PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 7 



Sherman also has noted many experiments on men with a much lower 

 indicated protein requirement for maintenance than the average cited 

 above, and has himself, in conjunction with his associates, reported 

 experimental observations of this description. Hindhede's work has 

 afforded further striking confirmation (^) of the adequacy throughout 

 prolonged feeding periods of amounts of protein only one-half the Sher- 

 man average. It is, therefore, clearly evident that the standard require- 

 ment of 0.6 part of digestible protein per 1000 of body weight includes 

 a large margin of safety. 



The excretion of urinary nitrogen hy fasting cattle or cattle on low- 

 protein rations. — However, if the maintenance requirement for protein 

 is to be measured in terms referable to the animal rather than to its 

 food, the values just considered are not directly pertinent to the problem. 

 Attention must rather be directed to the daily excretion of urinary 

 nitrogen of cattle on low-nitrogen or nitrogen-free rations. It is also 

 important to consider the nitrogen excretion of fasting cattle. Forbes, 

 Fries and KrissC) have found the average daily urinary nitrogen of two 

 fasting cows per 1000 pounds live weight to be 46.5 gms. and 43.6 gms., 

 for the sixth to the ninth day of fast, inclusive. This is equivalent 

 approximately to 0.1 gm. of urinary nitrogen per kilogram of body 

 weight, or to 0.6 pound of crude protein per 1000 pounds of body weight. 

 For two steers weighing approximately 600 kgms., taken from a sub- 

 maintenance ration and fasted for 10 days. Carpenter (8) found the 

 nitrogen excretion in the urine for the last 3 or 4 days to average .059 

 gm. and .067 gm. per kilogram per day, equivalent to 0.37 and 0.42 

 part of protein per 1000 of weight. 



It is, however, well known that the nitrogen excretion of fasting 

 animals includes a considerable fraction of nitrogen representing the 

 catabolism of tissue constituents serving as a source of energy. Since 

 this function may be served by the non-nitrogenous nutrients, it does not 

 represent a function of protein that should be considered in the deter- 

 mination of protein requirements. The protein requirement for main- 

 tenance, therefore, does not bear any constant relation to the excretion of 

 urinary nitrogen during fasting, but it is directly measured by the 

 excretion of urinary nitrogen during the adequate feeding of a nitrogen- 

 free ration, i. e., what might be called specific nitrogen starvation. 



Unfortunately no experimental data appear to be available on the 

 nitrogen output of cattle under conditions of specific nitrogen starvation. 

 It must, therefore, suffice to cite experiments in which this condition 

 is more or less closely approximated. Thus, reference may be made to 

 the experiments of Bull and Grindley(^) on the nitrogen metabolism of 

 steers on various rations. In these experiments, 2 steers were brought 



