MILK PRODUCTION 175 



of the animal and the yield of milk are quite independent. 

 The former determines the amount of coarse fodder, and the 

 latter the amount of concentrated food. The digestible 

 nutrients of the maintenance portion whichever food it may 

 be derived from undergo oxidation, and the value of the 

 food corresponds to the amount of thermic energy thereby 

 produced. The nutrients of the special food for milk pro- 

 duction undergo sundry changes, but they are not oxidised, 

 and the value of the food depends upon the readiness and 

 completeness with which they are converted into the con- 

 stituents of the milk. 



No advantage is to be gained by adding together the 

 nutrients of the two kinds of food, and for purposes of calcula- 

 tion the author thinks it desirable to keep them distinct. 



Influence of the Food. It is extremely difficult to ascer- 

 tain exactly what is the effect of any food upon the yield 

 and composition of milk, because so many disturbing factors 

 tend to obscure it. Two methods have been chiefly em- 

 ployed in experiments of this kind. They are known as 

 the group system and the period system respectively. ' 



In the group system a number of cows say twenty are 

 divided into two groups of ten each ; and they are so arranged 

 that the total yield of milk and all other circumstances in each 

 group are as nearly as possible alike. The cows in each group 

 are generally given the same maintenance ration, but different 

 food for milk production ; and any differences in the yield or 

 quality of the milk are attributed to this difference in feeding. 



In the period system, all the cows are fed alike, and the 

 time during which the experiment lasts is divided into three 

 equal periods of, say, twenty-eight days each. The cows receive 

 the same rations in the first and third periods, but a different 

 one in the middle or intermediate period. Both yield and 

 quality of the milk change continuously as the stage of lactation 

 advances, but both the direction and magnitude of the change 

 are approximately constant under constant conditions. Any 

 disturbance of the constancy of the change observed during 

 the middle period may, therefore, be attributed to the influence 

 of the food given at that time. 



