Oxytocin Levels in the Plasma of Holstein-Friesian Cows During Milking with and without a Premilking Stimulus

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Abstract
Blood oxytocin concentrations were deter- mined on 9 Holstein-Friesian cows during normal milking and during milking with- out a premilking stimulation at 1 to 2, 5 to 6, and 15 to 16 weeks postpartum. The average peak oxytocin concentration during normal milking was 139.2/zU per milliliter plasma and during milking without pre- milking stimulation was 122.9. The differ- ence was not statistically significant. The premilking stimulation caused an earlier release of oxytocin from the neurohypophy- sis into the blood and this resulted in a shorter machine-on time. The elimination of the premilking stimulation did not de- crease milk production. The afternoon milk yield during normal milking was 9.89 kg and that during milking without premilking stimulation was 9.95 kg. The stimulus from the application of the milking machine ap- parently caused an adequate release of oxy- tocin for maximum milk ejection and a normal milk yield. The oxytocin concentra- tion in the blood plasma declined rapidly after the peak concentration. By 5 minutes after teat cup application all values were below 2.5 ~U per milliliter plasma. By 5 minutes after the end of milking all values had dropped to less than 0.01 ~U. There was a tendency for the peak levels of oxy- tocin concentration to decrease with advanc- ing lactation; however, the differences were not statistically significant.