Thanks to the hot, dry summer we had here in Ontario, the weed Hoary Alyssum has been spreading more than ever this year and it is toxic to horses. Please be on the lookout in your horse’s pasture and in their hay. The weed is still toxic in hay for up to almost nine months after baling, possibly longer. Though it is not preferred by most horses, they will nibble on it when other plants are not abundant due to overgrazing or drought.
In horses, it can cause depression, stocking up, fever, diarrhea, founder, laminitis, intravascular haemolysis, and hypovolemic shock and can cause mares in foal to abort.
There isn’t a remedy for poisoning from this plant, so treatment efforts are aimed at treating the signs. Clearly, the first move is to remove all sources of exposure to the weed. Anti- inflammatory medications are often beneficial, and other specific treatments may be required to treat limb edema, laminitis, etc. Consult with your veterinarian for help with diagnosis and treatment plans.
OMAFRA fact sheet: http://www.omafra.gov.on.ca/english/crops/facts/ontweeds/hoary_alyssum.htm