While the precipitation received across Kansas has been wonderful for filling the soil profile and thus has increased yield potentials for some, it has been too much in other areas. Across the entire state it has created significant delays in planting both irrigated and dryland corn. This late planting situation raises several considerations for producers, particularly hybrid selection with respect to maturity and potential crop insurance implications of late planting from a risk management standpoint.
Corn Hybrid Maturity
Many growers are familiar with corn hybrid maturity being expressed as days of “relative maturity (RM)” or “comparative relative maturity (CRM). These systems, in place for many years, have generally been more effective at comparing hybrid maturities within a company as opposed to across companies. Fortunately, in recent years many seed companies have started providing maturity information expressed as growing degree units (GDU’s). Some companies provide both GDU’s to silking and GDU’s to maturity.
What is a GDU?
Growing degree units or growing degree days (GDD) are a weather-based scale to measure the progression of crop phenology in thermally-driven crops such as corn. GDD or GDU’s are calculated as:
GDU = (Daily Maximum Air Temperature + Daily Minimum Air Temperature)/2 – 50
In the case of corn, when the maximum air temperature is greater than 86 degrees F then the maximum air temperature is set to 86 degrees F, as the rate of growth for corn does not increase with increasing temperature above 86 degrees F. Similarly, when the minimum air temperature is less than 50 degrees F, we use 50 as the value.
In general, it takes 90-120 GDU’s for corn to emerge, residue and soil conditions contribute a great deal of variability to this range. A 110-day hybrid typically needs around 1500 GDU’s to reach silking and 2670 GDU’s to reach black layer or physiological maturity.
Probabilities of Corn Reaching Physiological Maturity based on Location, Planting Date, and Hybrid Maturity
Using historical weather data, the probability of reaching physiological maturity before a 28 degrees F freeze can be calculated. The threshold of 28 degrees F was used as long-term weather records only report the minimum, and not the duration of any given temperature. It takes multiple hours of 32 degrees F to kill corn, but only a few minutes of temperatures at 28 degrees F.
GDU’s were totaled for each year from each of multiple planting dates to determine cumulative GDU’s. This calculation was performed across 11 planting dates and 8 maturities for 36 locations in Kansas and 12 locations in bordering states. GDU’s to physiological maturity for a given days of relative maturity were determined by averaging the GDU’s of a given relative maturity across multiple hybrids from multiple companies.
As an example, the probability table for Colby is shown below in Figure 1. This is based on weather records from 1900 through 2016. If we were to plant a 113-day hybrid on May 22, we see that the probability of reaching physiological maturity before a 28-degree F freeze is 52.4%. Switching to a 108-day hybrid would improve that probability to 80%.
Figure 1. Historical probability of reaching black layer before a 28 degree F freeze for Colby, KS, 1900-2016.
Local data is important in evaluating these probabilities as relatively short distances can result in large changes in probability of success for a given hybrid x planting date combination due to changes in elevation and rate of in-season accumulation of GDU’s. For example, again looking at a 113-day hybrid planted on May 22, while the probability at Colby is only 52.4% (red circled value), it is 91.3% at Hill City, 64 miles to the east, and 88.5% at Hoxie, a mere 33 miles to the east (Figure 2).
Figure 2. Historically probability of reaching black layer before a 28 degree F freeze for Hoxie, KS, 1939-2016.
It is important to note that these tables likely represent the “worst-case” scenario for probabilities of success. Data from the eastern Corn Belt and some preliminary data in eastern Kansas suggest that when a hybrid is planted later than its optimal planting date it takes fewer GDU’s to reach physiological maturity. Their data suggest that GDU requirements for maturity are reduced by 6.8 GDU’s for every day that planting occurs after May 1. Unfortunately, I have not been able to find data from the Great Plains to either confirm that rate or suggest another rate.
It’s reasonable to believe that corn hybrids here will also reduce their GDU requirement with later planting. To get a full view of possibilities I would recommend you look at the probabilities for the maturity of the hybrid you are considering and then also the probabilities for a hybrid that is 3-6 day shorter. This will likely give you a realistic range of potential outcomes. For example, if you are considering the probabilities for a 110 day / 2670 GDU hybrid, you should consider the range of likely scenarios to be between the values listed for that hybrid and those listed for 108 or 105-day hybrid. So for a 110 day/2670 GUD hybrid planted May 29 at Hoxie, the range of probabilities would be 87.2 to 94.9% (green circled probabilities in Figure 2).
Probability charts for additional locations can be found at www.northwest.ksu.edu/agronomy
Locations included in the analysis with tables that can be found at the link include: Alton, Atwood, Belleville, Beloit, Big Bow, Bison, Brewster, Burr Oak, Cimarron, Colby, Concordia, Ellsworth, Garden City, Goodland, Hays, Hill City, Hoxie, Johnson, Lakin, Leoti, Lincoln, Ness City, Norton, Oberlin, Phillipsburg, Plainville, Quinter, Russel Springs, Scott City, Sharon Springs, Smith Center, St. Francis, Syracuse, Tribune, Ulysses, and WaKeeney in Kansas; Arapahoe, Bonny Reservoir, Burlington, Cheyenne Wells, and Wray in Colorado; and Benkelman, Cambridge, Culbertson, Harlan County Lake, McCook, Trenton, and Red Cloud in Nebraska.
The final planting date for corn in the majority of Kansas was May 15 in southeast, May 25, through central, northeast, and east-central, and May 31for western Kansas (Figure 3). After the final planting date there is a “late planting period” that extends for 20 days after the final plant date.
Figure 3. 2019 Final Planting Date for Corn in Kansas.
For corn acres that haven’t been planted by the final planting date growers have several options:
For example, consider a grower with a dryland corn APH yield of 105 bushels per acre who has signed up for Revenue Protection coverage with a 75% coverage level. Using the spring-projected price of $3.96/bushel, this grower would have a production guarantee of 78.8 bushels per acre and a revenue guarantee per acre of $311.85 (= 105 bu./acre x 75% x $3.96/bu.). An acre of corn planted five days after the final planting date, for example, would have its production guarantee reduced 5% (1% for each late day), meaning the revenue guarantee would decline 5% from $311.85 to $296.26.
Monte Vandeveer, K-State Ag Economist, outlines further details and examples on the economic implications of late planting and prevented planting in an article found at: http://www.agmanager.info/crop-insurance/risk-management-strategies/prevented-planting-options-2019-kansas-corn-growers.
Lucas Haag, Northwest Area Agronomist, Northwest Research-Extension Center – Colby