310 NUTRITION OF FARM ANIMALS 



animal on the exclusive hay ration, therefore, would have at 

 his disposal 1.32 Therms more heat than the other and accord- 

 ingly could withstand a lower temperature without drawing on 

 his body for fuel. 



396. Net energy below critical temperature. Down to the 

 critical temperature which corresponds to the particular amount 

 and kind of feed consumed, in accordance with the facts brought 

 out in the previous paragraphs, only part of the metaboliz- 

 able energy serves to maintain the animal. The remainder is 

 virtually expended in the " work of digestion " and converted 

 into heat, and this heat, since not needed by the animal, be- 

 comes an excretum and is gotten rid of. If, however, the ex- 

 ternal temperature falls below the critical point the case is dif- 

 ferent. Heat resulting from the ingestion of feed is just as useful 

 as heat from any other source for keeping the body warm. 

 Under these conditions, therefore, all the metabolizable energy 

 of the feed may be of use. Part of it (the net energy) is used 

 directly for supporting the necessary internal work of the body, 

 while the remainder prevents the necessity of katabolizing tis- 

 sue for the sake of heat production and is therefore indirectly 

 of use. In other words, the heat resulting from the consumption 

 of feed may be substituted for heat which would otherwise 

 have to be obtained by the katabolism of tissue. When the 

 external temperature falls so low that all the heat produced by 

 the digestive work is required for this purpose, obviously all 

 the metabolizable energy of the ration is of use directly or in- 

 directly to prevent loss of energy from the body and therefore 

 all of it appears to be net energy. 



Thus, if the ration of mixed grain and alfalfa hay used as an illus- 

 tration in the previous paragraph be fed to a steer whose surround- 

 ings are kept at the critical temperature for the fasting animal, the 

 6.0 Therms of net energy which the ration supplies will be used to 

 support the internal work of the body, and the heat thus produced 

 will be just sufficient to maintain the body temperature, while the 

 remaining 4.69 Therms of metabolizable energy will be expended in 

 superfluous heat production. Suppose, now, that the external tem- 

 perature falls to a point at which the fasting katabolism would be 

 10.69 Therms instead of 6.0 Therms, i.e., at which this amount of 

 heat is necessary to maintain the normal body temperature. The 

 necessary internal work of the body still yields 6.0 Therms, as before, 



