Growing degree days (GGDs), while not perfect, are a more reliable method of predicting crop and insect development than calendar days. Differing threshold temperatures and beginning accumulation dates are used to determine accumulated heat units for different crops. For example, a cool-season crop like alfalfa uses a threshold temperature of 41 degrees Fahrenheit and a starting date of March 1, whereas a warm-season crop like corn uses a threshold temperature of 50 F and the planting date as the starting date.
The threshold, or base temperature, is the temperature under which no significant crop development is expected. That is, it is assumed alfalfa does not grow at temperatures less than 41 F and corn does not grow at temperatures less than 50 F. Also, an upper-end cutoff temperature is used in the calculation. Crops don’t grow any more at temperatures greater than 86 F than they do at temperatures less than 86 F. Keep that in mind later when we talk about calculating GDDs.
There are two methods for calculating GDD. The first method is simpler and the second method involves higher level mathematics, but is more accurate, especially at cooler temperatures.
Method 1: Temperature averaging
Degree-day accumulation = [(Maximum Temperature + Minimum Temperature) / 2] – Base Temperature
There are a couple things to remember when using the temperature averaging method. Since plants don’t grow any more at temperatures greater than 86 F than they do at temperatures less than 86 F, we use 86 F as the maximum temperature for any temperature greater than 86 F. Also, negative values are recorded as zero.
Let’s try a couple examples. First, assume a high of 83 F and a low of 61 F on a given day.
Degree-day accumulation base 50 = [(83 + 61) / 2] – 50 = 22 accumulated growing degrees for that day.
Now let’s look at a cooler day, a day with a high of 57 F and a low of 33 F.
Degree-day accumulation base 50 = [(57 + 33) / 2] – 50 = -5, a negative number, so we record zero.
But wait—at least part of the day was greater than 50 F. Wouldn’t there have been some plant growth and development? Yes. As limited as it might have been, there would have been some plant growth and development, and that is the limitation of the temperature averaging method.
Method 2: Baskerville-Emin method
The Baskerville-Emin method fits a curve to the various temperature points that are greater than the base temperature, then calculates GDDs from the area under that curve. That’s a little more math than most people can do on a piece of note paper, but it does a better job of calculating heat accumulation, particularly during the beginning of the growing season when temperatures are still cool. When temperatures are still cool in spring, referencing tools such as Michigan State University Enviroweather and finding nearby weather stations can help get more accurate GDD totals.
An understanding of plant and pest development at various GDDs can be helpful in making a variety of management decisions, like cutting alfalfa and corn silage, as well as decisions on scouting for insect pests.
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