EVOLVED RESISTANCE TO GLYPHOSATE IN RIGID RYEGRASS (LOLIUM-RIGIDUM) IN AUSTRALIA

Following 15 yr of successful use, glyphosate failed to control a popu lation of the widespread grass weed rigid ryegrass in Australia. This population proved to be resistant to glyphosate in pot dose-response e xperiments conducted outdoors, exhibiting 7- to 11-fold resistance whe n compared to a susceptible population. Some cross-resistance to diclo fop-methyl (about 2.5-fold) was also observed. Similar levels. of cont rol of the resistant and susceptible populations were obtained followi ng application of amitrole, chlorsulfuron, fluazifop-P-butyl, paraquat , sethoxydim, simazine, or tralkoxydim. The presence of glyphosate res istance in a major weed species indicates a need for changes in glypho sate use patterns.