312 NUTRITION OF FARM ANIMALS 



To put the matter in another way, Zuntz and Hagemann con- 

 sider that when receiving the ordinary maintenance ration the 

 critical external temperature for the horse is comparatively high, 

 so that, for example, a ration which is sufficient for maintenance 

 in summer may be insufficient in winter, not^because it contains 

 any less available energy but because it fails to meet the de- 

 mand for heat. 



Tangl's experiments (377) showed that the critical tempera- 

 ture for swine is likewise comparatively high (68-73F.), 

 while the expenditure of energy in digestion by these animals, 

 especially when fed chiefly or wholly on concentrates, is likely 

 to be less than that of ruminants. Exposure to low tempera- 

 tures, therefore, may be expected to increase the actual main- 

 tenance ration of swine, and this belief seems to be confirmed by 

 the reported results upon the influence of exposure on the gains 

 of fattening swine. 1 It also seems possible that part of the very 

 wide variations observed in the amount of metabolizable energy 

 required for the maintenance of swine (378) may be due to dif- 

 ferences in the temperature at which the experiments were made. 



Experiments on cattle by Armsby and Fries have shown that 

 at temperatures in the neighborhood of 63 F., the feed may be 

 reduced very considerably below the maintenance ration with- 

 out any indication of an increased katabolism for the sake of 

 heat production. A single series of comparisons at a somewhat 

 lower temperature (56 F.) also showed no increase in the katab- 

 olism, even on rations much below maintenance. No exact 

 experiments at lower temperatures have been reported. Appar- 

 ently, the critical temperature of ruminants is rather low, 

 while the " work of digestion " is the source of a relatively 

 large amount of heat, so that, under ordinary conditions of feed- 

 ing, these animals are producing a surplus of heat and therefore 

 a ration supplying net energy sufficient for maintenance is also 

 ample as a source of heat. 



1 U. S. Dept. Agr., Bur. Anim. Indus., Bui. 108 (1908), pp. 84-86. 



