Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn

Yield-based approach to N recommendations has been questioned Poor relationship between recommendations and EONR from N rate response trials Uncertainly about how yield goals should be determined Assumption that NUE is constant across sites anyears Use of inadequate or inappropriate adjustments for nonfertilizer N sources

Data showed that optimum N rates increased over time at one site and decreased at the other site Results provide no clear indication of a change in N rates over time Reasons for similar or decreasing optimum N rates where yields have increased substantially include more efficient utilization of available N by the crop and increased soil N supplying capability