A large, diverse number of astragali and oxytropes endemic to North America are considered toxic to both livestock (cattle and sheep) 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". Representative species from each of these categories are listed in the table below (table based on information provided by USDA-PPRL, see below).
Aliphatic nitrotoxins, such as 3-nitro propanol, which are produced by a large number of species in the western North America (est. more 250 spp.), but only a relatively few species have high enough concentrations to actually poison livestock grazing on them (Williams and Barneby, 1977). 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, in sections Genistoidei, Ocreati, Albuli, Bisulcati, Oocalyces, Pectinati, Woodruffiani and Preussiani (Barneby, 1964), have been identified as selenium accumulators. These "selenophytes" or "seleniferous" species concentrate the element selenium (Se) in their tissues by actively absorbing it from selenium-rich soils, and return it to the soil in a soluble form that is readily taken up by other, more commonly grazed, herbaceous plants. Some species, such as Astragalus bisulcatus, are considered "indicator plants", species useful in identifying selenium-rich soils in the western United States.
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. Swainsonine was first discovered in Swainsona canescens (Benth.) F. Muell., a herbaceous legume native to Australia. When eaten, swainsonine - a poly-hydroxy-indolizidine alkaloid - which inhibits cellular enzymes (mannosidases), produces an intoxicating, addictive response, ultimately leading to weight loss and impaired locomotor functions, resulting in ataxia and death.
For more information about the poisonous locoweeds, their action and toxic affects on livestock, and an extensive bibliography on the subject, consult the Poisonous Plant Reference Database website at the US Department of Agriculture/ARS's Poisonous Plant Research Laboratory at Utah State University, Logan.
Table 11. Examples of
poisonous species of Astragalus and Oxytropis found in
North America
(nomenclature after Barneby, 1964, 1989).
Astragalus
atropubescens Coult. & Fish. Astragalus albulus
Woot. & Standl. Astragalus
allochrous Gray A. canadensis
L. A. beathii C. L.
Porter A. asymmetricus
Sheld. A. convallarius
var. convallarius Greene A. bisulcatus var.
bisulcatus (Hook.) A. Gray A. bisulcatus
(Hook.) Gray A. emoryanus
(Rydb.) Cory A. bisulcatus var.
haydenianus (Gray) Barneby (A. haydenianus) A. didymocarpus
var. didymocarpus H. & A. A. falcatus
Lam. A. crotalariae
(Bth.) Gray A. emoryanus var.
emoryanus (Rydb.) Cory A. miser var.
hylophilus (Rydb.) Barneby A. eastwoodiae
Jones A. flavus var.
argillosus (Jones) Barneby A. miser var.
oblongifolius (Rydb.) Cron. A. flavus var.
argillosus (Jones) Barneby A. humistratus
Gray A. miser var.
serotinus (Gray) Barneby A. flavus var.
candicans Gray (A. confertiflorus) A. lentiginosus
var. diphysus (Gray) Jones A. praelongus
Sheld. A. grayi Parryi ex
Wats. ap Parry A. lentiginosus
var. lentiginosus Dougl. ex Hook. A. tetrapterus
Gray A. moencoppensis
Jones A. lentiginosus
var. micans Barneby A. toanus
Jones A. oocalycis
Jones A. lentiginosus
var. nigricalysis Jones A. whitneyi
Gray A. osterhouti
Jones A. lentiginosus
var. wahweapensis Welsh A. pattersonii
Gray ex Brandegee A. lonchocarpus
Torr. A. pectinatus
Dougl. ex G. Don A. missouriensis
Nutt. A. praelongus var.
ellisiae (Rydb.) Barneby ap. B.L. Turner (A.
ellisiae) A. mollissimus
var. coryi Tidest. (A. argillophilus) A. praelongus var.
praelongus Sheldon (A. recedens) A. mollissimus
var. earlei (Greene ex Rydb.) Tidest. A. preussii A.
Gray A. mollissimus
var. mollissimus Torr. A. racemosus
Pursh A. nothoxys
Gray A. sabulosus
Jones A. oxyphysus
Gray A. toanus
Jones A. praelongus
Sheld. A. pubentissimus
T. & G. A. pycnostachyus
var. pycnostachyus Gray A. tephrodes
Gray A. thurberi
Gray A. wootoni
Sheld. Oxytropis
campestris (L.) DC. O. lambertii var.
lambertii Pursh O. sericea Nutt.
(O. saximontana)