Reducing Uncertainty and Effects of Fertilizer and Commodity Prices On Estimates of Economic Optimum Rates.

Economic optimum rates (EORs) have been used as the best references for estimating nitrogen (N) requirements for corn (Zea mays L.) during the last 50 yr. These estimates, however, include a large degree of uncertainty due to assumptions built in the economic theory used to calculate EORs: difficulty of selecting the best functional relationship between N rates and crop yields, large inherent spatial and temporal variability in yield response to N, and unpredictable effects of weather on N supply and losses. A sample of 54 small-plot yield response trials of corn after corn or soybean in Iowa collected over several years was used to describe how the uncertainty in EORs can be reduced by using a multistep procedure to estimate EORs for two management categories based on the previous crop, and using discrete marginal analysis to calculate two alternative benchmarks for EORs. The incremental break-even rates and incremental rates that give the desired rate of return were calculated based on various prices of grain and N fertilizer. To reduce model disagreements, we used only the near-optimal range of fertilization for each previous crop. The multistep procedure estimates category-based EORs by using information easily available (e.g. previous crop or timing of fertilizer applications) to growers before fertilizer applications.  The final step is to identify potential N management categories that are stable in time and enable growers to increase N use efficiency. The process needs to be repeated each time as new trials are collected and new information is available.