How to Calculate Growing Degree Days (Simple Formula)

This time of year, corn planting progress is all over the board — some farmers are weeks away from starting, some have planted a few fields and others are done. When the time comes to monitor emergence and development, consider the role heat measurement standards called growing degree days (GDDs), or growing degree units, play in the process.  

What Are Growing Degree Days?

People think in terms of calendar days and weeks while GDDs are a more reliable method of predicting plant and pest development, according to Ken Ferrie, Farm Journal field agronomist, based in central Illinois.
 
“The amount of GDDs a specific corn hybrid requires to reach each development stage during the growing season remains constant from year to year,” he explains. “However, the amount of time that hybrid needs to accumulate those heat units can vary considerably from year to year due to planting date, field conditions, weather and temperature.”

If you don’t know the GDDs your specific hybrids require, check with your seeds rep. Most seed corn companies can readily provide you with that information.

How Many Growing Degree Days Are Needed For Emergence?

Start the process by calculating GDDs daily beginning the day after planting the crop. Typically the range of GDDs needed for corn to emerge is between 100 to 150 GDDs.

For corn, GDDs are calculated by taking the average between the high and low soil temperatures and subtracting 50 (a base temperature). 

Here is the basic equation and an example for reference:

Growing Degree Days Formula
 
GDD = [(high °F + low °F)/2]-50
If the high is above 86°F, use 86°F in the equation.
If the low is below 50°F, use 50°F in the equation.
 
Example:

Recorded high temperature: 80°F

Recorded low temperature: 60°F

Corn base temperature: 50°F

Mean temperature = 80°F + 60°F/ 2 = 70°F

70°F – 50°F = 20 GDDs

Be aware that while most companies rate hybrids based on the timing between planting and maturity, some rate their hybrids from emergence to maturity. If the latter is what you find, add 150 to get the GDDs from when you planted.

Why Bother Calculating Growing Degree Days?

Keeping track of GDDs can help you predict when corn will reach various development stages which can help you make more strategic management decisions. 

Be mindful that stress can impact and alter the development process, Ferrie notes.

“If we stress corn prior to reproductive stages, we’ll slow its growth down,” he says. “If we stress corn after the reproductive stages, we speed that development process up.” 

Insect Growing Degree Units Are Different

As a side note, Ferrie says insect heat units are calculated a bit different from GDDs for corn. 

“Insect heat units are calculated on the hour or on the half hour,” he says. “Anytime we're above the minimum stage for an insect, you're going to have some growth in that insect that will move along its development.”

Using tools such as the Cornell Climate Smart Farming Growing Degree Calculator can help you predict a crop's growth stage relative to insect and weed life cycles to make more effective management decisions.

Ferrie explains how to calculate GDDs in this three-minute video: