136 THE FEEDING OF ANIMALS 



be sure, either, that the digestibUity of one hay applies 

 to another produced and ctu-ed under totally differ- 

 ent conditions. The truth of this latter statement is 

 clearly seen in the effect of the various factors upon 

 digestibility. 



198. The inacciu-acies of digestion coefficients. — ^The 

 inaccuracies of digestion coefficients are in part those 

 for protein and fats. The errors in the figiu-es for protein 

 are caused by the presence in the feces of nitrogen com- 

 pounds which are not a part of the undigested food pro- 

 tein. (See Pars. 173, 174.) These are waste compoimds 

 which are residues from the bUe and other digestive juices, 

 epithelial cells and mucus which are carried along from 

 the walls of the intestines diu-ing the passage of the food. 

 Their quantity seems not to be proportional to the pro- 

 tein fed, but appears to be influenced more or less by the 

 amount of food digested. Their source is in part the 

 "wear and tear" of the digestive apparatus. It follows then 

 that the less protein there is in a ration, the larger the 

 percentage error caused by these metabolic products. 

 In certain experiments with oat straw, the fecal nitrogen 

 has been more than that of the food, although without 

 question much of the straw protein was digested. It has 

 been found, using the best methods known for extracting 

 these waste products, that they cause a much larger 

 error for the protein of the straws than for that of the 

 legume hays. Under some conditions, at least, ten should 

 be added to the coefficients of digestibility of the protein 

 of coarse fodders as usually given in the tables that have 

 been compiled. 



Errors are caused in determination of the digestibility 

 of fat in much the same way. Certain of the bile residues 

 in the solid excrement are soluble in the ether which is 



