THE SELECTION PROBLEM IN ANIMAL BREEDING 



503 



given period is a logarithmic function of her age. His investigations 

 further show that the curve of milk production with respect to age is 

 of the general form 



Y = a + bX + cX 2 + d log X. 



In this equation, Y denotes the amount of milk produced in a given 

 time, X the age of the cow, and a, b, c, and d are constants the value of 

 which must be calculated for different breeds. Stated in general terms, 

 therefore, milk production increases with age until at about 5 years 

 a maximum is reached, after which it decreases with advancing age. 

 With reference to milk production within a given lactation period, it is 

 of course a matter of common experience that milk production decreases 

 gradually during the lactation period. These two factors have been 

 taken into account by Pearl in preparing a table of comparative effi- 

 ciency percentages for dairy cows. In this table the maximum efficiency 

 is set at 100 per cent, and the comparative excellence at any given age 

 or stage of lactation is given a value in per cent, of this maximum value. 

 Pearl has calculated such a table for 24 months of which 10 months 

 only are given in Table LXVI. This table provides a very satis- 

 factory method of comparing a set of individual records from cows of 

 different ages and in different stages of lactation. 



TABLE LXVI. EFFICIENCY PERCENTAGES FOR MILK PRODUCTION IN DAIRY CATTLE 



(Data of Pearl) 



