Frost damage depends on corn growth stage, frost severity, U of M expert says

ST. PAUL -- Much of Minnesota's corn crop was damaged by frost on the morning of Sept. 15, according to University of Minnesota Extension corn agronomist Jeff Coulter.

For corn, a killing freeze occurs when temperatures are 32 degrees for four hours or 28 degrees for just minutes. But according to Coulter, "A frost or killing freeze can still occur above 32 degrees, especially in low and unprotected areas when there is no wind."

Symptoms of Frost Damaged CornCorn leaves are more easily damaged by frost than stalks. In addition, leaves above the ears are more susceptible than leaves below the ear. Leaves damaged by frost initially have a water-soaked appearance, are light green to gray after drying, and later turn brown. A black layer will form prematurely when kernels are killed before maturing. "Wait a few days before scouting fields to assess the impact of frost," Coulter recommends. "If the frost is not severe enough to cause premature formation of the kernel black layer, the kernels will continue to accumulate dry matter."

Yield, Moisture and Quality Frost damage to corn reduces grain and silage yields, grain test weight, and silage quality. However, yield and quality reductions depend on the crop stage when frost occurs and the severity of the frost. If leaves above the ear are damaged by frost but leaves below the ear are unharmed, then yield losses are expected to be reduced. See Extension's Crop News article for tables that provide more detail at www.extension.umn.edu/go/1079.

The dry conditions that occurred during the last four to six weeks in much of Minnesota accelerated crop maturity. Most of the corn that was planted during the first three weeks of May was beyond the half-milk kernel stage (R5.5) when damaged, and therefore grain yield losses in these fields are expected to be low. However, in regions where corn was planted in late May or early June, the corn was around the R5.75 stage (25 percent milk) when damaged.

Corn that is severely damaged by frost often has kernels that are more susceptible to cracking, grain that is less digestible, and silage that has less energy (starch) and more fiber than normal. Grain with severe frost damage and light test weight should be monitored for mycotoxins before feeding to livestock.

Typical in-field dry-down rates for corn grain in Minnesota are available from the related Crop News article found at www.extension.umn.edu/go/1079.

Find more information on Extension's website at www.extension.umn.edu/frost.