112 NUTRITION OF FARM ANIMALS 



described in a subsequent paragraph (160) showed that out of 

 each 100 grams of protein in the clover hay eaten by the animal 

 53 grams were apparently digested and resorbed. The digesti- 

 bility of the protein in this case, therefore, is said to be 53 per 

 cent. Digestibility in this sense is obviously a conception 

 entirely distinct from that of rapidity or ease of digestion. A 

 feed may have no injurious nor disagreeable effects and yet 

 may have a low percentage digestibility straw, for example. 



Neither does the percentage digestibility alone determine 

 the effect produced by a feed. Two feeds may be equally di- 

 gestible and yet one may be more valuable than the other be- 

 cause its digested matter can be used to better advantage by 

 the body. Nevertheless, it is clear that the indigestible 

 portion of the feed can make no contribution to the nutrition 

 of the body. The first step, therefore, although by no means 

 the last one, in comparing the values of different feeds or rations 

 is to determine as accurately as possible what proportion of 

 each ingredient is capable of digestion. In other words, we 

 shall seek to add to the qualitative knowledge of the processes 

 of digestion and resorption outlined in the preceding sections 

 a quantitative knowledge of the extent of digestion. 



158. Method of digestion experiments. The percentage 

 digestibility of feeding stuffs can, as a rule, be determined only 

 by trial with an animal. Such trials are called digestion ex- 

 periments, and a brief outline of the way in which they are made 

 will aid in understanding just what is meant by digestibility. 



The method is substantially that originated by Henneberg 

 and Stohmann in their early investigations (707). Since it is 

 obviously impossible to collect and measure the substances 

 digested and resorbed from the feed, it is necessary to have 

 recourse to an indirect method, viz., to determine what portions 

 of the feed are not digested and compute by difference the 

 amounts digested. As already stated (155), the undigested 

 portions of the feed are excreted in the feces. A digestion 

 experiment really consists in determining as exactly as may be 

 how much of the feed is thus rejected, any portions of it which 

 disappear during its passage through the alimentary canal being 

 regarded as digested. 



If the feces consisted only of undigested feed residues, the 

 matter would be very simple, but they also include a greater or 



