In-Season Rate Determination From Plant Spectral Reflectance in Soft Red Winter Wheat.

Plant reflectance is determined by leaf surface properties, internal structure, and the concentration and distribution of biochemical components; therefore, remote analysis of reflected light can be used to assess plant biomass and the physiological status of a plant.  Current recommendations for intensively managed soft red winter wheat (SRWW) in Virginia call for N to be applied at Zadoks growth stage (GS) 25 based on tiller density thresholds and at GS 30 based on tissue nitrogen (N) concentration.  Determining wheat tissue N content requires physical sampling of multiple locations in each field as well as a time lag between sampling and return of laboratory analysis.  A system that can determine N needs at GS 30 as accurately as tissue N content but that can generate values in real time is needed.  The objectives of these studies were to evaluate the GreenSeeker as a tool to estimate GS 30 N needs of SRWW and to validate the performance of variable rate N applications to SRWW using the RT 200 system. Over 16 site years, grain yields were similar for wheat receiving N based on the Virginia wheat algorithm and the standard method of determining wheat N needs based on GS 30 tissue N concentration.  The Virginia algorithm treatment required 7 percent less N to reach these yields.