51 6 NUTRITION OF FARM ANIMALS 



energy supply is decreased below this point the milk yield will 

 tend to fall off while the maintenance requirement remains 

 practically constant. The maintenance, therefore, will consume 

 a larger percentage of the total feed energy so that, exactly as 

 in growth or in fattening, while the net energy requirement 

 for the formation of a unit of milk remains approximately con- 

 stant, the total net energy necessary to support both main- 

 tenance and milk production increases relatively per unit of 

 product. 



On the other hand, if the feed is increased so as to cause fat- 

 tening or to stimulate katabolism, it is clear that the energy 

 requirements per unit of milk produced will be apparently in- 

 creased for the reasons already explained. Such an increase in 

 the feed cost, however, may be economically justifiable for the 

 same reasons as in the case of any form of intensive production. 

 In average commercial milk production, it may be doubted 

 whether the rations should be made heavy enough to cause any 

 considerable fattening, and so far as this is the case, the esti- 

 mated net energy values per unit of milk in Table 145 may 

 serve as the basis for computing rations. If, however, feed is 

 relatively cheap and dairy products high in price, the diminish- 

 ing returns due to heavier feeding may still be profitable up 

 to a certain point even though more energy per unit of milk 

 must be supplied in order to support concurrent fattening, 

 while the fact that more or less of the fat stored in the body 

 may be utilized for the support of milk production in the early 

 stages of the next lactation is also to be considered. 



Fat requirement for milk production 



611. Is fat essential? It was noted in discussing the func- 

 tions of the nutrients (265) and also in connection with the re- 

 quirements for growth (498, 499) that the presence in the feed 

 of certain fats or of substances associated with them appears to 

 be essential to growth. Since milk production is in many re- 

 spects analogous to growth it is of interest to inquire whether 

 the fats of the feed exert any such specific effect, either on milk 

 production as a whole or on the production of milk fat. 



That milk fat as well as body fat may be manufactured in 

 the body in large amounts from other nutrients has been shown 



