Ag Instructor Vic Martin: The Speed Limit in Agriculture

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Great Bend Tribune
Published July 19, 2020

The six to ten day outlook (July 22 to 26) well-above normal temperatures and well-below normal precipitation so the recent rains were definitely a blessing for summer row and hay crops.  Looking out eight to fourteen days (July 24 to 30) indicates more of the same with well above normal temperatures and little in the way of rain.  The recent rains definitely helped save the milo crop if this outlook holds up. The drought monitor indicates receding dry and droughty soils conditions and that doesn’t include any rain since last Tuesday.  These recent rains and cooler temperatures are greatly helping the corn crop which is tasseling, silking and starting to develop kernels.  Also very beneficial to milo and soybean crops and the rains of the last several weeks came just in time.  Today, let’s take a look at the corn crop and why this timely rainfall and cooler temperatures came at just the right time.

The cooler than expected temperatures this past week, coupled with decent rainfall amounts for many, just as corn was pollinating and starting to develop seed was a welcome circumstance that should help corn production, both dryland and irrigated.  Naturally corn needs moisture, especially during the critical time from just before tasseling through the dough stages, but what helped just as much for many field were the cooler temperatures.  Here’s why.

A corn plant’s development is tied to temperature.  It accumulates GDUs (Growing Degree Units).  The formula is simple: GDU = (Tmax + Tmin)/2 – 50oF where Tmax is >86, use 86.  You don’t accumulate heat below fifty degrees as corn isn’t physiologically very active.  You don’t get added accumulation above eighty-six as that is about the upper limit above which the temperature is too hot and the plant slows down and goes into a protective mode.  As an example, if you high temperature was eighty-six and your low temperature was sixty-four, the plant accumulated twenty-five heat units.  The growth stage of corn is keyed to GDU accumulation.  So many DGUs are needed for each leaf, tasseling, silking, etc.  If you know how much heat the plant has accumulated, you can tell the growth stage without seeing the plant.  So why do these cooler temperatures matter?

After pollination, there are so many heat units allotted for grain fill, not days.  Warmer temperatures mean the plant has less time for grain fill, i.e. to fill the seed.  Cooler temperatures extend the number of days and allow for better grain fill.  Hotter days shorten this period.  A longer grain fill period increases yield.  The same holds true for wheat and to an extent milo.  And this holds true whether the corn is dryland or irrigated.  Perhaps the easiest way to think of this is as a road trip.  If you drive from Great Bend to Kansas City you can choose to obey the speed limit or go above it.  The further above the speed limit you go, the more quickly you reach your destination. Provided you don’t receive a ticket or become involved in an accident, it’s a good thing as you arrived faster.  However, for the corn plant, it is better to slow down and obey the speed limit or even go a little slower and enjoy the scenery.

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