Last week this column addressed the effect of frost on certain forage crops. Today, let's focus on corn and frost. This article is based, to a large extent, on information prepared by Emerson Nafziger, Extension corn specialist, that appeared in the October 9 issue of the University of Illinois Pest Management & Crop Development Bulletin.

The amount of yield loss from frost coming before corn is mature will be directly related to the amount of fill the kernels have left to do at the time of the frost.

Even in fields where enough Growing Degree Days (GDD) have accumulated to reach maturity, black layer does not seem to have formed as distinctly as it usually does. In some fields, leaves died early, bringing an end to photosynthesis and cutting off the supply of sugars to the kernels before they were completely filled. As a result, the cells that make up the black layer may not die or darken normally, so the black layer may be less distinct.

The potential yield loss due to premature end to grain-filling, such as a frost in some fields, decreases fairly quickly as the milk line moves down toward the kernel tip. The plant is more efficient, in terms of GDD, at increasing kernel dry weight early in the maturation process, so potential yield loss amounts decrease with time as grain filling slows.

As a general guide, corn that is between ¾ milk line and black layer has nearly accumulated 100 percent of its total dry matter. Corn that is at ½ milk line has accumulated about 92 percent of its dry matter.

What is the effect of frost on the grain dry-down rate and grain quality of immature corn? Kernels that stop filling when they still have a milk line visible have sugars at the tip. Sugars tend to "hold on to" water more than starch, so such kernels dry more slowly. Cobs are also wetter, which translates into slower kernel drying. On the other hand, frost that kills husks tends to make them dry faster and if they loosen when they dry, such kernels will probably be drier several weeks from now than if the weather had stayed warm and they had continued to fill. Lower kernel starch density, discolored kernels, and reduced test weight may result when kernels stop filling prior to black layer.

In conclusion, Nafziger notes that in northern Illinois corn planted late (after June 1) and with later-maturing hybrids are likely to be in early dent, at which stage a frost may reduce yield by 20 to 25 percent, and a hard freeze (28 degrees F or less for several hours) may reduce yields by a third or more.

The complete article can be viewed at http://ipm.illinois.edu/bulletin/article.php?id=1236