68 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



requirements of cattle derived above can be roughly used in estimating 

 the reqiiirements in terms of digestible protein. Laboratory animals 

 have given biological values for feed proteins of 60 or more, provided the 

 protein is not fed at a level higher than 10 per cent (^^' ^*). Will rumi- 

 nants give values of approximately the same order of magnitude? 



Hart, Humphrey and Morrison (^®) have reported nitrogen metabolism 

 studies on four growing calves consuming rations containing approxi- 

 mately 10 per cent of protein, either entirely from the corn plant or 

 largely from the alfalfa plant. The data obtained appear to be suitable 

 for estimating roughly the biological values of the dietary nitrogen upon 

 the basis of certain assumptions concerning the magnitude of the endog- 



* .\ssunied to equal 0.5 gni. per 100 gnis. dry matter consumed. All rations were assumed to 

 contain 90 pet. dry matter. 

 - Assumed to equal 0.030 gm. per kgm. body weight. 



, „. , . , , N intake — (Fecal N — met. N) — (Urine N — end. \) 



* Biological value = lOOx „ . ^ , t^^ r^r; , .„ 



N intake — (Fecal N — met. N) 



enous nitrogen in the urine and the metabolic nitrogen in the feces. 

 The former is taken to be 0,030 gm. of nitrogen per kilogram of body 

 weight, and the latter 0.5 gm. of nitrogen per 100 gms. of dry matter 

 consumed (^' p- ^^). The calculations will be found in Table 30. 



The biological values average 71 for the alfalfa rations and 69 for the 

 corn rations. These values are in good agreement with what would be 

 expected from laboratory animals, and may be taken to justify the tenta- 

 tive use of the value 50 in converting protein requirements in terms of 

 animal expenditures and storages to protein requirements in terms of 

 digestible protein. Probably this value includes a considerable margin of 

 safety, particularly with reference to the use of dietary ])rotein in milk 

 production. Future experimental work, it is hoped, will serve to differ- 

 entiate the different farm feeds and rations with respect to tlie biological 

 values of their protein constituents. 



