270 NUTRITION OF FARM ANIMALS 



requirements of the animal and the values of feeding stuffs and 

 rations for that purpose may be considered from two points of. 

 view: 



First, we may inquire how much energy is necessary to sup- 

 port the quiescent animal and what amounts of it the various 

 feeds and rations can supply in forms available for this purpose. 



Second, we may ask what specific materials and how much 

 of each must be supplied in the feed to make good the losses 

 due to the continual katabolism of body substances. It is 

 particularly the proteins, or rather the amino acids composing 

 them, and the ash ingredients and perhaps the so-called vita- 

 mines which need to be considered in this respect, the body ap- 

 parently possessing large powers of manufacturing other neces- 

 sary ingredients from those supplied in ordinary feeding stuffs. 



It will be convenient to consider these two general classes of 

 maintenance requirements in the order named, the present 

 chapter dealing with the energy requirements. 



362. Mutual replacement of organic nutrients. The dis- 

 cussion, in Chapter V, of the functions of the principal groups 

 of organic nutrients (262-267) showed that, besides certain 

 specific values as sources of particular chemical compounds, 

 they all serve as carriers of chemical energy for the needs of the 

 organism. It would be anticipated, therefore, that the various 

 digestible nutrients might mutually replace each other or the 

 ingredients of the body, and numerous experiments have shown 

 that such is indeed the case. 



Fats fed to a previously fasting animal diminish or suspend 

 the loss of body fat, while carbohydrates may be substituted for 

 the feed fat with a similar result. As has already been shown 

 (337), body protein may replace body fat in the katabolism of 

 the fasting animal, while when protein is given to such an ani- 

 mal the non-nitrogenous portion of the molecule serves as a 

 source of energy to the organism and can be substituted for body 

 fat. On the other hand, an excess of feed protein above the 

 minimum requirement can be replaced by fats or particularly by 

 the carbohydrates, and likewise by organic acids. 



In brief, the animal organism manifests a remarkable degree 

 of flexibility as regards the nature of the material which it can 

 utilize for its energy metabolism. Aside from the small min- 

 imum of protein required, the metabolic activities of the body 



