Toxic Weed & Horses

Dr. Joe Kendall, Veterinary Toxicologist, DVM, M.Sc., AAFRD
Agri-Food Systems Branch, Epidemiology, Toxicology and Technical Services Section, Longman Building

There are many poisonous plants but not many that cause a lot of problem. Below are some of the worst offenders that can be toxic if a horse were to eat one plant.

  • Water hemlock – it grows sparsely in swampy areas, most toxic after frost, it is a CNS stimulant that can cause epilepsy, and violent death.
  • Poison hemlock – has a distinct odor, grows up to 6’ high in dense stands, looks like wild or cow parsnip, doesn’t taste good so horses won’t normally eat it, causes depression of CNS like a stupor, horse passes out and is a calm death.
  • Flowering chokecherry – cianide concentrates in the leaves of plant after a pruning, frost or damage to leave, usually not a problem to horses.
  • Aero grass – grows in alkaline areas around dugout and streams. 
  • Ornamental Japanese or English Yew – leaves are very toxic to dogs, horses, people.
  • Blue- green algae – very toxic very quickly, doesn’t necessarily show upon visual inspection, use copper sulfate to control algae to reduce blue green algae forming.
  • Larkspur of delphinium – has a purple flower and is very common, horses need a lot to be toxic, causes muscular uncoordination, stupor, depression, domestic varieties are not as toxic.
  • Loco weed – grows in patches, is very high in selenium, horses must graze for quite awhile (weeks) to be affected, it is addictive.
  • Golden Rocket – is a member of the mustard family, toxin is in seeds.
  • Yellow Star thistle (Russian Knapweed) – grows densely, toxic exclusively to horses, need to consume quite a bit.
  • Alsike clover – can cause liver failure if enough is consumed, results in horse being sensitive to light (photosensitization).
  • Fescue – affects reproductive problems.
  • Nitrates – are not as much of a problem with horses as in cattle, blood is unable to transport oxygen to muscles, horses asphyxiate (can’t breath).
  • Garden produce – tomato, potato leaves and onion stocks are toxic, don’t throw this garden waste to the beasts!
  • Black walnut sawdust – sawdust or shavings from black walnut can harm horses.


For more information contact Dr. Kendall at 780 427 8389, fax 422 3438, e-mail