Skip to main content

In our last post, we talked about ornamental plants that could pose a problem to your horse. This time around, we’re going to discuss some weeds that you might find in your pasture that can cause serious issues if ingested. So let’s jump right into it!

Yellow Star Thistle: This brightly colored invasive species is more common in the west, but it does show up in areas along the east coast. While the flower does not seem very palatable, if grasses are sparse, horses may ingest this plant. It may also accidentally be ingested during general grazing.

After chronic ingestion (meaning they have to eat a lot of it over a period of time), signs may begin to appear. The toxins in yellow star thistle affect a specific area of the brain which affects the horse’s ability to take in food. They are able to use their incisors to pull food into their mouth, but they cannot move it back to their molars. They will show difficulty when trying to drink and may progress to further neurologic signs, such as depression, ataxia and circling. No specific treatment is available other than supportive care. Affected horses do not recover, however they may be able to learn how to deal with their difficulty.

Hoary Alyssum: A member of the mustard family, this plant sometimes grows in older alfalfa fields. It is somewhat palatable and can become incorporated into hay when a field gets baled. This weed is grows to a maximum height of about 30″ (2 1/2 feet), and is crowned by small, white, notched flowers. The flowering stems are often branched and the leaves are ovoid in shape with smooth edges.

Ingestion of hoary alyssum can lead to several generalized symptoms that begin to arise 18-36 hours later. These include lethargy, fever and occasionally diarrhea. Stiffness of the joints and laminitis are also possible sequelae. Symptomatic and supportive care are provided for treatment and signs typically resolve within 5 days after the removal of the plant from the diet.

For more information on hoary alyssum, check out this site from the MSU Extension.

Wild Cherry: While cherry trees can be quite beautiful, especially in the spring, they contain a toxin that can be extremely deadly to horses – cyanide. Cyanide is very prevalent within leaves (especially wilted ones) and the bark of the tree. Fully dried leaves do not appear to be as much of a problem. The highest risk comes 3-4 days after a branch is cut or falls into a paddock.

Cherry trees are easily identified in the spring, when they blossom. (Just think of all the tourists that go to Washington, DC in April!) The attractive white or pink flowers are present in clusters of three to five. The oval shaped green leaves are pointed at the tip and have serrated edges along their 3-5 inch length. They surface is shiny and smooth in texture.

Signs of cyanide toxicity can occur quickly, and often times it is too late by the time a veterinarian arrives. The cyanide interacts with enzymes in the body, which prevents oxygen from entering cells. Once the tissues are deprived of oxygen, the body begins to shut down. This can lead to an elevated heart rate, increased respiratory rate, and even seizures. While there is an antidote, death can occur within minutes of ingestion of the toxic principle.

White Snakeroot: While the toxic principle is not known, we do know that it can cause some serious signs. It is mainly a problem in late summer and early fall, so start watching for it now. It is approximately 3 feet tall, with coarsely toothed, opposite leaves which are rounded at the bottom. It is topped by small, flat clusters of white flowers.

Humans can be affected by this plant if they ingest milk from an animal that was grazing on it. Just ask Abraham Lincoln’s mother! If an animal (let’s say a horse, for example) eats it, they can become weak, depressed and develop trembling and sweating. In severe cases, it can lead to recumbency.

As with many of these plants, supportive care is the mainstay of treatment for white snakeroot ingestion. Horses usually recover after removal from the source and therapy, however there is the possibility of long lasting cardiac damage.

While this is not an extensive list of all the dangerous plants, it is a few of the worst offenders that are present within our area. The incidence of horses getting sick from these plants is low – most horses will choose to eat good quality forage (grass or hay) as long as it is available. That being said , it is always a good idea to walk your pastures at least once a year and try to identify any possible problems before they become serious.

In the next installment, we’ll be talking about some less scary plants which might be scattered around your fields.