Scott Anderson – County Extension Agent – Brownwood,Tx

I am receiving some reports where cattle have died soon after being put into new pastures. The most likely cause of death was prussic acid poisoning. Many forages, especially sudan type, produce cyanogenic glucosides (prussic acid) as they grow. Glucosides are sugar compounds that break down in the rumen, freeing the cyanide from the sugar and forming hydrocyanic acid. Hydrocyanic acid (HCN) is commonly known as cyanide. The HCN combines with hemoglobin to form cyanoglobin, which does not carry oxygen. Livestock poisoned by cyanide have respiratory stress similar to that caused by nitrate poisoning. A blood test can quickly distinguish between nitrate and prussic acid poisoning. If prussic acid is the toxic agent the blood will be cherry red, unlike the chocolate brown blood seen in nitrate poisoning. Horses, hogs and other nonruminant animals are less affected by prussic acid because their stomachs convert the prussic acid to less toxic formic acid and ammonium chloride.

Prussic acid poisoning can be treated effectively if the treatment is administered immediately after the first poisoning symptoms appear. Two common treatments are intravenous injection (125 to 250 ml) of 1.2% sodium nitrate or 7.4% sodium thiosulfate. Before administering the sodium nitrate treatment, have a veterinarian ensure that the symptoms are caused by prussic acid rather than nitrates.

Under normal conditions, when these plants are actively growing and healthy, they contain low levels of prussic acid because the compound breaks down over time, thus eliminating toxic accumulations. Unlike nitrate, prussic acid may be present for a while an then dissipate from plants properly cured for hay.
Prussic acid accumulation can happen when:
- There are poor growing conditions that prevent stems from developing properly.
- Recent hay harvest or grazing causes slow and stunted growth of new plant tissue.
- Nitrogen fertilizers are over-used or there are other soil fertility or nutrient imbalances.
- Plants develop new growth after a prolonged drought.
- Plants are injured by herbicides, frost, hail, or other events.
Prussic acid accumulates mainly in leaves, with highest concentrations in new growth. Concentrations in leaves are many times higher than in stems. Because livestock usually eat leaves before stems, samples taken for prussic acid analysis should be largely comprised of leaves. This is especially true when sampling fields where cattle will be allowed to graze. If grazing is limited, cattle probably will not consume stems.
We commonly see prussic acid problems when cattle are turned into new pastures. Feeding cattle immediately before turning them into new pastures can help prevent prussic acid poisoning. Successfully treating cattle which show symptoms of prussic acid poisoning can be done but has to be immediate. It is recommended to consult your veterinarian before changing pastures with cattle.