How Fast Can Heat Units Accumulate?

Even emerged crops need kick-start.

Published on: Jun 3, 2008

So just how fast could this slow-starting spring season, now in late spring, turn around. Behind on heat units, measured ion growing degree days, even fields that were planted looked like they were in slow gear. How fast can thy catch up?

Agronomists measure heat unit accumulation in terms of what's called growing degree days. The system is designed to start ticking like a clock at temperatures above what should make plants grow, not just maintain themselves. It stops at temperatures so high that growth likely ceases and stress begins. By counting the number of GDDs that add up each day, corn breeders determine how many it should take for a hybrid to mature. It's a running total. So once the crop emerges, the clock starts and doesn't stop.

Obviously, CDs that occur before the crop emerges are lost to that particular field. In a normal spring in Indiana, more than 500 Gs have accumulated by May 30. If the crop is planted then, those potential heat units are lost and can't be recovered. However, Bob Nielsen at Purdue University and Peter Thomison at Ohio State University have determined that if the crop is planted late, say in the May 30-June 5 time frame, the hybrid, no matter which one it is, somehow adjusts so it doesn't need as many heat units to reach maturity. They estimate that corn planted late can reach maturity on about 200 less heat units than the same hybrid planted at the normal time.

The difference less year is that since May was cooler than normal, about 3 degrees cooler as measured at Indianapolis in central Indiana, there weren't as many GDS accumulated by May 30 as usual. So crops in the ground and growing were running behind on heat units. Part of the slow growth and yellowing noted was likely because plants simply weren't getting enough heat energy to get off and going.

What about from here on out. Here is data based on 30 years, from 1971-2000. It's reported in the Purdue University Corn & Soybean Field Guide published by the Purdue Crop Diagnostics Center.

On average, about 2500 more heat units will accumulate in central Indiana between May 30 and October 3. In southeast Indiana, that number will be closer to 2700 GDD units, but in northeast Indiana it will be less than 2350. Not only do more total units accumulate during the entire growing season in southern Indiana, but they also add up at a faster rate, especially during the heart of a typical growing season.