W30 

 (leaf 2) 



of Mexican whorled milkweed is sufficient to kill a 100-pound sheep. 

 Fleming and coauthors 2 state that amounts of 5 pounds or more 

 apiece may reasonably be expected to kill yearling calves, and Marsh 

 and Clawson 1 report that, due to the similarity in effects of Mexican 

 whorled and horsetail milkweeds, it is fair to conclude that the 

 former is poisonous also to horses. 



Chemical analyses - indicate that the plant contains several poi- 

 sonous substances, but the symptoms appearing in range poison- 

 ing are probably attributable to a black, resinlike substance which 

 can be extracted from Mexican whorled milkweed with alcohol. 



The symptoms of poisoning are somewhat similar to, but usually 

 less severe than, those caused by horsetail milkweed. Sheep become 

 sick in from about 5 to 14 hours after grazing this poisonous species, 

 are generally depressed, refuse to eat, lack muscular coordination, 

 and walk with an unsteady, wobbly gait with paralysis usually 

 most marked in the hind legs. 1 2 In fatal cases the affected animal 

 soon falls, unable to rise. The pulse is fast and weak, and the 

 breathing is labored. Spasms are present but usually are neither 

 so marked nor as violent as those produced by horsetail milkweed. 

 No perceptible elevation of temperature may occur 1 or the tem- 

 perature may rise during the spasms, the maximum being reached 

 at the time of death. 2 The poisoned animals are often salivated 

 and bloated. No effective antidote is available. Since Mexican 

 whorled milkweed is easily recognized, livestock should be debarred 

 from heavily infested areas. Whenever feasible, the plant should 

 be eradicated from valuable pastures, fields, and hay lands, although 

 its prolific seeding habit and ability to grow from even very small 

 pieces of deep-seated, horizontal root make such eradication both 

 difficult and costly. The most efficient control similar to that used 

 in the overthrow of horsetail milkweed in such areas probably 

 would be to plow or grub the plants early in August before seed 

 maturity, repeating the process when the green shoots begin to ap- 

 pear in September. In some cases it is advisable to remove these 

 pestiferous plants from fencerows, roadsides, ditchbanks, and rocky 

 soils to prevent their serving as breeding centers for seed which 

 would later infest agricultural, range, pasture, or hay lands. 



Despite that it is poisonous to livestock, this species has several 

 possible economic uses. Analyses by Hall and Long 3 indicate its 

 potentialities as a source of commercial rubber, as much as 4.8 per- 

 cent of rubber having been obtained from latex in the leaves. These 

 authors also state: 



The largest plants are always found in moderately alkaline soil and are 

 often associated with such halophytes as Sporobolus airoides and Distichlis 

 spicata. These facts suggest that the proper place for the. cultivation of 

 the plant on a large scale would be the vast expanses of territory in the San 

 Joaquin Valley of California and the valleys of western Nevada which are 

 now uncultivated either because of alkaline conditions or the lack of water 

 for irrigation. 



' Hall, H. M., and Long, F. L. RUBBER-CONTENT OF NORTH AMERICAN PLANTS Carneeie 

 Inst. Wash. Pub. 313, 66 pp., illus. 1921. 



