Environmental legislation may impose limitations on the quantity of nitrogen (N) used in corn production on the basis of soil type and ground water flow. If N rates are reduced, this might influence the relative competitiveness of weed species. Therefore, the objectives of this research were to develop a surface response model to provide estimations of the effect of differing N rates on threshold values of green foxtail in corn and to use this model as a theoretical framework for hypothesis testing. Field experiments were conducted from 1999 to 2001 to examine the interaction of N rate and green foxtail density on corn grain yield. The experiment was designed as a two-factor factorial with N levels ranging from 0 to 200 kg N ha⁻¹ and targeted green foxtail densities ranging from 0 to 300 green foxtail plants m⁻². The addition of up to 200 kg N ha⁻¹ increased corn grain yield in both weed-free and weedy treatments. Corn yield loss attributed to green foxtail ranged from 35 to 40% at 0 kg N ha⁻¹ to 12 to 17% at 200 kg N ha⁻¹. Ridge analysis of the response surfaces indicated that optimal corn grain yield could be achieved at derived values of 131 to 138 kg N ha⁻¹ while maintaining a green foxtail density of 8 to 9 green foxtail plants m⁻² on a sandy soil with less than 2% organic matter. The analyses of simulation results led to the generation of hypotheses of practical relevance to N management. On the basis of the generated hypotheses, a legislated reduction in N or an increase in the cost of N fertilizer would result in a lower threshold value for green foxtail in corn. If legislation were to ban the use of all herbicides in corn production, higher N rates or an increase in mechanical weed control measures would be required to offset yield losses caused by green foxtail. The human health and environmental consequences of such legislation would be significant.

Nomenclature: Green foxtail, Setaria viridis (L.) Beauv., SETVI; corn, Zea mays L. ‘Pioneer 3905’.