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Cost-effective weed management requires accurate estimates of yield and the potential yield loss resulting from weed infestations. However, crop yield and the effects of weeds are highly variable across years and locations. Ecophysiological models may be useful for predicting the effects of environment and management on crop and weed growth and competitive ability. Ability of the model INTERCOM to predict corn (Zea mays) growth and yield, velvetleaf (Abutilon theophrasti) interference on corn yield loss, and single-year economic threshold velvetleaf density (Te) was evaluated using 13 data sets collected in four states. Predicted and observed monoculture corn total above-ground biomass and leaf area index were in close agreement for most of the growing season. Predicted and observed weed-free corn yields were in agreement for yields ranging from 8 to 13 Mg ha-1 but were over- and under-predicted under low-yielding and near-optimal production conditions, respectively. Predicted and observed corn yield loss agreed well across the full range of observed velvetleaf densities for five to nine location years, depending on the performance criterion used. Estimates of Te calculated from predicted weed-free yield and yield loss relationships were an average of 6% smaller than those calculated from observed data, indicating that the model predicts a conservative value of Te in most cases. Although results are encouraging, they indicate that further research is needed to improve the capacity of INTERCOM for predicting weed-free yield and corn–velvetleaf interference.