Nitrogen (N) responsiveness of crops can change with time since it is influenced by in-season environmental conditions. This study was conducted to determine the relationship of N responsiveness using a response index (RI) as a function of time at 5 locations over a 3-year period. The RI was plotted against days where growing degree days (GDD) were more than zero to determine the ideal stage for predicting N responsiveness. Sensor normalized difference vegetative index (NDVI) readings from a non-N limiting strip (N Rich Strip) divided by the farmer practice provides an estimate of the N responsiveness or RINDVI and that can be used to determine crop needs for mid-season N. Subplots (2.0 x 2.0 m) were established within five existing long-term trials employing a randomized complete block design at Efaw, Stillwater, Lake Carl Blackwell, Perkins and Lahoma, Oklahoma. The GreenseekerTM NDVI sensor readings were collected at various stages of growth where RINDVI was subsequently determined. At all sites, RINDVI increased with advancing stage of growth. Excluding Perkins 2005 and Stillwater 2006, the relationship between RINDVI and days where GDD>0 was positive and highly correlated. Severe moisture stress was encountered throughout the season at Perkins 2005 and Stillwater 2006. When the number of days from planting to sensing where GDD>0 (GDD=(Tmin+Tmax)/2–4.4°C) was less than 60, it is unlikely that a reliable estimate of RINDVI could be obtained since values were all small (close to 1.0), consistent with limited growth at the early stages of growth. Averaged over years and sites for all growth stages, the correlation of RINDVI and RIHarvest was positive and increased taking place up to the Feekes 7 growth stage, declining at later stages. Our results suggest that once RINDVI is collected it should be adjusted using the equation RINDVI adj=RINDVI X {1.87/(Days where GDD>0 X 0.00997)+0.5876}