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Weed & Pest

Poisonous plants can affect horses, other livestock through introduction of toxic chemicals

Written by Gayle Smith

Good pasture management can go a long way in preventing horses from becoming poisoned by toxic plants.

“Through good pasture management, recognizing toxic plants and understanding the effects of toxins on animals, plant poisoning can be largely avoided,” according to Tony Knight, who works with the College of Veterinary Medicine at Colorado State University (CSU).

“Plants contain a variety of toxic compounds that help to deter herbivores and insects from eating them,” he continued. “A classic example of this is milkweed that contains a milky sap that is an irritant and therefore distasteful. It is also poisonous.”

Plant poisonings not only causes death loss but can also cause economic and reproductive losses.

Producers also incur additional expenses associated with spraying herbicide to control these plants, he said.

Species susceptibility

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

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

Some plants, like locoweed, are poisonous because they have developed a mutually beneficial relationship with specific fungi that, when growing in the plant, produce a toxic alkaloid poisonous to horses and livestock, Knight said.

“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,” he explained. “Livestock poisoning occurs most often when rangeland is overgrazed, and animals are forced to eat whatever is available. Feeding hay that is full of weeds can have similar repercussions. Make sure you know what is put up in hay.”

Plants causing sudden or acute death will do so within a few hours of the onset of clinical signs. Most of the time, animals are found dead before physical symptoms appear.

Native plants causing sudden death are water hemlock, some species of milkweed, death camas, larkspur and choke cherry.

Water hemlock

“Poison hemlock is an invasive weed that, if eaten before it matures, can also cause fatalities,” Knight stated.

Water hemlock is a perennial that grows in wet or marshy environments. It can grow four to six feet by maturity. Water hemlock has hollow, compartmentalized stems with leaves that are compound, alternate and pinnate with leaflets five to eight centimeters long and one to two centimeters wide. It has many small, white flowers.

The tuberous roots produce a highly toxic, pungent, yellowish fluid when cut, Knight explained.

A lethal dose of water hemlock roots is less than 0.5 percent body weight.

“The potential for water hemlock poisoning is high in horses and livestock grazing pastures that have marshy areas or where animals have access to stream or river banks where the plants tend to grow,” Knight explained. “Animals find water hemlock palatable, especially the new leaves in the spring.”

Sorghum hay

Consuming sorghum hay also has the potential to be toxic for some animals.

“Although horses are rarely fatally poisoned by plants containing cyanogenic compounds, horses, cattle and sheep can develop a syndrome of posterior ataxia and urinary incontinence after consuming sorghum hay for a period of weeks,” Knight said.

Poisoning causes degeneration of the white matter of the spinal cord, hind leg ataxia, tail paralysis and urinary bladder paralysis leading to incontinence. The degenerative changes in the nerves are irreversible, and fetal limb deformities have been reported in foals and calves whose dams are fed sorghums during pregnancy.

Milkweeds

Knight also encouraged producers to watch out for milkweeds. A few species of milkweed can be poisonous to livestock, horses and domestic fowl, especially when other forages are scarce or milkweeds are incorporated into hay.

Milkweeds contain a variety of toxins that affect the heart, digestive system or the nervous system. The highest concentration of cardenolides is found in milky sap, but all parts of the plant are toxic. Milkweed remains toxic when dried, so animals consuming the dried plants in hay are at risk.

As little as one kilogram of green milkweed plant material is lethal to an adult horse. Clinical signs usually start about 12 hours after consumption and consist of depression and labored, slow respiration. Horses may show colic and diarrhea.

Death Camas

There are 13 species of Death Camas in North America that are poisonous to livestock, horses and alpacas. All parts of the plant are toxic, especially new growth and bulbs.

Death results from respiratory paralysis.

Most poisonings occur in the spring because Death Camas shoots appear before grasses and other plants and are considered particularly palatable to livestock.

Locoweed

Horses should be kept away from locoweed, added Knight.

At least 2,000 species of locoweed exist in western North America. However, few are toxic.

“A few species of locoweed have long been recognized as important poisonous plants affecting horses and livestock, causing more overall economic losses to the livestock industry than any other group of plants,” Knight stated.

Horses are particularly susceptible to locoweed poisoning.

“Animals find locoweed palatable and will graze the green plants readily in the spring before grasses emerge from dormancy,” he said.

Horses exhibit the most distinctive signs of locoism, characterized by changes in normal behavior including marked depression, drowsiness, blindness, abnormal gaits, marked difficulty in walking and backing-up. Abnormal attempts at chewing food and spastic jaw movement may also be observed, Knight said.

Affected horses also show unpredictable behavior, such as rearing on their hind legs and falling over backwards, along with sudden periods of sudden and extreme excitement.

Riding loco-affected horses is dangerous.

If clinical signs are recognized early and the horse is removed from locoweed, clinical improvement will occur, but it may take several months for abnormal behavior to resolve completely, Knight noted.

Horses should not be allowed to graze locoweed in the spring when other forages are unavailable. Since locoweed is palatable and nutritious, animals will readily consume it.

Locoweed can be controlled with herbicides, but dried locoweed stems remain toxic at the end of the growing season, Knight explained.

Overgrazing or overstocking pastures will increase the potential for locoweed poisoning.

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..