Prussic acid, hydrogen cyanide or hydrocyanic acid all are terms describing the same toxin or “poison.” A number of common plants, including sudangrass, johnsongrass, sorghums and sorghum-sudangrass hybrids easily accumulate the “cyanogenic glycosides” in the epidermal or outer cells of the plant. Further inside the leaf tissue in the mesophyll cells are the enzymes needed to convert these compounds to the actual 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 prussic acid. Once consumed, the toxin goes immediately to the bloodstream and blocks a necessary step in the release of oxygen from hemoglobin in the blood to the cells. The animal essentially suffocates from lack of oxygen. Ruminants are much more susceptible to prussic acid poisoning because they have enzymes in the rumen capable of converting the cyanogenic compounds in the plant into hydrogen cyanide.

Clinical signs of cyanide poisoning can occur in as quickly as 15-20 minutes and up to a few 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 then collapse. The mucous membranes (for example, the gums) are bright pink and the blood will be a bright cherry red color. A definitive diagnosis is usually made by testing the suspect forage for high levels of cyanide. A quick test may be performed in the field on forage samples using a Cyanide Field Test Kit, available from Forage Extension. The UKVDL no longer offers cyanide analysis, but other veterinary diagnostic laboratories (for example, Texas A&M) still accept properly collected and shipped samples. It is difficult to diagnose from blood, animal tissues or rumen contents because it disappears rapidly after death. If prussic acid poisoning is suspected in a live animal, a veterinarian has treatments available that can re-establish oxygen transport at the cellular level. 

Prussic acid is most often associated with sorghums (forage and grain) and sudangrass, but many plants can be cyanogenic including:

Plants with Cyanogenic Potential

Apple                                Johnsongrass

Apricot                              Lima bean

Arrow grass                      Peach

Birdsfoot trefoil                 Poison suckleya

Bahia                                Service berry

Cherry                               Sudangrass hybrids                                                                                                                

Elderberry                         Sorghum-sudangrass hybrids

Flax                                   Shattercane

Forage sorghums             Velvet grass

Grain sorghums                Vetch seed

Hydrangea                        White clover

Indiangrass

Important Points to Remember:

1. Leaves produce much more prussic acid than stems, especially young upper leaves. New shoots often contain high concentrations of prussic acid.
   1. Never graze sorghums less than 18” in height (“knee high”) to significantly reduce the potential for poisoning.
   2. Feed hungry cattle hay or grain before allowing them to graze forages which may contain high levels of prussic acid, therefore reducing the amount of cyanide consumed.
2. Drought increases the chance for prussic acid because slowed growth and the inability of the plant to mature favors the formation of cyanogenic compounds in the leaves.
3. Frost/Freezing is especially dangerous because the plant cells actually rupture, allowing prussic acid to be released.  Do not graze until well after the entire plant and new shoots are killed and have turned brown.
   1. NEW FORAGE GROWTH FOLLOWING DROUGHT OR FROST IS DANGEROUSLY HIGH IN CYANIDE. WAIT AT LEAST 7 DAYS TO GRAZE AFTER A KILLING FROST TO ALLOW CYANIDE TO DISSIPATE.
4. Plants grown in high nitrogen soil (and low in phosphorus and potassium) tend to have more prussic acid potential.  Splitting nitrogen applications will reduce the risk of toxicity. Herbicides such as 2,4 D can also increase prussic acid for several weeks following application.
5. Chopping or ensiling plants high in prussic acid will reduce toxin levels if properly cured. However, dangerous levels of prussic acid may remain if extremely high before cutting. If in doubt, analyze suspect forages before feeding.
6. Johnsongrass and sorghum-sudangrass hybrids also have the potential for nitrate accumulation, especially during drought conditions. Nitrate tends to accumulate in the lower stem, so cutting hay very short, or overgrazing so that cattle have to eat the lower stem bases (the “stubble”) can cause more intake of nitrate and signs similar to prussic acid poisoning.