Nitrogen Use Efficiency, Nitrogen Fertilizers, NUE, Nitrogen and the EnvironmentCritical Tissue N Levels
World Nitrogen Use Efficiency for Cereal Production is 33%,
Agronomy Journal 91:357
Forage Critical Levels for Cattle Consumption  
5000 ppm NO3 (pregnant cows)

10000 ppm NO3 (calves and steers)

Nitrate Toxicity in Livestock (OSU Fact Sheet 2903)

Trimble Pocket Sensor, Greenseeker, NDVI, nitrogen