Italian ryegrass (Lolium multiflorum Lam.) is a serious and widespread weed problem in grain fields of western Oregon and Washington. Field studies were conducted to measure effects of Italian ryegrass interference on winter wheat (Triticum aestivum L.) yields. Wheat cultivars used were ''Nugaines'', ''Druchamp,'' ''Hyslop,'' and ''Yamhill.'' Ryegrass densities ranging from 0 to approximately 100 plants/m2 were established by hand distribution on the soil surface, followed by harrowing. Urea fertilizer was applied in the spring at 56, 112 and 168 kg of N/ha. As ryegrass densities increased, wheat grain yields decreased. In one experiment, wheat grain yield was reduced by 4100 kg/ha when the ryegrass density was increased from 0.7-93 plants/m2. Interference caused greater percentage reductions in wheat yields at 168 kg N/ha than at 56 kg N/ha when ryegrass densities were high. The percentage of yield reduction tended to be higher in short wheat cultivars. ''Nugaines'' and ''Hyslop,'' than in tall cultivars, ''Druchamp'' and ''Yamhill.'' When no weed control measures were used, ryegrass infestations in the 1970 crop wheat yield reductions in the 1971 crop. This was due to higher ryegrass densities in 1971 in plots where the heaviest ryegrass densities occurred in 1970. When ryegrass was controlled with diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] in 1971, no carry-over effects from previous ryegrass infestations could be measured. Results obtained in these studies illustrate the importance of good ryegrass control in winter wheat management systems.
