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The relative competitive advantage of 12 commercially available wheat varieties was examined against Lolium rigidum Gaud. at a number of sites from 1995 to 1997 in south-eastern Australia. Nearly all the variation in crop grain yield was attributable to the variety × environment effects (81%), with only 4% due to variety × weed × environment effects. Some varieties exhibited an environment-specific competitive advantage, for example Katunga, Dollarbird and Hartog, whereas others like Shrike, Rosella and Janz were relatively poorly competitive in some situations. The introduction of greater genetic variability into wheat is required to significantly increase competitiveness. 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 selecting for competitive advantage with weeds when selecting for other traits such as early vigour. The varieties which showed competitive yield advantage also suppressed L. rigidum. A combination of short-term agronomic manipulations and a longer-term breeding effort is needed for increasing wheat competitiveness, and the increasing importance of herbicide-resistant weeds may facilitate this process. [ABSTRACT FROM AUTHOR]

The competitive abilities of a wide range of genotypes of wheat (Triticum aestivum L.) and durum wheat (Triticum durum Desf.) against Lolium rigidum Gaud. (annual ryegrass) were examined to determine the potential for breeders to select strongly competitive varieties. Considerable potential within the wheat genome to breed varieties with greater competitive ability was demonstrated. In 1993, 250 genotypes from around the world were screened and in 1994 a subset of 45 (mainly Australian) genotypes were further examined. A uniform density of L. rigidum reduced grain yield of wheat by up to about 80% in 1993 and to 50% in 1994, depending on wheat genotype. Reduction in grain yield was correlated with L. rigidum dry matter. Wheats varied in competitive ability with source, and durum wheats were less competitive than T. aestivum. The ‘old’ standard wheat varieties (released between 1880 and 1950) suppressed the weed more than all the current varieties, with the exception of eight F1 hybrids. A doubling of the crop seeding rate of 10 of the genotypes in 1994 reduced the biomass of L. rigidum by an average of 25% compared with the standard seeding rate. Ranking of competitive ability of varieties at high density was consistent at both seeding rates. The strongly competitive genotypes had high early biomass accumulation, large numbers of tillers, and were tall with extensive leaf display. The potential for breeding enhanced competitive ability in wheat is discussed. [ABSTRACT FROM AUTHOR]