Rain and cool weather continue to interfere with planting progress across much of Missouri. Many are on edge as we wonder when we’ll get to put corn in the ground. Understandably, this leaves some extra time to examine what management changes might be made as the growing season gets shorter with each day. However, we have faced this before with good results.

Yield potential has begun to decline slightly at this point and a good dose of drying will be needed to get in the field across much of the state, leading to further declines in yield potential. But yield potential is the key phrase. Many other factors besides planting date affect yield, particularly mid- to late-season drought. Given a good growing season, the opportunity for good yields still exists, and we recommend staying with plans for corn until the end of May. At that time we can expect around a 20 percent loss in yield potential and around 40 percent loss by mid-June.

Producers typically want to act if they can and one thing that could be changed until the seed is in the ground is the relative maturity rating of the corn hybrid planted. But this may not a good idea until very late in the corn planting calendar. This is counterintuitive to many because of the way we commonly rate the growing season length of hybrid seed. Typically we use the ‘relative maturity rating’ when referring to a hybrid and corn hybrids are usually sold as something like ‘110 day corn’. Naturally this leads to the literal interpretation of these numbers as a number of days. In fact relative maturity rating does not refer to calendar days, but is an index defined relative to a standard hybrid. Further terminology is more precise on the bag than in reality at later planting dates.

A shortened growing season does not necessarily translate into the need for a lower maturity rating. Relative maturity ratings apply to corn adapted or selected for a specific region that is planted ‘on time’. Studies indicate that late planted corn matures in fewer calendar days than early planted corn of the same hybrid. Researchers in Indiana and Ohio (Nielsen et al. 2002) planted corn at three dates between late April and June. It was found that later planted corn matured an average of 9 days before earlier planted corn. This average includes both short and long season hybrids.

Corn growth and development is largely controlled by temperature. The standard unit of measure used to mark this progress is the Growing Degree Unit (GDU), sometimes called ‘growing degree days’ or ‘heat units’. An older study from Minnesota (Sutton and Stucker, 1973) found that corn planted late matured about 90 Growing Degree Units (GDU) sooner than early planted corn. Relative maturity ratings, like the 110 relative maturity rating corn example above, are calculated from measurements of GDUs in corn development in hybrid trials. Some seed companies publish the GDUs required to reach maturity directly and seed companies differ in how they report hybrid maturity statistics. However, where they are used, both relative maturity rating and GDU are given for corn planted in the optimal window (between early-April and early-May for mid Missouri). These studies indicate that no change in relative maturity rating is required if planting happens within May and so long as adapted hybrids normal to the region are used.

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.