The latest weekly USDA crop progress report, based on conditions as of September 7, indicates 72% of Iowa's 2014 corn crop is in or past the dent stage with 5% of the corn acres mature. For soybeans, 26% of this year's crop has leaves turning color. Heat unit accumulation has been and continues to be slower than normal this growing season, resulting in a crop that is not maturing as early as planned. With a 50% probability of a 28-degree F frost generally occurring in mid-to-late October, there is some concern about a fall frost killing the crop before maturity is reached, especially for corn. That's the assessment of Mark Licht and Sotirios Archontoulis, Iowa State University Extension agronomists. They offer the following information and explanation of how you can assess the risk of a killing frost hitting your crop this fall.
Online tool can help you figure the odds of a killing frost
You can assess your risk of a killing frost by using the Corn GDD decision tool on the Useful to Useable website. This decision tool allows you to select a county and customize planting date, hybrid maturity and comparison year. The tool then outputs growing degree accumulation and predicted date of black layer as well as fall frost frequency and date of 50% probability of first freeze. This decision tool was used to calculate 2014 risk of frost for four different scenarios; planting a 102-day and 114-day hybrid on April 27, 2014 (Fig. 1) and planting a 102-day and 114-day hybrid on May 25, 2014 (Fig. 2). April 27 and May 25 were used because they represent when approximately 10% and 90%, respectively, of the Iowa corn acres were planted.
Figure 1. Useful to Useable Corn GDD decision tool scenarios for April 27 planting date with 102-day and 114-day hybrids. Green, yellow and red colors indicate low, moderate and high frost risk prior to reaching maturity.
Figure 2. Useful to Useable Corn GDD decision tool scenarios for May 25 planting date with 102 day and 114 day hybrids. Green, yellow and red colors indicate low, moderate and high frost risk prior to reaching maturity.
These four scenarios were run for select counties representing each of Iowa's nine crop reporting districts. These figures suggest that corn planted in April of well adapted maturities has little to no risk of a killing freeze hitting it before the corn reaches maturity. The figures also indicate that later planted corn, even at shorter maturities may be at moderate risk of a killing freeze in the northern two thirds of Iowa.
What does this mean for in-the-field grain dry down?
Corn drydown in the field is dependent on ear characteristics, date maturity is reached and weather (with temperature, humidity and wind to be the most influential). Ear characteristics that influence grain dry down are explained by Bob Nielson in Field Drydown of Mature Corn Grain. When maturity is reached later than normal not only are days suitable for drying reduced but the expected drydown rates are also reduced.
Drydown rates can range from 0% to 1% per day. Cool, wet weather delays dry down while warmer, drier conditions speed up dry down. A rule of thumb for Iowa would be 0.75% to 1% per day in September, 0.25% to 0.75% per day in October, and less than 0.25% per day in November. It should be expected that corn will reach maturity later than normal this fall. And likewise, in-the-field grain dry down rates will be lower. While it might be tempting to leave crops in the field anticipating better drying conditions this also increases the risk of field losses.