UNITED STATES DEPARTMENT OF AGRICULTURE 
j^sr^Lru 
| BULLETIN No. 969 
Xil^ Contribution from the Bureau of Animal Industry >X/ 
Contribution from the Bureau of Animal Industry 
JOHN R. MOHLER, Chief 
Washington, D. C. 
PROFESSIONAL PAPER 
October 6, 1921 
THE MEXICAN WHORLED MILKWEED (ASCLEPIAS 
MEXICANA AS A POISONOUS PLANT. 
By C. Dwight Marsh, Physiologist %n Charge of Investigations of Stock Poisoning by 
Plants, and A. B. Clawson, Physiologist, Pathological Division. 
CONTENTS. 
Page. 
Introduction 1 
Description of Asclepias mexicana 3 
Distribution and habits of A . mexicana 3 
Experimental work 4 
T ypical case, Sheep 610 5 
Results of experimental work and conclusions . 7 
Symptoms 7 
Delay in development of symptoms 7 
Duration of sickness 9 
Page. 
Results of experimental work, etc. — Contd. 
Autopsy findings 9 
Pathological changes in tissues 10 
Toxic and lethal doses 13 
Comparative toxicity of species of whorled 
milkweed : . 15 
Animals affected 16 
Summary 16 
Literature cited 16 
INTRODUCTION. 
Many of the milkweeds are said to have poisonous properties. 
Definite experiments, however, proving their toxic or nontoxic 
characteristics have been made in regard to only a few species. 
Reports by stockmen and others of the poisoning of livestock by 
these plants have been made from many parts of the Western States, 
and yet almost an equal number of reports have been made asserting 
that the plants are harmless. Generally speaking, the reports of 
harmful results have not indicated clearly what species of milkweed 
was supposed to be involved. For the most part the plants were not 
examined by botanists who could determine the species correctly. 
The United States Department of Agriculture, in making investiga- 
tions of these cases, has frequently been very much in the dark as to 
the actual plant suspected and has been led to make experiments on 
plants which proved to be nontoxic so far as livestock are concerned. 
Certain definite and rather startling results, however, have been 
obtained in the investigation of the whorled milkweeds. As shown 
in Bulletin 800 of the United States Department of Agriculture, 
48677°— 21— Bull. 969 
