Can they eat it? / St. John's wort
Can horses eat st. john's wort?
Careful. Causes sunburn on white skin and pink noses.
A photosensitising plant, fresh or in hay. Horses with white markings suffer most.
Red, peeling, painful skin on white areas after sun exposure.
Keep the horse out of the sun and call your vet.
Why
St. John's wort contains hypericin and related compounds that make the skin hypersensitive to sunlight. When a horse eats the plant, fresh or dried in hay, these compounds circulate and react with ultraviolet light in the skin, causing inflammation. The damage shows most on unpigmented areas like white legs, faces and pink noses, where there is little natural protection from the sun.
How much is too much
For a 1,000 lb horse, there is no clearly defined safe amount of St. John's wort, so the goal is to keep it out of the feed entirely rather than measure it. A few stray leaves in a flake of hay may cause little trouble, while a patch of it grazed or a hay load with a lot of the plant can cause real sunburn. Ponies and miniatures may react at smaller quantities because of their size, and horses with white markings are at greater risk regardless of body weight.
Horses
For full-size horses in work or at pasture, check fields and hay before turning out, especially in summer. Remove patches of St. John's wort and inspect hay for the yellow-flowered plant. Turn out during cooler hours if your horse has white markings, and use fly sheets or pink-nose protection with sun-block made for horses. If a horse shows red, painful skin after sun exposure, bring it into shade and call your vet.
Ponies and easy keepers
Ponies and easy keepers with white markings or pink skin are especially vulnerable, so keep them off pasture where the plant grows and inspect any hay carefully before feeding. Seniors and horses with metabolic issues may also be more sensitive to the stress of a sunburn, so act quickly. Provide full shade, a fly mask with UV protection and gentle, low-sugar forage. Call your vet at the first sign of swelling or peeling skin.
Safe alternatives
Offer a clean grass hay that you have checked by hand for weeds, or a timothy-based hay that is known to be free of this plant. Fresh, uncontaminated pasture grass is another good choice, along with a measured portion of a plain hay cube soaked briefly to soften it. Serve these in a clean feeder away from any suspect areas.
Questions people ask
Can St. John's wort be in hay?
Yes. It can dry in hay and remain toxic. Check bales for the yellow flowers and small leaves, and ask your hay supplier whether a load has been checked for weeds.
Is St. John's wort safe after frost?
Frost does not make it safe. The plant stays photosensitising whether fresh or dried, so keep horses from any patch regardless of the season or temperature.
What if a foal ate St. John's wort?
Call your vet promptly. Keep the foal out of direct sun and in shade, and watch for red, peeling skin on white areas. Do not wait for symptoms to appear before calling.
Careful for horses means: fine in a small amount, prepared as described, and not for every pet; the details say when to skip it.
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.