646 NUTRITION OF FARM ANIMALS 



the actual amounts of metabolizable energy supplied by the 

 digested portions of the feed irrespective of its secondary effects 

 i.e., to express its real metabolizable energy. Such figures 

 give a more accurate idea of the store of metabolizable energy 

 contained in the feeding stuff regarded by itself, while the ap- 

 parent metabolizable energy is better adapted for use in a dis- 

 cussion of questions of feeding. 1 The distinction is similar to 

 that already discussed in Chapter III (167) between real and 

 apparent digestibility. 



752. Computation of metabolizable energy from digestible 

 nutrients. While, in the absence of a respiration apparatus, 

 the metabolizable energy of a feeding stuff or ration may be 

 estimated with a fair degree of accuracy by the method out- 

 lined in previous paragraphs, not every experimenter is equipped 

 to determine the heats of combustion of the feed and the visible 

 excreta, and no satisfactory method of computing them is avail- 

 able. Various attempts have accordingly been made to compute 

 the metabolizable energy of feeding stuffs from chemical data. 



One such method is that employed by Rubner and by At- 

 water for estimating the metabolizable energy of the food of 

 man and of carnivora as described in Chapter VI (324), their 

 factors for protein, carbohydrates and fat being applied directly 

 to the digestible nutrients of feeding stuffs, and several tables 

 of energy values as thus computed have been published. Later 

 investigations, however, showed that the results thus obtained 

 were much too high in the case of herbivorous animals, es- 

 pecially of ruminants. To cite but a single instance, experi- 

 ments on cattle by the writer 2 gave the results shown in Table 

 189 for metabolizable energy as compared with those computed 

 by the use of Rubner's factors, and Kellner's somewhat earlier 

 results 3 led to the same general conclusion. 



There are two principal reasons for this discrepancy. The 

 first is the extensive fermentation of the carbohydrates in the 

 digestive tract of ruminants, leading to a relatively larger loss 

 of energy in the combustible gases excreted. The second rea- 

 son is the fact that the urine of herbivora carries off much 

 more non-nitrogenous material (224) than is the case with man 

 or carnivora. The results of direct determinations on swine 



1 Compare Armsby, Principles of Animal Nutrition, pp. 291-293 and 333-335. 



2 Penna. Expt. Sta., Bui. 71, p. 7. 



8 Landw. Vers. Stat., 53 (1904), 440-449. 



