318 NUTRITION OF FARM ANIMALS 



gen excretion has been determined, Table 57 may serve as an ex- 

 ample. 1 Two sheep were fed in periods i and 7 a basal ration of 

 hay and barley meal. To this ration were added in the intermediate 

 periods varying amounts of nearly pure protein in the form of con- 

 glutin (of lupins) or of flesh meal. A comparison of the nitrogen 

 digested from the ration with the urinary nitrogen shows that the 

 latter increased and diminished substantially parallel with the former. 



403. Utilization of protein limited. That the mere consump- 

 tion of protein cannot cause a large storing up of it is indeed 

 sufficiently obvious from daily experience. The muscles of 

 the weakling cannot be converted into those of the athlete by 

 feeding him upon a meat diet, nor the small man increased in 

 size by a very abundant protein supply. The protein tissues 

 of the mature animal have reached their natural limit of size 

 and consequently the capacity of the body to store up protein 

 is limited. Beyond the minimum required to make good the 

 necessary katabolism in the cells, protein can be utilized in 

 such an animal only to a small extent as protein, and it is there- 

 fore rapidly katabolized, its nitrogen appearing in the urine as 

 urea and other familiar end products. Nor is the situation 

 essentially different in the growing or the milk-producing 

 animal. While these animals are able to utilize consid- 

 erable amounts of feed protein, yet the limit to this utilization 

 is set by the normal rate of growth of the protein tissues or 

 the capacity of the mammary glands to manufacture the 

 casein and other proteins of the milk. Any surplus of pro- 

 tein over what can be used for this purpose is katabolized 

 precisely as is a surplus over the very small demand of the 

 unproductive mature animal. (Compare Chapter XI, 2 and 

 Chapter XIII, 4.) 



404. Protein as a source of energy. This increased katab- 

 olism of protein, however, is not to be regarded as the total 

 loss of so much feed material. In the presence of a surplus of 

 protein, the amino acids resulting from its digestion are in 

 large part deaminized (233), their nitrogen being excreted 

 chiefly as urea, while a non-nitrogenous residue is left which 

 contains the larger portion of the chemical energy of the protein 

 which it represents and is in condition to be oxidized as fuel 



1 Henneberg and Pfeiffer; Jour. Landw., 38 (1890), 215. 



