What the devil is a degree day and why do we care? Once again this season we will post accumulated degree days here throughout the growing season by crop reporting district. But to start things out, let's review why we use degree days and how we calculate them.

Living things need warmth to develop. For organisms that generate their own heat--dogs, cats, chickens, and humans, for instance--internal heat remains roughly constant throughout life, so development is consistent. Because of this, warm-blooded organisms can measure development by calendar days or months or years because they maintain fairly constant temperature.

However, nonheat-generating, "cold-blooded" organisms--insects, weeds, nematodes, etc.--depend on ambient temperatures; cold means slow growth, and warm means growth like gangbusters. Too cold and growth stops; too hot and other heat-related stresses reduce growth. Without a dormancy strategy (hibernation or aestivation, seed, pupation, etc.) if it is too hot or too cold, the organism dies. Nature is amazing by providing different organisms different ranges of temperatures to function well. That allows different organisms the ability to grow and compete in different conditions, making them adapted to different areas from one another.

Agronomy is a science that involves both managing cold-blooded organisms and guarding them against other cold-blooded organisms. We can track growth and development by understanding what temperature range applies to each species, and finding a way to measure the heat accumulated and relating it to development. This tracking system involves units of heat called degree-day.

Corn and soybean are good examples. Corn and soybean plants both start growing around 50 °F and reach a maximum growth and development rate around 86 °F. They grow twice as fast (and take half as long to advance a growth stage) at 70 °F (20 degrees above 50) than at 60 °F (10 degrees above 50). The rate change with each degree increase is not perfectly uniform from cold to hot, but it is close enough for us to use as a simple scale that we call growing degree days.

Calculating degree days

The concept of using accumulated degree days is pretty simple. In general terms, the degree days accumulated on a day represents the average temperature. Here is an example:

1. Determine the base temperature (the minimum temperature for development of the organism concerned) and the maximum temperature, if available.
2. If the low temperature is below the base, use the base temperature. This is because no development occurs below the base temperature, so the "low" stops at the base. If the high temperature is above the maximum for the species, reset it to the maximum.
3. For each day, take the adjusted high and low temperatures and average them.
4. Subtract the base temperature from the average in step 3, and that gives you the degree days accumulated that day.

Repeat this process for subsequent days, and add them together to give you accumulated degree days for that period.

Remember that each organism uses different bases and different biofixes (the start time for calculating development). Common ones we use in Iowa agriculture include:

We post accumulated base-50 degree days weekly throughout the growing season in the ICM Newsletter to follow corn and soybean development.