Plenty Of Heat Units During Growing Season

Totals mounted up even using formula with top cap.

Published on: Oct 11, 2012

One excuse you can't use for crops not maturing on time is a lack of heat units. Corn is obviously mature with a good portion of it already harvested, but soybeans seem to be moving ever so slowly now that it has shifted toward cooler, wetter weather. Some earlier beans are harvested, but many still have green to yellow leaves.

The My Rain Scout service we subscribed to this year recorded growing degree days from the date it started, May 10, through Oct. 2. Ten locations in central Indiana were monitored. The total growing degree days for that period ranged from 3,000 to 3,177.

NO HELP: High temperature readings in July didnt add to GDDs, they simply caused more stress to corn.
NO HELP: High temperature readings in July didn't add to GDDs, they simply caused more stress to corn.

Two things stand out. First, most corn hybrids only need 2,300 to 2,700 GDDs to reach maturity. Many were planted before May 10, and any GDDs that accumulated from planting to May 10 added to their total. So there was plenty of GDDs to move corn maturity along even if it had not had help drying up from disease and drought stress.

Second, although all but three days in July were above 90 degrees F, the GDD totals still accumulated at rapid but not outlandish rates. That's because the formula cuts off the top temperature at 86 degrees F. Above that reading, agronomists conclude extra heat doesn't help corn grow. Obviously, at some point, it becomes a detriment, as it did this year.

Finally, note the rather minimal range in growing degree days across these sites. Put on a percent basis, it's about 5%, with most falling much closer than that. This is just further proof that temperature is a more constant variable. While it may change some based on local topography or movement of weather fronts, it tends to stay rather consistent over a broad area, particularly compared to rainfall.

Soybeans are influenced more by photoperiod, but temperature can still have an impact. That's why some early-season beans matured quicker than you might expect.