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Frost Concern Forms in Northern Corn Belt

Bryce Anderson
By  Bryce Anderson , DTN Senior Ag Meteorologist
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Upper-air forecast maps through the end of August show no cold air pooling in northern North America. But there's still the entire month of September for producer concern. (Graphic courtesy of ECMWF)

For the first time in several years, frost concern is a part of growers' thinking as the season moves into its final phases. The issue is not necessarily with early frost potential -- forecast maps do not suggest that -- but with corn and soybean maturity being delayed enough that even an average first frost might still be too early for crops to reach full maturity.

During work on an article on fall prices and supplies of propane, my colleague Russ Quinn reached out to south-central Minnesota producer and dedicated DTN Weather Station user Mark Nowak for his thoughts on the situation. Mark offered so much detail on Growing Degree Days that some of those thoughts are in this blog item as well. Here's part of what he had to say:

"We got off to a later start planting than desired, getting most of the corn planted between May 6 and 10 -- two-and-one-half weeks late. Then, in late May-early June, temperatures were cooler than we would like. So, crop progress has been behind what we normally experience most years.

About 10 days ago, I started to realize Growing Degree Units (GDUs), were lagging enough to be concerned about an early frost... I calculate GDUs and the outlook every day now.

Most of the corn we plant in south-central Minnesota needs plus or minus 2,500 GDUs to blacklayer. As of this morning (Sunday, Aug. 20), we have accumulated 1,798 GDUs from midpoint of planting dates. Using my local 15-day forecast, 177 GDUs are forecast to the end of August. Then, an average GDU accumulation for September based upon the last three years, is 467 GDUs. So, add those three statistics together and by Sept. 30, we will have 2,442 GDUs. That's just short of full maturity, known as blacklayer.

Our normal first frost date is Oct. 2. So, if we have a frost on or before, Sept. 20, based upon the GDU outlook, we will lose yield."

Thanks to Mark for those comments. The bottom line weather-wise is this: Temperature forecasts and the upper-air pattern configurations will be very closely watched over the next five weeks. We have not been in this type of circumstance for a number of years -- probably since 2009. And that was a very wet harvest, with some growers not able to finish until the following spring.

At this time, forecast models do not suggest any sort of frost threat through Labor Day, and only slight coolness and not out of the ordinary through the first half of September. But, as noted, even normal may be too early when it comes to frost affecting crops this year.


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