Site-to-site and year-to-year variation in Triticum aestivum-Aegilops cylindrica interference relationships

Crop yield loss-weed density relationships critically influence calculation of economic thresholds and the resulting management recommendations made b y a bioeconomic model. To examine site-to-site and year-to-year variation i n winter Triticum aestivum L. (winter wheat)-Aegilops cylindrica Host. (joi nted goatgrass) interference relationships, the rectangular hyperbolic yiel d loss function was fit to data sets from multiyear field experiments condu cted at Colorado, Idaho, Kansas, Montana, Nebraska, Utah, Washington, and W yoming. The model was fit to three measures of A. cylindrica density: fall seedling, spring seedling, and reproductive tiller densities. Two parameter s: i, the slope of the yield loss curve as A. cylindrica density approaches zero, and a, the maximum percentage yield loss as A. cylindrica density be comes very large, were estimated for each data set using nonlinear regressi on. Fit of the model to the data was better using spring seedling densities than fall seedling densities, but it was similar for spring seedling and r eproductive tiller densities based on the residual mean square (RMS) values . Yield loss functions were less variable among years within a sire than am ong sires for all measures of weed density. For the one site where year-to- year variation was observed (Archer, WY), parameter a varied significantly among years, but parameter i did not. Yield loss functions differed signifi cantly among sites for 7 of 10 comparisons. Sire-to-site statistical differ ences were generally due to variation in estimates of parameter i. Site-to- site and year-to-year variation in winter T. aestivum-A. cylindrica yield l oss parameter estimates indicated that management recommendations made by a bioeconomic model cannot be based on a single yield loss function with the same parameter values for the winter T. aestivum-producing region. The pre dictive ability of a bioeconomic model is likely to be improved when yield loss functions incorporating time of emergence and crop density are built i nto the model's structure. Nomenclature: Aegilops cylindrica Host. AEGCY, jointed goatgrass; Triticum aestivum L., winter wheat.