Locoweeds and Milkvetches
Several hundred species of locoweed and milkvetch (Astragalus spp.) occur in North America, many of which are known to cause severe poisoning of livestock. This large group of legumes is taxonomically very difficult to identify, even by the experienced botanist. The genus Oxytropis, which like many Astragalus spp. is also commonly called locoweed, is a closely related genus and produces identical locoism in horses, cattle, sheep, and elk. However, not all species of Astragalus are toxic, and some are useful forage plants.
Astragalus spp. are perennial legumes growing to a height of up to 3 ft (1 m), with branching stems from a stout crown and extensive taproot. The leaves are alternate, pinnately compound, with each leaflet being elliptical or oval and minutely hairy. The racemes are usually produced at the ends of the branches and are densely covered with white to purple pea-like flowers, depending on the species. The leguminous seed pods vary considerably in shape and contain many bean-shaped seeds.
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Locoism, blind staggers, alkali disease, and a syndromecacterized by respiratory difficulty and incoordination have all been attributed to species of Astragalus and Oxytropis. Three distinct syndromes are now recognized by virtue of the different toxins that various species of these plants may accumulate. The Astragalus species can be categorized into the locoweeds that contain indolizidine alkaloids (swainsonine), the milkvetches that contain nitroglycosides (miserotoxin) and the milkvetches that, as discussed in the section in this chapter on lameness, accumulate toxic amounts of selenium. Some species may contain more than one of the toxins and consequently may cause a combination of clinical signs in affected animals.
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