Poisoning by "Locoweeds"

   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).


Nitro-toxin species


Selenium-accumulating species


Swainsonine-accumulating species




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)