6 BULLETIN 459, U. S. DEPARTMENT OF AGRICULTURE. 
FEED AS A SOURCE OF REPATR 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 
11-13 show the average percentages of digestible ‘‘crude’’ and ‘‘true”’ 
protein contained in a number of the more common feeding 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. 
