Prussic Acid Poisoning in Forages

Prussic acid, also called cyanide poisoning, is a fast-acting danger in certain forage sorghums and related plants. Awareness of when risk climbs is what keeps cattle safe.

Key points

Prussic acid is cyanide released from certain forage plants.
Frost, drought, wilting and young regrowth raise the risk.
Sorghum family forages are the usual culprits.
Do not graze risky fields until you have professional guidance.
Call the vet immediately for any sudden signs or deaths.

What prussic acid is

Some plants contain compounds that release hydrogen cyanide, known as prussic acid, when the plant tissue is damaged. Chewing, freezing, drought stress and wilting can all trigger the release. Once in the animal, cyanide stops cells from using oxygen, so death can come quickly.

The danger is in the forage itself, not in a disease. Nothing spreads from animal to animal.

The compounds are the plant's own chemistry, so the same plant can be safe one week and risky the next. That is why a rule based on calendar dates is not enough and why watching conditions on the ground matters.

Which plants are involved

The best-known examples are the sorghum family plants: sudangrass, sorghum-sudan crosses, forage sorghum, johnsongrass and shattercane. Some wild plants, such as wild cherry species and certain others, can also be a risk, especially wilted leaves.

Seed catalogs, extension publications and your agronomist can tell you which varieties are lower or higher risk. Ask before planting if you intend to graze.

Remember that fence-line plants and volunteer growth in old crop fields can look innocent. Walk fields before cattle go in and note anything unfamiliar so you can ask your extension agent to identify it.

When risk rises

New, young growth carries the most risk, especially fast regrowth after grazing or cutting. Stress events are the second driver: a killing frost, drought, and trampled or broken plants can all cause a spike. Regrowth from frosted stalks can be especially dangerous.

Weather watching is part of grazing management here. Plan so that no animal is unexpectedly grazing right after a frost or hard stress.

Young shoots and regrowth after cutting or grazing
Frost-damaged plants, whether standing or in new growth
Drought-stressed or wilted forage
Hungry cattle turned onto a risky field
Plants broken or trampled in storms

Signs in cattle

Signs often come on quickly after eating. Cattle may show rapid breathing, drooling, excitement, staggering, trembling or collapse. Some are found dead near the feed.

Because several poisonings and diseases look alike, only a veterinarian can tell what is going on. Never wait to see if the animals recover.

Ask your vet whether anyone in the area has had cases this season, since local reports help you gauge how careful to be right now.

Prevention in practice

Planning is the best tool. Ask your extension agent or nutritionist for guidance on when a field is safe to graze and how long to wait after frost or regrowth. Do not rely on rules you heard secondhand, because the answer depends on the plant and conditions.

Making hay or silage from risky forage may lower the hazard because curing and fermentation reduce the compounds, but testing and professional advice still apply. Do not feed suspect material without a plan.

Write a short field rule for your grazing plan, such as who decides when a field is open and what must be checked first. A clear owner for that decision prevents a rushed call after a cold night.

Do not turn hungry cattle onto risky forage
Stay out of fields after frost until you get guidance
Do not graze stressed or wilted growth
Rotate off before animals eat young regrowth
Keep water available and handle cattle quietly

If you suspect poisoning

Move cattle off the field calmly and call the veterinarian immediately. Time matters a great deal with this poisoning, and treatment must be done by a vet. Do not drench or force-feed animals in distress.

Record the field, the plant, the weather and how long the cattle were on it, and ask the vet whether samples should be taken. Observe all withdrawal times for treated animals.

Common mistakes

Turning cattle into a frosted sorghum field to use the feed before it is lost is a frequent and costly error. Others include grazing regrowth too soon, ignoring wild plants along fence lines, and delaying the call to the vet. Build a written rule for your farm, and make sure everyone who moves cattle knows it.

When to get help: Ask your extension agent or nutritionist when specific forages are safe to graze or feed, and call your veterinarian immediately if cattle drool, stagger or die after eating them. Any withdrawal times from treatments must be followed.

Sources and further reading

Organization homepages are linked so you can search them for current guidance on this topic.

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