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Follow Heat Units Instead of 'Days to Maturity' This Year

Follow Heat Units Instead of 'Days to Maturity' This Year
Most experts say heat unit method is more consistent any year.

Let's assume you still have corn to plant. You've got a 106-day hybrid from Company A and a 108-day hybrid from Company B. You can be absolutely sure the 106-day hybrid will mature quicker, so you ought to plant the 108 -day hybrid first and give it a little more time, right?

It may turn out to be right, but there's no way you can be absolutely sure. In the first place, seed companies rate hybrids on days to maturity on their own systems. Not every company's system is the same. There is no standardized 'days to maturity' rating system that carries over from one company to every other company.

Then there are other issues, such as weather and how the hybrid develops. Which will tassel first? Don't automatically assume it's the 106-day hybrid. If you're curious, ask your seedsman about days to flowering in each case. When a corn plant tassels and sheds pollen can be crucial depending upon the type of weather that develops while that's happening. Of course, there's no way to predict that in advance, other than play the odds. And at this stage in the game this season, the object is to get corn in the ground and take your chances, not get too cute or fancy with trying to outguess when the hottest week of the year might be that would negatively impact pollination the most.

Most corn experts who work with corn extensively prefer to talk about growing degree days. Most companies know how many growing degree day heat units it takes each of their hybrids to mature. Growing degree days accumulate over time, starting with when the corn is planted.

The Purdue Corn & Soybean Field Guide, 2012 edition, carries a table of expected accumulation of growing degree days, based on a starting date of March 1 and running until November 1. Provided by Ken Scheerigna, Indiana Assistant State Climatologist, the data is based on the 30-year period from 1970 through 2000. Here's how you can use it. Find it in the book on page 29.

Suppose you plant May 23 in central Indiana. On average, 458 GDDs have already accumulated. Suppose your hybrid typically needs 2,750 GDDs to reach black layer, which is about 35% moisture. Will it make it?

Grant the 200 GDD reduction for planting late, found by researchers a few years ago. So you need 2,550 GDDs. Or a total for the season of 3,008 since some were already used up when you planted. According to the chart, you should reach that number on or about Oct. 1. The first 32 degree F date in the fall there is about Oct. 10-15, and the first 28 degree F day is about two weeks later. So if the season is average after May 23, the hybrid should mature easily. How dry it will be at harvest and how long you want to let it stand in the field may be a different matter.

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