MAINTENANCE REQUIREMENTS OF MATTER 323 



katabolism upon the protein supply still holds true. Even 

 the most liberal supply of non-nitrogenous nutrients cannot 

 prevent the splitting-off and excretion of the nitrogen of surplus 

 protein which was illustrated in previous paragraphs, but simply 

 reduces it somewhat below the level which it would otherwise 

 reach. To that extent, it helps to bring about, and probably 

 to prolong somewhat, the temporary storage of protein men- 

 tioned on a previous page (406) and thus to bring the animal 

 upon a higher plane of protein nutrition. 



It is clear from the foregoing statements that no sharp dis- 

 tinction is to be conceived of between an insufficiency and a 

 sufficiency of non-nitrogenous nutrients, but rather a tendency 

 on the part of the latter to diminish the protein katabolism, a 

 tendency more or less marked according to their abundance in 

 the ration. It is not to be understood that no nitrogenous 

 material is katabolized for fuel purposes as long as sufficient 

 non-nitrogenous nutrients are present to supply the demands 

 for energy, nor that even the largest quantities of the latter 

 can prevent the katabolism of protein supplied in excess of its 

 possible constructive use by the body. 



Protein requirements of farm animals 



410. Minimum and optimum of protein. In considering the 

 protein requirements of the different species of farm animals, 

 it is important to distinguish between two points of view. On 

 the one hand, it may be sought to determine the least amount 

 of feed protein upon which the protein tissues of the animal can 

 be maintained. On the other hand, the endeavor may be to 

 formulate the most advantageous amount of protein to supply 

 when an animal is actually to be maintained for a time and this 

 amount may very possibly be greater than the physiological 

 minimum. The first point of view, however, is plainly the 

 fundamental one and should first receive consideration. Hav- 

 ing determined the lower limit of protein supply, it will then 

 be possible to consider intelligently the advantages, if any, of 

 a surplus. 



In. considering the results of experiments directed toward the 

 determination of the minimum of feed protein required by any 

 individual or species, it is essential to bear in mind the facts 



