations of five key weeds in Australian cotton systems

XtendFlexTM cotton with resistance to glyphosate, glufosinate and dicamba may become available in Australia. Resistance to these herbicides enables two additional modes of action to be applied in crop. The double knock strategy, typically glyphosate followed by paraquat, has been a successful tactic for control of glyphosate-resistant in fallow situations in Australia. Glufosinate is a contact herbicide, and may be useful as the second herbicide in a double knock for use in XtendFlexTM cotton crops. We tested the effectiveness of glufosinate applied at intervals of 1, 3, 7, and 10 d after initial applications of glyphosate, dicamba, clethodim and glyphosate mixtures with dicamba or clethodim on glyphosate-resistant and susceptible populations of Conyza bonariensis, Sonchus oleraceus, Chloris virgata, Chloris truncata and Echinochloa colona. Effective treatments for Conyza bonariensis with 100% control were dicamba and glyphosate+dicamba followed by glufosinate independent of the interval between applications. Sonchus oleraceus was effectively controlled in Experiment 1 by all treatments. However, in Experiment 2 effective treatments were dicamba and glyphosate+dicamba followed by glufosinate (99.3 – 100% control). Timing of the follow-up glufosinate did not affect the control achieved. Consistent control of Chloris virgata was achieved with glyphosate, clethodim or glyphosate+clethodim followed by glufosinate at 7 and 10 d intervals (99.7 – 100% control). Control of Chloris truncata was inconsistent. The best treatment for C. truncata was glyphosate+clethodim followed by glufosinate 10 d later (99.8 – 100% control). Echinochloa colona was effectively controlled with all treatments except for glyphosate on the glyphosate-resistant population. Additional in-crop use of glufosinate and dicamba should be beneficial for weed management in XtendFlexTM cotton crops, when utilising the double knock tactic with glufosinate. For effective herbicide resistance management, it is important that these herbicides be used in addition to, rather than substitution for, existing weed management tactics.