3i8 



Journal of Agricultural Research voi. x\ai. no. 6 



year is reached. From that point on, allowing for chance fluctuations 

 and the fact that the numbers dealt with get progressively smaller, the 

 fat percentage appears to remain about constant for the rest of the cow's 

 milking life. Consequently, it has seemed best to break the curve at the 

 lo-year point and fit the two parts separately, each with a straight line. 

 The resulting figures are given in the "calculated" column of Table XI, 

 and are shown graphically in figure 6. The equations to the two lines 



4,000' 



3.900 



FiQ. 6.— Showing the observed (zigzag line) and calculated (straight line) changes ia the mean fat per- 

 centage of the milk of Ayrshire cows with advancing age. 



are as follows, the fitting having been done by the method of least 

 squares. 



From 2 to lo years of age: 



;^' = 3.896-o.o343x. 

 From ID to i6 years of age: 



y= 3.610— 0.002 8x. 



SUMMARY 



This paper presents the results of a biometrical analysis of variation in 

 the quantity per unit of time, and the quality, as indicated by fat per- 

 centage, of the milk of Ayrshire cows. Its purpose is to establish normal 

 constants for interindividual variation in these characters, to serve as a 

 base of reference in future genetic studies on milk production. 



