Prussic acid is often referred to by several other terms, including; hydrocyanic acid, hydrogen cyanide, and cyanide. It is a very poisonous gas that sometimes occurs in sorghum, sudangrass, flax, birdsfoot trefoil, Johnson grass, or wild cherry or chokecherry leaves. As it occurs in these plants, it is bound in a chemical combination that is not toxic. When the plant is damaged by cutting, wilting, crushing, or freezing, an enzyme releases the prussic acid from this combination. The rate and amount of release is dependent on many factors, so the analysis for prussic acid gives only an estimate of the potential for the plant to cause poisoning.

The following guide may be useful in interpreting the results of a prussic acid analysis. It is a guide, only, and it must be used with common sense and with consideration of the below discussion of the problem.

Prussic Acid Content (wet basis) Comment
Less than 200 ppm This feed should not cause prussic acid poisoning
200 -600 ppm  This feed may be potentially toxic so it should be restricted rate. If pastured, animals should be put only during that part of the day when they can be observed on occasion and removed if they show any signs of discomfort.
Over 600 ppm This feed is potentially very toxic so it should be fed at a very restricted rate, if at all. Drying or ensiling or allowing to mature more fully should reduce its prussic acid content.

Prussic acid poisons by interfering with the utilization of oxygen by the animal tissues, resulting a form of asphyxiation. Thus, poisoned animals at first exhibit difficulties with breathing. This followed by staggering and spasm. Death often follows, usually within an hour after the first signs are observed. The blood becomes a brilliant cherry-red color. Prussic acid poisoning is sometimes confused with nitrate poisoning because of the similarity of their clinical signs. They can be distinguished by the color of the blood which, in the case of nitrate poisoning, is a dark chocolate-brown. Chronic prussic acid poisoning has not been found to occur.

Prussic acid accumulation in plants is enhanced by drought. Concentration of the poison is the highest in the leaves, and it decreases as the plant matures. On ensiling, freezing, or drying plants, it is freed from its combined form and eventually dissipated into the air. This may leave the plants essentially free of the poison. However, for a few days following freezing, cutting for dry forage, chopping for green chop or silage, the free prussic acid content may be very high so the potential for poisoning is increased. Regrowth following the cutting of sorghum or sudangrass is often very high in its prussic acid content.

The probability of the occurrence of prussic acid poisoning can usually be reduced if not completely avoided by: 1) restriction of the use of suspected forage based on a chemical anaIysis; 2) use of low prussic acid forages, which only under very unusual conditions contain potentially toxic concentrations of the poison; 3) cutting for dry forage or for silage and allowing to cure well before use; 4) avoiding the use of regrowth or frosted suspected plant materials, except possibly, after having an analysis made.

Although affected animals can be successfully treated by a veterinarian with a mixture of nitrate and thiosulfate, the poisoning is usually observed and identified too late for the treatment to be effective.

Source: South Dakota State University, Station Biochemistry, 1980