Can they eat it? / Horsetail (Equisetum)
Can horses eat horsetail (equisetum)?
No. Destroys vitamin B1 like bracken.
Grows in wet, sandy ground and ends up in hay from those fields.
Staggering, weakness, weight loss over weeks.
Call your vet; thiamine treats it early.
Why it is dangerous
Horsetail contains the enzyme thiaminase, which breaks down thiamine (vitamin B1) in the horse's body. Over time this leads to a thiamine deficiency that damages the nervous system. The plant also contains other compounds that may add to the harm. Because the enzyme keeps working even after the plant is dried, horsetail in hay is just as dangerous as fresh horsetail on pasture.
How much is too much
For a 1,000 lb horse, there is no amount of horsetail that is considered safe to eat, so no small serving should be offered. The harm builds up over days to weeks, so even a modest amount of contaminated hay or grazing can add up. Ponies, miniatures and metabolic horses are not safer with it and should be kept away from it just the same. If you suspect a horse has eaten any, contact your veterinarian promptly and do not wait to see if symptoms appear.
Horses
Horses in work or at pasture are most often exposed through wet, sandy fields and hay cut from them. Check your pasture for horsetail, especially along ditches, damp corners and areas with sandy soil. Inspect hay before feeding, looking for the jointed, hollow green or brown stems. Ask your hay supplier where the crop was grown, and keep horses away from field edges where horsetail tends to spread.
Ponies and easy keepers
Ponies, miniatures, seniors and easy keepers are just as vulnerable to horsetail toxicity as full-size horses, and they may be more likely to graze down a patch of it. Because these horses can be very food-motivated, they may eat plants that other horses leave alone. Keep these animals on fenced areas free of horsetail, and inspect any hay before offering it, since a smaller body can be affected by less.
Safe alternatives
Offer good quality grass hay from a trusted source that you have inspected. Fresh pasture grass grown in well-drained soil is a safe choice when horsetail is absent. For a treat, a few slices of carrot or apple are good options, served in small amounts and cut into pieces to prevent choking.
Questions people ask
Can horsetail be in hay?
Yes. Horsetail in hay is dangerous because the toxic enzyme stays active after drying. Inspect hay carefully and ask your supplier where it was grown.
What if my horse ate horsetail?
Call your veterinarian right away. Treatment with thiamine works best when started early, so do not wait for symptoms to appear.
Does frost make horsetail safer?
No. Frost does not remove the toxic enzyme, so horsetail remains dangerous after a freeze.
Toxic for horses means: do not give it; if they ate it, call your vet or a poison line.
General information from veterinary guidance, not a diagnosis. If you are worried, call your vet or a pet poison line: ASPCA Animal Poison Control (888) 426-4435. Charges may apply.
Written by Hoofdeck editors · updated Nov 13, 2026 · from ASPCA Animal Poison Control, Merck Veterinary Manual and university extension guidance.