Wheat Yield Loss in Response to Italian Ryegrass in Diverse Environments

Abstract

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. Linear 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 total plant population provided a better fit than that of yield loss with ryegrass density. Moreover, only two of the slopes relating yield loss and ryegrass proportion were significantly heterogenous from the combined data sets. The relatively high r2 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 the sites. Little relationship was found among the individual data sets between wheat yield loss and either accumulated heat units or precipitation. The most accurate equation describing the effect of Italian ryegrass interference was a simple linear regression: percentage wheat yield loss = 5.7 + (1.15 × percentage of ryegrass plants in the total plant population). Thus, it may be possible to predict potential yield loss in wheat fields from Italian ryegrass interference by scouting. These predictions may be used to assess the economic benefits of a postemergence herbicide application to control 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. Research Question This study sought to develop a simple assessment of wheat yield loss in response to Italian ryegrass infestations in a diversity of environments. Literature Summary Italian ryegrass is a widely distributed and an important weed of winter wheat in the USA. The effect of Italian ryegrass density on wheat yield was measured in Oregon, North Carolina, and Texas. Published data provided an opportunity to assess the reaction of wheat to Italian ryegrass across diverse wheat cultivars and environments. Study Description Two data sets from North Carolina, three from central Texas, and six from Oregon showed winter wheat yield loss from Italian ryegrass interference in the USA. To assess the response of wheat to Italian ryegrass interference, yield loss data from the individual data sets and the combined data were regressed against two measures of ryegrass infestation. One measure was density, the number of ryegrass plants per square foot. The other measure was the proportion of Italian ryegrass plants in the total plant population of the two species. Applied Questions Is wheat yield loss in response to Italian ryegrass better predicted by Italian ryegrass density or proportion of ryegrass in the plant population? Linear regression of wheat yield loss with Italian ryegrass proportion of the total plant population provided a better fit than that of yield loss with ryegrass density. What was the best description of the relationship between Italian ryegrass proportion and wheat yield loss? The equation that most accurately described this relation was: percentage wheat yield loss = 5.7 + (1.15 × percentage of ryegrass plants in the total plant population) Recommendation The simple linear relationship between wheat yield loss and proportion of Italian ryegrass in the plant population highlighted the potential of effective scouting of ryegrass in predicting wheat yield loss in diverse situations.