Beware the Hoary Alyssum!

Hoary Alyssum is a pasture weed that can cause toxic symptoms in horses. It is a member of the mustard family and has a slender tap root capable of deep soil penetration in dry climates. Hoary alyssum is adapted to the temperate continental climate characterized by cold winters and hot, dry summers. It thrives on dry and disturbed ground on limestone and calcareous substrata with poor fertility. It is commonly found growing along roads, and trails, gravelly stream and lake banks, in lawns, farmyards, and vacant lots. It can also be found in pastures and hay fields. Generally initial exposure will cause swelling of the lower limbs, sometimes with fever and/or loss of appetite. In more severe cases, laminitis may develop. In its most severe form, laminitis can result in the loss of the hoof structure and the death of the horse.

Most hoary alyssum toxicity in horses is the result of the plant being baled in hay. Generally, horses will refuse the plant in pasture if other more palatable options are available. However, there have been some horses affected by what appears to have been hoary alyssum poisoning from ingestion of the plant while on pasture. Hoary alyssum has white flowers. Each flower has 4 petals. The petals are deeply divided to form 8 half-petals.