Sept. IS, 1919 



Variation in Milk of Ayrshire Cows 



317 



Table X.— Comparison of observed mean weekly yields at different ages ivilh those cal- 

 culated on the assumption that the change is logarithm,ic 



With the equation relating to mean weekly yield and age in hand we 

 may consider the important problem of the age at which milk produc- 

 tion is at a maximum in these cows. To get an answer to this question 



we have obviously only to equate -j- to zero and solve for x. 



We have 



^=0. 6146-0. 0732x + -^-^. 



dy 



When -3^=0, we have 

 dx 



3^=10.4720. 



Or, we may say that in the large group of cows here dealt with the 

 maximum rate of milk production per unit of time is reached only when 

 the cow is io>2 years old. 



Turning next to the relation of fat percentage to age, we have the es- 

 sential data exhibited in Table XI, the values being taken from Table IV. 



Table XI. — Mean fat percentage at different ages 



From an examination of the observed figures it appears that in general 

 the fat percentage tends to decline with advancing age until the tenth 



