6 BULLETIN 459, U. S. DEPARTMENT OF AGRICULTURE. 



PEED AS A SOURCE OF REPAIR MATERIAL. 



For the reasons stated on page 5, the ash has generally been 

 omitted from consideration in discussing the feed as a source of repair 

 material. 



The value of a feeding stuff as a source of protein to the animal 

 body depends in the first place on the amount of protein which it 

 contains. Cottonseed meal, carrying some 43 per cent of protein, 

 is evidently, other things being equal, a better source of protein 

 than Indian corn, carrying about 10 per cent. 



In the second place, however, the protein of the feeding stuff must 

 be capable of being digested by the animal. Of two feeding stuffs 

 containing equal amounts of protein, that one is the more valuable 

 as a source of supply in which the larger proportion of the protein 

 is digestible. The second and third columns of the table on pages 

 1 1-13 show the average percentages of digestible ' 'crude " and "true " 

 protein contained in a number of the more common feedmg stuffs. 

 These figures are the average results of a considerable number of 

 analyses of the feeding stuffs and a smaller number of determina- 

 tions of their digestibility. Individual samples may vary more or 

 less, and sometimes considerably, from the average. 



A third question is at once suggested, viz, whether the digestible 

 protein from different feeding stuffs is equally valuable to the animal. 

 Recent investigations have shown that this is not the case. Single 

 proteins have been found to vary widely in nutritive value, and in 

 particular the mixed proteins of the cereals appear to be of some- 

 what inferior quality. In ordinary mixed rations, however, it 

 appears doubtful whether these differences are of very much practical 

 significance. At any rate, pending further investigation, the only 

 course which seems open at present is to assume the proteins of the 

 various feeding stuffs to be of substantially equal value. 



THE DEMAND FOR FUEL MATERIAL. 



Since the animal machinery is running continually, it requires a 

 continual supply of fuel material, the amount which is necessary 

 depending upon the amount of work done. This fuel material con- 

 sists chiefly of the carbohydrates and fats of the feed, although if 

 more protein be fed than is required for repair and construction 

 purposes it, too, may be used as fuel, while the worn-out portions of 

 the protein tissues are also utilized — that is, the bodily engine can 

 burn up its own waste products as fuel. The unnecessary use of pro- 

 tein as fuel material, however, is wasteful, because protein is ordinarily 

 more expensive to buy or to produce on the farm than are carbo- 

 hydrates and fats. 



