Recent work has shown that spectral measurements from a corn (Zea mays L.) canopy can be used to reliably predict differences in growth and nutrient status; however most researchers have found that the accuracy this assessment increases as the season progresses. In contrast, real differences upon which to base management decisions need to be measured as early in the season as possible due to the time restrictions associated with fertilizer and chemical application equipment and weather. The objectives of this research were to evaluate Normalized Difference Vegetative Index (NDVI) measurements as a predictor of corn biomass and grain yield and to define upper and lower limits for the effective use of NDVI measurements. Forage biomass and grain yield from eight field studies in the Coastal Plain of Virginia conducted in 2005 were compared to indirect measures of spectral reflectance and Leaf Area Index (LAI). The NDVI was found to be well correlated to vegetative forage biomass (R2=0.81) and LAI (R2=0.90) in a range from 0.27 to 0.82. This corresponds to 166 to 485 cumulative GDD, and a resultant developmental window of V6 to V9 when NDVI measurement are most useful and appropriate.