62 Maine Agricultural Experiment Station. 1919. 



The Relation of Milk and Fat Production to Age 



With the analyzed variation data furnished by this study, 

 it is possible to consider the problem of the changes in milk 

 production per unit of time and in mean fat percentage, with 

 advancing age of the cow. The great importance of a thorough 

 and comprehensive knowledge of these relationships, if one 

 is to make any adequate investigation of the inheritance of 

 milk and fat production is sufficiently obvious. It is a perfect- 

 ly well known fact, incorporated in all rules for advanced reg- 

 istry of dairy cattle, that milk production does change with 

 age, and to a marked degree. Until investigations on this sub- 

 ject were undertaken in the Biological Laboratory of the 

 Maine Station some years ago it has always been assumed by 

 those (such as advanced registry officials) who have had to 

 deal with the problem that the changes of milk production 

 with age were linear up to "mature" age, usually taken as 5 

 years, and that after that time there was no further change 

 with advancing age. How far wrong such an assumption is 

 will be shown below. It was pointed out four years ago by 

 Pearl in a preliminary paper based on calculations then com- 

 pleted that the fundamental law of change with milk flow 

 with age is logarithmic* 



Let us now examine the facts for Ayrshires, considering 

 first mean weekly yield. The mean weekly yields in gallons 

 for the combined distributions from age 2 to age 16 inclusive 

 are exhibited in Table 2. The calculated values are those 

 given by a logarithmic curve, of which the equation is 



y = 12.4766 + .6146:1' — .0366A' 2 + 3.6641 log x 



where y denotes mean weekly yield in gallons and x age in 

 years, taking origin from 1 year. 



It is very evident from Table 2 that the change here is 

 logarithmic. No better agreement between observation and 

 theory than that here shown could be expected. The law of 

 change may be stated in words in the following way. In these 

 Ayrshire cattle the absolute amount of milk produced per unit 

 of time increases with the age of the cow until a maximum is 



*See Bulletin 262 this Station. 



