Nitrogen Rates

Introduction In most crop rotations that include corn, nitrogen (N) applied to the corn phase is a proven and profitable practice. Corn in some rotations requires little to no N input, with first-year corn following established alfalfa as an example. Corn in other rotations requires substantial N input to meet plant requirements, with continuous corn (CC) typically requiring the greatest input. Other rotations or corn phases will be intermediate in N application requirement. With corn in the two most common crop sequences in the Corn Belt, corn following soybean (SC) and CC, if N is not applied yield will suffer. If N is not applied on an on-going basis, over time corn yield will often average around 50-60 bu/acre in CC and 100-110 bu/acre in SC, or less. The soil system typically cannot supply the full corn plant N requirement. On average the yield with no N applied is around 70% in a SC rotation and 55% in CC of the yield obtained at an economic optimum rate. Therefore, supplemental N is needed to reach economic yield potential. Research has been on-going for over 50 years measuring corn response to N application. Guidelines for suggested N rates based on that research have been derived using economic principles to determine economic optimum N rate (EONR) rather than maximum yield. Therefore, recommendations are guided by economic return to N application through corn yield increase. The expectation by many is that simply applying N at economic optimum rates will "solve" the issue of nitrate movement from fields in subsurface drainage. However, nitrate losses occur in corn production systems even when no N is applied, and N application at optimum rates increases loss. To date determination of EONR has not been modified to account for environmental costs resulting from increased nitrate loss to water systems when N is applied, largely due to lack of such cost information and societal decisions on where to partition those costs.