294 DOMESTIC ANIMALS, DAIRYING, ETC. 



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 

 food 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 to be rarely 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 care- 

 ful consideration. Protein is the characteristic ingredient of the 

 animal mechanism, and is broken down and destroyed in its opera- 

 tion 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, preg- 

 nant, 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 

 addition to that needed for repair purposes simply, and hence the 

 feed of these animals must contain a more liberal supply of this in- 

 gredient. This is important physiologically to secure proper nutri- 

 tion of the young and economically because the growth of milk pro- 

 duced is the principal object of the feeder. 



Feed as a Source of Repair Material. For the reasons stated 

 the ash has generally been omitted from consideration in discuss- 

 ing the feed as a source of repair material. The value of a feeding 

 stuff as a source of protein to the animal body evidently depends in 

 the first place on the amount of protein which it contains. Cotton- 

 iseed meal, carrying some 43 per cent of protein, is evidently, other 



