140 The Feeding of Animals 



and total organic matter represent, we have reason to 

 believe, very nearly the actual digestible matter in the 

 particular material studied. The proportions secured 

 for particular classes of nutrients may be less accurate, 

 for reasons that will appear. We cannot be sure, either, 

 that the digestibility of one hay applies to another 

 produced and cured under totally different conditions. 

 The truth of this latter statement is clearly seen in the 

 effect of the various factors upon digestibility. 



The inaccuracies of digestion coefficients are chiefly 

 in those for protein and fats. Let us see how and why 

 this is. The errors in the figures for protein are caused 

 by the presence in the feces of nitrogen compounds 

 which are not a part of the undigested food protein. 

 These are waste compounds which are residues from the 

 bile and other digestive juices, epithelial cells and mucus 

 which are carried along from the walls of the intestines 

 during the passage of the food. Their quantity seems 

 not to be proportional to the protein fed, but appears 

 to be influenced more or less by the amount of food 

 digested. Their source is the "wear and tear" of the 

 digestive apparatus. It follows then that the less pro- 

 tein there is in a ration, the larger the percentage error 

 caused by these metabolic products. In certain experi- 

 ments 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. 

 It is probably safe to affirm that at least ten should 



