150 Maine Agricultural Experiment Station. 1920. 



differential action of the mechanism or effect of environment on 

 the milk production and butter- fat percentage at these different 

 ages. They do, however, lead to the important practical conclu- 

 sion that a cow commencing her lactations as a two year old with 

 a. high butter-fat percentage may be expected to duplicate this 

 relatively high performance within a small error in the next and 

 succeeding lactations. The first lactation records as to the butter- 

 fat percentage that a given cow will produce, are a good index of 

 what may be expected in future years of that cow, as will be 

 shown in a subsequent section. The selection of cows to remain 

 in the herd on the basis of these records is profitable to the dairy- 

 man. 



The comparison of these correlations with those on milk 

 production for the same date using the same divisions is of con- 

 siderable interest as showing the relative strength by which one 

 lactation governs the future production of another lactation. Table 

 2 gives this comparison for the average unweighted coefficients 

 of correlation for the records of each age with the records made 

 at another age. 



TABLE 2. 



Average Coefficients of Correlation for Lactation Records Made 



at a Given Age and Lactation Records Made at Other Ages 



for Milk Production and Butter-Fat Percentage. 





nich given 



Coefficient 



of Correlation 





Age at tv 







Difference 



record i 



s made 



Butter-Fat 



Milk Pro- 



Butter-Fat Per- 







Percentage 



duction 



centage Milk 

 Production 



2 years to 



3 vears 



+0.5696 



0.5491 



.0205 



3 years to 



4 years 



.5011 



.5694 



—.0683 



4 years to 



5 years 



.5526 



.5060 



.0466 



5 years to 



6 years 



.5514 



.5735 



—.0221 



• 6 years to 



7 years 



.5150 



.5661 



—.0511 



7 years to 



8 years 



.4967 



.5501 



—.0534 



8 years to 



10 vears 



.5156 



.5077 



.0079 



"10 years and older 



.4732 



.4597 



.0105 



Average of 



records at 









all ages 





.5215 



.5352 



—.0137 



These correlation coefficients range in value from -(-0.4702 

 to 0.5696 for the butter-fat percentage and from -f-0.4597 t0 

 0.5694 for the milk production. The average value of the butter- 



