Nitrogen Use Efficiency, Nitrogen Fertilizers, NUE, Nitrogen and the EnvironmentCalculation of Growing Degree Days and Heat Units
GDD = (Tmax + Tmin)/2 - Tbase   
   Tbase is 40F or 4.4C for Winter Wheat
   Tbase is 50F or 10C for Corn (Maize)
Pioneer Hi-Bred, GDU (their site)
How does OSU use "weather data for wheat?"
INSEY = in season estimate of yield = (NDVI / # of days from planting to sensing where GDD > 0)

Why INSEY?  Using the number of "days" where growth was possible is more robust for wheat simply because so many "days" from planting (mid October) to sensing (late February through March), do not exceed the average 40F temperature needed to trigger growth. 

Cummulative GDD have been used successfully in corn, but do not work well for winter wheat.
"Wheat Growth Day Counter"
.... counts the number of days where the average temperature was above 40F (4.4 C)
.... days where ((Tmin + Tmax)/2 - 40F) > 0

Degree-day Heat Unit Calculator (cummulative from planting to sensing)

Oklahoma:  Lower threshold of 40F and upper threshold of 86F
Iowa: Lower threshold of 50F and upper threshold of 86F
SBNRC Option 2 (Mesonet embedded algorithm)
Wheat, Feekes Growth Stages
World Nitrogen Use Efficiency for Cereal Production is 33%,
Agronomy Journal 91:357
Nitrogen Use Efficiency, Nitrogen Fertilizers,nitrogen algorithms, NUE, Nitrogen and the Environment