634 NUTRITION OF FARM ANIMALS 



prevent the use of a feed in sufficient amounts to be profitable 

 and yet the nutritive effect of the feed within the limits of 

 tolerance might be considerable. 



Production values, then, if determined by means of balance 

 experiments made under normal conditions, are to be regarded 

 as showing the potential values of feeding stuffs as sources of 

 matter and energy, i.e., their worth as constituents of a ration 

 which contains sufficient amounts of whatever " accessory in- 

 gredients " are necessary to ensure the normal course of metab- 

 olism. The study of " accessory substances " in the broadest 

 sense of the term has revealed an additional and apparently very 

 important group of factors influencing the extent to which the 

 potentialities of feeding stuffs are actually utilized. It is pos- 

 sible that in the future there must be added to the require- 

 ments already outlined for ash, protein (or amino acids) and 

 energy for the various purposes of feeding, the requirements for 

 the " accessory substances " necessary to secure the most 

 efficient functioning of the cells and organs of the body. 



2. PRODUCTION VALUES AS REGARDS ENERGY NET 

 ENERGY VALUES 



739. Recapitulation. The consideration of the processes of 

 nutrition in Part II, and in particular the study of metabolism 

 and of the balance of nutrition in Chapters V and VI, has 

 shown that the animal body is primarily a transformer of 

 chemical energy and that quantitatively the most important 

 function of the feed is to supply this energy. In the several 

 chapters of Part III the conception of net energy values was 

 developed and the requirements for net energy by different 

 species of farm animals and for different purposes were dis- 

 cussed. It is apparent from those discussions that the net 

 energy value is only another name for the production value of 

 a feeding stuff as regards energy as defined in the preceding 

 section. It appears desirable at this point, therefore, to re- 

 capitulate the general facts regarding the energetics of the 

 animal body which are contained in previous chapters, and 

 to consider in greater detail their bearing upon the production 

 values of feeding stuffs, even at the expense of a certain amount 

 of repetition. 



