Around the Farm – Critical Year for GDUs

Posted: August 14, 2019

Getting seed in the ground is only the beginning of the battle to growing a good crop. Farmers didn’t stand a chance against the monsoonariffic (yeah, I made it up) wrath of Mother Nature this spring. But now the looming thought that incessantly occupies farmers’ minds is weather – or more specifically – growing degree units (GDUs).

GDUs are the way the world of agriculture measures crop growth and development. It helps us keep track of how far along the crop has come and how much further it has to go before it reaches certain growth stages and finally full maturity.

A GDU for corn is measured by taking the daily maximum temperature (≤86°), adding it to the daily minimum temperature (≥50°), dividing it by 2 and subtracting 50. For example, today (July 29th) we had a high of 83° and low of 67° so to figure GDUs we would take (83°+67°)/ 2 – 50 = 25 GDUs. Luckily there are GDU calculators on the web so all a farmer needs to do is put in his location/planting date/maturity and the calculator will spit out how many GDUs the crop has accumulated since it was planted and will give a prediction for when it will reach physical maturity based on the given information (see chart below, maturity reached by Oct. 2).

This was a year that farmers very carefully considered which corn hybrids they put in the ground because if they planted one with a longer growing season they’d have to wait to harvest it and likely wouldn’t get their crop gathered in a timely manner. In addition to prolonging harvest, a late spring will likely pave the way to an early frost which is a terrifying prospect for farmers. This is why most midwest farmers planted hybrids with shorter-than-average growing seasons this year.

If you compare a 113-day late season corn (meaning it takes 113 days to reach physical maturity) that requires 2,760 GDUs to reach physical maturity to a 101-day early season corn that requires 2,450 GDUs you see that there’s a 310 GDU difference. When there is a delayed planting season such as the one we’ve had this year every GDU matters.