Corn maturity ratings and delayed planting
Faster development of late planted corn may in part be due to the ‘effective heat units’ corn experiences when planted in the optimum period versus mid- to late-May and early-June. Typically GDUs are calculated on a daily basis and are accumulated starting at the date of planting and ending at kernel black layer formation. Black layer formation marks physiological maturity of the corn kernel because it ends the plant’s ability to put more photosynthates into the seed. The calculation for GDU in a single day is produced in three steps as follows:
Tmax = 86° F, or the maximum daily temperature between 86° F and 50° F
Tmin = 50° F, or the minimum daily temperature between 86° F and 50° F
GDU = ((Tmax + Tmin)/2) – 50 ° F
It takes an accumulation of between 2400 and 2800 GDUs across the growing season for corn to reach physiological maturity. This range in GDU response accounts for the range in relative maturity ratings, typically between 98 and 120.
Growing degree units for a single day are somewhat like the average of the day’s temperatures. However, this calculation does not consider the temperatures throughout the day, or the length of time between min and max. It just limits the calculation to the average of the minimum and maximum temperatures so long as they are between 86° F and above 50° F. Temperatures above 86° F and below 50° F are considered ‘ineffective’ for plant growth. However, there is a big difference between the effective thermal units in a typical late April day than in a typical late May day. And the emergence lag means that later planted corn is developing in a different thermal environment than its relative maturity rating was designed for.
So back to the initial question, ‘should we plant earlier maturing corn for later planting dates?’ Probably not. Early maturing corn will also have accelerated development when planted late. We need to make use of whatever growing season is left with as late maturing hybrid we can plant, including late planting acceleration of corn development. Given the Nielsen study referenced above it seems reasonable to subtract 0.25 relative maturity days per days delayed.
For example if 110 day corn was bred and selected for planting by May 1, and not planted until May 30, a delay of 30 days, then we would subtract (0.25 x 30) = 7.5 days from the maturity rating. This would result in an estimated relative maturity rating of about 103.