Milk Yield of Jersey Cattle. 123 



deal of interest for comparison data with that presented here 

 as together they show the range of variation to be expected un- 

 der the different conditions of England and the United States, 

 a mixed herd and a pure bred herd, and a difference in the mea- 

 sure of the lactating capacity from lactation to lactation. 



Any adequate analysis of this problem should include a 

 study of the means and standard deviations of milk production 

 for the different age groups into which the lactations are di- 

 vided. The necessity for such analysis lies in the following 

 fact. If it can be shown that the milk production of the earlier 

 years in a cow's life has been used to select only high producing 

 cows to remain in the herd at later years it follows that the 

 correlations will be for this selected herd and not for the breed 

 as a whole. 



The analysis of the means and standard deviations for this 

 data show that no such selection took place. From this it fol- 

 lows that the rise and fall of mean milk production with advanc- 

 ing age shown in the preceding bulletin is due strictly to the 

 physiological changes brought about in the mammary functions 

 of the cow by age. The general equation to this physiological 

 change expresses the law by which it is governed in the same 

 way that Minot, Pearson and others have expressed the similar 

 law for the manner in which the metabolic functions producing 

 growth changes with increasing age. 



It further is established that the results to follow are free 

 from any influence of such selection. 



The Correlation of Eight Months Milk Production at a 



Given Age With the Eight Months Milk Production 



at Any Other Given Age. 



For a firm foundation of our practical agriculture, particu- 

 larly dairying, knowledge of the inter-relationships of the milk 

 yield at one age in comparison with the milk yield of another 

 age can scarcely be too exact. The existing practice is, as al- 

 ready point:d out, largely empirical in its nature often leading 

 to questionable results. The reflection of the questionable na- 

 ture of these practices is seen in the not uncommon practice of 

 dairymen neglecting the records of the first lactation as a mea- 

 sure of the cow's possibilities for future milk production. 



