Scientific Name: Equistem spp.
Distribution: Throughout the United States and Canada
Type of Poison: Thiaminase which causes thiamine deficiency in horses
Signs of Poisoning:
Growth Characteristics: Horsetail has two different appearances. In the spring, single, tan-colored, jointed shoots with a single cone-like structure at the top emerge. Later, green and sterile shoots emerge and have the appearance of a horse's tail or a miniature pine tree. Horesetail can grow 2 to 24 inches tall. Roots are tuber-bearing and rhizomatous, extending to depths of 6 feet.
Flowers and Seeds: Horsetail lacks flowers, but has a single cone, 0.75 to 1.5 inches long. It reproduces by spores, which look like a light yellow powder.
Leaves: Leaves are small and scale-like, often non-green, whorled, and united at the base to form a sheath around the stem.
Stems: Aerial, jointed stems, which occur in two different forms. A single, simple, cone-bearing stem grows in early spring, and a vegetative, non-fertile stem grows after the first. This second stem has many whorls of slender, green-jointed branches.
Horsetail thrives in many different habitats including wet, poorly drained areas of fields and grasslands, wet meadows, along streams and other sites with high water tables. It is equally at home in well-drained areas of farm fields, orchards, and nursery crops as well as sandy or gravelly soils along roadsides, railroad tracks, and beaches. In general, horsetail appears most commonly in acidic and wet soil conditions.
Horses are particularly sensitive and can be killed if large amounts of horsetail are consumed. Ruminants (cattle and sheep) are not generally affected by problems of thiamine deficiency because it is made in the rumen, although there are some reports of horsetail poisoning in ruminants.
Hay containing 20% or more horsetail produces symptoms in horses after two to five weeks. In fatal cases, death is preceded by quiescence and coma. Animals' appetites will remain unchanged during this period. The immediate removal of contaminated forage will bring about rapid recovery. Keeping poisoned animals out of rain and adverse weather helps. Cattle, sheep, and goats are rarely poisoned. If poisoning is suspected, consult a veterinarian.
L.F. James, R.F. Keeler, A.E. Johnson, M.C. Williams, E.H Cronin, and J.D. Olsen. Plants Poisonous to Livestock in the Western States. U.S. Department of Agriculture, Agriculture Information Bulletin 415, 90 pp. 1980.
Pratt, Mindy S. Horsetail. Range Plants of Utah. 2008. Utah State University Extension Service. 28 July 2009. http://extension.usu.edu/rangeplants/htm/horsetail