Our horses are surrounded by toxic plants. The good news is they tend to avoid them, provided there is enough decent food around to eat. If the pickings are slim, or they aren’t careful, though, horses may consume some bad plants. So, it’s best to be aware of which plants are poisonous and remove them from pastures and hay.
The plants I’ll go over here are ones that tend to be more common in Canada, though there are some excellent guides online that include others. Many of the severely poisonous plants are ornamental, such as azaleas, rhododendron and Japanese Yew, and are less likely to be in horse pastures, so they won’t be discussed here.
Hemlock (notably water hemlock, or poison hemlock) is one of the most common plants that causes sudden death. Plants are commonly found near water, such as ponds, streams or swampy areas. The toxins in water hemlock (cicutoxin and cicutol) and poison hemlock (alkaloids such as coniine) are found mostly in the roots or seeds of these plants respectively. Consuming about 1kg of these plants can kill a horse, and while seizures and respiratory failure may be observed, most owners simply find their horses dead.
Nightshade (aka belladonna) and most other plants of the nightshade (Solanum) family, including horse nettle, potatoes and tomatoes, are toxic to horses. Nightshade contains the compound solanine, which, when consumed, can affect the parasympathetic nervous system and digestive system. Symptoms of consumption include neurologic effects such as excitability or depression and incoordination, digestive issues such as colic or diarrhea, or seizures and sudden death. The whole plant is poisonous, particularly the green berries. Of note, the same toxic compound exists when you find a green spot on a potato or potato chip – don’t eat those!
Milkweed varieties contains cardiac glycosides and galitoxin in the plants, that causes both cardiac and neurologic problems. Horses develop seizures and abnormal heart rates, and typically die one to three days after ingestion.
Bloodroot (aka Indian Paint or Tetterwort) is a member of the poppy family and is commonly found in eastern Canada. Poppies produce the alkaloid opium (from morphine and codeine). When ingested by the horse, we see neurologic effects, respiratory failure, coma and death.
Field Horsetail (aka snake grass, mare’s tail or horserush) appears to have higher toxicity when it contaminates hay fed to horses vs. when it is consumed fresh. The toxin within horsetail is a thiaminase enzyme, which destroys vitamin B1 or thiamin, resulting in neurologic issues such as incoordination, depression and muscle tremors.
Pokeweed is another toxic plant found mostly in the eastern part of Canada. It is a tall plant with shiny purple berries that is not very palatable. If eaten, the toxins phytolaccatoxin and jaligonic acid can cause severe gastrointestinal and respiratory issues.
St. John’s Wort is an herb that is commonly found in equine pastures. It contains the toxin hypericin, that when ingested makes it way to the skin. When sunlight hits the hypericin in the skin, the toxin is altered and damages the skin, thus the effects of this plant is “phototoxicity.” The toxin can also cause damage to the red blood cells and liver. Initial signs of poisoning include agitation and depression, followed by inflammation of the skin, particularly around the eyes and face. White horses are particularly susceptible because they have less melanin (skin pigment), which helps to protect against UV light from the sun. Weeping areas of bleeding skin will develop, the horse may develop hyperthermia, and it can be fatal in high quantities. Because this herb is considered medicinal for some purposes, owners will want to make sure it is not included in any herbal supplement mixes.
Winterberry is a member of the Holly family, and contains saponins, theobromine (similar to caffeine) and cyanogenic glycosides, which produce cyanide. These are rarely consumed by horses, as they are mostly ornamental, but may found in pastures or on trails.
Bracken Fern (Eastern Bracken) also contains a thiaminase enzyme, and its rhizomes are particularly toxic. Consumption results in similar neurologic symptoms described for horsetail, but also coma or death if horses continue to consume bracken fern. Other types of ferns, Ostrich Fern and Sensitive Fern are also toxic.
Leaves from red Maple, Sugar Maple or Silver Maple trees are highly toxic to horses. Fresh leaves from these trees do not appear to cause problems, but once they start to wilt, they become increasingly toxic. The toxin gallic acid causes destruction of red blood cells and prevents oxygen from being transported, and can cause death 18 hours after consuming only 700g of wilted leaves. Pastures and surrounding areas should be free of these trees.
Tansy Ragwort and other members of the Senecio spp family produce hepatotoxins (liver toxins) called pyrrolizidine alkaloids. Toxicity causes the liver cells to be destroyed, resulting in liver failure and death. Early symptoms include weakness and jaundice. There is no known cure, so it is important to keep these plants from your pastures and hayfields.
Wild Parsnip is another plant that causes photosensitivity when consumed, and if the horse is exposed to UV light. It can also cause photosensitivity upon contact, similar to Giant Hogweed.
Alsike clover consumption also results in photosensitization and liver damage.
Giant Hogweed is an increasing problem in Canada. Its appearance is similar to hemlock, but can grow very tall (four to five meters), and is a member of the carrot family. This plant does not even need to be eaten by the horse; rather, contact with the plant delivers toxins to the skin, including xanthotoxin and angelicin, which are photoactive. Upon exposure to sunlight, areas affected develop dermatitis and severe skin lesions. Pastures should be monitored for these plants, and riders should look out for them on trails.
White Snakeroot is found in wooded areas and riverbanks across eastern Canada. Consumption causes “milk sickness” in cattle and goats and causes “trembles” in horses, with symptoms such as muscle trembling, depression and low head carriage, and progresses to heart damage and death in severe cases. This plant is managed with herbicides.
Hoary Alyssum is commonly found across most of Canada, particularly after times of drought, as it is adapted to dry conditions. It may be consumed as pasture, or mixed in with hay. Consumption of the plant causes swelling of the lower limbs, or “stocking up,”
as well as diarrhea. Pastures should be clear of the plant, and owners should ensure that it is not in hay at levels
above 30 per cent.
Buttercup is a small flowering plant that can be plentiful in pastures, particularly those that are poorly managed. Toxicity varies, but can cause gastrointestinal tract irritation and colic. Other signs of consumption include blistering in and around the mouth and swelling around the head and face.
Jimsonweed is found throughout southern Canada and it contains tropane alkaloids including atropine and scopolamine. These affect the central nervous system and cause behaviour issues such as depression or nervousness, dilated pupils and muscle twitching, and affects the cardiovascular system causing rapid pulse, coma and death. Jimsonweed has an offensive odour and is, therefore, not likely to be eaten unless other food is scarce.
