Finishing the Season

Charlie Tank, Agronomy Sales, Plainview (651) 558-6710

As we approach the end of summer and look forward to fall, a big question that many producers are wondering is when will my crop mature, and more importantly, will it fully mature before the first frost?  This past spring was extra challenging with the frequent rain we had and many corn fields were planted weeks behind the normal planting window.  Physiological maturity in corn, also known as black layer, is when our corn crop has reached full maturity and is safe from frost. 

In order to predict if we will reach physiological maturity, we have to first calculate how many growing degree units (GDUs) we have accumulated thus far and then estimate how many more we will accumulate the rest of the year.  The growing degree units to black layer for our 90 to 105 day corn maturities range from 2,150 to 2,550 GDUs.  As of July 17th in Rochester, MN, we are at 1540 GDUs.  GDUs are calculated by adding the high and low temperature (the high has a ceiling of 86 degrees and the low has a floor of 50) dividing that in half and subtracting 50.  Below are a few examples I referenced from Purdue on how to calculate GDUs.

  • Example 1: If the daily maximum temperature was 80 degrees and the minimum was 55 degrees, the GDU accumulation for the day would be ((80 + 55) / 2) – 50 or 17.5 GDUs.
  • Example 2 (Illustrating the limit on daily maximums): If the daily maximum temperature was 90 degrees and the minimum was 72 degrees, the GDU accumulation for the day would be ((86 + 72) / 2) – 50 or 29 GDDs.
  • Example 3 (Illustrating the limit on daily minimums): If the daily maximum temperature was 68 degrees and the minimum was 41 degrees, the GDU accumulation for the day would be (68 + 50) / 2) – 50 or 9 GDDs.   

The average frost dates in Minnesota from 1948 to present have been October 1st for 32 degrees, October 12th for 28 degrees, and October 28th for 24 degrees.  If we use October 1st as the goal for reaching black layer, we have approximately 70 days to get there or will need to accumulate around 12 GDUs per day.  By looking at the examples, you can see the types of temperatures we will need for the next two months to achieve black layer by October 1st

We don’t know what the weather will be like throughout the rest of the summer, but we can use the GDU calculator to help estimate the timeline of when we will reach black layer.  Thanks for your business this season and we wish you all a safe and prosperous fall! 

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