BULLETIN 221. 



I. CONSTANTS FOR NORMAL VARIATION IN THE 

 FAT CONTENT OF MIXED MILK.' 



By Raymond Pearl. 



The fundamental variation constants of characters which 

 are to be the object of genetic study are certainly highly desira- 

 ble, if not absolutely necessary. On this account the constants 

 to be presented here have been worked out, in connection with 

 the studies of the inheritance of milk production now in prog- 

 ress in this laboratory. It is expected that from time to time 

 further reports will be published recording normal variation 

 constants for other elements of milk and of milk production. 



The present paper deals with the variation in fat content, 

 both absolute and relative, of the mixed or composite milk pro- 

 duced by a large herd of cows. It is, of course, a well known 

 fact that the fat content of the milk of any individual cow fluc- 

 tuates, within usually rather narrow limits, from day to day. 

 Sometimes the range of such variation in the performance of a 

 single cow may be very wide. An example of this has recently 

 been furnished by Fraser in- which he cites the case of a 

 cow on an official two-day test, where, within a period of 48 

 hours, the butter fat varied from 2.7 per cent to 6.7 per cent, 

 and the absolute amount of butter fat from .08 to .9 of a pound. 



The causes of such fluctuations in the fat content of the 

 milk from an individual cow are various. Many of them be- 

 long in the general category of immediate environmental cir- 

 cumstances, including such things as kind and amount of food, 



'Papers from the Biological Laboratory of the Maine Agricultural Ex- 

 periment Station, No. 57. 



'Fraser, W. J. Variation in }^Iilk Yield. Breeder's Gazette, Vol. LXIV, 

 p. 562, 1913. 



