Examining Variability In Corn Nitrogen Response and Economic Return.

Over-application of nitrogen in the agronomic landscape of the Midwest has become of growing concern in recent years. The temporal and spatial variability in N response of corn based systems indicates that N rate determination entails both environmental and economic risk. Our objective is to address these concerns by characterizing economically optimal nitrogen rate variability and evaluating the maximum return to nitrogen (MRTN) rate recommendation system. A randomized complete block split-plot experiment was conducted at 7 sites in Illinois over 10 years from 1999 to 2008. A quadratic + plateau regression was fit to the nitrogen responses for each site year. Economically optimal nitrogen rates (EONR) and MRTN rates were calculated from the fitted regressions using an N to corn price ratio of 1:10. The distribution of the differences between the static ex ante MRTN (the rate the MRTN system would recommend before planting), and the annual ex post EONR (the economically optimal rate determined after the growing season is complete) and their economic returns were analyzed. EONR nitrogen rates showed a high degree of variation and were not statistically different from the static MRTN rate. The economic return of using the internally calculated MRTN rate was an average of $13.84/ha less for corn following corn and $10.13/ha less for corn following soy over all sites than if the ex post EONR was used. Sites with higher baseline corn yields following both corn and soybean (such as Monmouth and Dixon Bottomland) and less inter-annual nitrogen response variability had MRTN economic returns closer to economically optimal than sites with low baseline yields and higher inter-annual nitrogen response variability (such as Dixon Upland and Brownstown). Though the economic returns of the MRTN system were less than those of the ex post rate, given the high inter-annual variability and unpredictability of the EONR, the MRTN system is well suited to helping farmers maximize returns while reducing nitrogen over-application.