Forage Facts
Prussic acid is also known as hydrocyanic acid or hydrogen cynaide (HCN). Ingesting plants that producer excess cyanide causes prussic acid poisoning, Sorghums, sundangrass, sorghum-sudangrass crosses, and closely related species are most commonly associated with prussic acid poisoning. The mesophyll cells located below the epidermis contain an enzyme that under certain conditions converts dhurrin (a prussic acid precursor that most sorghums and sundangrasses contain). It is the prussic acid that is toxic to livestock.
Under normal growing conditions, the dhurrin concentration is low enough that animals can detoxify it before it causes toxicity. Dhurrin concentrations are highest in young plants, new regrowth, and following rapid regrowth after a period of stunted growth, such as rapid growth of drought-stressed plants following a rain, or regrowth following a frost or freeze. Under these conditions dhurrin concentration can be high enough to poison livestock.
Why Prussic Acid is Toxic
Once the prussic acid precursors are eaten, the rumen is an excellent medium for formation of cyanide, which is absorbed directly into the bloodstream and binds to enzymes in the cell. This cyanide complex prevents blood hemoglobin from transferring oxygen to individual body cells, and the animal dies from asphyxiation.
Prussic acid poisoning is related to the amount of forage consumed, the rate of consumption, and the animal's physiological condition, but HCN (prussic acid) levels in forages exceeding 100 parts per million on a wet weight (as is) basis are dangerous. On a dry weight basis, forages with more than 500 ppm HCN should be considered potentially toxic.
Level of prussic acid forage (dry matter basis) and potential effect on animals.
| ppm HCN | Effect on Animals |
| 0-500 | Generally safe; should not cause toxicity. |
| 500-1,000 | Potentially toxic; should not be the only source of feed. |
| 1,000 and Above | Dangerous to cattle and usually will cause death. |
Poisoning Symptoms
Prussic acid acts rapidly, often killing animals within minutes. Symptoms of poisoning include increased pulse rate and respiration, excessive salivation and foaming at the mouth, blue coloration of the lining of the mouth, difficult breathing, staggering convulsions and collapse. Death from respiratory paralysis follows shortly.
The clinical signs of prussic acid poisoning are similar to nitrate toxicity, but animals with cyanide poisoning have bright red blood that clots slowly whereas animals with nitrate have dark chocolate-colored blood. The smell of bitter almonds is often detected in animals poisoned with cyanide.
Because it occurs quickly the symptoms are usually observed too late for effective treatment. In the absence of a veterinarian, and if there is little doubt about the diagnosis, the animal can be treated with an injection of sodium nitrate and sodium thiosulfate. Sodium nitrate releases the cyanide from the cells, which binds with the sodium thiosulfate to form a nontoxic complex that is excreted. Animals still alive one to two hours after the onset of visible signs usually recover.
Avoiding Prussic Acid Poisoning
- Avoid grazing or green chopping your sorghum or sudangrass plants or new growth.
- Do not allow hungry cattle to graze where prussic acid may be a problem.
- Feed hay or grain before releasing cattle to pasture to slow intake and reduce the poisoning risk.
- Do not allow animals to graze potentially troublesome plants after light frost or after rain has ended a summer drought.
- Hay or ensile plants high in cyanide to reduce toxin levels.
- Have representative samples of any suspect forage analyzed before feeding.
Drought & Frost
Drought-stunted plants accumulate cyanide and can possess toxic levels at maturity. Freezing ruptures the plant cells and releases cyanide into the leaf tissue. After a killing frost, wait at least five days or until the frozen leaf tissue has completely dried out before grazing to allow the released HCN gas to dissipate. Prussic acid poisoning is most commonly associated with regrowth following a drought-ending rain or the first autumn frost. New growth from frosted or drought-stressed plants is palatable but can be dangerously high in cyanide.
Harvest Technique
Prussic acid concentrations are greater in fresh forage than in silage or hay because HCN is volatile and dissipates as the forage dries or ensiles. However, if the forage had extremely high cyanide content before cutting, or if the hay was not properly conditioned and cured, hazardous concentrations of prussic acid could remain. Hay or silage that likely contained high cyanide concentrations at harvest should be analyzed before it is fed.
Publication
Prussic Acid Poisoning