Use of alternative herbicide mixtures to manage glyphosate-resistant Lolium rigidum Gaud. along crop margins in South Australia

The repeated use of glyphosate to control annual ryegrass along fence lines and crop margins has resulted in the evolution of resistance to this herbicide in populations of annual ryegrass (Lolium rigidum) in cropping regions of Australia. Field trials were conducted between 2009 and 2011 at four fence-line sites in South Australia to identify suitable herbicide treatments for controlling glyphosate-resistant annual ryegrass. Annual ryegrass populations growing at all four sites were found to have 12–24-fold resistance to glyphosate compared with a standard susceptible population in dose-response experiments. Glyphosate alone (1080 g ha–1) did not effectively control glyphosate-resistant annual ryegrass at any location. Single applications of paraquat + diquat and paraquat + amitrole were effective where weed populations were low, with up to 99% reduction in seed-head production. Mixtures of paraquat + diquat + diuron, glufosinate ammonium + diuron and two applications of paraquat + diquat 14 days apart consistently provided high levels of control of glyphosate-resistant annual ryegrass at all locations, with >90% reduction in seed-head production. In 2011, glyphosate resistant individuals of annual ryegrass were identified up to 50 m inside the fields adjacent to the fence. Therefore, failure to control glyphosate-resistant annual ryegrass along crop margins risks movement of resistance into adjacent cropped fields.