Corn planting across the Midwest has been delayed by persistent wet weather during the last two springs. Delays this year have been much more widespread than in 2018, and delayed planting has put the corn crop at risk of not reaching physiological maturity or black layer before the first killing frost.
We generally expect that a complete tissue killing frost will occur when temperatures dip below 28 degrees Fahrenheit during a portion of evening. Corn that experiences frost before black layer will generally have lower test weight and, depending on the level of starch accumulated at the time of the frost, may not be marketable as grain.
Because we are expecting a feed shortage due to delayed and prevented planting in many parts of Michigan, corn that is not likely to reach physiological maturity may be better suited to be harvested as corn silage.
How quickly the crop will develop will depend largely on the accumulation of heat units, commonly referred to as growing degree days (GDD) or growing degree units (GDU). These are calculated by taking the average temperature of the day (high-low/2) and subtracting out the base temperature, which is 50 F for corn.
Because corn does not generally efficiently utilize temperatures above 86 F, some models establish an 86 F upper limit for GDD calculations. Most hybrids will list the number of GDUs needed to reach silking and black layer. This is how relative maturity (RM) group information is determined by the industry. The higher the RM, the more GDDs it takes to reach black layer.
Researchers at the University of Illinois, in cooperation with other land grant institutions from around the Midwest, built an online tool that can be used to predict corn growth and development and the potential for corn to reach maturity before a killing frost occurs: Corn GDD Decision Support Tool.
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This tool uses information compiled from weather stations across the region for a 30-year period to look at average GDD accumulations and provide an estimate of when the first killing frost is likely to occur.
The Corn GDD Decision Support Tool has the potential to be very valuable to producers that planted corn late this season as the year progresses. The model updates its projections as actual weather data becomes available. It can help growers to better understand the probability that a later planted field would be better suited for harvest as grain or corn silage.
This can allow producers to make arrangements with livestock producers to get a marketing agreement in place if the grain is likely to have low test weights.
What we like about the model is that it can show where the GDDs are in comparison to the average year. If 2019 starts turning cooler than normal, the likelihood that the latest planted corn will make good quality corn grain starts to dim quickly. As of July 1, we are experiencing above normal temperatures, especially during the overnight hours. These temperatures will help the later planted crops grow quickly.
However, the 6-10 day outlook shows a return to more seasonal temperatures, and the 8-14 day outlook shows a return to below normal temperatures. This tool is only an estimate based on weather data, but it can give you a quick visual estimate of when you crop is likely to mature.
The picture of the graph is from a field planted to 96-day RM corn that emerged on 7 in southern Van Buren County. It shows the silking date (red), the earliest potential black layer dates (dashed black vertical) and the projected black layer date (solid black vertical) for corn.
The graph shows a range of GDD accumulations from 20 years (the darker gray shaded area). The dark dashed line (following the curve) is the 2019 projected GDD accumulation. The blue bars on the right side of the graph show the potential for the first killing frost dates for the area.
The Corn GDD Decision Support Tool was developed by Useful to Useable (U2U), a team made up of faculty, staff and students from nine Midwestern universities working to develop decision support tools and resources to support resilient and profitable crop production. U2U was funded by a grant from USDA. Read more detailed information on the data the model uses to generate the estimation.