Herbicide Resistance in Summer Weeds –Northern Cropping Regions of Australia

Abstract

Weed resistance to herbicides is a major production challenge for Australian grain and cotton growers, resulting in fewer effective herbicides for in crop and fallow weed control. These industries support a regular (every 3 – 4 year) random survey of production fields to gain a better understanding of what herbicide resistance is present and where. The information helps farmers to better manage their weeds and informs industry as to which weed management challenges require further RD and E because of herbicide resistance. Herbicide resistance in summer weeds of the Northern Cropping Region of Australia (Queensland and New South Wales) is being evaluated to commonly used herbicides including glyphosate and haloxyfop. The current evaluation is the third resistance evaluation conducted in this region with previous surveys taking place in 2016/17 and 2021/22. A survey of >700 fields across the Northern Region resulted in the collection of 183 feathertop Rhodes grass (Chloris virgata), 136 awnless barnyard grass (Echinochloa colona), 46 sweet summer grass (Moorochloa eruciformis), and 116 fleabane (Conyza spp.) populations. Seed collected from populations is being sown and seedlings treated at the upper label recommended rate to determine resistance status. Evaluation is currently underway focussing on glyphosate and haloxyfop. Future evaluation will be made on resistance to Group 22 herbicides paraquat and paraquat + diquat, several Group 1 herbicides on the summer grasses and 2,4-D on fleabane. In addition, cotton field in Northern Australia (northern Queensland and Northern Territory) will be surveyed in May/June 2026. This survey will be the first of its kind and will identify key weeds. The paper will report on findings on resistance to glyphosate and haloxyfop, the change in resistance over time and the implications for the management of weeds in Northern Region farming systems. Results of the northern Australia survey will also be presented.