in the urine, while ruminants, and to a certain extent horses, also 

 give off combustible gases, arising from fermentations in the diges- 

 tive tract. Thus about 22 per cent, of the energy of corn meal and 

 fully 55 per cent, of that of average hay has been found to escape 

 in these ways. 



Second, the animal body has to extract its real fuel material 

 from its feed, separating it from the relatively large proportion of 

 useless material which it excretes. To effect this separation re- 

 quires work and consumes energy, and this energy, of course, is 

 not available for other purposes. The case is somewhat as if the 

 gasoline engine had to distill its own gasoline and separate it from 

 impurities. Moreover, when the animal eats more feed than is re- 

 quired simply to furnish energy to run its machinery, and hence 

 is able to produce meat or milk, the process of converting the 

 food into suitable forms to store up in the body may require a fur- 

 ther expenditure of energy. 



It is not, then, the total energy contained in a feeding stuff 

 which measures its value for maintenance, but what remains after 

 deducting the losses in the unburned materials of the excreta and 

 the energy expended in extracting the real fuel materials from the 

 feed and transforming them into substances which the body can 

 use or store up. For example, while 100 pounds of corn contain, 

 as stated, about 170.9 Therms of chemical energy, only about 88.8 

 Therms remain, after all these deductions have been made, to repre- 

 sent the actual value of the corn as a source of energy to the organ- 

 ism. 



Determination of available energy : The amount of energy in 

 available form which a feeding stuff contains is determined by the 

 same sort of experiments as those which serve to determine indi- 

 rectly the maintenance requirement. Thus in the example on page 

 8, the steer in both periods was on a sub-maintenance ration, i. e., he 

 was breaking down daily more or less of his own tissue, especially 

 his fat, to supply energy. As determined directly with the respi- 

 ration calorimeter, the actual daily amount of energy thus derived 

 from the destruction of body tissue was as follows : 



Hay eaten Energy supplied 



from body tissue 

 Ibs. Therms 



Period IV n.68 0.371 



Period III 7.05 2.495 



Difference 4.63 2.124 



(13) 



