Response index computed by dividing the mid-season NDVI measurement from the Non-N Limiting (90 to 120 lb N/ac preplant in wheat, and 150 to 250 lb N/ac in corn) divided by the common farmer practice where less N is applied preplant.  In the example above, the response index is determined by dividing the NDVI from the 90 lb N/ac preplant plot by the NDVI value in the 45 lb N/ac preplant.

Figure 1.  Wheat grain yield from 1971 to 2003 from long-term experiment #502, near Lahoma, OK from the check plot where no N has been applied since 1971 (0-40-60), and the non-N limiting plot that has received 100 lb N/ac every year (100-40-60).  Note that in 2002, near maximum yields were produced in the check plot, a product of an extremely warm and wet winter resulting in significant quantities of N mineralized from soil organic matter.

Definitions

INSEY                  In Season Estimated Yield = NDVI (Feekes 4 to 6)/days from planting to sensing (days with GDD>0) = YP₀

GDD                    Growing Degree Days = (Tmin + Tmax)/2 – 4.4°C

RI_NDVI                         = NDVI from plots receiving adequate but not excessive preplant N, divided by NDVI from the farmer check (specific to that field) where preplant N may or may not have been applied.

RI_HARVEST                 = Maximum observed grain yield (treatment average with N fertilizer) divided by the observed grain yield from plots where no N was applied either preplant or topdress

RI_SV                      = Estimate of RI (spatial variability), using the average NDVI value from a random sensor sample (100 m in length) collected in a farmers field + 1 standard deviation, divided by the average NDVI value minus 1 standard deviation.

YP_MAX                  = Maximum obtainable yield level for a specific environment determined by the farmer, or previously defined as a biological maximum by research agronomists for that crop, and for that region (units: Mg/ha)

YP₀                       = Predicted or potential yield based on growing conditions up to the time of sensing, that can be achieved with no additional (topdress) N fertilization  (units: Mg/ha)

YP_N                             = Predicted or potential yield that can be achieved with additional (topdress) N fertilization based on the in-season response index (RI_NDVI) (units: Mg/ha)

                             = (YP₀)/ (1/RI_NDVI)

                             = (YP₀)*(RI_NDVI)

PNG                     = Percent nitrogen in the grain estimated from multiple observations, over years, using various different wheat varieties (likely to be region specific) (units: %)

GNUP                  = Predicted grain N uptake (predicted percent N in the grain multiplied times YP_N/1000) (units: kg/ha)

FNUP                   = Predicted forage N uptake estimated from the calibrated relationship with NDVI  (units: kg/ha)

FNR                     = Fertilizer nitrogen requirement estimated by subtracting predicted forage N uptake (FNUP) from predicted grain N uptake (GNUP) (units: kg/ha)

Restrictions

                     YP_N cannot be > YP_MAX

Figure 1.  Predicted yield achievable with added N fertilization (YP_N) based on RI_NDVI as a function of predicted yield that can be achieved with no added N fertilization (YP0), where YP_N = (YP₀)/ (1/RI_NDVI).