29. FIELD HORSETAIL SCOURINGRUSH

Equisetum arvense Equisetum hyemale

(horsetail family)

TOXICITY RATING: High for horses, moderate for other species.

ANIMALS AFFECTED: Horses are the species most affected, cattle and sheep may be affected, but this occurs rarely.

DANGEROUS PARTS OF PLANT: All parts, both fresh and dried.

CLASS OF SIGNS: Weight loss, weakness, gait abnormalities, abnormal heart rate and/or rhythm, inability to rise, death.

PLANT DESCRIPTION: Two types of shoots 1 to 3 feet tall merge from horsetail's underground rootstock. Both types are round, hollow, stiff, and jointed. The stem sections easily pull apart. The first type of shoot (fig. 29A) is tan, appears early in spring, and ends in a terminal, cone-like structure. The later, green, sterile shoot (fig. 29A) bears whorls of pine-needle-like branches and looks like a horse's tail. Scouringrush sends up long, tapering, cane-like shoots 1-6 feet tall. These stiff, evergreen shoots terminate in spore-producing cones. Leaves are reduced to teeth-like scales arranged in whorls around the joints of the stems (fig. 29B). The plants commonly grow on shaded, moist soil in meadows, along roadsides, in ditches and thickets, along stream banks, at the borders of swamps, and on railroad embankments.

SIGNS: The toxic signs associated with horsetail are essentially the same as for bracken fern, since the toxin is the same: thiaminase. Horsetail does not contain the bone marrow toxin found in brackenfern. See the section on bracken fern (horses) for more details.

FIRST AID: A veterinarian needs to be called to assist in the treatment of thiamine deficiency. For more discussion, see brackenfern.

SAFETY IN PREPARED FEEDS: Consumption of horsetail in feeds is the most likely route of poisoning, but fresh plants are also toxic (but considered unpalatable).

PREVENTION: Do not allow horsetail to be incorporated into feeds, especially if these feeds are intended for horses. Do not feed contaminated hay.