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The Heat Unit Count Is On!

The Heat Unit Count Is On!

It's what accumulates after the crop is planted that matters most.

One year ago the imaginary clock that accumulates growing degree days once corn is planted didn't start for weeks. Hundreds of potential growing degree days that could have been captured by plants and used to make food for plants slipped by while soggy soils kept seed in bags and boxes.

This year the clock has already started running on many locations. Once a site is planted, then growing degree days begin accumulating. Hybrids are still rated by relative days to maturity by most companies. But companies also know how many growing degree days a hybrid needs to reach maturity. That's the clock that is running.

For example, if you planted corn April 18 near Oolitic, then you accumulated 20 days by April 20. Numbers vary somewhat across the state. However, compared to moisture determinations, temperature typically remains fairly constant from southern to northern Indiana. Some season bias is built in for the areas that are further south.

Growing degree days is simply a measure of how much solar energy plants have access to in a given period. It utilizes temperature to estimate how fast a plant might grow. The system used officially by Purdue University Extension agronomist sis the 86/50 system. If it's hotter than 86 or cooler than 50, it's assumed that no growth is occurring.

A hybrid typically takes 2,500 to 2,700 growing degree days to mature. The reason last year's crop couldn't catch up after a late start is because temperatures were cool in July. There was an abundance of days where the maximum number of growing degree days that would have favored corn growth were not reached. In the final analysis, the result was corn that developed and yielded well, but which matured more slowly. That's why dryer sales are a hot ticket item now.

The only saving grace a year ago was that both Bob Nielsen at Purdue and Peter Thomison, his counterpart at Ohio State University, have determined that when corn is planted late, it automatically adjusts and reduces the number of days required to reach maturity. Often the change can be as much as 200 growing degree days. So if hybrid A usually needs 2,500 days, it may only need 2,300 days if planted in very late May or early June.

However, typically yield is also affected. That wasn't true last year, when yields were high even though corn was planted late.

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