Ranking the ability of wheat varieties to compete with Lolium rigidum

The relative competitive advantage of 12 commercially available wheat varie ties was examined against Lolium rigidum Gaud, at a number of sites from 19 95 to 1997 in south-eastern Australia. Nearly all the variation in crop gra in yield was attributable to the variety x environment effects (81%), with only 4% due to variety x weed x environment effects. Some varieties exhibit ed an environment-specific competitive advantage, for example Katunga, Doll arbird and Hartog, whereas others like Shrike, Rosella and Janz were relati vely poorly competitive in some situations. The introduction of greater gen etic variability into wheat is required to significantly increase competiti veness. Alternatively, manipulating crop agronomy, such as increasing crop seeding rate, may be a practical alternative. The grain yield of weed-free wheat was highly positively correlated with grain yield of the weedy plots, suggesting that local adaptation is important for strong competitiveness, and that wheat breeders in southern Australia may be inadvertently selectin g for competitive advantage with weeds when selecting for other traits such as early vigour. The Varieties which showed competitive yield advantage al so suppressed L. rigidum. A combination of short-term agronomic manipulatio ns and a longer-term breeding effort is needed for increasing wheat competi tiveness, and the increasing importance of herbicide-resistant weeds may fa cilitate this process.