260 Maine Agricultural Experiment Station. 1919. 



the daughters in determining the. merits of a sire. The bull at 

 service in a herd of high producing dams would consequently 

 be more likely to lower the milk production of his daughters 

 than a bull at service in a herd of low producing dams . It is 

 true however that the man who owned the high producing herd 

 would probably be wise to get rid of the bull failing to maintain 

 this herd's production in its daughters. 



The Variation of the Milk of Ayrshire Cows in Quantity 

 and Fat Content of Their Milk. 



As previously pointed out the study of existing milk records 

 has been actively pushed. These studies on Scottish Ayrshires 

 include a study of the relation which exists between age and 

 milk yield and age and butter- fat percentage. It was found 

 that in these Ayrshire cattle the, absolute amount of milk pro- 

 duced per unit of time increases with the age of the cow until 

 a maximum is reached, but the amount of increase diminishes 

 each year with advancing age until the absolute maximum of 

 production is reached. After the time of maximum productiv- 

 ity the absolute production per unit of time decreases with ad- 

 vancing age, and by a continually increasing amount. 



The butter-fat percentage was found to decrease regularly 

 from two years to ten years. From ten years on the rate of 

 decline was somewhat less than it was from two to ten years. 



The variability of the mixed, milk of a large herd was com- 

 pared with that of the individual milk yield of the Ayrshire. It 

 was found that the variation of the mixed milk was 9 whereas 

 the variability was 17 to 25 when individuality of the Ayrshire 

 cows was at play. These figures as they stand suggest that 

 roughly about one-half of the variation (measured by the coef- 

 ficients of variation) in milk yield results from the varying in- 

 herited constitution for milk pioduction of the animals in re- 

 spect of this character, and the other half results from the 

 varying external circumstances to which cows are subjected 

 during lactation and which have an effect upon the flow of 

 milk. Or, to put the matter in another way, if the conclusion 

 just stated were true it would mean that if a large number of 

 cows were placed in environmental circumstances which were 

 at once ideal and uniform we should expect the variation ex- 





