SELENIUM — HURD-KARRER 291 



definitely traced it to the grain in the diet of the affected animals 

 (Evans, Bushey, and Kuhlnian, 1925; Franke, 1934a). When a soil 

 specialist (Rice) with other scientists of the Department of Agri- 

 culture made a survey of the region in 1931, he added the further bit 

 of knowledge that the toxic vegetation occurred on soils derived from 

 Pierre shales (Franke et al., 1934). This discovery was the clue 

 that led the soil chemists to suspect selenium, an element that might 

 reasonably be supposed to occur in the shales, and known to be 

 poisonous to animals (Knight, 1935). 



A search for selenium in the toxic soils, grain, and affected animals 

 followed, and by careful painstaking analyses its presence in them in 

 minute quantities was soon proved beyond a doubt (Robinson, 1933). 

 At the same time it was shown that while rats with sclenious acid 

 added to their food developed symptoms of poisoning similar to those 

 in rats fed the naturally toxic wheat (Munsell ^). The gluten of the 

 wheat was found to be the toxic fraction and to contain most of the 

 selenium (Robinson, 1933; Franke, 1934b; Nelson^). Then wheat 

 was grown in the greenhouse on soil to which a selenium salt (sodium 

 selenate) had been added. The amount of this added salt was so 

 small that the selenium constituted but one one-millionth of the 

 weight of the soil. The plants grew normally, and the grain gave no 

 sign of having anything the matter with it. But, amazingly enough, 

 rats fed on it died while those fed on grain from the same soil without 

 the selenium grew normally (Nelson, Hurd-Karrer, and Robinson, 

 1933). In more prolonged feeding tests the symptoms of the disease 

 were produced by grain grown in quartz sand cultures to which 

 sodium selenate was added (Munsell 2). Finally, to clinch the evi- 

 dence, inorganic selenium salts were fed to pigs (Schoening *). When 

 their hair fell out and their hoofs developed abnormally just as they 

 did in pigs that were fed the naturally toxic grain, it was concluded 

 that the cause of the disease had been found. 



It seems surprising that grain that looks normal in every respect 

 should be so toxic. But this is consistent with reports from the 

 toxic-soil areas that the plants there give no outward sign of abnor- 

 mality. It is evident that animals are far more sensitive to selenium 

 than are plants. Plants absorb relatively large amounts without 

 visible injury and yet may kill animals. The reverse is true of 

 boron. Plants may take up enough of this element to be fatally 

 injured yet they are harmless to animals. 



2 Report to be published by the Bureau of Home Economics, U. S. Department of 

 Agriculture. 



•Report to be published by the Bureau of Chemistry and Soils, U. S. Department of 

 Agriculture. Referred to in the annual report of the Chief of the Bureau, 1935. 



* Report to be published by the Bureau of Animal Industry, U. S. Department of Agri- 

 culture. Referred to in the annual report of the Chief of the Bureau, 19.35. 



36923—36 20 



