ENERGY VALUES OF RATIONS FOR FARM ANIMALS. 5 
THE DEMAND FOR REPAIR MATERIAL. 
The repair material for any machine must be of the same kind of 
which the machine is made. We have just seen that the machinery of 
the body is composed of protein, ash, and water. These, then, are 
the materials which must be supplied to keep it in repair. 
Water, of course, is or should be abundantly supplied in the drink 
and scarcely need be considered in a discussion of rations. 
Ash.—The ash supply has received less attention in the past than 
its importance deserves. In the ordinary operation of the bodily ma- 
chinery its ash ingredients are being continually excreted and the 
feed must supply ash sufficient in amount and of the right kinds to 
make good the loss, while the growing animal needs an additional 
supply for building up its new tissues. Fortunately, normally con- 
stituted rations appear rarely to be deficient in ash. Usually it is 
only when large amounts of certain by-product feeds are used or 
when there is a misrelation between grain and coarse fodder that 
special attention needs to be given to the ash supply. 
Protein.—The protein supply, on the other hand, calls for careful 
consideration. Protein is the characteristic ingredient of the animal 
mechanism and is broken down and destroyed in its operation at a 
fairly regular rate. Moreover, since the bodily machinery is running 
all the time, whether any external work is done or not, this loss is 
continually going on. 
The body differs from a machine in being self-repairing, but it 
can not manufacture protein for repair purposes out of the carbo- 
hydrates and fats of its feed any more than it is possible to make 
repairs for an automobile out of the gasoline which supplies the 
power. For its protein the body is absolutely dependent on the 
protein of the feed. This protein is needed for two purposes: 
First. It is required for repair purposes in the strict sense; i. e., 
for making good the wear and tear of the bodily machinery. The 
amount needed for this purpose is comparatively small, and is no 
greater under normal conditions when the animal is doing work than 
when it is not. Like a good engine, the body makes relatively small 
demands for repair material and requires chiefly fuel. 
Second. Protein as well as ash is needed in the growing, pregnant, 
or milking animal to furnish the material for enlarging the working 
machinery of the body of the animal itself or of its young. The 
amount of protein required for this purpose is just so much in addi- 
tion to that needed for repair purposes simply, and hence the feed of 
_ these animals must contain a more liberal supply of this ingredient. 
This is important physiologically to secure proper nutrition of the 
young and economically because the growth or milk produced is the 
principal object of the feeder. 
