Can they eat it? / Locoweed
Can horses eat locoweed?
No. Permanent brain damage with repeated grazing.
A western range plant (Astragalus and Oxytropis) horses develop a taste for. The damage does not reverse.
Staggering, head bobbing, strange behavior, weight loss.
Remove the horse from the range and call your vet.
Why it is dangerous
Locoweed contains swainsonine and, in some species, selenium accumulators and related alkaloids. Swainsonine inhibits alpha-mannosidase, a lysosomal enzyme, so cells in the brain and nerves store damaged material over time. The result is a progressive neurological condition that horses can develop after repeated grazing, and the damage does not reverse. Some horses acquire a taste for it, so it is not a plant to ignore even when other forage is available.
How much is too much
The guidance is that there is no safe amount of locoweed for a horse. Even a small handful eaten repeatedly can build up effects over weeks or months, and a large amount can cause problems sooner. A 1,000 lb horse and a pony are both at risk, and the smaller animal does not get a safer margin. Horses with other health problems may show signs faster. Because signs can be slow to appear, the safest position is zero access.
Horses
Full-size horses on range, pasture or trail are the most common locoweed cases. Walk your fence lines and pastures in spring and early summer when the plant is growing and most palatable, and dig out any plants by the root before they seed. Hay that contains locoweed can also cause problems, so check bales carefully. A horse showing staggering, head bobbing or odd behavior should come off the grass and be seen by your vet promptly.
Ponies and easy keepers
Ponies, miniatures, seniors and easy keepers can be affected just as readily, and they often graze more closely to the ground where locoweed grows. Keep these animals fenced away from any patch of it, and check that hay is clean before feeding. Older horses may show subtle weight loss first, so watch body condition closely. Any stagger or unusual head movement needs a vet call, not a wait-and-see approach.
Safe alternatives
Offer good-quality grass hay or a pasture with confirmed clean, mixed grasses, and provide a salt block and fresh water at all times. For variety in treats, use a small amount of plain carrot or apple cut into thin slices. Make sure every feed and forage source is inspected before it reaches your horse.
Questions people ask
Is locoweed dangerous in hay?
Yes. Dried locoweed in hay stays toxic, so inspect bales carefully and avoid hay from fields where the plant grew.
Can a horse recover from locoweed?
The neurological damage is generally considered permanent, so the goal is prevention. Remove the horse from access and call your vet right away.
What if my horse ate a little locoweed?
Call your veterinarian promptly, even if the horse seems fine, and keep it off the range while you wait for advice.
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.