308 NUTRITION OF FARM ANIMALS 



4. THE RELATION or THE MAINTENANCE REQUIREMENT 

 TO EXTERNAL TEMPERATURE 



While the temperature to which an animal is exposed is but 

 one among other factors which may affect its maintenance re- 

 quirement, the somewhat complicated relations involved seem 

 to warrant a separate discussion. 



395. Feed consumption lowers the critical temperature. In 

 discussing the influence of temperature upon the fasting katab- 

 olism (350-357) it was shown that for the fasting animal there 

 is a certain external temperature (or more strictly, thermal en- 

 vironment), called the " critical temperature," at which the 

 heat production incidental to the necessary fasting katabolism 

 just balances the unavoidable loss by radiation, conduction and 

 evaporation, so that the body temperature is just maintained. 

 Above this temperature, the fasting animal has a surplus of 

 heat which it gets rid of by means of the physical regulation. 

 Below the critical temperature, on the other hand, its katab- 

 olism is increased beyond that necessary for the internal work 

 of the body in order to supply the necessary amount of heat ; 

 i.e., the energy expenditure is augmented. 



As has been shown (365), however, the consumption of feed 

 results in increasing the heat production of an animal. When 

 an animal is fed, therefore, it has two sources of heat : first, as 

 in the fasting state, the heat resulting from the katabolism inci- 

 dent to the necessary internal work of the body, and second, in 

 addition to this, the heat generated by the so-called " work of 

 digestion." Under these conditions, with more heat being 

 produced in the body as the result of feed consumption, it is 

 obvious that the animal can withstand a greater cooling effect 

 of its surroundings without being compelled to katabolize body 

 substance to maintain its body temperature. In other words, 

 the " critical temperature " is lowered. Furthermore, the 

 greater the amount of feed consumed the lower is the point to 

 which the external temperature can fall without reaching the 

 critical point, so that animals receiving heavy rations in pro- 

 ductive feeding can withstand more cold than those on simple 

 maintenance. Conversely, for any particular temperature 

 there will be a definite amount of any given feed the consumption 

 of which, together with the katabolism required for the internal 



