Careful consideration should be given prior to switching to an earlier corn product. Full-season corn products for a given area typically have the highest yield potential, which may help offset the cost of drying higher moisture corn. As planting is delayed, corn product maturities come closer together. Corn generally requires 1.6 growing degree units (GDUs) less each day to reach flowering and 6.8 GDUs less each day to reach physiological maturity (black layer) as planting is delayed beyond about May 1.1 Therefore, corn planted in late May compared to an optimum date could actually take 110 to 210 fewer GDUs to reach black layer.
The yield for late-planted corn will vary greatly depending on the remainder of the growing season. The decision to switch maturities can be a difficult one because of variations in the growing seasons relative to available GDUs, first frost date, and fall drying conditions.
Table 1 provides average available growing season GDU accumulations for various planting dates to average first frost (28ºF). For example, if planting is delayed until the week of May 25 in the Galesburg area, a seed product with a GDU to black layer rating of 2800 can still be planted because its GDU to black layer rating is below the 2949 potential. Additionally, a product with a GDU to black layer rating of 2800, planted on May 25, should only require 2637 GDUs to black layer [2800 GDU Requirement - (6.8 GDUs less/day X 24 days)].
The numbers provided are based on averages and should only be used as a reference. The main reason for switching to an earlier maturity corn product is not so much for potential yield, but to reduce the risk of immature and wet grain in the fall. Quite often, the increased yield potential of full-season seed products can outweigh the increased cost of drying in the fall. Based on GDU ratings, full-season products for an area can usually be safely planted until the later part of May. Through the years, products with high yield potential remain products with high yield potential regardless of the planting date. Therefore, the decision to switch to an earlier product should not be made quickly.
The best one can do is review long term averages and future forecasts for late planting information. There have been growing seasons, such as in 2009, that GDUs were slow to accumulate as temperatures remained on the cool side throughout the growing season. However, that is usually not the case and one must use the long-term averages. As an example, from April 27 to May 15 at Rockford, an average of 6.9 GDUs accumulate per day. That increases to 15.0 per day for the period May 16 to June 15, 22.2 per day for June 16 to July 15, 23.2 per day for July 16 to August 15, and 19.2 per day for August 16 to September 15 (Table 2).
With insect protection and crop safety becoming more important with later planting, corn with traits that offer insect protection and herbicide tolerance should be considered. Additionally, even with delayed planting, it is still important to try to minimize the risk of adverse weather during critical growth stages by planting a package of products that range in GDU requirements to flowering as well as maturity. Selecting products that flower early for their maturity may help reduce the risk of damage from an early frost.