Death in 15 minutes: why post-drought Johnsongrass is killing cattle

Prussic acid poisoning can kill a cow within 15 minutes of it eating toxic forage, and clinical signs can show up in five. That's the blunt warning from Tim Evans, University of Missouri Extension state specialist in animal health and veterinary toxicology, aimed at producers whose drought-stressed pastures have just received rain.

The rain itself isn't the problem. The regrowth it triggers is. New shoots of Johnsongrass and other sorghum species, especially the leaves, can carry high concentrations of prussic acid precursors — hydrocyanic acid, or hydrogen cyanide — according to MU Extension livestock specialist Gene Schmitz.

In other words, the pasture that finally looks green again may be the most dangerous one on the farm.

What should producers do before turnout?

Evans's advice is simple and cheap: don't turn hungry animals onto lush regrowth of these species. Fill them first.

• Feed hay or grain supplements before turnout, so animals are less likely to gorge on toxic forage.
• Introduce suspect pastures gradually, with careful monitoring.
• Keep supplemental feeding in place for valuable, pregnant animals until the danger passes.

Schmitz gives a concrete threshold: wait until plants reach roughly 24 inches high before grazing. Prussic acid concentrations decline as plants mature, and young leaves carry the highest levels — more than the stems of older growth.

The toxin doesn't linger indefinitely. Cyanide dissipates by gassing off 10 to 14 days after a frost, or during the hay curing process. Hay made from these forages is therefore a different risk proposition entirely from fresh, young regrowth.

What does poisoning look like?

Early signs include rapid or laboured breathing that might resemble pneumonia — a misdiagnosis that could cost valuable time. Other symptoms are muscle tremors, excessive salivation, staggering, convulsions and collapse. Gums may turn bright cherry red or bluish. Pregnant cattle exposed to prussic acid can abort.

Timing varies. Evans says signs can appear within five minutes of eating affected forage and death can follow in as little as 15 minutes, though in some circumstances it takes a few hours for both to develop.

If you suspect poisoning, call a veterinarian immediately. Prompt treatment can reverse the clinical signs, Evans says — one of the few intoxications where speed genuinely changes the outcome.

Then move all animals, including seemingly unaffected ones, calmly to a different pasture or corral. Stress makes things worse. Monitor every exposed animal over the next 12 to 24 hours for symptoms taking longer to emerge.

What does this change for grazing management?

Schmitz takes the longer view. He encourages producers to maintain diverse forage mixtures and rotate livestock among pastures, rather than depending on sorghum-family species that flourish precisely when conditions turn risky.

Where regrowth looks suspect, confine animals and feed hay instead. There's a side benefit: resting pastures can improve the persistence of desirable forages in drought-stressed fields, so the cautious option is also the agronomically sensible one.

The practical checklist for the coming weeks: check sorghum-family regrowth height before turnout, fill stock with hay or grain first, and mark the date of the first frost. Once 10 to 14 days have passed post-frost, the cyanide has largely gone. Until then, the greenest field on the farm deserves a second look.

Photo: Johnsongrass, a common forage, can build up prussic acid, harmful and sometimes deadly to animals. (Photo courtesy of Tim Evans.)