Northern Prairie Wildlife Research Center
A true Great Plains dweller, slender milk-vetch occurs west of the Missouri River in North Dakota. The plant is restricted to the belt of dry grasslands that extend from the southern Prairie Provinces to Texas and northeastern New Mexico at elevations under 9,000 ft.
Slender milk-vetch is perennial from a knobby underground caudex that may be 6 inches or more wide. Stems can be few to several, up to 2 ft long, and erect or decumbent. Leaves are about 2 inches long, with up to 21 leaflets arranged as two combs held back-to-back. At the ends of peduncles up to 6 inches long are found clusters of up to 40 small, pale to dark purple flowers. Fruits are 1/4 inch-long legumes (pods) that bend down slightly at maturity.
Look for slender milk-vetch during July or August in native prairie pastures that are lightly or moderately grazed. Unlike many milk-vetches, this plant is not listed as a selenium-accumulator. Such plants may cause blindness, hemorrhaging, or paralysis in livestock, but acute poisoning is rare and only caused by single, massive doses. Many milk-vetches are used for foods, medicines, and gums, but I found no mention of economic uses for slender milk-vetch.
This plant is a member of the economically important bean family (Fabaceae). Fab means "bean" in Latin. The generic name Astragalus is an old Greek name of some leguminous plant, also of the ankle bone. Worldwide, there are about 1,500 species of Astragalus; most are found in the North Temperate and Arctic zones. The specific epithet gracilis means "slender" in botanical Latin. Slender milk-vetch was described for science in 1818 by the famous English-American explorer, botanist, and ornithologist Thomas Nuttall (1786-1859). He visited the Mandan villages in North Dakota in 1810-1811.