CHAPTER III 



MEASUEING THE USEFULNESS OF FEEDS 



I, Digestibility op Feeds 



In determining the relative usefulness of different feeding stuffs to 

 the animal, it is necessary to find a means of measuring the amount of 

 nutrients which each actually furnishes. The most simple method is 

 to determine the digestibility of the various nutrients ; i. e., the per- 

 centage of the total crude protein, fiber, nitrogen-free extract, and fat 

 in the feed which is digested by the animal. The digestible matter is 

 obviously the only portion of the feed which is of use, since the 

 remainder passes out in the feces without ever having really entered 

 the body. 



A digestion trial. — In studying the digestibility of a given feed the 

 chemist first determines by analysis the percentage of each nutrient it 

 contains. The animal is then fed the feed to be tested for a pre- 

 liminary period of a few days, in order that all residues of former feed 

 may pass from the alimentary canal. Weighed quantities of the feed 

 are then given to the animal and the feces voided during a stated 

 period are collected and weighed, and samples are analyzed. The dif- 

 ference between the amount of each nutrient fed and that found in 

 the feces resulting therefrom represents the digested portion. 



To show the manner in which the digestibility of a feed is deter- 

 mined, let us suppose that during a 10-day trial a cow was fed 20 lbs. of 

 clover hay each day, containing the amounts of nutrients shown in the 

 table. During this time she excreted, on the average, 47.3 lbs. of feces, 

 containing the amounts of undigested dry matter, crude protein, fiber, 

 nitrogen-free extract, and fat shown in the table : 



Digestion trial with coiv fed clover hay; average for 1 day 



Fed 20 lbs. hay, containing 



Excreted 47.3 lbs. feces, containing 



Digested 



Per ct. digested 



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