Improving Early Season Sidedress Nitrogen Rate Prescriptions for Corn

Corn requires the most nitrogen (N) of cereal grain crops and N supply is correlated with grain yield.  Canopy reflectance has been used to assess crop N needs and to derive optimum application rates in mid-season corn.  Canopy reflectance has not been useful for N rate determination in early season corn because of low biomass and the sensing background can interfere, or overwhelm crop canopy reflectance measures.  Widespread adoption of canopy reflectance as a basis for generating in-season corn N rates would be more likely if N rate recommendations could be made early, i.e. by the V6 growth stage.  The objectives of this research were to: i) examine the influence of soil color, soil moisture, surface crop residues, and sensor orientation on normalized difference vegetation index (NDVI) readings from corn from planting through the V6 growth stage; and ii) evaluate the effect of sensor orientation and field of view at early corn growth stages on the relationship between NDVI and corn biomass, N uptake, and chlorophyll meter readings.  Soil color, soil moisture, crop residue type, and sensor orientation influenced reflectance and these factors were much more influential when sensing plants with low biomass.  Canopy reflectance was capable of differentiating between N rates in the field and altering sensor orientation did not minimize sensing background influence or improve the ability of the sensor to distinguish plant N status.  Even when canopy reflectance detected differences in crop N status, N rate prescription based on NDVI was consistently below the profitable estimated sidedress N rate.