Endocrine responses of cows subjected to controlled voltages during milking.

For the relative influence of central and of peripheral inhibitive mechanisms on the milk-ejection reflex, dairy cows were subjected to control voltages during milking. Integrated 1-min jugular vein samples were taken at -20, -12, -9, -2, -1, 2, 4, 6, 10, and 20 min relative to the start of milking for measurement of norepinephrine, epinephrine, dopamine, oxytocin, and prolactin. Cows were subjected to voltages that resulted in body currents of 5 mA. For the response of cows to continuously applied voltage, the voltage remained on for 20 min starting 10 min prior to milking. For the response to intermittently applied voltage, the voltage was on for 5 of every 30 s. Milk yield and milking time were decreased in cows subjected to stimulation by intermittent voltage. This decrease could not be explained by changes in peripheral concentrations of either oxytocin or catecholamines. Plasma prolactin did decrease; however, it seems unlikely the prolactin would affect the milk-ejection reflex. We hypothesize that this decrease in milk yield resulted from neural mediated changes within the mammary gland.