230 NUTRITION OF FARM ANIMALS 



may fail to be incorporated into the body, or second, they may 

 be incompletely katabolized. 



317. The feces. Since a greater or less proportion of the 

 organic matter of most feeding stuffs fails of digestion and re- 

 sorption by farm animals and so does not enter into the body 

 proper (148), a considerable amount of unused feed energy 

 escapes in the feces, while the excretory products which they 

 carry (154) contain chemical energy which has failed of com- 

 plete conversion in the body. The chemical energy of the feces 

 of farm animals constitutes a very considerable item in their 

 total outgo of energy. Its amount can be determined as in 

 the case of feeding stuffs by burning a sample, after drying 

 with suitable precautions, in a calorimeter and measuring the 

 heat evolved. 



318. Combustible gases. The combustible gases produced 

 by fermentation in the digestive tract also carry off relatively 

 large amounts of unused chemical energy, the loss in this 

 way being precisely analogous to that in the undigested 

 matter of the feces except that it escapes in invisible products. 

 These gases cannot well be separated from the other gaseous 

 excreta for the purpose of making a direct determination of their 

 energy. The amounts of carbon and hydrogen excreted in 

 them, however, can be determined with the aid of the respiration 

 apparatus and on the well-founded assumption that only meth- 

 ane and hydrogen are produced the amount of each excreted 

 may be calculated. The heats of combustion of both these 

 gases being known, the amount of chemical energy which they 

 carry off can be readily computed. 



319. Products of incomplete katabolism. The heat of com- 

 bustion of a substance, as already denned, is the amount of heat 

 evolved when it is completely oxidized, that is, in the case of 

 substances prdinarily occurring in feeding stuffs, when it is 

 burned to CO 2 , H 2 O, N 2 , and SO 3 . If the katabolism in the 

 body stops short of these end products, the quantity of chemical 

 energy transformed is clearly less than the gross energy of the 

 substance by an amount equal to the heat of combustion of 

 the incompletely oxidized products. 



The proteins of the feed constitute the most important in- 

 stance of this sort. All the nitrogen of the digested protein 

 and part of its carbon, hydrogen and oxygen, are excreted in 



