2 BULLETIN 969, U. S. DEPARTMENT OE AGRICULTURE. 
Asclepias galioides, the whorled milkweed of Colorado, Utah, Arizona, 
and New Mexico, is one of the most poisonous of the weeds affecting 
our livestock, and under some circumstances causes very heavy 
losses. Especial interest, therefore, attaches to the other closely 
allied plants. The whorled milkweeds form a very distinct group of 
the milkweed family, Asclepiadacex, being easily recognized by the 
narrow leaves which are frequently arranged in a whorled manner. 
The number of species of these plants depends upon the opinions 
of the botanists who study them, but for our purposes four may be 
recognized which have, a fairly definite geographical range. The 
relationships of these species are briefly characterized by Eggleston in 
Bulletin 800, United States Department of Agriculture, pages 5 and 6: 
Asclepias galioides is found in Colorado, Utah, Arizona, and New 
Mexico, west of the Continental Divide, and so far as known does not 
extend west of Utah; Asclepias pumila grows in the plains east of 
the Rocky Mountains ; Asclepias verticillata is found in the Mississippi 
Valley, and eastward; Asclepias mexicana is a western species, 
being especially abundant in California. 
In Bulletin 942, United States Department of Agriculture, Asclepias 
pumila and Asclepias verticillata var. geyeri were shown to have 
properties similar to those of Asclepias galioides, but differing in 
degree. It is evidently a matter of considerable importance, as well 
as of interest, to find out definitely the character of Asclepias mexicana, 
which has been frequently reported in California as a sheep-poisoning 
plant, although published statements are very few. 
Chesnut (1898) says that it is reported from California that sheep 
and calves were poisoned by eating the growing plant, and cows by 
eating hay containing' it. 
Pammel (1910) quotes the statement of Chesnut. 
Jepson (1911) states that it is said to poison cattle. 
Hall (1912) says: 
It is much dreaded by sheepmen on hot days, when they are obliged to drive their 
flocks over dry districts, where there is little other vegetation and no "water. Under 
these conditions the sheep are tempted to eat the milkweed, which contains an active 
poison and causes many deaths. This is sometimes prevented by cutting the plants 
down a few days before the sheep are driven through, the object being to dry the 
herbage and so render it less tempting. 
Hall and Yates (1915) speak of the three species, Asclepias mexicana, 
Asclepias eriocarpa, and Asclepias speciosa, as being equally poisonous, 
treat of their distribution, and give symptoms as reported to them. 
These symptoms do not correspond to those appearing in sheep 
poisoned by Asclepias mexicana. It may be noted, too, that the 
authors of this bulletin have not only failed to get any clear evidence 
of poisoning by Asclepias speciosa, but have also failed experimentally 
to poison animals by it. 
