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Understanding Cyanide Poisoning

cyanide poisoning, grazing sorghum

Submitted by Craig Askim, Extension Agent/Agriculture and Natural Resources

Producers have been calling about grazing sorghum this fall. The following article is on cyanide poisoning, which is a concern with grazing sorghum. Nitrates can also be an issue. It is highly recommended to test forages before cattle are allowed to graze any forages this fall.

A number of common plants may accumulate large quantities of cyanogenic compounds. Sorghums and related species readily accumulate these compounds. These cyanogenic compounds are in epidermal cells (outer tissue) of the plant, while the enzymes that enable cyanide production are in the mesophyll cells (leaf tissue).

Any event that causes the plant cell to rupture, allowing the cyanogenic compound and the enzyme to combine, will produce cyanide. Plant cells can be ruptured by cutting, wilting, and freezing, drought, crushing, trampling, chewing or chopping.

Once plants containing cyanide have been consumed, the toxin rapidly enters the blood stream and is transported throughout the body of the animal. Cyanide inhibits oxygen utilization by the cells in the animal’s body. In essence, the animal suffocates.

Ruminant animals (cattle and sheep) are more susceptible to cyanide poisoning than no ruminant animals because the ruminal microorganisms have enzymes that will release cyanide in the animal’s digestive tract. Cyanide is a potent, rapidly acting poison. Signs of cyanide poisoning can occur within 15 to 20 minutes to a few hours after animals consume the toxic forage. Animals often are found dead.

Clinical signs, when noticed, occur in rapid succession. Excitement, rapid pulse and generalized muscle tremors occur initially, followed by rapid and labored breathing, staggering and collapse. Signs also may include salivation (drooling), lacrimation (runny eyes), and voiding of urine and feces. The mucous membranes are usually bright pink, and the blood will be a characteristic bright cherry red.

The plants most commonly associated with cyanide poisoning are sudangrass, Johnsongrass, sorghums and sorghum-sudangrass hybrids. Grain sorghums are potentially more toxic than forage sorghums or sudangrass. Indiangrass and chokecherry also can cause cyanide poisoning. Hybrid pearl millet and foxtail millet generally have very low levels of cyanide.

Young, rapidly growing plants generally have high levels of cyanide. Higher concentrations of cyanide are found in young leaves than in old leaves or stems. New forage growth following drought or frost is dangerously high in cyanide.

Plants grown in soils high in nitrogen but low in phosphorus and potassium tend to have high cyanide concentrations. Cyanide poisoning commonly is associated with plant regrowth following a drought-ending rain or the first autumn frost. Wait at least seven days after a killing frost before grazing to allow cyanide to dissipate.

Most livestock losses occur when hungry or stressed animals graze young sorghum growth. Do not graze new growth or regrowth in sorghum or sorghum-sudan pastures. Feeding grain or hay before turning animals into pasture may reduce the rapid intake of forage and, thus, the amount of cyanide consumed. Animals do not develop immunity to cyanide, but they can detoxify low levels of cyanide.

Fresh forages have higher concentrations of cyanide than silages or hay. However, if the forage had extremely high concentrations of cyanide before cutting or if the hay was not properly cured, dangerous levels of cyanide can remain. If you have any doubts about the level of cyanide in a forage, get suspect hays and silages analyzed before feeding.

Cattle are more susceptible to cyanide poisoning than sheep. Horses also can be affected. Keep the following guidelines in mind when feeding forages such as sorghum and sorghum-sudan hybrids:

  • Never graze sorghum less than 18 inches in height.
  • Feed hungry cattle before allowing them to graze forages that may contain high levels of cyanide.
  • Do not allow animals to graze troublesome plants after a light frost or after rain has ended a summer drought. Wait several days after a killing frost before grazing.
  • Chop or ensile plants high in cyanide to reduce toxin levels.
  • Analyze suspect forage samples before feeding.

Source: Charlie Stoltenow – Former NDSU Veterinarian

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