Experiments were made to determine the effects on yield and butterfat percentage of three times a day milking and of inefficient milking using cows whose milk secretion rates had been shown to be unaffected by the milk accumulation involved. The milking treatments were applied to half-udders of the same cow to ensure that there was no difference between each pair of experimental units in the amount of oxytocic hormone released by the pituitary gland.

Milking three times a day compared with twice a day over a 39-day period resulted in an increase of 12% in both milk and butterfat yields. Increasing the residual milk by 1 lb in each half-udder for a 39-day period caused a 15% decrease in both milk and butterfat yields. It was concluded that since the cause of these findings could be neither the direct effect of differences in milk accumulation nor the effect of different amounts of oxytocic hormone, some local cell effect was involved, activated either by differences in the amount of residual milk or by long-term effects of differences in milk accumulation.