Development of field-specific spring N rate recommendations for winter wheat

Optimum spring N fertilizer rates for winter wheat in Virginia vary widely from field to field, but traditionally spring N is applied at a uniform rate to all fields. A recently-developed tissue test procedure provides a field-specific evaluation of crop N status and predicts optimum N rate for the second spring N application in a split spring application management system. However, this procedure is based on a small number of researcher-planted experiments utilizing a single cultivar; it fails to provide field-specific rate recommendations for the first spring N application; and it is not accessible to farmers who are unwilling to split their spring N applications. Our objectives were: to evaluate the reliability of the tissue test procedure in a large number of farmer fields; to develop a method for making field-specific N rate recommendations for the first spring application in a split-application management system; and to develop a method for making field-specific N rate recommendations in a single-application management system. Forty-five spring N rate experiments were established in farmer fields over a five-year period. A range of N fertilizer rates was applied in early spring (Zadoks growth stage 25) and again in mid-spring (Zadoks growth stage 30) in all possible combinations. Yield data were used to calculate economic optimum N rates at growth stage (GS) 25 and GS 30 with split-application management, and at GS 25 with single-application management. These optima were regressed against a variety of predictor variables measured in the same fields. The reliability of the previously-developed tissue test procedure for making GS 30 N rate recommendations was confirmed. Tiller density was the best predictor of optimum GS 25 N rate with split-application management, while soil nitrate to 90 cm was the best predictor of optimum GS 25 N rate with single-application management. These three relationships fit together to form a flexible and powerful system for making spring N rate recommendations for winter wheat. This system increased estimated profit and apparent fertilizer efficiency in these experiments.