BUTTEEFAT PEODUCTIOX OF CATTLE 
23 
The data on the Guernsey cows studied indicate that they reach 
their maximum producing ability at 5 years of age, and the period 
of maximum production continues until they are 11 years old. 
The results of this study do not furnish a basis for interbreed 
comparisons, since it embodies Jersey records with and without 
calving requirements; whereas the Guernsey records were all of the 
sso 
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400 
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Fig. 1.— Improvement in butterfat production of Jersey cows, as shown by the average production of all 
records for each class compared with the average production of each thousand records, 1911 to 1921. 
Dotted line: Average production for entire period. Solid line: Average production by groups of 1,000 
records 
