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

Correct N application rates on winter wheat (Triticum aestivum L. emend. Thell) are needed to minimize NO3 Leaching to ground and surface waters and to maximize profit. A system for predicting optimum spring N fertilizer rates for winter wheat in the U.S. mid-Atlantic was previously developed from a small set of experiments using a single wheat variety and in which all cultural operations were conducted by researchers. The objective of the research reported here was to evaluate whether this system could be successfully applied to the more varied environment 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 relationship 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 strong enough to be useful. Economic analyses indicate that tissue test-based N rate recommendations increased profit by an average of $36 ha-(1) relative to traditional N application practices. Apparent fertilizer efficiency was measured in 10 of the experiments and was significantly higher for N applications based on the tissue test system than for traditional N applications.