46 FEEDS AND FEEDING 



(64) These have fuel value which is lost to the body. Again we may 

 liken this loss to that which occurs in the boiler thru the creosote 

 which, tho having fuel value, is not burned in the fire box but escapes 

 or is deposited in the chimney. 



The available energy of any feed, which is the only portion that is 

 of use to the animal, is the amount of energy left after deducting from 

 the gross energy of the feed these three losses; i. e., (1) the energy 

 lost in feces, (2) the energy lost in urine, and (3) the energy lost in 

 combustible gases. Some authorities prefer the term metabolizable 

 energy, instead of available energy. 



78. Net energy. The available energy of the food measures its value 

 for heat production in the animal, but does not represent its true value 

 for other purposes. The animal must spend a part of the total avail- 

 able energy of any food in the work of masticating and digesting the 

 food and of assimilating the digested nutrients. The energy so expended 

 finally takes the form of heat and may help to keep the animal warm 

 (88), but it is not available for other purposes in the body, since the 

 animal has no power to convert heat into other forms of energy. Only 

 the net amount of energy left after deducting these losses can be used 

 for other functions in the body. 



The net energy of any feed is the amount of energy left after deduct- 

 ing from the available energy the losses of energy in mastication, 

 digestion, and assimilation of the feed. These three losses are often 

 called the "work of digestion." They may be compared to the losses 

 which would occur if a gasoline engine had to distil its own gasoline 

 from crude petroleum and then get rid of the impurities which it could 

 not use. Attention should be called to the fact that the loss of energy 

 due to the fermentations of carbohydrates in the digestive tract in 

 which carbon dioxid is produced, is included in the "work of digestion/' 

 as this heat which is evolved aids in warming the body. On the other 

 hand, the loss of energy in the combustible gases produced in these 

 fermentations is one of the losses previously deducted to get the avail- 

 able energy of the feed, for this is a total loss to the body. 



The net energy is used by the animal, first of all, in the work of the 

 heart, lungs, and other internal organs, and in case a surplus of net 

 energy remains after satisfying the requirement of the animal for 

 merely maintaining the body, such surplus may be used for producing 

 growth, fat. milk, or wool, or in the performance of external work. 



To fix the definitions for available energy and net energy in mind 

 they may be stated thus in the form of equations. 



Available energy=gross energy minus energy lost in (1) feces, (2) 



urine, (3) combustible gases. 

 Net energy^ available energy minus energy lost in work of digestion. 



79. Net energy of feeding stuffs. Our knowledge of the net energy 

 values of different feeds has been obtained largely thru the pains- 



