UNITED STATES DEPARTMENT OF AGRICULTURE 



ll BULLETIN No. 969 



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Contribution from the Bureau of Animal Industry 

 JOHN R. MOHLER. Chief 



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Washington, D. C. 



PROFESSIONAL PAPER 



October 6, 1921 



THE MEXICAN WHORLED MILKWEED (ASCLEPIAS 

 MEXICANA AS A POISONOUS PLANT. 



By C. DwiGHT Marsh, Phtj biologist m Charge of Investigations of Stock Poisoning by 

 Plants, and A. B. Clawson, Physiologist, Pathological Division. 



CONTENTS. 



Page. 



Introduction 1 



Description of A aclepias mezicana 3 



Distribution and habits of A . meiicana 3 



Experimental work 4 



Typical case, Sheep 610 5 



Results of experimental work and conclusions . 7 



SjTnptoms 7 



Delay in development of sj-mptoms 7 



Duration of sickness 



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 aSected IG 



Summary 16 



Literature cited 16 



INTRODUCTION. 



Many of the milkweeds arc 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 startlmg results, however, have been 

 obtained in the investigation of the whorled milkweeds. As shown 

 in Bulletin 800 of the Ignited States Department of Agricultm-e, 



48677°— 21— Bull. 969 



