818 THE VARIABILITY OF COWS' MILK,!., 



suggesting means by which the variation might be controlled and 

 made to proceed in a desired direction, or, at least, predicted. The 

 researches which were luidertaken to acquire this information were 

 at first confined almost entirely to determinations of the amounts of 

 butter-fat yielded by cows. 



The two outstanding factors which have been recognised to play 

 the chief parts in determining the amount of fat which a cow yields 

 in its milk are, firstly, the breed and individual peculiarities of the 

 animal; and, secondly, the method of milking. The factors were 

 recognised vaguely before any quantitative work was done on the 

 subject, and the investigations so far carried out have doi^ little 

 more than confirmed and made precise the prevailing impression. 

 Unfortunately, the investigators have concerned themselves chiefly 

 with the output of herds, and not of single cows. For the herd is 

 the commercial unit: it is the mixed milk from his herd that a 

 dairyman usually sells. 



Certain breeds of cow, such as the Jersey, habitually yield a milk 

 which is rich in fat, while other breeds, such as the Holstein, give 

 a more copious flow of a poorer milk. It has been pointed out by 

 von Wendt (1903), that cows yielding a rich milk are, in general, 

 small. Their calves will, therefore, lose heat more rapidly in pro- 

 portion to their weight than the calves of larger animals, and will 

 require a relatively greater supply of heat-producing material in 

 their food. Conversely, the larger breeds of cows produce a greater 

 quantity of milk poorer in fat, since their larger offspring require 

 more nutritive material, but do not lose heat so rapidly, in propor- 

 tion to their weight, as those of the smaller animals. 



Although the breed of a cow is, for practical purposes, the most 

 important factor determining the quality of its milk, since herds 

 are usually dealt with, it must be remembered that the individual 

 peculiarities of an animal are of even more fundamental import- 

 ance. The formation of a breed is, after all, only a means adopted 

 to perpetuate the qualities of certain individuals, which are 

 superior in some way to their fellows, either in their suitability to 

 environment, as in the natural breeds, or in their economic value, 

 as in the breeds produced by human selection. 



