136 Maine Agricultural Experiment Station. 1920. 



the herd's history by the use of the tubercular test. All rec- 

 ords 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 dairy milk sheet for the given cow which are kept in the 

 barn. The milking taking 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 bookkeeper. 

 The total production for a given month is found together 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 rec- 

 ords the author has extracted 1741 complete 8 months records of 

 healthy cows for milk production. Of these 1741, 1713 have 

 records for the butter-fat per cent. The weighted monthly 

 averages of the bi-monthly test have been used to obtain the 

 weighted 8 months average for the 8 months lactation period 

 chosen for study." 



Variation of Fat Percentage in Jersey Milk With the 

 Age When the Test Was Made. 



The records for the mean butter-fat percentage for the 8 

 months of lactation have been calculated by the author for all 

 cows and for each lactation. 



The chief physical constants for these data are presented in 

 Table I. The four constants presented are the mean, standard 

 deviation, coefficient of variation and skewness. 



Several features of general interest concerning butter-fat 

 secretion are evidenced by this table. These points can only be 

 touched hurriedly as it is planned to take up most of them in- 

 dividually in later sections of this paper. 



