638 VETERINARY PHYSIOLOGY 



The percentage of fat and carbohydrates are not markedly 

 different from those of milk. 



Colostrum is valuable to the newly born animal. Its 

 food value is high, and it stimulates evacuation of the bowels. 



(o) Factors affecting Milk Production in Cows — (1) Breed- — 

 The amount and composition of the milk produced is partly 

 dependent upon the breed of cows. Thus the milk of the 

 Jersey is especially rich in fat, frequently containing over 

 5 per cent., while that of the Friesian Holstein and the 

 Holderness has a comparatively low fat content. In general, 

 the breeds noted for a large yield give milk with a low 

 percentage of fat. The differences due to breeds are, how- 

 ever, comparatively small — less than what is often found 

 between individuals of the same breed. 



(2) Milking. — It has been definitely proved that in a 

 cow with a large yield the cavities of the udder have not 

 the capacity to contain all the milk that can be obtained at 

 a milking. There must be rapid secretion during the time 

 of milking. In the interval between the milkings secretion 

 is most active when the udder is empty. As the cavities 

 get filled the distension appears to inhibit secretion. The 

 more frequent the emptying of the udder therefore up to the 

 point where the prejudicial influence of over-action of the 

 gland cells appears, the greater the quantity of milk obtained. 

 It is estimated that three milkings per day may yield nearly 

 5 to 20 per cent, more milk than two, and two may yield 

 nearl}^ 50 per cent, more than one. 



When the intervals between the milkings are of an 

 unequal length, milk with a higher fat content is got after 

 the shorter period. It has been suggested that this is due 

 to the distension of the ducts, which occurs in the longer 

 interval, preventing the extrusion of the fat globules from 

 the secreting cells and consequently slowing the synthesis of fat. 



Marked differences occur in the fat content of the milk 

 drawn at different stages of milking. The first portion may 

 contain less than 1 per cent., the last portion drawn, " the 

 strippings," may contain over 10 per cent. The fat only is 

 involved, the other constituents remain uniform throughout 

 the period of milking. 



