PROTEIN REQUIREMENTS OF CATTLE: MITCHELL H 



posit protein " ' in the tissues is only eliminated after a considerable period 

 of nitrogen-free feeding. In the case of the pig, McCollum has found that 

 oftentimes 2 or 3 weeks of such feeding are required before a minimal 

 excretion of urinary nitrogen is established. Thus it is to be expected 

 that in the search for a minimum value the success of any particular 

 attempt may be considered the greater, the lower the result obtained 

 becomes. 



A tentative estimate of the minimum protein requirement for main- 

 tenance. — For the reasons given, the lower values reported of the ratio 

 between urinary nitrogen and body weight are in all probability more 

 significant than the higher values, a conclusion that receives considerable 

 support from the fact that for pigs, sheep and cattle, as well as for men, 

 the minimum values obtained agree within narrow limits, i. e., 0.024 gm. 

 to 0.035 gm. per kilogram of weight. It is probably not far from the 

 truth to conclude that the maintenance requirement of nitrogen by these 

 species is not far from 0.030 gm. per kilogram of body weight.* This is 

 equivalent to 0.19 pound of protein per 1000 pounds body weight. 



That this value may be subject to fluctuation with age, vitality, fat- 

 ness, sex, and other factors seems probable, but the extent and direction 

 of these effects cannot be predicted definitely in the absence of experi- 

 mental observations bearing directly upon these questions, except with 

 respect to fatness. It appears reasonably certain that increasing fatness 

 will decrease the ratio of endogenous urinary nitrogen to body weight, 

 and since the recorded observations of this ratio do not relate to animals 

 in a highly fat condition (in most cases quite the reverse), the minimum 

 ratio indicated is probably well above the truth when applied to fat 

 animals. The effect of age is not so readily predicted, but if the endog- 

 enous metabolism may be presumed to bear some relation to the basal 

 heat production, then the young animal, particularly the very young 

 animal, would be expected to possess a larger ratio of endogenous nitro- 

 gen to body weight than would the mature animal. 



* This teiyn is applied by Lusk to the unorganized protein in the body which is 

 retained in the cellular fluids in amounts proportionate to the level of protein 

 intake. With a change in protein intake, the lag in nitrogen excretion is due to 

 the repletion or depletion of deposit protein. 



^ However, there is no implication that this value has a general application to 

 all animals. The experiments of Underbill and Goldschmidt on dogs (J. Biol. 

 Chem., 15: 341 (1913)) indicate a much higher level of endogenous metabolism for 

 this species, the minimum approximating 0.1 gm. of urinary nitrogen per kilogram 

 of body weight. For rabbits, similar high values have been obtained (Mendel and 

 Rose, J. Biol. Chem., 10: 226 (1911)) ; Meyers and Fine, Ibid., 15: 305 (1913) ; Scrio, 

 Biochem. Z., 142: 440 (1923)), while a large number of experiments in the author's 

 laboratory have shown that for rats the minimum value ranges from 0.2 gm. to 

 0.1 gm. per kilogram of weight, depending upon age, apparently. 



