6 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



in terms of digestible true protein. Statements thus expressed are ambig- 

 uous, since they are not related to protein of any definite or known 

 biological value. In this report, protein requirements are expressed in 

 terms of animal expenditures or storages of nitrogen, converted for 

 convenience into conventional protein (N x 6.25). They may be considered 

 as representing the requirements for digestible crude protein possessing 

 a biological value of 100. They are thus definite in their significance, 

 minimal in the truest sense of the tvord, and adaptable to any protein 

 mixture the biological value of which may be satisfactorily estimated. 



THE PROTEIN REQUIREMENTS FOR MAINTENANCE 



Existing protein standards for maintenance. — From a study of Ameri- 

 can and foreign investigations on farm animals in which low-protein 

 rations were used, Armsby(2) has concluded that the protein require- 

 ments of swine, cattle, sheep and horses are very closely the same per 

 unit of weight. This evident similarity between animals differing so 

 widely in size and dietary habits is probably more significant than the 

 actual numerical requirement resulting from this study, i. e., 0.6 pound 

 digestible crude protein per 1000 pounds live weight. It is of some sig- 

 nificance also that Sherman (^), in a summary of similar, though more 

 extensive and exact, investigations on adult human subjects, also arrived 

 at approximately the same relation of protein requirement to body 

 weight, i. e., 0.6 gram per 1000 grams. 



But there is abundant evidence that these requirements are too high, 

 even in terms of digestible protein. Armsby himself has noted certain 

 Danish experiments on two dry cows in which nitrogen equilibrium was 

 attained on intakes of 0.21 pound and 0.25 pound of digestible protein 

 per 1000 pounds live weight, and another experiment on steers in which 

 similar quantities of digestible protein sufficed for maintenance. The 

 long-continued feeding experiments of Perkins (*) on dairy cattle may 

 also be cited to the same effect. Subtracting the protein secreted in the 

 milk of these cows from the digestible crude protein intake, left balances 

 of crude protein varying from 0.43 to 0.68 pound per 1000 pounds live 

 weight, averaging 0.52 pound. Since ■ a 100 per cent conversion of 

 digestible crude protein into milk protein would probably never be 

 realized, the actual amounts of protein used for maintenance by these 

 cows were probably distinctly less than the values above cited.* The 

 recent results of Buschmann(^) possess much the same significance. 



* Much the same experience has been more recently reported by Maynard, 

 Miller, and Krauss in Memoir 113 of the Cornell University Agricultural Experi- 

 ment Station (p. 17). 



