Milk of Different Breeds. 31 
milk,—that is, milk containing from 3 to 4.5. per 
eent 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. 
The percentage of fat in the milk is also af- 
fected by the age of the cow. During the first 
and second periods of lactation the young cow 
usually gives milk poorer in fat than when she is 
mature. During the years of greatest vigor the 
percentage of fat is fairly uniform. In cows of 
advanced age it may sometimes again fall to a low 
point. The breed of the cow also influences very 
largely the percentage of fat in the milk. Cows 
of certain breeds normally give milk much _ richer 
in fat than others. The variation due to breed 
includes not only the amount of fat, and the color 
and melting point of the fat, but the size of the 
milk globules. In some breeds the milk globules 
are uniformly large, in others extremely small, and 
in still others both large and small globules are 
found. 
While there is a distinct difference in the qual- 
ity of the milk of the different breeds, the dif- 
ferent individuals in the breed also vary largely in 
the quality of the milk. The difference in the 
percentage of fat in milk from different cows of 
the same breed is quite as great as the average 
differences between the breeds; that is to say, the 
difference between the highest and lowest percent- 
age of fat in the milk of different individuals of 
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