Technical Abstract: Crop variety effect on herbicide performance is not well characterized, particularly for sweet corn, a crop that varies greatly among hybrids in competitive ability with weeds. Field studies were used to determine the effect of crop competitive ability on season-long herbicide performance in sweet corn. Two sethoxydim-tolerant sweet corn hybrids were grown in the presence of Panicum miliaceum and plots were treated postemergence with a range of sethoxydim doses. Significant differences in height, leaf area index, and intercepted light were observed between hybrids near anthesis. Across a range of sublethal herbicide doses, the more-dense canopy hybrid Rocker suppressed P. miliaceum shoot biomass and fecundity to a greater extent than hybrid Cahill. Yield of sweet corn improved to the level of the weed-free control with increasing sethoxydim dose. The indirect effect of herbicide dose on crop yield, mediated through P. miliaceum biomass reduction, was significant for all of Cahill¿s yield traits but not Rocker. These results indicate a less competitive hybrid requires relatively more weed suppression by the herbicide to not only reduce weed growth and seed production, but also to maintain yield. Sweet corn competitive ability consistently influences season-long herbicide performance.