Interpreting corn hybrid maturity ratings
The relationship between "days to maturity" and GDD ratings is close but not always exact because each is based on a different definition of "maturity", the difference being the time period between physiological and harvest maturity. If two hybrids require the same number of GDD to reach physiological maturity, but field dry at different rates, they may be assigned the same GDD hybrid rating, but different "days to maturity" ratings. Neither method is perfect, either, because of the influences of climatic conditions and plant stress on the grain maturation process.
Another "fly in the ointment" is the fact that there are no agreed upon standards within the seed industry for the application of either method for assigning relative hybrid maturities. Minor differences in methodologies among seed companies often result in the farmer's frustration in comparing maturity values among different brands of hybrids.
Exception: By law, the labeled relative maturity of commercial corn hybrids sold in Minnesota must be within three days of the actual relative maturity as determined by comparative trials conducted by the Minnesota Ag. Expt. Station.
Unfortunately, the lack of industry standardization can make it difficult for growers who need to make a hybrid maturity decision for late planting situations and want to base that decision on the remaining available GDDs. Fortunately, at least one of the larger seed corn companies clearly states that their hybrids are rated according to GDD accumulations from planting to kernel black layer. Consequently, one can compare their relative hybrid maturity ratings against their growing degree day ratings to kernel black layer and develop a mathematical formula for predicting the GDD requirement of a hybrid using its relative hybrid maturity rating (Fig. 2)
Disclaimer: Reference to any seed company in this article does not constitute an endorsement of said seed company by me or Purdue University. The public availability of said seed company's hybrid maturity ratings simply facilitates the mathematical modeling described in this article.
click image to zoomFigure 2. Relationship between company ratings for growing degree units (GDUs) from planting to kernel black layer with company ratings for relative hybrid maturity (CRMs). Adapted from data listed in "Characteristic Ratings of Pioneer® brand Corn Hybrids for 2011", Pioneer Hi-Bred, Int'l. One can use this relationship to estimate the GDDs from planting to black layer for other companies’ hybrids of similar relative maturities. For example, if the relative maturity of a hybrid is known to be comparable to a 110-day (CRM) Pioneer™ brand hybrid maturity, then Figure 1 suggests that the GDDs from planting to black layer would be approximately 2650. With this estimate in hand, growers can then begin the process of determining safe hybrid maturities for late planting situations.