68 NUTRITION OF FARM ANIMALS 



of feeding stuffs in general are indirect ones based upon a 

 determination of their characteristic element nitrogen. 



104. Crude protein. In the method of feeding stuffs anal- 

 ysis inherited from the early investigations of Henneberg and 

 Stohmann, the protein is estimated from the amount of total 

 nitrogen upon two assumptions: first, that all the proteins 

 contain 16 per cent of nitrogen and, second, that all the nitro- 

 gen of feeding stuffs exists in the protein form. On the basis 

 of these assumptions, the protein is, of course, equal to total 

 nitrogen multiplied by 6.25. The protein as thus determined 

 is designated as crude protein to indicate the approximate na- 

 ture of the determination. 



Subsequent investigations by Scheibler, E. Schulze, Kellner 

 and others have shown the presence in many feeding stuffs 

 of relatively large amounts of non-protein nitrogenous com- 

 pounds, so that it is desirable to distinguish at least between the 

 nitrogen present as true protein and that present in the simpler 

 compounds grouped under the general term non-protein 

 (60-67) , and all analyses of feeding stuffs for scientific purposes 

 should at least make this distinction. Logically, too, the term 

 crude protein should be dropped altogether, but when, as in 

 the case of the older analyses, this is impracticable, care should 

 be taken to retain the adjective, reserving the term " protein " 

 for use in the sense given it in the next paragraph. 



105. True protein. As a means of effecting an approximate 

 separation of the true protein from the other nitrogenous 

 compounds present in plants, advantage is taken of the fact 

 that most of the latter class of substances are soluble in water. 

 An aqueous extract of a feeding stuff, therefore, contains 

 by far the larger share of its non-protein. Such an extract, 

 however, contains also any water-soluble proteins existing 

 in the substance. These are removed in part by coagulation by 

 heating, i.e., by boiling the solution, and in part by the addition 

 of some reagent with which they form insoluble compounds. 

 Various substances have been used for this purpose but the 

 present official method of analysis, based upon Stutzer's inves- 

 tigations, uses copper hydrate as the precipitant. In practice, 

 the feeding stuff is boiled with water, the precipitant added 

 and the soluble matter filtered off. The nitrogen of the in- 

 soluble residue is regarded as being protein nitrogen and from 



