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Pasture management, plant recognition decreases chances of livestock poisoning

by Wyoming Livestock Roundup

When a Colorado rancher went out to check the yearling calves he had purchased the day before, he never expected to find several of them laying dead on the range. With grass and other desirable forbes in short supply, the young calves had consumed an abundance of pigweed throughout the night.

Although the plant isn’t harmful in smaller amounts, if a ruminant animal consumes large quantities, it can be poisonous. Pigweed can also be high in nitrates, which can make it hard for ruminant animals to digest. The nitrates turn into nitrites and then ammonia in the rumen, poisoning the animal.

Tony Knight, who works with the College of Veterinary Medicine and Biomedical Sciences at Colorado State University (CSU), told producers that plant poisoning can occur during any time of the year. However, with good pasture management, recognition of toxic plants and an understanding of how toxic plants effect an animal, plant poisonings can be avoided, he said during the Nebraska Grazing Conference.

Species’ susceptibility

Each species of animal has different susceptibility levels to plant poisoning, Knight said.

“Sheep and goats have more carbonic acid in their saliva, so they can consume more poisonous plants,” Knight said. “Horses have an entirely different digestive system than cattle, so they can consume feed with nitrates.”

“Livestock poisoning occurs most often when rangeland is overgrazed and animals are forced to eat whatever feed is available,” he explained.

Because of all the types of poisonous plants, Knight cautions producers to ensure hay quality.

“Feeding hay that is full of weeds can have similar repercussions,” he said. “Make sure you know what is put up in the hay.”


One common toxic plant that hay producers should watch out for is Arrow grass, which grows in irrigated grass. Arrow grass contains a form of cyanide that will remain toxic even in the hay bale.

Poison suckleya is another plant that produces cyanide and is typically found around ponds and dams. Plant-induced cyanide poisoning in horses is rare, but it is more common in cattle because they have a highly alkaline stomach that can’t process the plant enzymes to break down the hydrogen cyanide, or prussic acid.

Sorghum hay also contains cyanide. Horses, cattle and sheep can all develop posterior ataxia and urinary incontinence if they consume too much sorghum hay over several weeks, Knight said.

These syndromes are irreversible. In addition, if animals consume this hay while pregnant, their fetus can be born with limb deformities.

“There are numerous native range plants that are potentially poisonous to livestock, but rarely is an animal poisoned by eating a few mouthfuls of these plants,” Knight said.


One of the most common toxic plants is larkspur, which can cause respiratory paralysis, a staggering gait, bloat and sudden death in infected animals. While the plant can be poisonous to cattle, horses and sheep, sheep are the least susceptible to poisoning.

“Larkspur is most toxic in the spring during the pre-flowering stage,” Knight said. “Still, it takes consumption of five pounds of leaves for a 1,000-pound cow to be poisoned.”

If caught in time, the animal can be treated with the antitoxin Physostigmine or Neostigmine at 0.04 to 0.08 milligrams per kilogram of body weight intravenously.


With 370 species and counting, Knight said locoweed is the number one plant blamed for cattle poisonings by the cattle industry. Cattle who consume too much locoweed may exhibit symptoms like stumbling, abortions, crooked leg calves and low fertility in bulls. These symptoms are untreatable in cattle, but a horse may recover if it is removed from infested pastures.

“If producers see a cow eating locoweed, she needs to be removed from that pasture so she isn’t teaching other cattle to eat this plant,” Knight stated.

Producers can also control locoweed by knowing when the plant is most palatable, then rotationally grazing to safe pastures. Locoweed can also be controlled with herbicides.

“Plants like locoweed have developed a mutually beneficial relationship with specific fungi or endophytes. So when it is growing in the plant, it will produce a toxic alkaloid poisonous to horses and livestock,” he explained.

Poisonous threshold

How much of a toxic plant can a cow eat before she will be poisoned by it? Knight says this is a question he is commonly asked, and it varies by which plant they are eating.

For example, the lethal dose of water hemlock roots is less than 0.5 percent of body weight. Water hemlock is considered the most poisonous plant to all animals, Knight said.

“It doesn’t take very much. It is a very potent toxin that can enter the body through open sores, as well as consumption,” he explained. “It causes muscle paralysis, seizures and death in a matter of minutes.”

Avoiding risk

“The animals most susceptible to plant poisoning are those that are placed in a high-risk pasture,” Knight continued. “For the most part, animals are selective grazers and will avoid poisonous plants unless there is nothing else there for them to eat.”

Some plants also contain various toxic compounds that will deter animals from eating them.

“A classic example of this is milkweed, which contains a milky sap that is an irritant, and therefore distasteful. It is also poisonous,” he explained.

The toxins from this plant affect the heart and nervous system, and if it is fed through hay during the winter, it will kill the animal, he added.

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to

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