Corn nitrogen (N) applications are still done on a field basis in Kentucky, according to previous crop, soil tillage management and soil drainage. Oklahoma State University has developed an optical sensor based variable rate N application system for wheat. This technology could be useful for corn production, and results from the University of Missouri indicate that the technology reduces N use while slightly increasing yields, giving a strongly positive economic margin over normal practice. Because early N deficiency can reduce yield potential that can not be regained with later N applications, early detection is critical. The initial objective was to determine just how early in the maize growth cycle a N deficiency could be detected using an active proximal sensor. A long-term monoculture corn trial, with two tillage systems (no-tillage and moldboard plow) and four at-planting N rates (0, 84, 168 and 336 kg N/ha) were used. The sensor was used to determine crop Normalized Difference Vegetative Index (NDVI) every two days, beginning at growth stage V4 and continuing until V13. A strong yield response to N was observed, in both tillage systems. Sensor NDVI at V5 was positively related to grain yield in moldboard plow corn and by V7 in no-till corn.