Italian ryegrass interference in broccoli was measured in field experiments and the data fit a rectangular hyperbolic competition model. The model predicted 58% of broccoli yield loss related to Italian ryegrass density when pooled over three y. An economic threshold value of 4.9 Italian ryegrass plants m−1 of crop row was determined to be the density required to cause a 3.6% yield loss, equal to postemergence weed control costs. Italian ryegrass densities of 600 to 1000 plants m1 of broccoli row caused 100% yield loss.