244 NUTRITION OF FARM ANIMALS 



that the conclusions as to the nutritive value of any material 

 drawn from even the most elaborate chemical and physio- 

 logical investigations will need finally to be checked and con- 

 firmed by the methods of the balance experiment. 



The value of the balance experiment in relation to stock 

 feeding, however, is far from being limited to the summary 

 determination of the total nutritive values of feeding stuffs, 

 although it has rendered important service in that field. 



As will become apparent in Part III, the feed requirements 

 for animals for various purposes, as well as the general physio- 

 logical laws governing the processes of maintenance, growth, 

 fattening, milk production, the performance of work, etc., can 

 be successfully studied only with the aid of balance experi- 

 ments, and the results obtained in such experiments are of 

 general scientific value independent of the particular feeding 

 stuff used. A striking illustration of the importance of such 

 investigations on farm animals is afforded by the results ob- 

 tained by Zuntz and his associates, by Kellner and others re- 

 garding the expenditure of energy in the digestion and assimi- 

 lation of the feed (365-370). The marked differences between 

 these animals and man or carnivora as regards the character 

 of the feed and of the digestive processes have served to make 

 prominent certain factors of the so-called " work of digestion " 

 which were inconspicuous in the latter subjects and thus the 

 investigations have yielded important contributions to com- 

 parative physiology. 



332. Comparison with practical experiments. Finally, it 

 should be observed that the methods of exact feeding experi- 

 ments based on a determination of the balance of matter and 

 energy do not differ in their ultimate logical basis from those of 

 so-called " practical " experiments. In both cases, the meas- 

 ure of the nutritive value of a feeding stuff, of the influence of 

 changed conditions, or of the efficiency of the animal as a food 

 producer, is the effect upon the animal. The difference lies 

 in the accuracy and degree of detail with which that effect is 

 determined. The reasons for the inadequacy of the live weight 

 as a measure of nutritive effect have already been considered 

 (281-283), while the experience of more than 50 years has 

 sufficiently demonstrated that the attempt to measure nutritive 

 effects by changes in the weight of the animal or by the gross 



