The Variation of Milk Secretion with Age. 51 



Such an accumulation of exact statistical data on the cows 

 of the Jersey breed for problems of so much interest, both 

 biologically and economically, warrants the application of ade- 

 quate biometrical methods in their analysis. The necessity of 

 such analysis is now well recognized by most investigators as 

 of the utmost importance to our understanding of the funda- 

 mental principles of physiology which underlie the process of 

 milk secretion itself. 



The general problems attacked are those of the individual 

 variation between the lactating functions of different dairy cows 

 at a given age. What is the true type of the frequency distribu- 

 tions of milk production? What relation exists between the 

 mean productions of the successive ages in a true random sam- 

 ple of the Jersey breed? An understanding of these and simi- 

 lar questions is necessary to the fuller utilization of the data 

 found in the herd book of the registry association. 



The data used for study are all from normal, healthy cows. 

 Two diseases have been present in the herd, tuberculosis and 

 abortion. The tuberculous animals were all eliminated early in 

 the herd's history by the use of the tuberculin test. All records 

 from animals which were proven to be tubercular or which 

 aborted were not used. Records from animals normally healthy 

 but sick during a given lactation were not used. All of the 

 cows have been kept in climatic conditions similar to those of- 

 Western Virginia. 



A word as to the method of keeping the data and its trans- 

 fer to this Station. All records are made at the time of milking 

 on the daily milk sheet for the given cow which are kept in the 

 barn. The milking takes place twice a day, the records are 

 for night and morning. The weekly production taken from 

 these sheets is transferred to the herd ledger by a trained book- 

 keeper. The total production for a given month is found to- 

 gether with the yearly production by adding the weekly totals. 

 All records are recorded to pounds and tenths. The cows are 

 tested bi-monthly by the Babcock test and the percentage of 

 butter-fat is recorded beside its corresponding monthly milk 

 yield. All weighings and readings are recorded immediately 

 after they are made so there is little chance of inaccuracy. From 

 these records the author has extracted 1741 complete 8 months 

 records of healthy cows for milk production. Of these 1741, 



