Losses in grain yield of winter crops from Lolium rigidum competition depend on crop species, cultivar and season

The competitive abilities of eight winter crops were compared against Lolium rigidum Gaud. (annual ryegrass), an important weed of southern Australia, as a potential strategy to suppress weeds and reduce dependence on herbicides. Two cultivars of each species were chosen to represent the range of competitive ability within each crop and grown in field experiments in 1992 and 1993. The order of decreasing competitive ability (with the ranges of percentage yield reduction from L. rigidum at 300 plants m(-2) in parenthesis) was as follows: oats (Avena sativa L.), 2-14%; cereal rye (Secale cereale L.), 14-20%; and triticale (x: Triticosecale), 5-24%; followed by oilseed rape, (Brassica napus L.), 9-30%; spring wheat (Triticum aestivum L.), 22-40%; spring barley (Hordeum vulgare L.), 10-55%; and, lastly, field pea (Pisum sativum L.), 100%, and lupin (Lupinus angustifolius L.), 100%. Differences in competitive ability of cultivars within each species were identified, but competition was strongly influenced by seasonal conditions. Competition for nutrients (N, P and K) and light was demonstrated. L. rigidum dry matter and seed production were negatively correlated with grain yield of the weedy crops. More competitive crops offer the potential to suppress grass weeds while maintaining acceptable grain yields. Ways of improving the competitive abilities of grain legume crops are discussed.