78 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



Another opportunity for future research relates to the nitrogen metabo- 

 lism of the cow after parturition throughout her puerperium. Crowther 

 and Woodman have shown that their one subject was in a condition of 

 negative nitrogen balance for several weeks after parturition. Might not 

 the extent of this excess nitrogen catabolism measure the involution of 

 the uterus (^^) and hence the nitrogen needed for the hypertrophy of this 

 organ during pregnancy? Obviously the initiation of lactation during 

 this period will complicate the study of this important question. 



Is the optimum requirement of protein different from the minimum? — 

 The significance of " deposit protein " in animal nutrition also has a bear- 

 ing on the method of computing protein requirements that has been 

 advocated and exemplified in this report. When the nitrogen intake of an 

 animal is suddenly raised, a temporary increase in the nitrogen balance 

 may occur extending over one, two or three days; if it be suddenly de- 

 creased, the reverse temporary disturbance in nitrogen balance occurs. 

 The phenomenon is explained by assuming that there is a variable and 

 labile form of protein in the body, different from that of the organized 

 tissues, and probably retained in the cellular fluids. This circulating 

 reservoir of protein rises and falls with the level of protein feeding. How- 

 ever, this explanation is not based upon the results of direct experimental 

 inquiry. 



It is interesting to speculate whether the amount of deposit protein 

 in the tissue bears any relation to the well-being of the animal or to its 

 physiological efficiency. If it does, then an animal should thrive better 

 when nourished on amounts of protein greater than its requirements, 

 as judged by a thorough study of all possible ways in which protein is 

 utilized. This conception implies that there is an optimum intake of 

 protein greater than the minimum, which in turn implies that the mere 

 presence of an excess of protein (or amino acids) in the body fluids 

 exerts a favorable physiological effect. The apparent existence of an 

 optimum requirement cannot be postulated until it has been shown that 

 all of the requirements for protein have been included in the minimum 

 requirement with which the assumed optimum has been compared. 



The theory of optimum nutrition has been applied particularly to the 

 dairy cow. Although early investigations in this country and in Europe 

 may be cited in support of the view that an excess of protein above the 

 requirements of maintenance and the production of milk stimulates milk 

 secretion, even up to very high levels of protein intake, and although 

 current practice in feeding cows on test is based upon the correctness of 

 this assumption, more recent investigations have not afforded any con- 

 siderable support for it. Besides establishing the fact that the protein 



