In-Season Corn Nitrogen Measurement and Fertilization Via Image Analysis.

Corn (Zea mays L.) requires higher rates of nitrogen than any other major U.S. crop partly because N fertilizers are subject to loss through various mechanisms.  Consequently, corn may not receive economically optimum nitrogen rates mid-season.  Using appropriate software, hue, saturation, and brightness values of digital images can be combined in a dark green color index (DGCI), which is closely associated with leaf nitrogen concentration.  Previous research has found that digital-image analysis can be used to determine the nitrogen status of corn leaves at tasseling.  Our aim was to develop tools to allow farmers to make informed decisions regarding mid-season nitrogen applications.  Our objectives were:  (1) to develop quantitative relationships among yield, corn leaf nitrogen concentration, and DGCI measurements taken in the mid-vegetative stages of growth development; and (2) to determine the amount of nitrogen to apply to recover yield based upon DGCI measurements from 8-leaf corn plants (V8).  Commercial corn hybrids with 120 day maturity ratings were planted at four Arkansas locations in fields treated with three different nitrogen fertilizer rates (0, 84, and 168 kg N ha^-1) at emergence.  At the V8 development stage, subplots were measured for DGCI, SPAD, and leaf nitrogen concentration, then treated with one of six additional nitrogen fertilizer rates (0, 28, 56, 84, 112, 168 kg N ha^-1).  SPAD, DGCI, and leaf nitrogen concentration measurements were also taken at the tasseling stage.  DGCI, SPAD, and leaf nitrogen concentrations were found to be closely associated.  For all treatments, yield response to N rate at V8 followed a quadratic response; maximum yield for this response was estimated as the first derivative.  Combining data from all locations, we were able to determine the amount of N to apply at V8 to recover 90% and 95% of the maximum yield based upon DGCI measurements at V8.  The study is in now in its second year and analysis in ongoing.