Effect of Prices and Moisture Stress on Nitrogen Rates that Maximize Two Economic Criteria for Corn

Fertilizer rates, which maximize certain economic criteria for crop production, are affected by the yield response to fertilizer, the ferti1izer:crop-price ratio, the crop price, and the cost of production. The purpose of this study was to evaluate the effects of different fertilizer N:crop-price ratios (P,), corn (Zea mays L.) prices (Py), and moisture-stress levels (S) on the rates of fertilizer N that maximize net return (m) and rate of return (M) to total investment in corn production per unit of land area. Fifteen site-years of data were used from two continuous-corn fertilizer rate experiments conducted under different crop management and environmental conditions. Moisture stress had a different effect in each experiment on yield and yield response to N. Net return from corn production and the rate of return to investment per hectare at the economic optimum N rate were affected by all factors considered. They were affected more by decreasing Py by half from 13.77 to 6.88¢/kg than by increasing the Pᵣ ratio fourfold from 2.8 to 11.2 on a kg N:kg corn-price basis. Only a severe deleterious effect of moisture stress on yields and yield reponse to N decreased the net return and rate of return more than decreasing the price of corn. At a constant PᵣM, decreasing the corn price caused the N rate that maximized M to increase and approach the economic optimum N rate. At a constant N price and a decreasing corn price, the economic optimum N rate decreased, but the N rate that maximized M remained static. In either case, these two N rates approached each other. A deleterious moisture-stress effect also caused both N rate to decrease but approach each other. These results suggest that, if there are increasing production costs and(or) decreasing crop prices, an overreduction of a divisible responsive input such as fertilizer N on corn could markedly affect the return to total investment on per-land-area basis.