Crop Sensors As in-Season Nitrogen Management Tool for Winter Wheat in Wisconsin.

In Wisconsin, current winter wheat (Triticum aestivum L.) nitrogen (N) rate guidelines are determined by soil type, previous crop and pre-plant soil nitrate test (PPNT).  Nitrogen management may be improved through site-specific assessments of N need in the spring, offering a more effective use of top-dressed N.  The study objective was to determine if crop reflectance measurements could be used to determine optimal in-season N rates on silt loam soils in eastern Wisconsin.  This study evaluated the effect of ammonium nitrate applied at 0 to 167 kg N ha^-1, in 33 kg N ha^-1 increments, top-dressed at green-up (GU) or Zadoks growth stage 30 (GS 30), on canopy reflectance, whole plant and grain N uptake, and yield.

Field experiments were conducted at two and three farms, in 2014 and 2015, respectively, where crop canopy reflectance measurements, using the Holland Scientific Crop Circle ACS-430 (Holland Scientific, Inc., Lincoln, NE), were collected from May (approximately GS 25) to mid-June (GS 40). Whole plant samples were collected at GS 30 in 2014.  In 2015, whole plant samples were collected at GU, GS 30, and GS 40 and soil samples 0 to 30 cm were collected at GU.  All plant samples were analyzed for total N and all soil samples were analyzed for NO₃-N and NH₄-N.  In 2014, both normalized difference vegetation index (NDVI) and normalized difference red-edge index (NDRE) at GS 30 were significantly positively correlated with GS 30 N uptake.  A quadratic-plateau regression model had the best fit for yield, NDVI, or NDRE response to N rate at both locations.