MAINTENANCE THE ENERGY REQUIREMENTS 279 



(367 e), that is, causing little " work of digestion," will have a 

 relatively high net energy value; while, on the other hand, 

 materials of low digestibility, which undergo extensive fer- 

 mentations, require much muscular work in their mastication 

 and digestion, or stimulate the body metabolism, will have 

 correspondingly low net energy values. 



371. Net energy values for different purposes. The net 

 energy value of the same feeding stuff may differ according to 

 the species of animal by which it is consumed and the purpose 

 for which it is used. 



The structure of the digestive organs of different species varies 

 and, as is shown in Chapter XVI (713-717), specific differences 

 in digestive capacity exist. In other words, the proportion of 

 the gross energy of a feeding stuff which is lost in the feces 

 differs as between different species, and therefore its net energy 

 value tends to vary accordingly. Similarly, the extent to which 

 bacterial fermentations occur in the digestive tract of an animal 

 tends to influence the net energy value of its feed in two ways. 

 The more extensive these fermentations, the less of the chemical 

 energy of the feed is rejected in the feces but, on the other hand, 

 the more chemical energy is given off in the combustible gases 

 excreted or is transformed into heat in the process of fermenta- 

 tion and increases the "work of digestion." Finally, it appears 

 not unlikely that the mechanical work required in mastication 

 and digestion may vary as between different species. 



The materials resorbed from maintenance or submaintenance 

 rations may be regarded chiefly as fuel to be oxidized more or 

 less directly, while in the fattening or growing animal a part of 

 the digested nutrients is transformed into flesh or fat, or in 

 the milking animal into butter fat, lactose, casein, etc. The 

 net energy values for these purposes would evidently be equal 

 to the amounts of energy contained in the gains made and 

 might very well differ from the values for simple maintenance. 



The net energy values of feeding stuffs for different species 

 and for the various purposes for which farm animals are kept, 

 together with the methods for their determination or estimation, 

 are discussed in Chapter XVII and the average results for a 

 considerable number of feeding stuffs are tabulated in the Ap- 

 pendix. What is essential at this point is to acquire a clear 

 idea of the general conception. 



