Wheat yield loss in response to Italian ryegrass in diverse environments

Eleven data sets, including two from North Carolina, three from Texas, and six from Oregon were used to assess wheat (Triticum aestivum L.) yield loss in response to Italian ryegrass (Lolium multiflorum Lam,) infestations. Li near regression of yield loss from data in the individual data sets and the combined data from all the sets with Italian ryegrass proportion of the to tal plant population provided a better fit than that of yield loss with rye grass density. Moreover, only two of the slopes relating yield loss and rye grass proportion were significantly heterogenous from the combined data set s. The relatively high r(2) value of 0.88 from regressing wheat yield loss against Italian ryegrass proportion of the combined data was quite striking considering the differences in wheat cultivars, soils, and weather among t he sites. Little relationship was found among the individual data sets betw een wheat yield loss and either accumulated heat units or precipitation, Th e most accurate equation describing the effect of Italian ryegrass Interfer ence was a simple linear regression: percentage wheat yield loss = 5.7 + (1 .15 x percentage of ryegrass plants in the total plant population). Thus, i t may be possible to predict potential yield loss in wheat fields from Ital ian ryegrass interference by scouting, These predictions may be used to ass ess the economic benefits of a postemergence herbicide application to contr ol Italian ryegrass. They may also help to determine whether a wheat field with an Italian ryegrass infestation is better used for forage rather than grain production.