Mexican whorled milkweed, a perennial herb often called narrow- 

 leaf milkweed, is poisonous to livestock, having, in particular, caused 

 serious sheep losses. The Latin specific name mexicana, as well as the 

 Mexican of the English name, allude to the fact that the plant was 

 first collected in Mexico ; whorled refers to the arrangement of most 

 of the leaves in circles (whorls) of 3 to 6 at the stem joints. Milk- 

 weed is a reminder of the milky juice which exudes from wounds of 

 this and other plants of the milkweed family (Asclepiadaceae), when 

 injured. This species is confined to western North America, where 

 it ranges from southern Mexico northward through California to 

 Washington, western Idaho, Utah, and western Arizona. It is most 

 abundant in California and Nevada. Rather extensive patches grow 

 practically throughout California, but chiefly in the dry ground of 

 valleys and foothills, except along the coast in the northern part. 



Mexican whorled milkweed often develops in small colonies on dry 

 plains and foothills and may occur at altitudes up to 6,000 feet, al- 

 though it usually appears at much lower elevations. It demands an 

 open, sunny site; once established the deep-set and extensive root 

 system enables this species to withstand extensive drought. However, 

 it makes a more luxuriant growth and spreads faster in such moist 

 situations as along watercourses, where shade is not excessive. 



This plant usually inhabits sandy, rocky, clayey or gravelly soils, 

 and frequently attains abundance on bedgrounds and newly dis- 

 turbed or eroded areas, where the original cover has been depleted. 

 It spreads rapidly, extending its range by invasions into disturbed 

 soil areas along irrigation ditches, streams, roadsides, and f encerows, 

 as well as into pastures, washes, and on abandoned agricultural land, 

 where it frequently develops dense, extensive stands. Occasionally, 

 this herb also occurs in hayfields and meadows, and either lowers the 

 hay grade materially or makes the contaminated crop worthless or 

 even dangerous. This obtains because Mexican whorled milkweed, in 

 drying, loses some of its disagreeable taste, but not its poisonous 

 properties, and is rather readily eaten in hay. 



Animals grazed where Mexican whorled milkweed is intermixed 

 with good forage, spurn it until the other feed is utilized. Although, 

 as above intimated, most reported fatalities have been among sheep, 

 some cattle losses have occurred, but practically no mortality has 

 resulted among horses. The maximum sheep losses have been ex- 

 perienced along sheep trails or on bedding areas, and in overgrazed 

 pastures and fields where the animals were forced to eat this plant due 

 to the scarcity of better forage. While losses are usually few and 

 scattered, occasionally almost entire bands have been wiped out at 

 one time. 



Although Mexican whorled milkweed is very poisonous, it is not 

 nearly as serious a menace as its sister species, horsetail milkweed 

 (A. galioi'des), being only one-fourth as toxic in fact, the amount 

 required for a lethal dose is six times as much. 1 Investigations * 2 

 indicate that ordinarily about V/2 to %y 2 pounds of green material 



1 Marsh, C. D., and Clawson, A. B. THE MEXICAN WHORLED MILKWEED (ASCLEPIAS 

 MEXiCANA) AS A POISONOUS PLANT. U. S. Dept. Agr. Bull. 969, 16 pp., illus. 1921. 



2 Fleming, C. E., and Peterson, N. F., assisted by Miller, M. R., Vawter, L. R., and 

 Wright, L. H. THE NARROW-LEAVED MILKWEED (ASCLEPIAS MEXICANA) AND THE BROAD- 

 LEAVED OR SHOWY MILKWEED (ASCLEPIAS SPECIOSA) PLANTS POISONOUS TO LIVESTOCK IN 



NEVADA. Nev. Agr. Expt. Sta. Bull. 99, 32 pp., illus. 1920. 



