The Effect of Complete Evacuation of the Mammary Gland by Pitocin upon Milk and Fat Production

Abstract

It is generally recognized that incomplete removal of the milk from the udder over a period of time will cause a decline in lactation. Partial re- moval of the milk has been used by many as a means of "drying off" cows. Miller and Petersen (4) observed marked downward trends in the lactation curve when cows were stimulated to "let down" their milk 20 minutes before milking began, resulting in incomplete evacuation of the gland. While the reason for this effect of incomplete evacuation of the gland is speculative at the present it can best be explained by increased intra-alveolar pressures created by the retained milk. Petersen and Rigor (5) and Gar- rison and Turner (3) showed a decreased rate of milk secretion with in- creased pressures. The former observed complete stoppage of milk secre- tion when these pressures reached 30 ram. Hg while the latter noted that 40 ram. ttg pressure was needed to completely stop milk secretion. The in- creased milk production observed with increased frequency of milking can also be explained by the hypothesis that this practice prevents the develop- ment of as great pressures or the maintenance of high pressures over as long periods of time as does less frequent milking. Since it has been established that incomplete evacuation of the udder over a period of time will cause a drop in the lactation curve, the question arose as to what part natural incomplete emptying of the gland plays in the decline of lactation with the advance in lactation. A second question is that of the effect upon the lactation curve of cows that are erratic in their let down of milk and if incomplete let down of milk might not be the cause of the rapid decline in the lactation observed in many cases. As the oxytocic principle (2), when injected intravenously, has been shown to practically completely evacuate the alveoli, use of this hormone following milking enables one to ascertain the amount of milk remaining in the gland after a normal milking has been completed. Complete evacuation of the gland at each milking, by use of this hormone, will also make it pos- sible to determine the effect of this procedure upon the lactation curve. EXPERIMENTAL