When Does it Become Too Late for My Current Choice of Hybrids?

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• For each day past May 1^st corn needs 6.8 GDU’s less to reach black layer.
• Know the average number of GDU’s accumulated for your area from the prospective planting date to the first killing frost.
• Consider the amount of time between the killing frost date and the estimated maturity date and its impact on hybrid dry-down.
• Generally, changing hybrid choices before the first week of June reduces yield potential more than sticking with original hybrid choices appropriate for a given maturity zone.

A temptation to change crop management plans always exists in late planting seasons. Often the first thought is, “The planting season is getting late and I need to change hybrids.  If I don’t, my current choices won’t reach black-layer prior to the killing frost date in my area.”  Is this thinking correct?  This article will examine corn hybrids, planting dates and how to determine if there is a need to switch hybrids. 

Corn mainly responds to heat unit accumulation (growing degree units or GDU’s) to move through its growth stages; however, in late planting situations hybrids can adjust their maturities.  This is because corn does exhibit some sensitivity to the number of dark hours in a 24-hour period, which allows for adjustments in maturity.  Research from Purdue University found that for every day past May 1^st, a corn hybrid decreases its GDU requirements to black-layer by 6.8 GDU per day.  Refer to the table below for GDU adjustments for various GDU Black Layer requirements for hybrids sold throughout the Midwest (Table 1).

The next piece of information needed to make an informed decision is the average number of GDU’s accumulated for your location.  You can calculate this by going to the U2U  website (https://mygeohub.org/groups/u2u/purdue_gdd) (Figure 1).  Here you can choose your location on the opening map and click “create GDD (growing degree day) Graph”. You then can change a start date for calculating GDD and select the three comparison years you’d like.  Leave everything else alone and let the graph generate. 

The location for the image above is Macon County, Illinois. The tallest light blue vertical line on the graph is the killing frost date (28⁰F for two hours) and this occurs around October 25th on average.  The number of heat units accumulated on average by that date from 1981-2010 is 3134 GDU’s.  Going back to the table of values (Table 1), a hybrid usually rated 2715 GDU’s to black layer with a planting date of May 13^th would need approximately 2612-2662 GDU’s to black layer.  In this illustration, the long-term average of 1981-2010 provides 2881 GDU’s or an excess of 472-522 GDU’s, approximately 24-26 days in real-time at 20 GDU’s per day.  The first black vertical line is the actual maturing date for a hybrid requiring 2715 GDU’s, ahead of the killing frost date.

The information in this example shows that even with a planting date of May 1^3th in Macon County, Illinois, a 113-day hybrid requiring 2715 heat units to black-layer would likely reach maturity over three weeks ahead of the first killing frost to dry-down.  Experience tells us that the original hybrid choice is appropriate for the growing zone you farm in and that, based on the information shown above, changing hybrids is not necessary. Doing so could jeopardize yield at the expense of ROI per acre prior to the first full week of June in many areas.