196 NUTRITION OF FARM ANIMALS 



looked upon in the light of stored energy, a still simpler ex- 

 pression of the nutritive effect may be obtained by determining 

 the effect of the feed upon the store of protein and of chemical 

 energy in the body. 1 



Experiments directed to the determination of the gain or loss 

 of matter and of energy by the body have been of two general 

 kinds, viz., comparative slaughter tests and what are called 

 balance experiments. Both have played an important role in 

 the study of nutrition. 



281. Live weight as a measure of nutritive effect. At the 

 very outset, however, the question arises whether the simple 

 and obvious method of weighing an experimental animal is not 

 sufficient to determine the aggregate effect of a ration, without 

 the necessity for any elaborate experimental devices. 



The answer to this question depends largely upon the object 

 of the experiment. If it be one undertaken to answer a com- 

 mercial question, the increase in live weight during a considerable 

 period, when determined with the necessary precautions, may 

 be entirely adequate as a measure of the results obtained. If, 

 for example, the question under investigation is the relative 

 profits of two methods of fattening cattle, the gains made by a 

 considerable number of animals, together with the judgment of 

 the market regarding the quality of the finished animals, will 

 substantially determine which method is to be preferred. The 

 use of more elaborate experimental methods would not only be 

 a needless refinement but might actually interfere with the 

 settlement of the economic question involved. So, too, in the 

 handling of young stock or in milk production, the general 

 appearance and condition of the animals, together with the gain 

 in live weight or the yield of milk, furnishes a sufficiently ac- 

 curate indication of the practical results obtained, provided a 

 sufficient number of individuals be employed. 



If, however, the purpose of the investigation is to study some 

 question relating to the fundamental principles of nutrition, 



1 To make the demonstration absolutely complete, of course, it would be neces- 

 sary to show that the stock of each different kind of protein in the body had been 

 maintained and that all the energy containing material derived from the feed was 

 actually capable of yielding up its energy to the organism. Usually, however, 

 especially on a mixed diet, it may be assumed that if the body maintains its stock of 

 protein, each particular kind is practically maintained, while no considerable storage 

 of unavailable energy in the body has been recognized. 



