192-3

The traditional methodology for calculating economic optimum rates (EORs) of nitrogen (N) fertilization for corn (*Zea mays* L.) requires fitting models to describe relationship between yields and N rates. However, different models fitted to the same yield response data can result in disagreements in EORs, and the disagreements can be large enough to have economic and environmental importance.* *The objective of this study was to explore an alternative method for calculating optimal rates without fitting models when analyzing a large number of yield response observations. We analyzed a sample of 54 trials for corn after corn and corn after soybean with seven N rates applied to plots in a randomized complete block design with three replications. We used discrete marginal analysis where marginal physical products (MPPs) of N fertilization expressed as yield responses per kg of N applied were calculated for each fertilizer increment and segmented lines were used to interpolate MPPs between N rates applied. Economically optimal rates were calculated for maximizing profit per unit of area and for receiving the desired rate of profit on the last unit of N applied. The results showed that MPPs tended to be near linearly related to N rates in the near-optimal range. Discrete marginal analysis and linear interpolation in the near-optimal range was shown to be a simple and reliable method for calculating optimal rates. The utility of the alternative method is also in ability to provide better visual effects of changes in price of N and grain on optimal rates.