PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 67 



application to feeding problems may be devised, but the tentative nature 

 of the data must be constantly kept in mind and the existence of certain 

 interrelations between animal functions occurring simultaneously must 

 be pointed out, even though nothing quantitative can be said about them 

 at present. The need for further information on many points throughout 

 this scheme of measuring the protein requirements of cattle suggests con- 

 tinually a number of profitable lines of reasearch. 



Using the factors in any particular case. — An 800 pound Holstein 

 heifer requires for maintenance an amount of nitrogen equivalent to 0.030 

 gm. per kilogram of weight per day (see page 11), or 1.36 gms. per 100 

 pounds, or a total of 10.88 gms. For growth it would require daily 4.11 

 gms. of nitrogen (Table 23), making a total requirement of 14.99 gms. 

 of nitrogen, or 0.206 pound of protein (N x 6.25). If the heifer is in calf 

 and is at the end of her sixth month of gestation, her daily nitrogen 

 requirement should be increased by 4.0 gms. daily (Table 27), and if she 

 is in addition producing 15 pounds of 4 per cent milk," a further consider- 

 able quota of 39.1 gms. of nitrogen must be allowed daily (Equation 28), 

 thus raising her total nitrogen requirement to 58.1 gms. of nitrogen, or 

 0.80 pound of protein (l^xQ.25). 



The composite nitrogen (or protein) requirements obtained in this way 

 for any combination of conditions represent the amounts of nitrogen that 

 are expended by the animal and used by the animal in the elaboration of 

 new tissue or of milk. The amounts of digestible dietary nitrogen needed 

 to cover these requirements must allow in addition for the wastage of 

 dietary nitrogen in metabolism. 



The biological value of a given source of nitrogen measures the mini- 

 mum wastage of absorbed nitrogen in tissue syntheses. With information 

 concerning the biological values of the digestible nitrogen of different 

 feeds and rations, it would be possible to convert the nitrogen require- 

 ments of the animal, compounded in the fashion just illustrated, into re- 

 quirements for digestible nitrogen or protein. Thus, a requirement of 

 0.8 pound of protein, would, for a source of nitrogen possessing a biologi- 

 cal value of 50 at approximately the level of protein feeding appropriate 

 for the specified conditions, call for 1.6 pounds of digestible protein. It 

 should of course be realized that the biological value of a given source of 

 nitrogen may vary, depending upon the purpose for which it is to be used 

 in the animal body. 



The biological value of feed protein for ruminants. — In a tentative way, 

 can it be safely assumed that the biological value of ordinary sources of 

 nitrogen in cattle rations is 50 or more ? If so, the values Cor the nitrogen 



^ Admittedly an extreme, though not an impossible, illustration. 



