Identification of Nitrogen Management Categories to Reduce Uncertainty in Estimates of Economic Optimum Rates.

Estimating economic optimum rates (EORs) of nitrogen (N) fertilizer for corn allows estimates of how changes in prices affect recommended rates, but it does not allow further fine tuning of N management due to large uncertainty in EORs and the high cost of collecting sufficient yield response data. In Iowa, about 30 groups of growers have started testing an alternative approach, adaptive management, to identify potential N management categories based on corn stalk nitrate surveys guided by remote sensing of the corn canopy. An annual survey of 600-800 corn fields was conducted during the last three yr.  Aerial images of corn canopy in late August were used to select three sampling areas, one within each predominate soil type within a field. Survey categorical analysis was used to calculate the cumulative probability of a stalk sample to test in a higher stalk test category and to identify important factors affecting stalk test outcomes. The analysis revealed statistically significant differences in corn N status among management categories based on forms and timing of fertilizer and manure applications, previous crop, soil drainage, and variability in early-season rainfall across Iowa. Organized groups of growers that use guided corn stalk nitrate surveys have the ability to identify relatively efficient N management practices within large geographic areas. Strip trials with three rates of N can be used to calculate incremental break-even rates and incremental rates that give the desired rate of return for a category, which are better indicators of N rates than EORs.