Secale cereale interference and economic thresholds in winter Triticum aestivum

Secale cereale is a serious weed problem in winter Triticum aestivum-produc ing regions. The interference relationships and economic thresholds of S. c ereale in winter T. aestivum in Colorado, Kansas, Nebraska, and Wyoming wer e determined over 4 yr. Winter T. aestivum density was held constant at rec ommended planting densities for each site. Target S. cereale densities were 0, 5, 10, 25, 50, or 100 plants m(-2) Secale cereale-winter T. aestivum in terference relationships across locations and years were determined using a negative hyperbolic yield loss function. Two parameters-I, which represent s the percent yield loss as S, cereale density approaches zero, and A, the maximum percent yield loss as S, cereale density increases-were estimated f or each data set using nonlinear regression. Parameter I was more stable am ong years within locations than among locations within years, whereas maxim um percentage yield loss was more stable across locations and years. Enviro nmental conditions appeared to have a role in the stability of these relati onships. Parameter estimates for I and A were incorporated into a second mo del to determine economic thresholds. On average, threshold values were bet ween 4 and 5 S. cereale plants m(-2); however, the large variation in these threshold values signifies considerable risk in making economic weed manag ement decisions based upon these values.