The Richmond Register

October 3, 2012

Beware of cyanide (prussic acid) poisoning

By Brandon Sears
Extension Agent

RICHMOND — With the start of fall comes the risk of cyanide poisoning in ruminants.

Cyanide, prussic acid, hydrogen cyanide or hydrocyanic acid poisoning are all terms describing the same condition.

A number of common plants, including sudangrass, johnsongrass, sorghums and sorghum-sudangrass hybrids contain cyanogenic glycosides in the outer cells of the plant.

Further inside the leaf tissue are the enzymes needed to convert these compounds to the cyanide poison.

When the plant undergoes a stressful event such as cutting, wilting, freezing, drought, crushing, trampling, chewing or chopping, the plant cells rupture which allows the cyanogenic compounds and the enzymes to combine and produce hydrogen cyanide gas.

Ruminants also have microflora in the rumen capable of converting the cyanogenic compounds in the plant into cyanide.

The toxic gas goes to the bloodstream and blocks a necessary step in the release of oxygen from red blood cells.  The animal essentially dies from lack of oxygen.

Clinical signs of cyanide poisoning can occur within minutes to hours after consuming the toxic forage.

Usually, the affected animals are found dead but, if observed early, may show rapid, difficult breathing, frothing at the mouth, muscle tremors, staggering and then collapse.  The mucous membranes (such as the gums) are bright pink and the blood can be a bright cherry red color.  

It is important to recognize and avoid situations in which these forages pose a danger to livestock.

Cattle and other ruminants should only graze sorghum, sorghum hybrids or johnsongrass when the plants have reached at least 18-24 inches in height.

Do not graze plants with young tillers. Do not graze these plants during drought periods when growth is severely reduced or the plant is wilted or twisted and wait at least one week after rainfall to resume grazing.

Do not graze at night when frost is likely. Frost allows conversion to hydrogen cyanide within the plant. Do not graze for two weeks after a non-killing (greater than 28 degrees) frost.

It is best not to allow ruminants to graze after a light frost as this is an extremely dangerous time and it may be several weeks before the cyanide potential subsides. Do not graze after a killing frost until plant material is completely dry and brown.  

If a high cyanide is suspected in forages, do not graze or feed as green chop.

If cut for hay, allow at least 72 hours or longer before baling so that the cyanide will dissipate. Allow thorough drying because toxicity can be retained in cool or moist weather. Delay feeding silage 6 to 8 weeks following ensiling.

(Source: Dr. Ray Smith, UK Plant and Soil Sciences Dept., Dr. Cindy Gaskill and Dr. Michelle Arnold, UK Vet Diagnostic Lab)