380 VETERINARY PHYSIOLOGY 



lent energy value from being catabolised merely as fuel for 

 heat production. At low temperatures more than the amount 

 of heat liberated by increased metabolism due to taking food 

 may be necessary, and an amount of food varying with the 

 temperature must be consumed to yield heat, leaving a 

 variable surplus for production. The productive value of a 

 ration therefore is not a fixed quantity applicable to all 

 conditions. 



Whatever feeding standard or unit ot production value is 

 used, it should constantly be kept in mind that a food-stuff 

 has only two real energy values: — (1) The amount that can 

 be transformed to heat by complete combustion in a bomb 

 calorimeter (p. 256) — \A\q ]}hysical energy; and (2) the amount 

 that can be transformed by the animal body — the physiological 

 energy. What proportion of the physiological energy is 

 available for production depends upon — 



(1) The maintenance requirement of the animal, which 

 is partly determined by an external condition — the tempera- 

 ture. 



(2) The nature of the product, e.g. milk, fat, meat, or 

 work. 



These factors are not qualities of the food, and con- 

 sequently the absolute productive value of a food cannot be 

 determined by any system of calculation based solely on per- 

 centages of digestible nutrients. The productive values 

 given are merely hj'pothetical, and error arises unless they 

 are regarded as such. 



4. Manurial Values. 



In farm animals the further question of the manurial 

 value of the food has to be considered. The constituents of 

 the excreta which are of special manurial vaUie are nitrogen, 

 phosphorus, and potash. Since so little nitrogen is fixed in 

 the body of the adult animal as proteins, the greater quantity 

 of the nitrogen of the food is recoverable in the urine and 

 faeces. In ruminants the phosphorus is chiefly excreted in 



