Relation of Fat to Casein 31 



oily fat, while cotton -seed meal, the seeds of the 

 various legumes and wheat bran make a hard fat. 

 Constituents other than the fat are not so readily 

 affected in this way. 



Of the constituents of milk, the ash and the 

 sugar are the least variable, the fat and albumin 

 the most variable, while the casein usually bears a 

 nearly constant ratio to the fat. The percentage 

 of water also varies considerably. The causes of 

 the variation of the fat have already been noticed. 

 The proportion of albumin is very largely in- 

 fluenced by the physical condition of the cow, and 

 it has been shown, notably by Van Slyke (see 

 Chapter X ) , that with what may be called normal 

 milk, — that is, milk containing from 3 to 4.5 per 

 cent of fat, — the proportion of casein rises or falls 

 in almost exact ratio with the fat, but when the 

 fat rises above this point the casein does not 

 follow in the same proportion. 



A notion is prevalent that the percentage of fat 

 in the milk is also affected by the age of the cow ; 

 that during the first and second periods of lactation 

 the young cow usually gives milk poorer in fat 

 than when she is mature. During tlie years of 

 greatest vigor the percentage of fat is supposed to 

 be fairly uniform ; but in cows of advanced age it 

 may sometimes again fall to a low point. Recently 

 some records have been published* that go to show 

 that the age of the cow has little, if any, influence 



*Holstein Friesian Herd Book, vols. 13, 14, 15, 16, 17, Cornell University 

 Agr. Expt. Station, Biillet'n No. 169, 



