204 THE FEEDING OF ANIMALS 



consider experiments in testing rations for growth and 

 milk production. The usual method of procedure with 

 such feeding trials is either to feed two lots of animals 

 on the rations to be compared and note the compara- 

 tive growth or milk yield, or to feed the same lot 

 on one ration for a time and then change to another 

 ration. 



If these tests are made with growing or fattening 

 animals, the increase in live weight is taken as the meas- 

 ure of the relative efficiency of the rations compared. 

 It should be said of these experiments that then* appar- 

 ent verdict is to be accepted with great caution, and 

 definite conclusions are not justified until repeated 

 extended trials of two rations or of two systems of feed- 

 ing, made with the use of all possible precautions against 

 error, and under a variety of conditions, give uniform 

 and consistent results in the same direction. There are 

 several reasons why this is so, the main one being that 

 the increase in the weight of an animal is an uncer- 

 tain measure of actual growth. Variations in the con- 

 tents of the alimentary canal due to the irregularity of 

 fecal discharge and to a lack of uniformity in the water 

 drank may cause temporary variations in the live weight 

 of considerable magnitude. Moreover, the nature of the 

 growth of body substance is revealed neither by the mere 

 weighing of an animal nor by his general appearance. 

 Even if the changes in weight are due to an increase of 

 body tissue, this may be more largely water in one case 

 than in another, so that the real contribution of the food 

 to the dry substance of the body may not be shown. 

 Nor is the character of the solids deposited in the animal 

 discovered by merely weighing him. In fact, by such 

 practical experiments we simply learn that one set of 



