PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 51 



W being expressed in grams, and t, the age from breeding, in months. 

 The agreement between observations and corresponding values derived 

 from this equation is given in Table 26. 



Having obtained a continuous mathematical function descriptive of 

 the relation between the age and the nitrogen content of the fetus, in so 

 far as the limited amount of available data will permit, it is possible to 

 derive a second mathematical function or equation from which the rate 



dW 



of nitrogen deposition, — j— ? may be computed for any age The dif- 

 ferentiation of Equation (26) will give 



dW 



■!^ =0.611 (t- 1)3-29 (27) 



from which, by substituting any desired value for t, the monthly rate of 

 nitrogen retention in pregnancy at the particular time chosen may be 



obtained. In Table 27, this has been done for successive months from 

 1 to 9, the results being divided by 30 to give the daily retentions of 

 nitrogen in grams, 



A tentative estimate of the protein requirements of pregnancy. — In 

 this table an attempt has been made to adapt these values to difEerent 

 breeds of cattle. The values themselves are considered as applying to 

 the Holstein breed, since the fetuses themselves were largely of Holstein 

 breeding and since the value at full-term was taken from a Holstein 

 fetus. The gestation periods of different breeds of cattle are the same, 

 but their birth weights vary considerably. It seems reasonable to assume 

 tentatively, therefore, that the rate of nitrogen retention at any age will 

 vary among the different breeds of cattle in proportion to their birth 

 weights. The values given in Table 27 for the Jersey, Ayrshire, Guern- 

 sey and Dairy Shorthorn have been computed on this basis from the 

 average birth weights given by Eckles(5*), i. e., 89 pounds for Holstein 



