526 PRINCIPLES OF ANIMAL NU TRITION. 
available energy of the food. These determinations by Zuntz and 
his co-workers, however, do not bring the energy recovered as 
mechanical work into direct relation with the energy of the food; 
that is to say (aside from such computations of available energy as 
those made by Zuntz & Hagemann * for the food of the horse), 
they do not tell us how much of the energy contained in a given 
feeding-stuff we may expect to recover in the form of mechanical 
work, but only what proportion of the stored-up energy resulting 
from the use of this feeding-stuff is so recoverable. 
It is the former question rather than the latter, however, which 
is of direct and immediate interest to the feeder of working animals. 
The feeding-stuffs which he employs are comparable to the fuel of 
an engine, and the practical question is how much of the energy 
which he pays for in this form he can get back as useful work. 
Meruops oF DETERMINATION.—I wo general methods are open 
for the determination of the percentage utilization of the energy 
of the food. 
It is obvious that if we know the net availability of the energy 
(gross or metabolizable) of a given food material we can compute 
its percentage utilization in work production from the data of the 
foregoing paragraphs with a degree of accuracy depending upon 
that of the factors used. For example, if we know that the net 
available energy of a sample of oats is 60 per cent. of its gross energy, 
then if the oats are fed to a draft horse utilizing, according to Zuntz 
& Hagemann, 31.3 per cent. of the net available energy, it is obvious 
that the utilization of the gross energy of the oats is 60X0.313= 
18.78 per cent. An entirely similar computation could of course be 
made of the percentage utilization of the metabolizable energy of 
the oats. 
Unfortunately, however, as we have already seen, our present 
knowledge of the net availability of the energy of feeding-stuffs and 
nutrients for different classes of animals is extremely defective, and 
extensive investigations in this direction are an essential first step 
in the determination of the percentage utilization of the energy 
of feeding-stuffs in work production by this method. Until trust- 
worthy data of this sort are supplied, results like those of Zuntz & 
Hagemann can be applied to practical conditions only on the basis 
* Landw. Jahrb., 27, Supp. III, 279 and 429. 
