Hoary alyssum: the most common poisonous plant to horses in Minnesota

Hoary alyssum is common throughout Minnesota, the upper Midwest and Western states.

Hoary alyssum is most abundant in disturbed sites. It’s also found in meadows and pastures and is a common weed in hay fields. Hoary alyssum is more adapted to dry conditions on sandy or gravely soils. It prefers direct sunlight but tolerates shade.

Horses usually react differently to hoary alyssum toxicity. You can usually see signs 12 to 24 hours after the horse ingests the plant.

About 50 percent of horses eating hoary alyssum have the following signs:

• Swelling and fluid build-up in the lower legs, “stocking up”

• A fever of 103 F or higher

• Warm hooves

• Pronounced digital pulse (laminitis)

• Stiff joints

• Not willing to move

• A “camped out” stance

• Rarely death

Death didn’t occur in any horses dosed experimentally with hoary alyssum, but researchers observed all the other signs.

Mild stocking up was most common in horses on pasture or eating hay with less than 20 percent hoary alyssum. More severe signs occurred in horses eating hay with more than 20 percent hoary alyssum. These cases tended to be rodeo, race, dressage or other fit horses.

Don’t transport horses with laminitis during the acute phase of poisoning.

Clinical signs normally go away with supportive treatment 2 to 4 days after removing the weed source. Horses that foundered may take more time to recover. Horses may not return to full performance for a few months after the onset of clinical signs.