SPRING NITROGEN ON WINTER-WHEAT .1. FARMER-FIELD VALIDATION OF TISSUETEST-BASED RATE RECOMMENDATIONS

Correct N application rates on winter wheat (Triticum aestivum L. emen d. Thell) are needed to minimize NO3 Leaching to ground and surface wa ters and to maximize profit. A system for predicting optimum spring N fertilizer rates for winter wheat in the U.S. mid-Atlantic was previou sly developed from a small set of experiments using a single wheat var iety and in which all cultural operations were conducted by researcher s. The objective of the research reported here was to evaluate whether this system could be successfully applied to the more varied environm ent of farmer fields. Thirty-nine N response experiments were carried out in farmer fields over five growing seasons encompassing a wide and representative range of soil types and wheat varieties. The relations hip between wheat tissue N content at Zadoks growth stage (GS) 30 and optimum N rate at os 30 was weaker in farmer fields (r(2) = 0.51) than in the researcher-planted experiments (r(2) = 0.59) but was stilt str ong enough to be useful. Economic analyses indicate that tissue test-b ased N rate recommendations increased profit by an average of $36 ha-( 1) relative to traditional N application practices. Apparent fertilize r efficiency was measured in 10 of the experiments and was significant ly higher for N applications based on the tissue test system than for traditional N applications.