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)

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Nitrate Toxicity in Livestock (OSU Fact Sheet 2903)

Trimble Pocket Sensor, Greenseeker, NDVI, nitrogen