Physiological
Feekes Growth Stages in Winter Wheat Critical level for GDD (NDVI versus Yield) Large, E.C. 1954. Growth stages in cereals. Plant Pathol. 3:128129. 
According to Large (1954) cereals develop as follows via Feekes Growth Stages Stage
S T E M E X T E N S I O N H E A D I N G F L O W E R I N G (WHEAT) R I P E N I N G Reference Large, E.C. 1954. Growth stages in cereals. Plant Pathol. 3:128129.

•Exp. 222, r2= 1.305+0.0204(GDD>0), when GDD<106; plateau for r2 at 0.87 when GDD>=106

Coefficients of determination
(r2) were recorded for linear regression equations from NDVI data collected
over the entire growing season (Figure x).
This included a range in GDD>0 of 50 to
126.
These coefficients were then paired with the
number of days from planting to sensing where GDD>0.
Once the r2 values were plotted against
GDD>0, the objective was to determine the point at which r2 values were
maximized and to simultaneously determine a point where changes further into
the season were capped.
A linear plateau model was
then used to determine the point at which r2 values no longer increased or
no longer showed a benefit in terms of predicting yield.
This question can go the other direction in
that it helped to determine how early yield could be accurately predicted
and not sacrifice precision.
Using the linear plateau model, the earliest number of days from planting to sensing where GDD>0, and that would be required to accurately predict wheat grain yields was 87. It should be noted that the
data also suggests that yield should be predicted prior to reaching 112 days
(GDD>0) where after that time yield prediction was diminished.
