1. Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn

2. 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

3. 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