42 FEEDS AND FEEDING 



When the total digestible nutrients (including fat X 2.25) in a feed 

 or ration are given, as in Appendix Table III and this sample table, the 

 nutritive ratio may be computed by simply subtracting the digestible 

 crude protein from the total digestible nutrients, and dividing the re- 

 mainder by the digestible crude protein. For example, the nutritive 

 ratio of dent corn may be found thus: (85.7 7.5)-r-7.5=10.4, second 

 term of nutritive ratio. 



71. Carbonaceous feeds; nitrogenous feeds. The term carbonaceous 

 feed, which has recently come into common use, is a convenient designa- 

 tion for a feeding stuff having a wide nutritive ratio. Similarly, the term 

 nitrogenous feed designates a feeding stuff having a narrow nutritive 

 ratio ; i. e., one rich in protein. 



72. Limitations of digestion trials. The data secured in digestion 

 trials concerning the digestibility of feeds form the general basis for the 

 computation of balanced rations for all classes of live stock. Even the 

 net energy values of feeds, which, as we shall see later, are theoretically 

 more accurate measures of the relative values of different feeds than are 

 the amounts of digestible nutrients, have been computed for the most 

 part from tables of digestible nutrients, such as Appendix Table III of 

 this volume. These values for digestible nutrients thus furnish for most 

 feeds the most accurate information we have concerning the usefulness 

 of each feed to farm animals. However, in studying tables of digestion 

 coefficients and digestible nutrients it is well to bear in mind the follow- 

 ing facts concerning the limitations in securing data of this sort: 



In digestion trials it is commonly assumed that all matter appearing in the 

 feces represents the part of the food which is actually indigestible. This is only 

 approximately correct, for the feces always contain in addition some waste from 

 the body itself, such as unabsorbed residues from the bile and other digestive 

 juices, worn-out cells and mucus from the membranes lining the digestive tract, 

 and waste mineral matter. The feces also include innumerable bacteria and 

 their dead remains. In herbivora, such as cattle and horses, which eat much 

 roughage, these products form but a small part of the feces, while in carnivora, 

 such as dogs, they form a considerable portion. All these constituents of the 

 feces are waste products. Therefore, while they do not represent undigested 

 food, it is entirely correct from a practical standpoint to deduct them, along 

 with the food which is actually undigested, in determining the digestible nutri- 

 ents which are of use to an animal. These intestinal waste products which are 

 excreted in the feces are a part of the cost of digesting the feed, as they repre- 

 sent the "wear and tear" of the digestive organs. 2 



In nutrition studies, when it is desired to determine how much of the protein 

 of the feed has actually been digested, the feces may be treated with an acid 

 solution of pepsin. This dissolves practically all the protein compounds in the 

 feces except the true undigested food protein. 



It has been pointed out in the preceding chapter (40) that in the digestion 

 of cellulose and pentosans by bacteria in the paunch of ruminants and to a less 

 extent in the large intestine of other animals, more or less of the carbohydrates 

 are broken down into carbon dioxid and methan gases, which have no nutritive 

 value. Yet, due to the method of computing digestible nutrients, these are 

 commonly included in the amount of carbohydrates considered digestible. This 

 does not involve any serious error even with ruminants, and with other classes 

 of stock the discrepancy is usually negligible. 



Errors are apt to occur in determining the digestibility of the ether extract, or 

 so-called "fat," for fat is usually present in feeding stuffs in relatively small 



2 Armsby, Nutrition of Farm Animals, p. 120. 



