172 THE CHEMISTRY OF CATTLE FEEDING 



as such in the milk) is given in the food, the yield of milk 

 rapidly declines. 



Milk fat may also be formed from the fat of the food ; but, 

 on comparison, it will be found that, in general, the milk 

 contains more fat than could have been formed from the fat 

 and excess of protein in the food taken together. No doubt 

 is entertained that milk fat may be, and commonly is, formed 

 also from the carbohydrates. 



Now, though milk fat may be formed from all or any of 

 the digestible nutrients of the food, these are not all of equal 

 value for this purpose (fat formation). The dynamic values of 

 milk fat, protein, and starch are 9*4 kt, 4*93 kt., and 376 kt. 

 respectively ; and experiments show that fat and starch have 

 much the same relative value (2-5 to i) for the formation of 

 fat in the animal's body. The relative value of protein as a 

 fat former is not proportional to its dynamic value; it is 

 probably no greater, or perhaps actually a little less, than that 

 of starch (p. 197); and, as it costs much more, the production 

 of fat from protein cannot be regarded as an economical 

 process (in the pecuniary sense), though it may be to a certain 

 extent a necessary one. At any rate, it is important to 

 remember that it is only excess of protein that is available for 

 fat formation. 



Milk Sugar. Under ordinary circumstances there can be 

 little doubt that the milk sugar is formed mainly from the 

 digestible carbohydrates of the food. A certain amount may 

 be formed from protein when the latter is present in excess. 

 Milk sugar is probably not, as a rule, formed from the fat of 

 the food. On the contrary, fat is formed from the carbo- 

 hydrates (p. 113), and it is probable that the fat formed from 

 protein passes through the stage of carbohydrate. 



Since the fat and carbohydrates of the food perform similar 

 functions, and have a relative value of 2*5 to i, they may be 

 conveniently considered together on this basis. Thus, we find 

 that [0-038 X 2 -5 -j- 0-045 =] ' T 4 lb. of total digestible 

 non-nitrogenous nutrients, reckoned as starch, is the minimum 

 required to produce the fat and sugar in each pound of 

 milk. 



