• Before changing hybrid choices consider that for each day past May 1^st corn needs 6.8 GDU’s less to reach black layer.
• Take into consideration the average number of GDU’s accumulated for your area from the prospective planting date to the first killing frost.
• Generally, changing hybrid choices before the first week of June reduces yield more than sticking with original hybrid choices appropriate for your maturity zone.
• Consider the amount of time between the killing frost date and the estimated maturity date and its impact on hybrid dry-down.
• The three-month weather outlook indicates slightly above average to normal precipitation and temperatures for much of the corn-belt.

The temptation to change crop management plans always exists in late planting seasons. Often the first thought is that the planting season is getting late and I need to change hybrids because my current choices won’t reach black-layer prior to the killing frost date in my area.This article will examine corn hybrids, planting dates and how to determine if there is a need to change our plans.

GDU TO BLACK LAYER

While corn mainly responds to heat unit accumulation (growing degree units or GDU’s) to move through its growth stages, in late planting situations corn hybrids have the ability to adjust their maturities.The theory is that while corn is predominately heat sensitive, it does exhibit some sensitivity to the number of dark hours in a 24-hour period to make growth stage adjustments to maturity.  Bob Nielsen et. al. at Purdue University conducted research on this topic in 2002 and 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)

Table 1 (below): GDU reductions to black-layer by planting date.

GROWING DEGREE DAYS

The next piece of information needed to make an informed decision is what is the average number of GDU’s accumulated for your location.You can calculate this by going to the following link, Useful to Usable website (Image 1 below). 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.I leave everything else alone and let the graph be generated because I want to look at the 1981-2010 average by clicking on the check boxes on the top left corner of the graph.

Image 1:  This web calculator can help you estimate corn growing degree days.  Below is an example calculation using the Useful to Usable online decision tool for a 113-day hybrid in McLean Co. IL.

The location I chose for the image above is McLean County IL.The second light blue vertical line that appears on the graph is the killing frost date (28⁰F for two hours) and is October 3^rd on average.The number of heat units accumulated on average by that date from 1981-2010 is 2881 GDU’s.Going back to the table of values (Table 1), a hybrid usually rated 2700 GDU’s to black layer with a planting date of May 10^th would need approximately 2632 GDU’s to black layer.In this illustration, the long-term average of 1981-2010 provides 2881 GDU’s or an excess of 249 GDU’s, approximately 12 days in realtime at 20 GDU’s per day.While this is not a lot, the first black vertical line is the actual maturing date for a hybrid requiring 2787 GDU’s, ahead of the killing frost date.

Even though the GDU is just an example, the information above shows that even with a planting date ten days out in McLean County, IL a 113-day hybrid requiring 2715 heat units to black-layer would reach maturity nearly 2 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.

WEATHER

Additional good news!  The National Weather Service 3-month outlook for the corn-belt states (Figures 1 & 2) shows good news with normal to slightly above normal temperatures.

Figure 1: Temperature Outlook (below)

Figure 2: Precipitation Outlook

While these maps may feel intimidating at first glance here is an example on how to read them.  Illinois is tan colored for most of the state indicating a 33% probability of slightly above normal temperatures for the next 3 months.  With regards to precipitation and the map to the right, the entire state has a 33% probability of slightly above normal rain. Both of these environmental factors will make the corn crop mature at a slightly above average rate than normal depending on where you reside.