This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Native. Perennial. Propagates by seeds. Time of bloom: April in the southern limit of its range, to June at the northern limit. Seed-time: June to August. Range: South Dakota and Wyoming, southward to Texas, New
Mexico, and Arizona. Most abundant in Colorado and western
Kansas and Nebraska. Habitat: Open prairies; lower mountain slopes; wild meadows.
The Loco-weed Disease in horses, cattle, and sheep is every year the cause of enormous losses to persons engaged in the business of raising live-stock in many of the Western States. The symptoms indicate much cerebral disturbance and affected animals are commonly said to be crazy. There are a number of plants that cause the disease, all of them Legumes and nearly related; but this and the following species range most widely and are credited with the greatest amount of injury to the stock-raising industry. Horses and sheep are the chief sufferers from the poison, though cattle also are frequently "locoed."
Woolly Loco has a large, tough, woody, deep-boring root, sometimes penetrating to a depth of six or more feet, from the crown of which spring tufts of short, branching stems, a foot or less long, some erect and some reclining on the ground for most of their length. The whole plant is densely covered with long, white, silky hair. Leaves alternate, odd pinnate, five to eight inches in length, composed of seventeen to twenty-nine leaflets, pointed-ovate, and about a half-inch long; petioles slender, with membranous pointed-ovate stipules united to their bases. The peduncles spring from the lower axils and are longer than the leaves, so that the short, dense spike of deep purple or violet flowers is held above them; each flower is a little more than a half-inch long, and has a tubular calyx with five nearly equal teeth, an erect, oblong standard, narrow wings, and a blunt keel. Like the rest of the plant, the flower is hairy; the pods, however, are smooth, dry, leathery, about three-fourths of an inch long, slightly incurved, grooved at the sutures, two-celled, each cavity containing a number of seeds, which have very long vitality when in the soil. (Fig. 171.)
1 Manual of Poisonous Plants, page 558.
During the years 1881 to 1885 the State of Colorado paid a bounty of 21 dollars a ton, dry, "for any Loco or poison weed dug up not less than three inches below the surface of the ground, during the months of May, June, and July." After about two hundred thousand dollars had been spent, the law was repealed. But the experiment proved that if these plants are cut off at the root, well below the crown, when they are in full bloom, they never sprout again, but die. And a man with a sharp spade or a sharp and heavy hoe can destroy the plants very rapidly, cleansing a large extent of ground in a day. Dormant seeds may furnish another crop; but if successive germinations are cut off before developing seed, the ground will be cleansed in the course of two or three seasons at a much less expense than is now suffered in losses of live-stock in a single year.
Fig. 171. - Woolly Loco-weed (Astragalus mollisimus). X 1/3.