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Plasma Oxytocic Activity and Intramammary Pressure in Lactating Dairy Cows¹

Virginia Polytechnic Institute, Blacksburg

ABSTRACT

Oxytocic activity was determined in unextracted plasma of blood drawn from catheterized jugular veins of seven lactating cows, before, during, and after stimulation for milk ejection. Intramammary pressures were recorded concurrent to blood sampling.

Prestimulation levels of oxytocic activity were less than 1 x 10^–3 mU/ml with the exception of one cow in which levels of 1.92 and 2.19 mU/ml were observed. Stimulation for milk ejection resulted in increases in plasma oxytocic activity within four minutes poststimulation. The secretion of high levels of plasma oxytocic activity during the poststimulation period varied between cows. Usually the peak of activity was reached before intramammary pressures reached a peak. Levels as high as 6.41 mU/ml plasma oxytocic activity were observed poststimulation.

Specific levels of plasma oxytocic activity were not related to specific increases in intramammary pressures during the milk ejection period. This may be due to the lower levels of oxytocic activity requiring a longer period of time to initiate an increase in pressures than higher levels, possibly because the lower levels eject less milk per unit time. It is also possible that the plasma oxytocic activity level in jugular vein blood is not necessarily related to the activity in the blood that reaches the udder.

¹ This paper is part of a thesis submitted by the senior author in partial fulfillment of the requirements for the M.S. degree at Virginia Polytechnic Institute.

² Present address: Department of Animal Science, Cornell University, Ithaca, New York 14850.