4QO NUTRITION OF FARM ANIMALS 



is subject to rapid katabolism, so that high protein rations will 

 necessarily show a low apparent utilization of the protein for 

 milk just as they do for growth (468) . On the other hand, too 

 small a supply of protein may cause the tissue proteins of the 

 body to be mobilized and utilized as a source of milk protein so 

 that a direct comparison of feed protein and milk protein would 

 give too high a result. To determine the utilization of feed 

 protein, therefore, it is necessary, while maintaining a sufficient 

 energy supply, to reduce the protein content of the ration 

 as nearly as possible to that which is just sufficient to prevent 

 a loss of body protein and then to compare the feed protein 

 minus the maintenance 'requirement with the milk protein. 



Such an experiment obviously requires a determination of 

 the nitrogen balance of the animal, and relatively few of the 

 reported investigations on milk production include such a 

 determination, while in none yet reported has the sufficiency 

 of the energy supply been demonstrated by means of respira- 

 tion experiments. There are, however, a not inconsiderable 

 number of experiments on record in which the live weights of 

 the animals have been well maintained and in which amounts 

 of digestible protein but little greater than those found in the 

 milk plus those estimated to be necessary for maintenance have 

 been adequate for the production of at least moderate amounts 

 of milk without drawing on the body protein. 



Naturally an exact balance of the income and outgo of nitrogen 

 will rarely be secured. In most cases it is necessary to compare the 

 feed protein with the algebraic sum of the milk protein and the gain 

 or loss of body protein, the comparison being more nearly correct as 

 the latter factor becomes smaller. 



Table 131 shows the computed utilization of the protein 

 of a number of low protein rations, the daily maintenance 

 requirement of crude protein being estimated as 0.6 pound per 

 1000 pounds live weight in direct proportion to the latter. It 

 includes the experiments by Jordan upon the sources of milk 

 fat, the results of one of which as regards protein have just been 

 cited, an experiment by Hayward 1 the results of which as 

 regards the nitrogen balance are still unpublished, the ex- 

 tensive experiments upon the minimum protein requirements 



1 Penna. Expt. Sta., Rpt. 1901-1902, pp. 314 to 396. 



