THE FEEDING OF MILCH CATTLE 319 



with 17 g. fat. Between the preliminary and final 

 periods there are sometimes two, three, or more 

 periods with different food-stuffs interpolated, and 

 the calculation of the results is carried out in 

 exactly the same manner as in the above example. 

 It is best before and after each period with a differ- 

 ent food to again have a period with the original 

 ration (basal ration), e.g. in periods 1, 3, 5, 7 to use 

 the same food, and in periods 2, 4, 6 to feed the 

 ration which is to be compared with the basal 

 ration. 



A factor which can influence the result, and is not 

 always to be avoided in this kind of investigation, 

 is the change in the condition of the animals under 

 the influence of the various kinds of food. If, in 

 the above example, the average weight of the 

 animals underwent much alteration in the second 

 period, this change would make itself apparent in 

 the yield of milk in the third period, and the calcula- 

 tion of the natural decrease in the flow of milk 

 would be incorrect. This source of error is avoided 

 by the adoption of the next method — 



(b) The group system. 



In its main features this kind of investigation 

 has already been mentioned (p. 240). In each of 

 the comparative groups ten cows are placed, and 

 all the groups are then given the same food in order 



