XtendFlex^™ 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 cotton 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 XtendFlex^™ 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 glyphosate-susceptible populations of flaxleaf fleabane, common sowthistle, feather fingergrass, windmill grass, and junglerice. Effective treatments for flaxleaf fleabane with 100% control were dicamba and glyphosate+dicamba followed by glufosinate independent of the interval between applications. Common sowthistle 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% to 100% control). Timing of the follow-up glufosinate did not affect the control achieved. Consistent control of feather fingergrass was achieved with glyphosate, clethodim, or glyphosate+clethodim followed by glufosinate at 7-d and 10-d intervals (99.7% to 100% control). Control of feather fingergrass was inconsistent. The best treatment for windmill grass was glyphosate+clethodim followed by glufosinate 10 d later (99.8% to 100% control). Junglerice 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 XtendFlex^™ cotton crops, when using 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.

Nomenclature: Glyphosate; glufosinate; dicamba; cotton; Gossypium hirsutum L.; flaxleaf fleabane; Conyza bonariensis (L.) Cronquist; common sowthistle; Sonchus oleraceus L.; feather fingergrass; Chloris virgata Sw.; windmill grass; Chloris truncata R. Br.; junglerice; Echinochloa colona (L.) Link