Can they eat it? / Hoary alyssum
Can horses eat hoary alyssum?
No. Common in hay in the upper Midwest; swollen legs and fever.
A weed of poor, dry fields that gets baled with hay. Most horses recover once it is removed, but it can trigger laminitis.
Stocked-up legs, fever, depression, sometimes laminitis, 12 to 24 hours after eating.
Stop the hay, call your vet.
Why it is dangerous
Hoary alyssum (Berteroa incana) contains compounds that interfere with the body's handling of fluid and blood flow, and it is thought to affect the kidneys and the circulation to the legs. The result is swelling in the lower legs, fever, and dullness, sometimes progressing to laminitis. Because the plant is a common contaminant of hay in dry, poor soils, horses often take in enough without anyone noticing it in the bale.
How much is too much
For a 1,000 lb horse there is no amount known to be safe, so even a handful of plant material in hay is a concern. A few stems mixed through a flake may cause no visible harm, but a steady run of contaminated hay can. Ponies, miniatures, and easy keepers are more prone to laminitis, so the same exposure carries more risk for them.
Horses
Horses in work or at pasture are most often exposed through baled hay, especially cut from fields with poor, dry soil. Check each bale for grayish-green, branching plants with small white flowers and seed pods. Watch for stocked-up legs, fever, or reluctance to move after feeding, and call your vet promptly if they appear.
Ponies and easy keepers
Ponies, miniatures, seniors, and insulin-resistant or laminitis-prone horses face a higher risk of lasting harm, since the laminitis this plant can trigger is hard to reverse in these animals. Keep them off any suspect hay entirely and test their hay sources carefully. Early signs like warm feet or a reluctance to turn should prompt an immediate vet call.
Safe alternatives
Feed hay from a trusted source that you have inspected, such as a supplier who grows in fertile, well-managed fields. Timothy, orchardgrass, or a grass-legume mix that you have looked through by hand is a dependable base. Offer a balanced ration of forage and a plain salt block to round out the diet.
Questions people ask
Can horses eat hoary alyssum in hay?
No. It is toxic and can cause swollen legs, fever, and laminitis, so remove any hay containing it and call your vet.
How do I spot hoary alyssum in a bale?
Look for grayish-green, branching plants with small white flowers and flat seed pods. Break open bales and check, especially hay cut from poor or dry fields.
What should I do if my horse ate it?
Stop feeding the hay right away and call your veterinarian. Signs can appear 12 to 24 hours after eating, so keep watching the legs and temperature.
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.