Multiple weed species in the field combine to cause yield losses and can be described using one of several empirical models. Field studies were conducted to compare observed corn yield loss caused by common sunflower and shattercane populations with predicted yield losses modeled using a multiple species rectangular hyperbola model, an additive model, or the yield loss model in the decision support system, WeedSOFT, and to derive competitive indices for common sunflower and shattercane. Common sunflower and shattercane emerged with corn and selected densities established in field experiments at Scandia and Rossville, KS, between 2000 and 2002. The multiple species rectangular hyperbola model fit pooled data from three of five location–years with a predicted maximum corn yield loss of 60%. Initial slope parameter estimate for common sunflower was 49.2 and 4.2% for shattercane. A ratio of these estimates indicated that common sunflower was 11 times more competitive than shattercane. When common sunflower was assigned a competitive index (CI) value of 10, shattercane CI was 0.9. Predicted yield losses modeled for separate common sunflower or shattercane populations were additive when compared with observed yield losses caused by low-density mixed populations of common sunflower (0 to 0.5 plants m−2) and shattercane (0 to 4 plants m−2). However, a ratio of estimates of these models indicated that common sunflower was only four times as competitive as shattercane, with a CI of 2.5 for shattercane. The yield loss model in WeedSOFT underpredicted the same corn losses by 7.5%. Clearly, both the CI for shattercane and the yield loss model in WeedSOFT need to be reevaluated, and the multiple species rectangular hyperbola model is proposed.