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Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061.
* Corresponding author.
A recently developed system to predict the optimum N fertilizer rate for winter wheat (Triticum aestivum L. emend. Thell) at Zadoks growth stage (GS) 30 is based on the relationship between measured economic optimum N rate at GS 30 and wheat tissue N content measured at GS 30. However, winter wheat often needs an earlier application of spring N to achieve optimum yield. We therefore developed a test to determine which fields need this earlier (GS 25) application, and to predict the optimum N rate with split-application management using the tissue test or as a single spring application. The optimum N rate at GS 25 was measured over 5 yr, both with and without GS 30 N applications. These measured optimum N rates were regressed against a variety of possible predictor variables measured in the same fields. Tiller density at GS 25 was a good predictor of optimum N rate at GS 25 in a split spring application program. Using this relationship along with the GS 30 tissue test to make N recommendations for winter wheat increased estimated profit relative to using the tissue test alone. Soil NO3 measured to 0.9 m depth was the best predictor of optimum N rate at GS 25 when that is to be the only spring N application, and improved estimated profit relative to applying 90 kg N ha–1 at all sites; however, the economic performance of split spring N applications was substantially better than for any single spring applications. The recommendation system developed by integrating these component relationships is powerful and flexible, and provides field-specific N rate recommendations for all spring N applications to winter wheat, regardless of management decisions about splitting spring N applications.
Received for publication March 22, 1993.
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