Hoary alyssum (Berteroa incana) is an erect, densely haired, annual to short-lived perennial forb which is toxic to horses if ingested. Hoary alyssum can cause lameness resulting from limb edema and in severe cases, laminitis if the plant is consumed in the pasture or contaminated hay.
Hoary alyssum has gray-green, hairy stems which branches out near the top and stands about 1 to 3 feet in height. It has gray-green, hairy, oblong, narrow, alternate leaves with smooth edges that are roughly 0.5 to 3 inches in length. It's flowers appear as small white notched petals, blooming from early spring to late fall. Hoary alyssum has hairy, oblong seedpods with short beaks on the end. Its seeds can germinate while the plant is in bloom. Seedlings which are late to establish themselves will remain as rosettes and produce flowers and seeds the following year, reproducing as winter annuals or biennials. Sometimes it can form a seed bank which causes seeds to remain dormant for up to nine years. It's seeds are a frequent contaminate of forage and lawn seed, as well as contaminated hay.
Contamination of more than 30% or more of hay with hoary alyssum is toxic to horses, causing clinical signs of lameness, limb edema, haemolysis, hypovolemic shock, and laminitis. The severity depends on the amount of the plant consumed and on the horse, as just under 50% of horses ingesting hoary alyssum show clinical signs. It usually takes 12 to 24 hours following ingestion of the plant for the horse to begin to show signs of toxicity.