MILK PRODUCTION 501 



animals. Feeding, therefore, while in a sense a secondary 

 factor is nevertheless an important one. 



596. Feed requirements. Regarded solely as a source of 

 material for the formation of milk, the daily ration must, of 

 course, contain an adequate amount of protein and ash and 

 a quantity of non-nitrogenous nutrients sufficient to furnish 

 material for the manufacture of the non-nitrogenous ingredi- 

 ents of the milk, while it must also supply enough energy for 

 the physiological activities of the body, including maintenance 

 and the energy expended in the processes of milk formation. 



In addition to this, however, there must be taken into con- 

 sideration the possibility of the presence or absence in the feed 

 of substances which may have a specific effect on the milk 

 gland, either by stimulating or depressing its action as a whole 

 or by affecting qualitatively the character of its action and so 

 the composition of the milk. There is, of course, a possibility 

 of such specific effects in other forms of production, but it is 

 most obvious in milk production for evident reasons and has 

 been most studied in that connection. 



Protein requirements for milk production 



597. Milk rich in protein. The physiological purpose of 

 milk production is, of course, to support the growth of the young. 



The essential feature of growth, however (462), is the pro- 

 duction of new protein tissue, which, in the suckling animal, 

 is relatively rapid, and in order to support this growth the milk 

 must contain protein in amount more or less proportional to 

 the rate of growth of the species. Cow's milk is decidedly 

 protein in character, the ratio of protein to non-nitrogenous 

 ingredients corresponding roughly with that of the increase 

 made by an animal three months old (556) . Moreover, in the 

 case of the cow, man has been able to increase greatly the natural 

 milk-producing capacity, with, of course, a corresponding in- 

 crease in the total amount of milk protein formed. Even the 

 moderate daily yield of 20 pounds of milk of average composi- 

 tion contains over 0.6 pound of protein, while the extraordinary 

 yields of champion cows contain several times this amount. 



598. Minimum protein requirement. Just as in the case of 

 growth, it is evident that the least amount of digestible protein 



