Prussic acid poisoning is caused by cyanide production in several types of plants under certain growing conditions. Sorghums and other plants possess a cyanogenic molecule called dhurrin in their epidermal cells. In healthy, intact leaf tissue dhurrin is nontoxic. However, mesophyll cells located beneath the epidermis have an enzyme that removes HCN from dhurrin. If the leaves become damaged, dhurrin and its hydrolyzing enzyme will intermix and release cyanide.

Once eaten, cyanide is absorbed directly into the bloodstream and binds to enzymes in the cell. This cyanide complex prevents hemoglobin from transferring oxygen to individual cells and the animal dies. Prussic acid acts rapidly, frequently killing the animal within minutes. Symptoms include: Excess salivation, Difficult breathing, Staggering, convulsions, and collapse. Animals alive one to two hours after the onset of visible signs usually recover.

Causes of High HCN in Forage crops

Plant Species:- Crop species most commonly involved with prussic acid poisoning are sorghums, Johnsongrass, and sudangrass. Potential cyanide production among varieties and hybrids of most summer annual forages varies widely. Grain sorghums are potentially more toxic than forage sorghums or sudangrass, whereas hybrid pearl millet and foxtail millet generally have very low cyanide levels.

Plant age and condition: - Young, rapidly growing plants are likely to contain high levels of prussic acid. New sorghum growth following drought or frost is dangerously high in cyanide. Pure stands of Indian grass that are grazed when the plants are less than 8 inches tall can possess lethal concentrations of cyanide. Generally, any stress condition that retards normal plant growth may increase prussic acid content. Hydrogen cyanide is released when plant leaves are physically damaged by trampling, cutting, crushing, chewing, or wilting.

Drought and Frost: - Drought-stunted plants accumulate cyanide and can possess toxic levels at maturity. Freezing ruptures the plant cells and releases cyanide. After a killing frost, wait at least four days before grazing to allow the released HCN 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 dangerously high in cyanide.
Soil Fertility: - Plants growing in soil that are high in nitrogen and low in phosphorus and potassium tend to have high cyanide concentrations. Split applications of nitrogen decrease the risk of prussic acid toxicity.

Harvest Technique:- Prussic acid concentrations are higher in fresh forage than in silage or hay because HCN is volatile and dissipates as the forage dries.

Measures to avoid HCN toxicity

1. Do not allow hungry cattle to graze where prussic acid may be a problem.

2. Do not allow animals to graze potentially troublesome plants after a light frost or after rain has ended a summer drought.

3. Always harvest the fodder crops at proper maturity stage.

4. Do not harvest the crop when it is in any type of stress.

5. Chop or ensile plants high in cyanide to reduce toxin levels.

6. Have representative samples of any suspect forage analyzed before feeding, even of silage.

About Author / Additional Info:
Senior Scientist(Agronomy) at National Dairy Research Institute, Karnal. My area of work is Forage Agronomy.