AVAILABILITY OF ENERGY FOR CATTLE. 



21 



On the lighter ration, the steer supplemented the energy derived 

 from its feed by 2.377 therms derived from the katabolism of its own 

 fat and protein, but when 2.1 kilograms of timothy hay was added to 

 the ration, the amount of energy which had to be furnished by the 

 body tissues was reduced to 0.357 therm. In other words, 2.1 kilo- 

 grams of timothy hay supplied 2.020 therms of energy which was 

 available to support the necessary bodily activities and which, there- 

 fore, could replace an equal amount which would otherwise have been 

 derived from the katabolism of body substance. This was the con- 

 tribution which this amount of hay made to the maintenance of the 

 steer. 



But the 2.1 kilograms of timothy hay added to the ration supplied, 

 as the table shows, 3.575 therms of metabolizable energy. Clearly, 

 then, a unit of metabolizable energy supplied by the digestible matter 

 of the hay was less efficient than the same amount supplied by body 

 substance. Only 56.5 per cent of it could be substituted for that pre- 

 viously supplied by the katabolism of the fat and protein of the body 

 of the steer, while the remaining 1.555 therms, or 43.5 per cent, sim- 

 ply increased the heat production of the animal, the latter being as 

 follows : 



It is customary in such a case to speak of the 2.020 therms as the 

 available energy of the hay added to the basal ration of Period III 

 and to say that 56.5 per cent of the metabolizable energy of the hay 

 was available. Such a method of statement does not necessarily 

 imply that the remaining 43.5 per cent served no useful function in 

 the body, but simply asserts that the net result to the organism was 

 the same as if 56.5 per cent of the metabolizable energy were sub- 

 stituted unit for unit for energy derived from the katabolism of body 

 substance and as if the remaining 43.5 per cent were useless. What 

 the experiment really shows is that a unit of metabolizable energy 

 in the hay had only 56.5 per cent of the value for maintenance of a 

 unit of metabolizable energy in the body substance (chiefly fat) 

 previously katabolized, but the first method of expression is both 

 common and convenient and may be retained. 



Experiments by the same authors on several other feeding stuffs have 

 given results of the same general character as those just quoted. Of the 

 metabolizable energy of these feeding stuffs, as directly determined in each ex- 



Daily heat production. 1 



Therms. 



On the heavier ration 

 On the lighter ration. 



9. 619 

 8. 064 



1 Corrected to 12 hours standing. 



