Beware of Frost-damaged Forages

Sandy Smith, OSU Extension Educator ANR, Carroll County
(Previously published in Farm and Dairy: October 25, 2018)

Last week, we experienced our first frosts of the season in some areas of Ohio, but I don’t think anyone has experienced the real killing frost yet.

When some forages freeze, changes in their metabolism and composition can be toxic to ruminant livestock. The two problems that can occur are prussic acid (hydrogen cyanide) poisoning and bloat.

Beware of poison

First, I want to write about prussic acid or cyanide poisoning. Sorghum-related plants such as grain sorghum, sorghum-Sudan grass and Sudan grass varieties can contain toxic levels of cyanide after a frost. Johnsongrass, black cherry and elderberry can also develop toxic levels of prussic acid after a killing frost.

Light frost can stress plants, but do not kill them entirely can also cause cyanide poisoning.

The following is what happens when ruminants consume forages with high levels of cyanide-producing compounds. The prussic acid is absorbed into the bloodstream, where it binds with the hemoglobin and interferes with oxygen transfer within the body.

The acid acts quickly, killing the animals by asphyxiation within minutes. Symptoms of prussic acid poisoning include excess salivation, difficult breathing, staggering, convulsions and collapse.

Ruminants are more susceptible to the poisoning than other species, such as horses or swine, because cud chewing and rumen bacteria increase the release of cyanide.

Take precautions

Some prussic acid poisoning precautions when grazing include:

  • Grazing should be avoided on nights when frost is most likely to occur, because high levels of prussic acid produced within hours after a frost can be toxic.
  • After a killing frost, avoid grazing forages until the plants are dry. This usually takes five to seven days.
  • Do not graze wilted plants or plants with young regrowth, because these can still be toxic after a killing frost.

Beware of bloat

The second problem that can occur after a frost is pasture bloat.

Frothy bloat or legume bloat is the most common type of pasture bloat and is a concern […] after the first frost.

Consumption of forages containing high levels of soluble protein, such as alfalfa, white clover and winter wheat, can cause foam production. Pasture bloat results from the formation of foam in the rumen that minimizes the animal’s ability to expel rumen gases. This foam can cover the esophageal entrance from the reticulorumen and prevent eructation of gases.

[Ruminants] suffering from bloat swell rapidly on the left side and can die within an hour. Some early signs of discomfort in [ruminants] are animals kicking at their sides or stomping their feet before going down.

Grazing legumes following a killing frost can still cause bloat in the forage remains green and succulent. The occurrence of bloat declines as the legumes dry. About one week is usually required to dry down frost-killed legumes before the risk of bloat is reduced.

Reduce risk

Recommendations to reduce the incidence of bloat include:

  • Bloat risk is at its highest when grazing pure legume stands. Pastures containing over 50 percent grass will have a minimal danger of bloat.
  • Bloat can be reduced by supplementing grass hay to [ruminants] grazing bloat-provoking pasture.
  • The stage of legume maturity is also a factor in preventing pasture bloat. Bloat potency is highest in young plants and decreases as the plants mature.
  • Waiting to graze legumes until well after a hard freeze is a safe management practice.

Cover crops and forages are excellent pasture options for cold weather, but remember that proactive risk management can reduce or eliminate the dangers of prussic acid poisoning and bloat in your [flocks and herds].


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