The first killing frost of the fall nipped corn, soybeans and forage crops in areas of the northern Corn Belt this past weekend. As long as corn and soybeans have reached maturity, you have nothing to worry about with those two crops. Yields won’t be hurt. But with certain forage crops — if you are grazing cattle — a hard frost can cause problems for the livestock.

“You’ll need to take some precautions if forage crops are hit by a killing frost and you are grazing them,” says Brian Lang, Iowa State University Extension field agronomist in northeast Iowa. He provides the following information and guidelines for managing crops that have been struck by the first hard frost in the fall.

How cold must it get to be a killing frost?
How low does the temperature have to go to kill a crop, or to cause frost-related problems? Generally, it’s 28 degrees F or below for corn and soybeans. And whether or not that causes problems depends on whether or not the crop had reached maturity before the frost occurred. For forages, the answer is a little more complicated.

For corn and soybeans, a 28-degree temperature is considered a killing frost for these crops. Lang believes this year’s corn and soybean crops in northeast Iowa were mature before frost hit this fall, so no worries there for corn and beans. “The killing frost should also kill the annual weeds, so maybe harvesting this fall will run smoother as the combine moves through the field,” he notes.

What about grain sorghum, forages, sorghum-sudan and sudangrass?
A 28-degree temperature is considered a killing frost for these crops. “However, you need to be cautious with these forages if you are feeding them even with a light frost,” warns Lang.

Prussic acid accumulates in the frosted tissue within a few hours after thawing and wilting, he says. A light frost may damage just the tops of plants. If this occurs, you should delay grazing or harvesting by a few days after frost. That will allow the prussic acid to dissipate from the plant tops.

When can livestock be returned to graze?
Livestock can be returned to graze a field of frost-injured sudangrass (18 inches or taller) and sorghum-sudan (28 inches or taller) once the frost-damaged parts of the forage dry out, usually in seven to 10 days, says Lang. Sometimes a "light" frost enhances development of young shoots from the base of the plants. If this occurs, you should delay sending livestock to graze this forage since new shoots would be high in prussic acid. 

Ideally, you wait for the new shoots to grow to a proper grazing height (sudangrass 18 inches or taller and sorghum-sudan 28 inches or taller), but more than likely you should wait for a complete killing frost. Once a complete killing frost occurs, wait until the frosted tissue is drying out (usually seven to 10 days) before grazing or harvesting the forage, he advises.

What if you are making hay, or green-chopping?
If you are haying sorghum-sudan forage, the curing process of the hay decreases the prussic acid content as much as 75%, which removes the feeding concern. However, haying these forages this late in the season is nearly impossible because of poor drydown conditions. 

If you are green-chopping the forage, chop only as much as cattle will consume in four to five hours. Never green-chop forage and let it sit on a wagon overnight. If ensiling, harvest the forage at proper moisture for your storage structure to ensure good fermentation. Good fermentation takes a minimum of four weeks. The fermentation process will reduce the prussic acid content. Since immature plants can contain higher prussic acid levels, leave this forage to ferment for at least eight weeks before feeding.

Lang adds this note: Never allow horses to graze sorghums or sudangrass at any time.

For alfalfa and clovers, a killing frost is considered to be when the temperature drops to around 24 degrees for a few hours, he explains. However, a frosty morning can increase the chance of bloat. A 26 degree to maybe 28 degree temperature can cause some frost damage. Combine that with heavy morning dew and you have a short-term risk of a higher bloat potential. For an explanation of this and suggested management, Lang recommends reading the following article:

“Alfalfa is not bloat-safe after a killing frost” by Dennis Cosgrove, University of Wisconsin River Falls; Peter Jeranyam, South Dakota State University; Dwain Meyer, North Dakota State University; Paul Peterson, University Of Minnesota; and Dan Undersander, University of Wisconsin at Madison.

Several factors are known to affect the bloat potential in any legume, says Lang. They include: 1) amount of soluble protein in the legume and possibly the type of protein, 2) presence or absence of condensed tannins, 3) the release rate of the soluble protein. 

Is alfalfa safe from causing bloat?
Alfalfa has a reputation of being bloat-safe after a killing frost. However, as long as the alfalfa remains green and succulent, there is a risk of bloat, he notes. In fact, the first frost ruptures plant cells producing small plant cell wall fragments and increasing the amount of potassium, calcium and magnesium all of which can increase risk of bloat. Not until standing alfalfa forage actually dries substantially does bloat incidence decline. At least one week is usually required to dry down frost-killed alfalfa before bloat risk is reduced.

“Bloat is not a concern in alfalfa that has been field-cured for baling,” he says. “The claim that the risk of bloat may be reduced by waiting until dew is off the alfalfa before allowing cattle to graze has been substantiated by several research studies. However, the claim that creeping-rooted alfalfa is bloat-safe is unfounded.”

Stage of maturity key to prevent pasture bloat
The stage of alfalfa development and maturity is an important factor in preventing pasture bloat, say Lang. Bloat potency is highest at the vegetative (pre-bud stage) and decreases progressively as alfalfa plants grow to full bloom. Moving cattle to a new pasture in the afternoon reduces the risk of cattle to bloat. Pasture management systems that promote continuous and rapid clearance of the rumen are most likely to reduce the incidence of bloat. 

Grazing alfalfa plants that have been swathed and wilted is another strategy for reducing bloat. Lang says frothy bloat potential of alfalfa is increased by frost but it is lessened: 1) if alfalfa has begun to flower; 2) if cattle are moved into new pasture in the afternoon; 3) if grazing is continuous and not interrupted; 4) if bloat reducing supplementary products are used as the alfalfa plant dries.