Ohio State University Extension
Department of Horticulture and Crop Science
2021 Coffey Road, Columbus, Ohio 43210-1044

Growing Degree Days as a Method of Rating the Maturity of Corn Hybrids

AGF-101-90

Because of differences in the growing season for corn across Ohio, producers need a method of accurately rating the maturity of their crops to assure maximum yield and quality with low moisture at harvest. For this reason, many hybrid seed corn companies rate hybrid maturity on the basis of "Growing Degree Days" (GDD) or "Heat Units." Because a corn hybrid requires a specific number of GDD to reach maturity regardless of the number of days taken to accumulate them, this method is more accurate than the traditional "Days to Maturity" method and should be understood by anyone producing corn.

Though there are several ways of calculating GDD, the most used method is to subtract 50 degrees F from the mean daily temperature. The following adjustments are necessary: 1) temperatures below 50 degrees F are set at 50 degrees F, and 2) temperatures above 86 degrees F are set at 86 degrees F. This method of calculating GDD is often referred to as the (86,50) system. The Agricultural Weather Center of the National Weather Service reports GDD based on this system in the Ohio Weekly Crop and Weather Summary. Following is the formula used to calculate GGD along with some sample calculations.

Formula: GDD = (T High plus T Low) divided by 2, minus 50

Examples of GDD Calculations:

For High = 80 degrees F, Low = 60 degrees F:
GDD = 80 plus 60 divided by 2 minus 50 = 20;

For High = 60 degrees F, Low = 40 degrees F:
GDD = 60 plus 50(40) divided by 2 minus 50 = 5;

For High = 90 degrees F, Low = 70 degrees F:
GDD = 86(90) plus 70 divided by 2 minus 50 = 28

Growing Degree Days or heat units are calculated for each day starting the day after planting. The GDD accumulations for the growing season in Ohio range from less than 2500 GDD in northeastern counties to more than 3000 GDD along the Ohio River in southwestern Ohio. Growing Degree Day accumulations for specific locations in the state may be found in the following table.

The "length of growing season" of various hybrids is directly related to their GDD requirements - long-season hybrids requiring more GDD to reach maturity than shorter season ones. Since GDD accumulations per year increase as one moves southwestward across the state, the relative maturity lengths of adapted hybrids increase in the same direction (i.e., hybrids classed as long season in Wayne County will be a short or mid-season maturity in Clinton County). Mid-season hybrids require approximately 100 fewer GDD than full season hybrids to give a two-week spread in maturity between the long and mid-season selections.

Often GDD information on hybrids can be obtained from company information booklets and salesmen. In using company information, however, the buyer should be sure that company ratings are made using the (86,50) system, because this is the basis for published weather data. Most companies use the time from planting to maturity to rate hybrids. If the rating is from emergence to maturity, add 150 to get the GDD from planting.

Growing Degree Days (GDD) accumulated at various sites in Ohio from several dates in May through the 10% frost date in the Fall.

Prepared by:
Donald J. Eckert
Assoc. Professor, Agronomy Dept.

This publication was prepared and funded through a cooperative effort of the Ohio Department of Energy and Ohio State University Extension.