The response in milk energy output to increasing metabolisable energy intake (MEI) is curvilinear and obeys the law of diminishing returns (Blaxter, 1966). This curvilinearity can be explained by the increased partitioning of MEI to body tissue as intake increases. Rationing dairy cows for cost-effective milk production requires prediction of the extent and nature of this partitioning phenomenon. However, the current rationing system in the U.K., the ME system, does not address this issue. The objective of this study was to examine two of the possible animal factors which might influence the partitioning of increments of MEI given above requirements i.e. days in milk (DIM) and physiological state (number of mammary glands milked) of the animal.