Yield Target Approaches to Nitrogen Recommendations for Corn.

An expectation of corn yield (expressed as yield goal, expected yield, or proven yield) is a major determinant of the N rate recommendations made by Land Grant Universities in 65% of states. However, among recommendations there is substantial variation in how the yield target is defined, ranging from no definition to prescriptive approaches for calculating multiyear averages with adjustments for unusual years and expected yield increases in future years. Implicit in this recommendation system is the generalization that increased corn yield is correlated to increased crop N content. Thus, the crop N requirement is calculated by multiplying the yield target by a factor (Nf) of pounds of N per bushel of corn. The Nf factor ranges from 1.0 to 1.6 lb/bu among state recommendations, with most using a factor of about 1.2 lb N/bu. Most yield target recommendations consider that some of the crop N requirement will be derived from sources other than fertilizer N applied for the growing season. Sources of N subtracted from the crop N requirement to calculate the fertilizer N requirement include legume N credits (85% of states making yield target-based N recommendations), mineralized N from organic matter (25% of states), residual inorganic-N (40% of states), and/or irrigation water nitrate-N (20% of states). Other factors affecting yield target-based N recommendations include surface soil texture, soil moisture status, drainage class, tillage, and cation exchange capacity. In relatively few of the yield target-based N recommendation systems is the fertilizer N requirement explicitly adjusted for estimated fertilizer use efficiency and/or economic considerations. Although the accuracy of yield target-based N recommendations has not been recently evaluated in many states, one would expect considerable differences in accuracy among the states given the diversity of approaches used to calculate the fertilizer N requirement.