PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 5 



studies. The present report is an attempt to apply this method to a 

 determination of the protein requirements of beef and dairy cattle by 

 the use of experimental data already available. 



The essential features of the method of measuring protein require- 

 ments that will be exemplified in this report are as follows : 



1. The protein requirements of animals, as well as the protein 

 values of feeds, are best expressed in terms of nitrogen, or of a conven- 

 tional protein, such as the ordinary N x 6.25, rather than in terms of 

 true protein. Throughout this report, the term " protein " is used in 

 this conventional sense. 



2. The problem of protein requirements must be considered as separate 

 and distinct from the problem of the protein values of feeds and rations, 

 if permanent progress is to be made in respect to either. Hence the 

 attempt to measure protein requirements in terms of digestible feed 

 protein should be abandoned. Such expressions of protein requirements 

 have served a useful purpose in the past, and may necessarily serve for 

 several years to come until the information required for a more rational 

 expression becomes available; but future investigational work may well 

 be planned along other lines. 



3. Protein requirements may be conveniently and rationally ex- 

 pressed in terms referable to the animal rather than to its feed. Although 

 the percentage of nitrogen in the different nitrogenous compounds of 

 the animal body varies greatly, the need for them, or for their precursors, 

 the dietary amino acids, may be satisfactorily measured by the total 

 nitrogen content of the tissue constituents catabolized endogenously, in 

 the case of maintenance, or by the total nitrogen content of the new 

 tissues formed in gro\vth, fattening and reproduction, or by the total 

 nitrogen content of the milk produced in lactation. These nitrogen 

 values may for convenience be converted into conventional protein. 



4. The use of such measures of protein requirements can be made to 

 the best advantage only in conjunction with measures of the protein 

 values of feeds based upon (a) the total nitrogen content of the feed, 

 (b) the wastage of nitrogen in digestion, (c) the necessary wastage of 

 absorbed nitrogen in the process -of its conversion into tissue constituents 

 or the constituents of body secretions, and (d) the supplementary 

 relations between the available nitrogenous constituents of the feed and 

 those of the other feeds with which it is commonly fed. 



It follows, therefore, that the results obtained in the folloivmg study 

 relative to protein requirements are not to he compared with values 

 already current in the literature. Heretofore, protein requirements have 

 been expressed in two ways, either in terms of digestible crude protein or 



