The Influence of Oxytocin-Induced Udder Evacuation on Milk and Butterfat Production in a Complete Lactation

Summary One set of identical twin cows was observed through two complete lactations and the first 10 weeks of their third lactation. In the first lactation, milk production was extremely low for both mates under like conditions. In the second lactation, one mate was used as a control while oxytocin was administered to the other in amounts varying from 30 to 1.5 I.U. per injection immediately following each regular milking, starting shortly after freshening. The residual milk was then removed. In the third lactation the mates were reversed as control and experimental subjects. In the second lactation, the experimental animal produced 5.7 times as much milk and 5.6 times as much butterfat as her mate. In the third lactation, the experimental animal again out-produced the control in the first 10 weeks in approximately the same manner as in the second lactation. Oxytocin appeared to be quite effective in amounts down to 1.5 I.U. per injection based on the early assay, although fright or excitement raised the threshold level to more than four times this amount, as noted by lacks of response to repeated injections on certain occasions. The exact mechanism whereby this increase in milk production was brought about is unknown. Not only did the complete ejection of milk from the mammary gland allow for the maximum space for accumulation of milk before secretory pressures were balanced by intra-alveolar pressures; it appeared to be stimulatory beyond the space factor alone.