278 NUTRITION OF FARM ANIMALS 



to just that extent the quantity of energy which the engine 

 can deliver in other forms of work, so that the effect is 

 virtually the same as if the energy content of the gasoline 

 as delivered at the cylinder were diminished by the same 

 amount. 



In a precisely similar way, the energy expended in the so-called 

 " work of digestion " and eliminated as heat does not serve the 

 general purposes of the body. It cannot be said to be waste 

 energy, like the chemical energy of the feces, for example, since 

 part at least is expended for necessary purposes. The feed must 

 be eaten and assimilated, just as the gasoline for the engine must 

 be hoisted. The energy spent in so doing, however, consti- 

 tutes virtually a deduction which must be made from the meta- 

 bolizable energy of the feed in order to obtain the net amount 

 of energy which it can contribute to the performance of the 

 necessary internal work of the body (i.e., to its maintenance) 

 or to such processes as the performance of external work or the 

 storage of meat or fat. 



370. Net energy values. By means of balance experiments 

 like that with a steer used as an illustration in a previous para- 

 graph (364), the effect of a feeding stuff upon the heat produc- 

 tion of an animal or the amount of energy which it contributes 

 towards the maintenance of the body may be determined. 

 The latter result has been called the net energy "value of the sub- 

 stance because it shows the net result as regards energy obtained 

 by its use.- The net energy value of the hay in the illustration 

 cited was 502 Cals. per pound of dry matter. Net energy might 

 be denned, therefore, as metabolizable energy minus the work of 

 digestion, the latter term, of course, being understood in the 

 very general sense already indicated as equivalent to the 

 additional heat production caused by the consumption of 

 the feed. 



Stated in a slightly different way, the net energy value of a 

 feeding stuff is the energy remaining after the losses of chemical 

 energy in the various excreta and also the energy expended in 

 the processes incident to the consumption of the material 

 have been deducted from its gross energy. The amount of these 

 deductions naturally varies as between different feeding stuffs. 

 One containing much digestible matter, readily masticated and 

 exerting little stimulating effect on the metabolic processes 



