Recently there has been increased interest in the effects of changing the milking frequency of dairy cows because of the possibilities of either increased milking frequency, through the use of robot-milkers or of once-daily milking as a part of a programme of extensification. Archer (1983), reviewing the literature and data from herds monitored by Farm Management Services, found milk yield responses on changing from twice- to thrice-daily milking from -0.14 to +0.49. These differences represent large between-animal differences in responses to thrice-daily milking and there is a need to identify sources of this variation.

Milk is stored in the udder in both the cistern and the secretory alveolar tissue itself. Since milk contains a protein which is inhibitory to secretion, storage within the secretory tissue is disadvantageous and reduces the efficiency of secretory tissue. Work at the Hannah Research Institute has shown that the effect of the inhibitor can be overcome by milking more frequently and that the response to thrice-daily milking of goats was greater with animals having smaller cisterns.