CHAPTER III 



MEASURING 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 each class of nutrients 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. 



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

 the chemist first determines by analysis the percentage of each nutrient 

 it contains. An animal is then given 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 digestive tract. 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 amount of 

 each nutrient which is digested is found by subtracting the amount 

 which is excreted in the feces from the total amount in the feed. The 

 amount voided in the urine is not deducted in addition, due to the fact 

 that the compounds in the urine are not undigested food, but are waste 

 products which have come from the breaking down within the body 

 proper of substances which have at some previous time been digested 

 and absorbed from the digestive tract. 



To show the manner in which the digestibility of a feed is determined, 

 the following results are given from an actual digestion trial conducted 

 by Armsby at the Pennsylvania Station. 1 To find the digestibility of 

 red clover hay he fed a steer 5,198.2 grams (about 11.5 Ibs.) of clover 

 hay daily during a 10-day trial, after the steer had already been fed 

 the same ration for several days previously. By analyzing the feed, 

 it was found that the hay fed each day contained the amount of nutri- 

 ents shown in the table on the next page. The feces were collected by 

 means of a rubber duct and a harness which is used in such trials. 

 On analysis, they were found to contain the amounts indicated of un- 

 digested dry matter, crude protein, fiber, nitrogen-free extract, and fat. 



The table shows that the hay fed daily contained 4,459.0 grams of 

 dry matter. Of this amount there was excreted in the feces 1,822.3 

 grams, leaving 2,636.7 grams, or 59.1 per ct., as the digested part of the 



'Perm. Bpt. 1904-05, Part II, pp. 124-126. 



37 



