The Collaborative International Dictionary
loco weed
Loco \Lo"co\, n. [Sp. loco insane.]
(Bot.) A plant ( Astragalus Hornii) growing in the Southwestern United States, which is said to poison horses and cattle, first making them insane. The name is also given vaguely to several other species of the same genus. Called also loco weed.
(Bot.) Any one of various leguminous plants or weeds besides Astragalus, whose herbage is poisonous to cattle, as Spiesia Lambertii, syn. Oxytropis Lambertii.