Refinement of Feeding Experiments for Milk Production 217 



for milk secretion is reached. Thus, on heavy rations both flesh pro- 

 duction and milk production take place, while on light rations the 

 former may not occur. 



A review of these factors, which specifically cause variation in feeding 

 experiments for milk production, must convince one of the especial need 

 of careful selection of animals and control of experimental conditions 

 in such trials. The necessity of the application of statistical methods 

 to the interpretation of results must also be evident. 



methods of conducting feeding experiments with milch cows 



In attempting a study of the probable error attendant on feeding 

 experiments with milch cows, it is necessary first to consider two rather 

 distinct procedures that have been used in such experiments. In carrying 

 out feeding trials, usually either the alternation or the continuous system 

 has been employed. A considerable survey has been made of the litera- 

 ture of the subject, in order to determine the extent of the use of each of 

 these systems and the evidence regarding their relative merits. 



The alternation system 



In the alternation system competitive rations are fed to the same 

 group of cows in successive periods — Ration A followed by Ration B, 

 this in turn followed by Ration A, and so on. This system appears to 

 have had the widest use both at the American experiment stations and 

 abroad. Hills has conducted extensive experiments with dairy cows 

 since 1887. The extensive nature of these experiments as regards time, 

 coupled with the large number of animals that have been available, has 

 enabled this investigator to make a thoro study of experimental methods. 

 Detailed accounts of this work are given in the annual reports of the 

 Vermont station from 1887 to 1908. Among other features an exhaustive 

 study has been made of the alternation system and modifications of it. 

 The results of the observations of twenty years are embodied by Hills 

 in a paper published in 1907. 



Hills has made considerable study relative to the most satisfactory 

 length of the feeding period in the alternation system. He believes that 

 periods of from two to three weeks are too short. He points out, how- 

 ever, that making the periods as long as eight weeks seriously curtails 

 the number of periods. A feeding period of five or six weeks is favored. 



For the sake of eliminating the influence of seasonal variations. Hills 

 has used two groups of animals — Group I receiving Ration A during 

 the period when Group II received Ration B, and vice versa. 



327 



