FOOD NUTRIENTS 45 
in unusual abundance. In this manner the shortage of 
one is balanced by the abundance of others. 
3. Digestibility defined.—Every feeding stuff contains 
protein, carbohydrates, fat, ash and water. A distinct 
portion of each is absorbed, but the remainder is rejected 
and excreted in the feces. The part so absorbed is spoken 
of as the amount digested. Digestibility refers to the true 
food value of any nutrient. Every food, regardless of 
the balance of its proximate principles, contains both 
digestible and indigestible matter. A feeder must be 
familiar with the digestible nutrients of feeding stuffs if 
he is to use them to best advantage. 
4. How digestibility of a 
food is determined.—The gen- 
eral method of ascertaining 
the digestibility of the various 
constituents of a feeding stuff 
is to supply an animal with 
weighed quantities of food, the 
composition of which is known 
by chemical analysis. During 
the period of experiment the 
solid excrements are collected, 
weighed and analyzed by the 
same chemical methods applied dicts pleniacmamarcas 
previously to the food. In this ae eagle Sei eg re about 
manner the amount of each [° “eestibility of food. 
constituent of the food which passes through the animal 
unabsorbed is determined. It is a simple matter now to 
subtract this quantity from the amount found to have 
been present in the food originally. The difference is the 
amount digested and absorbed. 
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aa 
5. First step is to obtain composition.—Chemists have 
