Frost-covered Forage Dangerous to Livestock

As the season’s first frosts approach, take steps to protect your livestock from toxins produced by frost-damaged forages.

by Dani Yokhna
During the season's first frosts, be aware that the forages your animals are grazing could produce toxic prussic acid. Photo courtesy iStockphoto/Thinkstock (
Photo courtesy iStockphoto/Thinkstock
During the season’s first frosts, be aware that the forages your animals are grazing could produce toxic prussic acid.

Fall frost is an annual concern for livestock producers because of the potential for prussic-acid poisoning, but the potential for toxicity in livestock is of wider concern this year because of drought.

The 2012 drought has been one of the worst on record, leaving many livestock producers short on hay and silage supplies. Extreme heat and dryness and the lack of substantial rainfall has left many producers looking for alternative forages they can plant to make up for the shortages, according to Mark Sulc, an Ohio State University Extension forage specialist.

As a result, some farmers have chosen to grow sudangrass, sudangrass hybrids, forage sorghums or sorghum-sudangrass crosses, which can produce excellent forage yields in a short time but are capable of becoming toxic to livestock after a frost, Sulc says. These species contain compounds called cyanogenic glucosides that convert quickly to prussic acid in freeze-damaged plant tissue.

“Animals can die within minutes if they consume forages, such as the sorghum species that contain high concentrations of prussic acid in the plant tissue soon after a frost,” Sulc says. “Ruminants are more susceptible to prussic-acid poisoning than horses or swine because cud chewing and rumen bacteria help release the cyanide from plant tissue.”

The signs of prussic-acid poisoning appear rapidly after the animal eats forage high in prussic acid because it interferes with oxygen transfer in the animal’s bloodstream, causing it to die of asphyxiation. Symptoms include staggering, labored breathing, spasms, foaming at the mouth and convulsions.

The concern is growing now since some parts of the Midwest have already experienced frost, with the larger threat for widespread frost in early October, according to Jim Noel, with the National Weather Service.

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Making hay and silage from these forages after a frost reduces the risk of poisoning, Sulc says, because prussic acid in the plant decreases during the wilting and hay-drying process. But hay or silage that isn’t properly cured and dried before bailing or ensiling should be tested for prussic acid before feeding to livestock.

“Because prussic acid is a gas, the longer the gas has to dissipate out of the plant, the less dangerous it is for livestock,” Sulc says.

Here are other tips farmers can use to avoid prussic-acid poisoning in their livestock:

  1. Don’t graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost.
  2. Don’t graze after a killing frost until plants are dry, which usually takes five to seven days.
  3. After a non-killing frost, do not allow animals to graze for two weeks because the plants usually contain high concentrations of toxic compounds.
  4. New growth may appear at the base of the plant after a non-killing frost. If this occurs, wait for a hard killing freeze, then wait another 10 to 14 days before grazing the new growth.
  5. Don’t allow hungry or stressed animals to graze young growth of species with prussic-acid potential.
  6. Graze or green chop sudangrass only after it is 18 inches tall. Sorghum-sudangrass should be 30 inches tall before grazing. Never graze immature growth.
  7. Don’t graze wilted plants or plants with young tillers.
  8. Green chopping the frost-damaged plants will lower the risk compared with grazing directly, because animals will be less likely to selectively graze damaged tissue. However, the forage can still be toxic, so feed green chop with great caution after a frost.
  9. Feed green-chopped forage within a few hours, and don’t leave it in wagons or feed bunks overnight.


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