Jim Schwartz, CCA 
ICORN IGRONOMIST
1-800-240-0101
jschwartz@icorn.com

 Will this corn crop make it?

How much time is needed?

How much time/heat is left?

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Will this corn crop make it?

As the temperatures remain below normal (and quite pleasant for us humans) many of you may be left wondering about whether or not the corn crop will reach maturity before time runs out.  This is especially true for those of you who faced planting delays due to the wet May weather.  Let’s examine some time lines for crop development and any mitigating factors that might affect this process.  I’m going to spend most of the time discussing corn.

How much time is needed?

Below are some charts that list the amount of time required for corn to go through the various grain fill stages.  It’s interesting to note that the length of time is pretty much similar for all hybrids in the maturity range we deal with.  In other words, regardless of maturity, hybrids move through the grain fill process at roughly the same pace (real early <100 day and real late > 114 day hybrids vary more).  Maturity differences are derived mainly from the length of time required to flower. 

There is a mitigating factor to these numbers; this data was created using standard planting dates.  Most of you are probably aware that joint research conducted by Nielsen (Purdue) and Thomison (Ohio State) from 1991-1994 indicates that a corn plant adjusts its thermal requirements as planting is delayed.  Their research suggests that for each day delay from optimum planting dates (late April-early May) a hybrid’s total GDU requirement decreases by about 7 GDU’s.  So a 25 day delay in planting would result in a hybrid maturing with 175 fewer GDU’s (25 X 7 = 175).  If you figure a September day with a high of 78 and a low of 56, that would equate to roughly 10 fewer days in the fall to reach black layer.  The calculation for GDU’s is ( GDU=[78 + 56] divided by 2 =67; 67-50=17 GDUs for that particular day).  Fewer days would be saved if you figure it out using a warm August day, which is why it's better to use GDU’s instead of calendar days. 

The other interesting point uncovered in this research is that the majority of the shortened GDU requirement seemed to occur after pollination.   Somehow or some way, the corn plant senses (perhaps a type of photoperiodic response) the difference and moves through the grain fill process more quickly.  So having said all that, go back and look at the charts above and subtract some GDU’s for the later planted corn.

How much time/heat is left?

This is a little tougher to figure since every year is different.  Obviously the answer to this question depends on the weather from here on out and the timing of the first killing freeze.  The only thing we can do is look at 30-year averages.  Depending on where you live, average first frost dates (50% chance) range from the 1st week of October to the 3rd week of the same month (see attached maps).  Keep in mind that the average first frost generally refers to the first occurrence of 32-degree temperatures.  In reality this would not be a killing freeze, which typically requires 28 degrees for four consecutive hours.  Although a light frost will kill some leaf tissue and reduce a plant’s photosynthetic capacity, a killing freeze is what really hurts.  In any event, using average first frost dates we should still have the months of August and September to accumulate heat units.  Depending on where you live in our three state area, those of you in the north should expect around 1,000-1,100 more GDU’s to accumulate during that time, those in the central areas 1,100-1,200 and those in the south 1,200 –1,300 more GDU’s.  Plus you can count on a few in the first part of October.  (Here comes my lawyer-like disclaimer) Of course these numbers reflect historical averages and are no predictor of the future.  They also do not take into account things like elevation or a lake affect phenomenon.  If you go back and look at the chart, it indicates 1,200-1,300 heat units are required from silking/pollination to reach maturity, but remember that late-planted corn will require fewer GDU’s.  With all this in mind, if your corn pollinates within the next week, we have average or higher temperatures from here on out, and we don’t have an early killing freeze, corn should reach physiological maturity or black layer.

Ohio first frost date map

Indiana first frost date map

Illinois first frost date map

Of course after that, we still need good drying weather to force moisture out of the grain, but unless we have a very warm/dry fall I would expect that harvested grain moisture and drying costs will both be higher this fall.

As always, call or e-mail with any questions.

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