432 PRINCIPLES OF ANIMAL NUTRITION. 
tion and assimilation and the energy expended in converting the 
resorbed material into permanent tissue. 
- The Maintenance Ration.—As already defined, net available 
energy is that portion of the metabolizable energy of the food 
which serves to make good the losses of potential energy arising 
from the internal work plus the work of digestion and assimilation, 
or, in other words, which contributes towards the maintenance of 
the stored-up capital of energy. We may, therefore, appropriately 
consider the bearings of the known facts regarding availability 
upon the amount of food required for maintenance. 
RELATIONS TO AVAILABILITY.—Not a little effort has been 
expended in determining the maintenance requirements of farm 
animals on the more or less tacit assumption that this quantity is 
a constant for the same animal, and the same assumption has even 
more largely controlled in computations based on the experimental 
data obtained. 
By the maintenance ration, of course, we understand a ration 
just sufficient to prevent any loss of tissue—that is, of potential 
energy—by the animal. To accomplish this we must give a ration 
containing net available energy equal in amount to the potential 
energy lost when no food is given. Expressed thus in terms of net 
available energy, the maintenance requirement under given condi- 
tions is a constant and is equal to the energy of the fasting metabo- 
lism. 
The maintenance requirement, however, particularly in the case 
of farm animals, has not usually been expressed thus, since the 
necessary data are lacking, but in terms of total digestible matter 
or of real or supposed metabolizable energy. When thus expressed, 
however, it is apparent in the light of the foregoing discussion that 
the maintenance requirement must be a variable, depending upon 
the availability of the metabolizable energy of the food. Referring 
again to the graphic representation on p. 410, it is evident that, 
under the conditions there represented, with an availability ex- 
pressed by tan DAC, the amount of metabolizable energy required 
for maintenance will be equal to OS. Furthermore, it is equally 
evident that as the availability decreases and the angle DAC con- 
sequently becomes more acute OS will increase. Only when the 
critical amount of food, OM, is greater than the fasting metabolism 
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