COMPOSITION AND PROPERTIES OF MILK 291 



The composition of the milk of any given cow varies 

 according to the stage of lactation and time of milking ; and 

 it may also be affected, in greater or less degree, by the age, 

 health, and condition of the animal. 



It was previously shown (p. 178) that the composition of 

 milk is not directly affected to any considerable extent by the 

 food of the animal ; and the fact has been quoted in support 

 of the view that milk is not directly secreted from the blood. 



Many practical dairymen believe that the youngest cows 

 give the richest milk, but this has not been confirmed. In 

 fact, some of the evidence points to an opposite conclusion. 

 At present it is uncertain whether the age of the cow affects the 

 composition of the milk at all. If it does, it is probably only 

 to a very slight extent. It is generally agreed, however, that 

 the yield increases each time the cow " comes in " up to the sixth 

 or seventh calf, and that after that it tends to decline again. 



The health of the animals may affect the composition of 

 the milk to a considerable extent. When they are sick, the 

 milk is almost invariably of poorer quality than under normal 

 circumstances. The percentage of fat, and sometimes also 

 but more rarely of non-fatty solids, is diminished. Similar 

 effects are often produced by excitement, as when the cows 

 are taken to shows and fairs ; and also when they are in heat. 



The period of lactation, i.e. from the date of calving until 

 the cow goes dry, is generally about ten months. During the 

 first two or three months the yield is usually well maintained 

 it may even increase to a maximum during that time but 

 afterwards it steadily declines, and towards the end it becomes 



