Horse owners need to keep eye out for hoary alyssum

Krishona Martinson
University of Minnesota Extension

Hoary alyssum is a perennial toxic weed commonly found in sandy pastures and hay fields in the Midwest.

The weed stands about 1 to 3 feet tall and is grayish green with oblong-shaped leaves. The flowers are petite and white with four deeply divided petals. Seed pods are oblong and can appear swollen with a point at one end. As the plant matures, the seed pods become translucent and small black seeds can be seen. Hoary alyssum tends to spread rapidly due to the high number of seeds produced.

Hoary alyssum is toxic to horses when grazed fresh in pastures or consumed dried in hay. Horses usually prefer other, more palatable forages over hoary alyssum, but cases of hoary alyssum poisoning can still occur in pastures. Most hoary alyssum poisonings occur when horses accidentally consume it in hay. Prior to feeding, horse owners should examine hay for toxic weeds, especially hoary alyssum. There is zero tolerance for hoary alyssum in hay intended for horses; do not feed hay infested with hoary alyssum to horses. Hoary alyssum does not affect other livestock, so hay infested with hoary alyssum can be fed to cattle and small ruminants (e.g. goats and sheep).

Horses can react differently to hoary alyssum toxicity, but most horses will show signs of toxicity within 12 to 24 hours after ingesting the plant. Common signs of toxicity include:

  • Swelling and fluid build-up in the lower legs or “stocking up.”
  • A fever of 103F or higher.
  • Warm hooves.
  • Pronounced digital pulse (laminitis).
  • Stiff joints and an unwillingness to move.
  • A “camped out” stance.
  • Rarely death.

In most horses, clinical signs normally disappear with supportive treatment two to four days after removing the weed source. However, some horses may be slow to return to full performance or may never completely recover. Do not transport horses with laminitis as stress related to transportation can result in a more severe case of laminitis.

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Hoary alyssum in hay. The weed is toxic to horses.