Crop competitive ability contributes to herbicide performance in sweet corn M M WILLIAMS II*, R A BOYDSTON  & A S DAVIS* *USDA-ARS, Invasive Weed Management Research, Urbana, Illinois, USA, and  USDA-ARS, Vegetable and Forage Crops Research, Prosser, Washington, USA Received 29 March 2007 Revised version accepted 21 September 2007 Summary Crop variety effects on herbicide performance is not well characterised, particularly for sweet corn, a crop that varies greatly among hybrids in competitive ability with weeds. Field studies were used to determine the effects of crop competitive ability on season-long herbicide per- formance in sweet corn. Two sethoxydim-tolerant sweet corn hybrids were grown in the presence of Panicum miliaceum and plots were treated post-emergence 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 sub-lethal herbicide doses, the denser canopy hybrid Rocker suppressed P. miliaceum shoot biomass and fecundity to a greater extent than the 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 the CahillÕs yield traits but not Rocker. These results indicate that 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 con- sistently influences season-long herbicide performance. Keywords: competition, cultivar, dose–response, herbi- cide, integrated weed management, risk, sethoxydim, Zea mays. WILLIAMS II MM, BOYDSTON RA & DAVIS AS (2008). Crop competitive ability contributes to herbicide performance in sweet corn. Weed Research 48, 58–67. Introduction Weed management in North American sweet corn (Zea mays L.) production is characterised by reliance on soil applications of triazine and chloroacetamide herbicides (Anonymous, 2003) and risk of crop injury from several post-emergence herbicides (Pataky et al., 2006). Recent surveys of commercial production fields in the United States reported extensive weed abundance and fecundity at harvest and 58% of the fields suffered yield loss because of weed interference (Williams et al., 2006c). Weed interference differentially affects yield and ear traits important to both processing and fresh markets (Williams & Masiunas, 2006). In light of these chal- lenges, weeds present in sweet corn production and particularly in a growing organic market mean that improved weed management systems are a top priority to the industry (Anonymous, 2003). Commercially grown sweet corn germplasm varies widely in competitive ability, defined here as the collective measure of both crop tolerance to weed interference and the cropÕs ability to suppress weeds. Large differences among hybridsÕ ability to suppress Panicum miliaceum L. (wild proso millet) have been shown to be negatively correlated with leaf area index (LAI) and intercepted photosynthetically active radia- tion (PAR) after crop anthesis (Williams et al., 2007). Factors driving yield loss varied among sweet corn hybrids; more competitive hybrids established canopy dominance, restrained weed growth and experienced less yield loss (Williams et al., 2008). While crop competitive ability is likely to have practical implications in sweet Correspondence: MM Williams II, USDA-ARS, Invasive Weed Management Research, University of Illinois, 1102 S. Goodwin Ave., Urbana, Illinois 61801, USA. Tel: (+1) 217 244 5476; E-mail: mmwillms@uiuc.edu Ó 2008 European Weed Research Society Weed Research 48, 58–67 No claim to original US government works