The GDD maturity rating system is based on heat units. It is more accurate in determining hybrid maturity than the days to maturity system because growth of the corn plant is directly related to the accumulation of heat over time rather than the number of calendar days from planting. The GDD system has several advantages over the days to maturity system. The GDD system provides information for choosing hybrids that will mature reliably, given a location and planting date, allows the grower to follow the progress of the crop through the growing season, and aids in planning harvest schedules.
The GDD calculation method most commonly used for corn in the U.S. is the 86/50 cutoff method. GDD are calculated as the average daily temperature minus 50.
GDD = ((Tmax+ Tmin)/2) -50
If the maximum daily temperature (Tmax) is greater than 86 degrees Fahrenheit, 86 is used to determine the daily average. Similarly, if the minimum daily temperature (Tmin) is less than 50 degrees Fahrenheit, 50 is used to determine the daily average. The high cutoff temperature (86 degrees Fahrenheit) is used because growth rates of corn do not increase above 86 degrees Fahrenheit. Growth at the low temperature cutoff (50 degrees Fahrenheit) is already near zero, so it does not continue to slow as temperatures drop further. GDDs are calculated daily and summed over time to define thermal time for a given period of time. The cumulative GDDs associated with different vegetative and reproductive stages are shown in Table 4-4.
Each corn hybrid requires a certain number of accumulated GDDs to reach maturity. Most seed corn dealers have information on specific hybrids. Table 1-1 (see Chapter 1) lists average GDD accumulation for several Ohio locations from several dates in May to the 10 percent frost date at the particular location (late September). To monitor GDD accumulations during the growing season, the grower should follow the weekly report Ohio Crop Weather provided by the United States Department of Agriculture National Agricultural Statistics Service (NASS), available online at: www.nass.usda.gov/.
As with any system, the GDD system has several shortcomings. GDD ratings of hybrids with similar days to maturity ratings don’t always agree, especially if the hybrids are from different companies. Some seed companies start counting GDDs from the day of planting, while others begin from the day of emergence. When this occurs, similar maturity hybrids may vary by 100 to 150 GDDs―the average GDDs required for emergence. Some companies use entirely different mathematical methods to calculate GDD. Although most companies use the 86/50 cutoff method described above, others use different methods to calculate GDDs. Also, under certain delayed planting situations and stress conditions, GDD requirements for maturity may be reduced significantly (see the section on Date of Planting).