Frost and Potential Yield Losses

According the October 4, 2009 edition of the Crop and Weather Report, about 86% of the corn crop was mature. The first occurrence of a fall freeze is normally mid to late October in Kentucky. A freeze event on corn that is not mature can reduce yields, depending on the stage of kernel fill.

According to research conducted by Drs. TeKrony and Egli at the University of Kentucky, a freeze event at 1/4 milk line will cause about a 40 to 60% yield loss. A freeze event at 1/2 milk line (see image) will cause about 5 to 10% yield loss, while a freeze event at 3/4 milk line will cause virtually no yield loss. These measurements come from corn that matured in August and September, months that normally accumulate growing degree days (GDDs) much more quickly than October.

According to a great summary on grain fill stages by Bob Nielsen, about 280 GDDs are needed to get corn from 1/2 milk line to blacklayer. If corn were currently at 1/2 milk line, then it would take until about November 2 to reach the required GDDs in Kentucky. If corn were only at 1/4 milk line, then corn would not mature until about November 13, assuming a freeze does not occur first.

This has been a highly unusual year, and using 30-year averages to predict GDDs or first freeze events almost seems silly. Farmers can inspect corn to determine where the milk line is and make judgements on yield when and if a freeze event occurs.

Resources:
Hunter. J.L., D.M. TeKrony, D.F. Miles and D.B. Egli. 1991. Corn seed maturity indicators and their relationship to uptake of carbon-14 assimilate. Crop Sci. 31: 1309-1313.