Variation in herbicide cross-resistance among populations of annual ryegrass ( Lolium rigidum ) resistant to diclofop-methyl

A population of annual ryegrass (Lolium rigidurn), known to be susceptible to diclofop-methyl, and eleven populations known to be resistant to the herbicide, were evaluated for their cross-resistance to fluazifop-butyl, haloxyfop-methyl, sethoxydim, chlorsulfuron, glyphosate and propham, applied postemergence as a spray treatment. None of the populations were cross-resistant to glyphosate or propham. All the populations showed some level of cross-resistance to the other herbicides, but there was considerable variation between populations in this resistance. The variation could not be related in any simple manner to the origin of the populations in Australia, nor to their past histories of herbicide applications. The results indicate the complexity that will be faced in resolving the biochemistry and genetics of the phenomenon and the formulation of advice to farmers.