Because of differences in the growing season for corn across Ohio, practitioners need a method of accurately rating crop’s maturity to ensure maximum yield, quality, and adequate grain moisture at harvest. For this reason, many seed corn companies rate hybrid maturity based on “Growing Degree Days” (GDD) or “Heat Units.” Because a corn hybrid requires a specific number of GDD to reach maturity—a number that is independent of the crop’s total number of growing days—the GDD method is more accurate than the traditional days to maturity method and should be considered by today’s practitioners.
The growth of a corn plant (Figure 1) is directly related to the accumulation of heat units over time rather than the number of calendar days from planting. The GDD system provides information for estimating crop stages and phenology (e.g., tasseling, maturity), given site-specific conditions throughout the season (e.g., daily temperatures), and planting dates. This information helps practitioners plan crop management during the season.
Though GDD can be calculated in several ways, the most used method is to subtract 50 degrees Fahrenheit from the mean daily temperature. This method of calculating GDD is often called the “86/50 corn system.” This GDD calculation method is primarily used for corn in the United States. The equation used to calculate GDD and example calculations are provided below.
GDD equation: 
The GDD calculation method has two conditions for determining Tmax and Tmin values:
- If the maximum daily temperature (Tmax) is greater than 86 F, 86 is used to determine the daily average.
- If the minimum daily temperature (Tmin) is less than 50 F, 50 is used to determine the daily average.
The maximum temperature (86 F) is used because corn growth rates typically do not increase above 86 F. When temperatures exceed 86 F, plant development slows as leaf stomata close. Corn growth at the minimum daily temperature of 50 F is near zero. No further reduction in growth is expected at lower temperatures. Accounting for the maximum and minimum temperature conditions in the GDD equation is necessary to prevent a negative GDD calculation.
Examples of GDD calculations:
Tmax = 80 F, Tmin = 60 F: 
Tmax = 60 F, Tmin = 40 F*: 
*change 40 to 50 to meet minimum temperature condition
Tmax = 90 F**, Tmin = 70 F: 
**change 90 to 86 to meet maximum temperature condition
Begin calculating GDD or heat units the day after planting. GDD temperatures are calculated daily and are summed over time for a cumulative reading. Table 1 shows a timeline relating corn growth and development to GDD accumulation during the growing season.
Table 1. A Timeline for Corn Growth and Development (click image to download complete PDF).
The length of the growing season of various hybrids is directly related to their GDD requirements — long-season hybrids require more GDD to reach maturity than shorter-season hybrids. Because GDD accumulations generally increase as one moves toward southwest Ohio, adapted hybrids usually have longer relative maturity. As result, hybrids classified as a long season maturity in the northeast (i.e., fewer GDD accumulation) may exhibit a short or midseason maturity in the southwest of the state.
As with any system, the GDD approach has shortcomings. One potential issue is the difference between accumulating GDD from the planting date and the emergence date. Information on GDD requirements for specific hybrids can often be obtained from company information booklets or a seed dealership. You can also request this information from the dealer if it is not readily available. In using company information, however, confirm that company ratings are made using the 86/50 method because this is the basis for published weather reports (e.g., USDA Weekly Crop Progress & Condition Report). Most companies use the time from planting to maturity to rate hybrids. If the hybrid rating is from emergence to maturity, add 100–150 (the average GDD required for emergence) to determine the GDD from planting.
To monitor GDD accumulation during the growing season, practitioners can follow the weekly USDA Crop Progress reports for Ohio. An open-source tool, Useful to Usable (U2U), can also help monitor GDD accumulation. The tool helps to develop different scenarios; it provides county-level estimates based on historical GDDs accumulation, planting dates, relative hybrid maturities, GDDs to black layer, and historical freeze temperature dates (Spring and Fall). Be aware that U2U assumes the same GDD are needed to reach the black layer for the same hybrid, regardless of when it is planted. Under delayed planting situations, research has suggested that GDD requirements for maturity may be reduced, which is often referred to as “growing degree compression.” More information can be found in “Hybrid Maturity Decisions for Delayed Planting,” published by the Corny News Network, Purdue University).
Other tools that track crop development or identify key stages in crops may be available from industry groups. However, verifying the stage in the field before implementing management practices is still recommended to ensure that those practices are used efficiently and effectively.
The GDD system has important applications for tracking crop progress and decision-making on the farm to ensure higher corn productivity and profitability.
Original authors, 2017: Donald J. Eckert and Peter R. Thomison, Professors Emeritus, Agronomy Department and Department of Horticulture and Crop Science, The Ohio State University. This publication was originally prepared and funded through a cooperative effort of the Ohio Department of Energy and Ohio State University Extension.