It has to do with the Milk Quantity, Its shown on the cow when you look at the Info.
Also there is the some info on the wiki about it
''Milk
Cows produce Milk at a rate of Milk Quantity * .01 per 10 minutes. That is .1L/10 minutes at Quantity 10. .4L/10 minutes at Quantity 40, 2.0L/10 minutes at Quantity 200. Cows can only store a maximum of 10L Milk at a time.''