Calculating GDDs for Corn

By Troy

Growing Degree Days (GDDs) is very closely tied to the development of the various growth stages in corn.  Calculating GDDs is a mathematical approach to accurately track and predict crop development based on daily temperatures, starting the day after planting and ending when the corn reaches physiological maturity. Accumulated GDDs are called heat units.

Using degrees Fahrenheit, the formula to calculate how many heat units have accumulated each day is:

 

ffc calculating GDDs

Example: 

Maximum temp = 80 degrees, minimum temp = 68 degrees.

ffc calculating GDDs example 1

This formula is fairly straightforward. However, there are two important rules that apply to this calculation: If the maximum temperature for the day is above 86 degrees, then that value is set to 86.  Likewise, if the minimum temperature for the day is less than 50 degrees, then that value is set at 50.  This is because corn grows & develops very little when the temperature is above 90 degrees or below 50 degrees.

Example: 

Maximum temp = 90 degrees, minimum temp = 72 degrees.

ffc calculating GDDs example 2

ffc calculating GDDs example 2.2

Why is this important?

Corn plants go through a few growth stage milestones once they hit certain heat units. These milestones will vary somewhat depending on the corn plant and the latitude at which it was planted.

  • Emergence (VE): 125 heat units
  • V6: 476 heat units
  • Tassel emergence (VT): 1135 heat units
  • R2: 1660 heat units
  • Physiological maturity (R6): 2700 heat units

You can see why it’s important to talk about growing degree days when we talk about the growth stages of corn. Corn will not reach its maximum potential if it doesn’t experience warm and sunny weather conditions. We can water the plants, apply nitrogen and use other inputs to help the corn grow well, but if the weather is too cool or if the corn doesn’t get enough sun, then it probably won’t grow well. So next time you’re complaining about the heat of the summer, remember: that might mean farmers in your area will get a decent corn yield!

ABOUT TROY

Troy is formerly a high school Agriculture teacher and FFA advisor who is passionate about teaching Agronomics, Ag Science, and Plant Biology. Now the manager of the Monmouth Learning Center, Troy has led the Fantasy Farming Challenge since 2013, helping hundreds of high school students to understand the choices farmers make against the challenges of weather, disease, weed management, and insect pressure.

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