TALL WHITETOP
Hoary Alyssum

Berteroa incana

COMMON NAME Tall whitetop

NATIVE RANGE Southeastern Europe and southwestern Asia

IDENTIFICATION Regional Noxious. Biennial (rarely perennial). annual to short-lived perennial in the Mustard Family growing erect to 0.7 metres tall; the whole plant covered with star-shaped hairs; upper leaves are elliptic and clasp the stem; white flowers with deeply notched petals; oval seedpods are 5 to 8 mm long, somewhat flattened and held close to the stem Oval seedpods are flattened and held close to the stem. Leaves are grey-green in colour. Upper leaves are small and also clasped close to the stem. The entire plant is covered in star-shaped hairs.

DAMAGE Hoary alyssum tends to increase in forage crops following drought or winterkill. Horses consuming this plant may be troubled with fever, limb edema, and laminitis.

HABITAT Most common on sandy or gravelly soils, hoary alyssum establishes in dry, disturbed habitats, such as roadsides and railway embankments. It is also found on meadows, pastures, and hayfields. It occurs in south central and southeastern BC in the Okanagan, Thompson, and Kootenay agricultural regions.

SPREAD It is spread by seed. The plant emerges early in spring and continues to flower and produce seed until frost.

CLINICAL SIGNS AND TOXICITY The toxicity of hoary alyssum to any species of animal has not previously been reported. Therefore, our current understanding is based on field reports and recent feeding studies. No cases of hoary alyssum toxicity in ruminants (dairy, beef, sheep or goats) have been reported to date. Some horses, however, have shown a response to hoary alyssum.

• Clinical signs Horses experience depression and a "stocking up," or swelling of the lower legs, 12 to 24 hours following ingestion of hoary alyssum in hay or on pasture. A fever and occasionally short term diarrhea have also been observed. These clinical signs normally subside 2 to 4 days following removal of the 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 clinical evidence of founder may take several additional days.
• In very rare cases, where hoary alyssum comprised extremely high percentages of the hay (30 to 70%), circumstantial evidence exists associating the plant with the death of a few horses. To date, death has not occurred in horses fed hay containing hoary alyssum under experimental conditions.
   
• Toxicity Definitive statements about the toxicity of hoary alyssum when eaten by horses are difficult to make, considering the fact that many horses do not appear to be affected by it.
• In field cases where a large number of horses were known to be involved, only approximately 50% of the animals ingesting hay containing 30 to 70% hoary alyssum demonstrated any of the clinical signs of toxicity. The cases of severe "stocking up," apparent founder, and death have only been observed in horses ingesting hay containing 30 to 70% hoary alyssum. Only mild "stocking up" has been observed in horses on pasture or those ingesting hay with low percentages of alyssum.

For more information about toxicity to horses and control of this weed please see : http://www.pestid.msu.edu/profiles/hoaryalyssum.pdf