General: perennial, short-flat-hairy, with a branched crown and slender, erect to creeping-based stems 40-70 cm long.
Leaves: alternate, 4-12 cm long, very short-stalked, the 15-21 leaflets linear-oblanceolate to oblong, 5-22 mm long, often short-stalked at base, rounded to notched, finely flat-hairy on the lower surface but usually hairless on the upper, the axis often flexile. Lower stipules 2-4 mm long, ovate-lanceolate, and slightly joined, the upper ones more narrowly lanceolate, 3-5 mm long, and not joined.
Flowers: about 10-30 in elongate loose clusters which in fruit are usually at least equal to the 5-10 cm main stalks. Individual flower stalks 1-3 mm long. Calyx grayish-flat-stiff-hairy, 4-5 mm long, the slender teeth 1/4-1/3 as long as the tube. Corolla nearly white and merely purplish-tinged to light lavender-purple, 6-10 mm long, the rounded keel 5-7 mm long, much shorter than the wings.
Fruits: pods, with a stalk-like support scarcely 1 mm long and concealed by the calyx tube, the body narrowly oblong, 12-21 mm long, 2-3.5 mm wide, pointed at each end, almost straight and round in section.
Plains and hills, often on rather strongly alkaline soil, in w. and c. parts of MT. Also along the e. slope of the Rocky Mts., from Alberta and Sask. to NM, MN, and KS.
A large, diverse number of milkvetches endemic to North America are considered toxic to both livestock and wildlife, often producing behaviors in poisoned animals described as "crazy", hence the common name "locoweed" ("loco" is Spanish for "crazy") often given to many species. The poisonous species have been historically divided into three groups: those species that produce nitrotoxins, those that accumulate the element selenium, and those that produce alkaloids known as "locoine" or "swainsonine". Nitrotoxins, such as 3-nitro propanol, are produced by a large number of species in the western North America, but only a relatively few species have high enough concentrations to actually poison livestock grazing on them. The toxic nitro-containing compounds as well as their glycoside derivatives disrupt normal functions of the central nervous system, often leading to paralysis and death. Some 25 North American species of Astragalus have been identified as selenium accumulators. These species concentrate the element selenium (Se) in their tissues to toxic levels. The third type of poisoning and probably the most severe, called "locoweed poisoning" or "locoism", is caused by several species of Astragalus and a few species of Oxytropis which synthesize the alkaloid swainsonine. When eaten, swainsonine inhibits cellular enzymes (mannosidases) and produces an intoxicating, addictive response, ultimately leading to weight loss and impaired locomotor functions, resulting in ataxia and death.
Our specimen belong to var. flexuosus (Hook.) Dougl. ex G. Don.
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