678 NUTRITION OF FARM ANIMALS 



the digestible nutrients of roughage as compared with those of grain. 

 That such is the case is doubtless explained in part by the rather 

 limited variety of feeding stuffs employed in the experiments. The 

 roughage was meadow hay with, in some cases, a small addition of 

 straw, while the grain was usually oats, partially replaced in some 

 instances by other feeds. Whether the same relation between fiber- 

 free nutrients and work done would hold in widely different rations 

 is not apparent. 



Wolff's results are relative only. They do not show the actual 

 amount of net energy in the rations but only that it was proportional 

 to the fiber-free nutrients. The energy content of the latter would 

 differ considerably from the net energy as computed by Zuntz and 

 Hagemann's method, first because it does not include the deduction 

 of 9 per cent of the metabolizable energy, and second, because it 

 assumes a uniform value of zero for crude fiber, while Zuntz and Hage- 

 mann's method gives the crude fiber a negative value if it has a di- 

 gestibility of less than 55 per cent. Values computed according 

 to Wolff's method from the fiber-free nutrients would therefore con- 

 siderably exceed Zuntz and Hagemann's figures. 



4. PRODUCTION VALUES AS REGARDS PROTEIN 



Relative values of proteins 



780. Differences in proteins. As appears from the discus- 

 sions of the preceding section, the production values of feeding 

 stuffs as regards energy may already be formulated with some de- 

 gree of accuracy, although further investigation is much needed. 



Concerning the production values as regards protein, the 

 situation is far less satisfactory. For years the protein of 

 feeding stuffs has been treated as if it were a single chemical 

 substance ; i.e., the different proteins known to exist in feeding 

 stuffs have been assumed to have substantially equal nutritive 

 values. The more recent investigations into the chemistry 

 and physiology of the proteins, however, have resulted in an 

 entire change in the point of view. As has been fully shown 

 in previous chapters (340, 398, 465, 552), it is the constituent 

 amino acids into which the proteins are split in digestion which 

 are the materials out of which body protein is constructed, and 

 the processes of maintenance, growth or milk production re- 

 quire for their support, not proteins as such, but certain 

 amounts and proportions of such of the amino acids as cannot 



