CKITICAL TEMPERATURE. 



73 



physical regulation already described. The heat produced is then in 

 a sense an excretum, and under these conditions obviously the ex- 

 ternal temperature does not materially affect the maintenance ration. 

 The latter, as already shown, is measured by the amount of available 

 energy necessary to support the vital processes, i. e,, by the total 

 fasting katabolism. 



Below the critical temperature, however, the conditions are differ- 

 ent. At relatively low temperatures all the metabolizable energy of 

 the feed is used directly or indirectly to keep the animal warm, and 

 as the external temperature falls, either more feed must be given or 

 more tissue burned to supply the additional heat required to main- 

 tain the body temperature. 



FEED CONSUMPTION LOWERS THE CRITICAL TEMPERATURE. 



Since feed consumption is itself a source of heat, the animal con- 

 suming feed can, other things being equal, withstand a lower tem- 

 perature than when fasting, and the larger the amount of feed 

 consumed the lower is the corresponding temperature. The matter 

 may also be put in the reverse way. For any particular (low) tem- 

 perature there is a certain amount of feed the digestion and assimila- 

 tion of which will yield an amount of heat sufficient to supplement 

 that derived from the fasting katabolism, so as to just maintain the 

 body temperature. This particular external temperature, then, is 

 the critical temperature for that amount and kind of feed, and, con- 

 versely, that particular ration may be called the critical amount of 

 feed for the particular external temperature. 



CRITICAL TEMPERATURE FOR FARM ANIMALS. 



The critical temperature for farm animals has not been definitely 

 determined. In the case of cattle and probably of sheep, however, it 

 is apparently rather low for animals consuming an ordinary ration. 

 Thus Armsby and Fries have found that at about 18° C. the ration 

 of cattle can be reduced considerably below the maintenance require- 

 ment without any evidence of increased oxidation of tissue for the 

 sake of heat production. In the case of fattening animals consuming 

 heavy rations and therefore producing a large amount of heat as a 

 result of digestive work, the critical temperature would be still lower 

 and experiments upon such animals have shown that they may be 

 exposed to comparatively low temperatures, as in an open shed or 

 yard, without causing them to oxidize any more food material. As 

 already stated (p. 56) the critical temperature for the horse appears 

 to be relatively higher. 



