Estimating corn N status during the growing season with active optical reflectance sensors can improve N management efficiency by estimating the effects of early season management practices and environmental conditions on N availability.  Utilization of crop sensor-based N management will increase if crop sensing can be performed early in the growing season as N applications can be made with traditional sidedress equipment.  A field experiment was conducted to determine the earliest growth stage that N stress occurs and can be detected with sensors.  High N reference areas (251 kg N/ha) were established with different methods – [pre-planting banded anhydrous ammonia (Pre-AA), pre-planting broadcast then incorporated urea ammonium nitrate (Pre-UAN), and post planting banded UAN (Post-UAN)] to evaluate their performance to detect N stress compared to a starter-only control (27 kg N/ha).  Total aboveground biomass was collected ten times throughout the growing season beginning at V4 and continuing through R6 for dry matter (DM) and N concentration measurements.  Crop reflectance was collected with the Greenseeker RT200 and Crop Circle ACS-210 at V4, V7, V8 and V10.  All high N methods other than Post-UAN had no effects on plant DM, N content, or crop reflectance at V4 compared to the starter-only control.  At V7, Pre-AA reduced DM and N content compared to all other treatments.  Whereas DM, N content, and crop reflectance for Pre-UAN and Post-UAN were no different than the starter-only control.  At V8, all high N reference methods increased N content equally (avg. 42%) and crop reflectance compared to the starter-only control.  At V10, all high N methods increased DM and N content equally compared to the low N control, 22% and 67% respectively while differences in crop reflectance values were also evident.  After V7, all high N reference treatments were effective in distinguishing N stress compared to the starter-only control as each sensor detected N stress.