A 1,200-pound cow can be fatally poisoned by as little as 2 to 14 pounds of high-cyanide Johnsongrass. That wide range depends on how concentrated the toxin is in the plant at the time of grazing, which shifts dramatically based on weather, plant height, and growing conditions. The poison involved is hydrogen cyanide, sometimes called prussic acid, and it can kill within minutes of ingestion.
Why the Lethal Amount Varies So Much
Johnsongrass contains a compound called dhurrin that breaks down into hydrogen cyanide when the plant’s cells are damaged, whether by chewing, frost, or drought stress. The lethal dose of cyanide for cattle falls between 0.5 and 3 milligrams per kilogram of body weight. Sorghum-type grasses like Johnsongrass average around 250 milligrams of cyanide per kilogram of plant tissue, but that number can spike dramatically under stress.
This is why the range is so broad. A cow grazing tall, healthy Johnsongrass on a warm afternoon might consume far more forage without issue than one eating short, frost-damaged regrowth. The concentration of cyanide in the plant at that exact moment is what determines whether a few pounds are deadly or whether a cow can graze all day without symptoms.
Conditions That Make Johnsongrass Most Dangerous
Young, rapidly growing shoots carry the highest cyanide concentrations. New growth under 18 inches tall is considerably more toxic than mature plants. Regrowth after cutting or grazing is similarly concentrated. The leaves hold more toxin than the stems, so lush, leafy stands pose the greatest risk.
Frost is the single most dangerous trigger. When temperatures dip low enough to damage plant cells but not kill the entire plant, cyanide is released rapidly. Toxin levels can spike within hours of a frost event, making nighttime grazing especially risky during fall and early spring. After a light frost (temperatures above 28°F), the recommendation is to keep cattle off Johnsongrass pastures for a full two weeks. After a killing frost, the toxin typically dissipates within 72 hours, but the safest practice is to wait until all the plant material has turned completely dry and brown.
Drought stress works through a similar mechanism. When the plant stops growing but stays green, cyanide accumulates in the tissue. A sudden rain after drought can trigger a flush of new growth that is extremely high in toxin.
How Cyanide Poisoning Kills Cattle
Hydrogen cyanide blocks cells from using oxygen. Even though the blood is fully oxygenated, the body’s tissues cannot absorb it. The animal essentially suffocates at the cellular level while its lungs keep working. This is why one of the hallmark signs of cyanide poisoning is bright cherry-red blood, a color that persists for several hours after death. The blood stays red because the oxygen in it was never consumed.
Symptoms appear fast and escalate quickly. The first signs are excitement and muscle tremors, followed almost immediately by rapid, labored breathing. The animal goes down, gasps for breath, and may convulse. The pupils dilate and the mucous membranes turn bright pink. From the first symptoms to death can be a matter of minutes, which is why cyanide poisoning is so difficult to catch in time.
Nitrate Toxicity: The Other Risk
Johnsongrass also accumulates nitrates, which cause a different type of poisoning. While cyanide prevents cells from using oxygen, nitrates convert the oxygen-carrying molecule in blood into a form that can no longer transport oxygen. The result is similar (the animal suffocates) but the mechanism and timeline differ. Nitrate poisoning tends to develop more slowly and is more associated with hay or green-chop feeding than fresh grazing.
Forage with nitrate levels below 1,500 parts per million is generally manageable if cattle also receive supplemental energy feeds like shelled corn at a minimum of 2 pounds per head per day. Above that threshold, the risk climbs. Nitrate levels in Johnsongrass are highest in the lower stems, so raising your cutting height helps reduce exposure.
Is Johnsongrass Hay Safe?
Properly cured Johnsongrass hay poses little cyanide risk. The prussic acid breaks down during the drying process and does not persist in well-made hay. Ensiled Johnsongrass also loses its cyanide content over time. The key word is “properly cured.” Hay that was baled too wet or stored improperly may not have fully off-gassed. Nitrates, unlike cyanide, do persist in hay, so testing for nitrate levels before feeding Johnsongrass hay is still worthwhile.
What Treatment Looks Like
If you catch cyanide poisoning early enough, treatment is possible but requires immediate veterinary intervention. The standard approach involves an intravenous injection that converts some of the animal’s blood chemistry to bind and neutralize the cyanide. In some cases, an oral supplement can also help detoxify any remaining cyanide still being released in the rumen. The treatment can be repeated if the animal doesn’t respond within minutes. Realistically, though, the speed of onset means many animals are found dead before treatment is possible, especially in large pastures where cattle aren’t under constant observation.
Reducing the Risk on Your Pasture
The most practical safeguards are about timing and plant maturity. Never turn hungry cattle onto Johnsongrass pasture, because they’ll eat faster and consume more before their rumen can process the toxin. Wait until plants are well above 18 inches before grazing. Avoid grazing during or immediately after drought, frost, or any event that stunts or damages the plant.
If frost is in the forecast, pull cattle off Johnsongrass pastures before nightfall. After a light frost, wait two full weeks before returning animals to the field. After a hard killing freeze, wait until every bit of plant material is dry and brown. When in doubt, have your forage tested. County extension offices can run both prussic acid and nitrate analyses, and the small cost of testing is trivial compared to losing animals.