164 The Feeding of Animals 



. In the second place, the digested proteids are never 

 fully burned. A portion of these compounds always 

 passes off in the urine unoxidized, the fuel value of 

 which is lost to the animal. For this reason the avail- 

 able energy of the digested proteids is about one -fourth 

 less than the total. 



In the third place, there is, with ruminants and 

 horses at least, an escape from the alimentary canal 

 of unconsumed gases, due to the fermentations which 

 take place during digestion. These gases, mostly 

 methane (marsh gas), have their source in the carbo- 

 hydrates, and Kellner found them to represent from 

 10 to 20 per cent of the total energy value of the 

 dry substance digested from various materials. From 

 twenty experiments, upon five different animals, Kiihn 

 found the loss in methane to be over one -seventh the 

 energy of the digested crude fiber and nitrogen -free 

 extract. 



We are to understand, then, that the available 

 energy of a ration is represented by the fuel value of 

 the dry matter which is digested from it, minus the 

 dry matter of the urine and that lost in gases. 



If, however, we wish to know the actual energy 

 gain to the animal from a particular ration, we must 

 go farther than a determination of its available energy. 



Net energy. Within a comparatively short time we 

 have begun to speak of the net energy of foods, and as 

 this is a practical consideration which is likely to be 

 the subject of much future discussion, it is well to no- 

 tice it in an explanatory way. As we have learned, 

 food is not applied to use until it reaches the blood. 



