324 NUTRITION OF FARM ANIMALS 



regarding the influence of the feed supply upon the protein 

 katabolism which have just been considered. 



411. The plane of protein nutrition. It has been shown 

 in previous paragraphs that the protein katabolism adjusts 

 itself more or less promptly to the supply in the feed. A surplus 

 above the minimum requirement, while causing a small storage 

 of protein, results chiefly in raising the plane of protein nutrition 

 and so increasing the katabolism until income and outgo of 

 nitrogen come into equilibrium. The mere fact, therefore, 

 that an animal is in equilibrium with a certain supply of protein 

 in its feed by no means proves the latter to be the least amount 

 necessary for the maintenance of the animal, since it may be 

 living upon an unnecessarily high plane of protein nutrition. 



412. The supply of non-nitrogenous nutrients. It has 

 also been shown that the sufficiency of a given amount of pro- 

 tein depends not only upon the plane of protein nutrition of 

 the body, but also, within certain limits, upon the amount of 

 non-nitrogenous nutrients supplied with the protein. With 

 an abundant supply of the former an amount of protein equal to 

 the fasting katabolism, or perhaps even less, appears to be a 

 sufficient minimum for maintenance. As the supply of non- 

 nitrogenous materials is reduced a larger supply of feed protein 

 seems to be required to reach equilibrium because more and 

 more of it is diverted for use as fuel, so that in the total absence 

 of non-nitrogenous nutrients a large excess of protein must 

 be fed before equilibrium between income and outgo of nitro- 

 gen is reached. In interpreting experiments or formulating a 

 maintenance ration, therefore, it is not sufficient to consider 

 simply the amount of protein, but account must also be taken of 

 the supply of non-nitrogenous materials, and only when the 

 net energy content of the ration is ample for maintenance can 

 it be concluded that a loss of body protein shows the protein 

 supply to be insufficient. 



413. Value of non-protein. The crude protein of the feed 

 of farm animals includes not only true protein but a great 

 variety of other nitrogenous substances, grouped for con- 

 venience under the designation non-protein. In considering 

 the results of experiments upon the protein requirements of 

 these animals, therefore, it is necessary to determine whether 

 the true protein should be the basis of comparison or whether 



