July 8, 1918 
Percentage of Fat in Cow's Milk 
73 
In no one of the four groups classified as to age is there a significant 
correlation, meaning that for these classes of individuals studied the 
milk yield and percentage of fat are independent. It is curious that this 
breed should have a different relation existing between fat and milk than 
that of the Jerseys, Guernseys, and Holstein-Friesians. This result is 
probably due to the fact that they are a much more highly selected group 
of individuals than in the case of any of the other breeds, owing to the 
requirements for registry imposed by the Ayrshire Association. The 
minimum requirements are as follows: 
Age. 
Weight of milk. 
Weight of fat. 
Years. 
2 tO 3 . 
Pounds. 
6 , OOO 
6, 500 
7, 500 
8, 500 
Pounds. 
214.3 
C36.O 
279. O 
322. O 
5 to 4 . 
4 to . 
5 and over. 
This would mean that upon the basis of these specified amounts the 
percentages of fat for the different ages would be as follows: 
2 to 3 years.. 3. 572 
3 to 4 years. 3-631 
4 to 5 years.... 3. 720 
5 years and over.... 3. 788 
A gradual increase in the percentage of fat as the age increases is to 
be noted. This is contrary to what was found to exist for the other 
breeds. For Table XX, constructed from Tables XVI to XIX, a corre¬ 
lation of —0.138 ±0.020 is found. This is practically the same corre¬ 
lation found for the Holstein-Friesian cows. This negative correlation 
resulting from combining the subgroups of Ayrshires is to be expected, 
since there is an increase in the milk yield with age and a decrease in 
the percentage of fat. (See Table E.) It may be that the require¬ 
ments for entry in the register of the Ayrshire Association tend to elim¬ 
inate high-yielding cows with a low percentage of fat and low-yielding 
cows with a high percentage of fat. 
The correlation between the yield and percentage of fat for 98 Ayr¬ 
shire cows which were tested and failed to meet the requirements was 
found to be 
r = —0.226 ±0.065. 
A larger number of such tests would be necessary to determine whether 
a difference exists between cows which meet the requirements and those 
which do not. 
