Q: Should I be concerned about the weed hoary alyssum?
A: Hoary alyssum is mostly a concern in hay, as horses will not choose to eat it in pastures if adequate forage exist. When ingested, horses experience depression and a "stocking up," or swelling of the lower legs, 12 to 24 hours following ingestion. A fever and occasionally short term diarrhea have also been observed. These clinical signs normally subside 2 to 4 days following removal of the hoary alyssum source.
In more severe cases, an apparent founder with a stiffness of joints and reluctance of the animal to move has been observed, recovery of animals with these signs may take several additional days. In very rare cases, where hoary alyssum comprised 30 to 70% of the hay, circumstantial evidence exists associating the plant with the death of a few horses, however, death has not occurred in horses fed hay containing hoary alyssum under experimental conditions. In field cases, only half of the animals ingesting hay containing more than 30% hoary alyssum demonstrated any signs of toxicity. The cases of severe "stocking up," apparent founder, and death have only been observed in horses ingesting hay containing more than 30% hoary alyssum. Usually, only mild "stocking up" has been observed in horses on pasture or those ingesting hay with less than 30% hoary alyssum.
Bottom line: hay containing 30% or more of hoary alyssum, or any weed for that matter, should not be fed to horses. If you think your horse is suffering from hoary alyssum toxicity, contact your vet, as these clinical signs can be produced by many other diseases, including strangles.
By Krishona Martinson, PhD, University of Minnesota