FUNCTIONS OF THE NUTRIENTS 167 



Table XXIX 



Calories Calories 



Mixed hay 4.39 Com meal 4.47 



Alfalfa hay 4.40 Linseed meal . 5.04 



Oat straw 4.48 Flaxseed meal .... 6.93 



Sugar-beets . . . 3.93 Rice meal . 4.40 



These figures mean that when a gram of each of 

 these materials is wholly burned the heat produced 

 is as stated. 



244. Metabolizable energy. — ^We must distinguish, 

 however, between the heat produced when any food 

 substance is wholly oxidized in a calorimeter and the heat 

 or energy which is available (metabolizable) when the 

 same material is applied to physiological uses. It never 

 happens that the combustible portion of a ration is 

 entirely oxidized in the animal. 



245. Loss of food energy in feces. — In the first place, 

 the food of domestic animals is practically never all 

 digested and, as only the digested portion furnishes 

 energy, the available fuel value of a ration must be based 

 primarily, not on the total quantity of dry matter it 

 represents, but on the amoimt which is dissolved and 

 passes into the blood. If all feeding-stuifs or rations 

 were digested in the same proportion and with the same 

 ease, their total fuel values might show their relative 

 energy worth, but as digestion coefficients for dry matter 

 vary from less than 50 per cent with the straws to nearly 

 90 per cent with some of the cereal products, it is evident 

 that the fuel waste in the feces is not uniform. 



246. Loss of food energy in urine. — In the second 

 place, the digested proteins 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. 



