This section is from the book "A Guide To The Poisonous Plants And Weed Seeds Of Canada And The Northern United States", by Robert Boyd Thomson, H. B. Sifton. Also available from Amazon: A guide to the poisonous plants and weed seeds of Canada and the northern United States.
The resemblance between these plants and the stem-less Loco Weeds is marked. They may, however, usually be distinguished by the presence of leafy stems, but always, as stated above, by the absence of any spur at the tip of the keel in the blossoms. Of the seventeen species listed by Rydberg, a number are undoubtedly responsible for loco disease and are as poisonous as the Stemless Loco Weeds. One species, Astragalus mollissimus Torr-, has caused great losses in parts of the western United States. Chesnut says the damage done to the live-stock business by this weed is immense. The State of Colorado paid out nearly $200,000 in bounties between 1881 and 1885, to check its ravages.
Barium salts have been suspected as the toxic substance of the Loco Weeds.