Defining useful limits for spectral reflectance measures in corn

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. Most researchers have found that the accuracy of 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 the relationship between Normalized Difference Vegetative Index (NDVI) measurements and corn biomass and grain yield and to define upper and lower limits for effectively using NDVI measurements to make in-season management decisions in corn. Forage biomass and grain yield from eight field studies conducted in the Coastal Plain of Virginia in 2005 were compared to indirect measures of spectral reflectance and leaf area index (LAI). The NDVl was well correlated with vegetative forage biomass (R 2 = 0.8 1) and LAI (R-2 = 0.90) within the range 0.27 to 0.82. This range in NDVI values corresponds to 166 to 485 cumulative growing degree days (GDD), and a resultant developmental window of V5 to V9 when NDVI measurement are most useful and appropriate for making in-season management decisions for corn production.