Weeds & Plants That Are Poisonous to Horses

By: Nicole Salo

- A list of weeds and plants that are commonly found throughout Canada and the United States that are poisonous to horses and ponies. For further info and/or a list of poisonous weeds and plants that can be harmful to livestock in your area, please contact your local Ministry of Agriculture department, they can help you with any concerns and even safe ways to dispose of toxic weeds and plants.

Buttercups (Ranunculus spp.)
Buttercups are often found in run down pastures, by the road side or along the edges of pastures. This wildflowers juices (mainly the leaves and stems), if ingested cause severe blistering of the mucous membrane lining the entire gastrointestinal tract, which will be seen as bloody diarrhea. Luckily this plant is rendered harmless after cutting, and is safe to be ingested by horses if it happens to be cut into hay.

Horsetail (Equisetum arvense L.)
Horsetail (also known as Mares Tail and scouring rush) are found in boggy areas or by streams. Horsetail poisoning is very similar to poisoning by Bracken Fern due to the Thiaminase found in both plants. Horsetail poisoning occurs when Horsetail is cut into hay and fed to the horse, Horsetail also stays green into the winter months and horse may nibble on it if other forage isn’t provided. All varieties of Horsetail are poisonous to horses and ponies.

Saint-John’s Wort (Hypericum perforatum)
St. John’s Wort is often found in pastures, along the road side, edges of the woods, abandoned areas and boggy/water areas. St. John’s Wort causes photosensitization in horses, causing them to become hyper sensitive to the sun, sun burning of areas not covered by thick fur under normal exposure to the sun. Signs of ingestion include inflammation of unpigmented areas, burnt areas may become red and peel, the mouth and nose area being affected the most.

Nightshade (Solanum sp.)
Nightshade, also known as Deadly Nightshade, is found along the edges of fields and fences, along roadsides, in open woods and sometimes in hedges and gardens. Signs of Nightshade poisoning include colic like symptoms, disorientation, loss of muscle control, dilated pupils. If Nightshade poisoning is not treated it can lead to death.

Milkweed (Asclepias sp.)
Milkweed is a common pasture weed, tolerating various soil types they are found in sunny locations. Signs of Milkweed poisoning include rapid and weak pulse, loss of muscle control, disorientation and respiratory paralysis. If not treated horses can die within 1-3 days of consuming even a small amount of Milkweed.

Poison Hemlock (Conium maculatum)
Poison Hemlock is found along roadsides, boggy areas and ditches. Signs of Poison Hemlock ingestion include dilated pupils, frothing at the mouth, nervousness, weak and rapid pulse, clamping of the jaws and convulsions. Poison Hemlock contains various piperidine alkaloids that cause respiratory failure in less then 3 hours of ingestion and as swiftly as within 15 minutes of consuming the poisonous weed. Water Hemlock, which is found near water ways or wet pastures is just as deadly as Poison Hemlock.

Lupine (Lupinus)
Lupine loves dry open pastures and prairie/wood areas and is even the State Flower of Texas, often being a favourite of gardeners. Levels of poisoning vary within the Lupine varieties, signs of poisoning include nervousness, laboured breathing, frothing at the mouth, convulsions, frenzy/aimless running. Some varieties of Lupine can be deadly if not treated.

Tansy Ragwort (Senecio sp.)
Tansy Ragwort is often found in pastures, hay fields, along roadsides and water ways. Tansy Ragwort contains a liver damaging alkaloid that causes liver cells to expand and die. Signs of Tansy Ragwort poisoning include weakness, high temperature, in-coordination, yellow mucous membranes and liver failure. There is no known anti-dote for Tansy Ragwort poisoning, so all efforts should be made to reduce any exposure to this weed.

Yew (Taxus baccata)
Yew is often used an ornamental plant, and the majority of horse poisoning reported have been by ingestion of the Yew plant. Yew is highly toxic to horses, only taking a handful of consumption to kill a horse — so be careful what you decorate with, and where! Often ingestion of this plant renders the horse dead before any signs or symptoms occur.

Take the time to look around your horse’s paddock or pasture to identify any plants or weeds that don’t belong there. Often plants and weeds that are toxic to horses are also harmful to other animals (livestock and pets) as well.

Nicole Salo is an equine entrepreneur with experience in various parts of the industry, including: training, breeding, management, social media and online content. Nicole currently holds an Equine Science Certificate and Diploma in Equine Studies through the University of Guelph while aspiring to finish her Certificate in Equine Business Management.