Factors Affecting Endogenous Plasma Oxytocic Activity in Lactating Holstein Cows

Abstract

Jugular blood samples were obtained through indwelling catheters from each of 14 Holstein cows on days 58, 115, 177, 223, and 262 of lactation. One sample was taken prior to stimulation for milk ejection, and nine samples were taken at consecutive 30-second intervals following the initiation of stimulation. The reproductive status and residual milk yield of each cow at each sampling period were recorded. Plasma oxytocic activity in international units per milliliter increased, reached a maximum, and declined in a quadratic response to advancing time from initiation of stimulation. Both prestimulation and peak post-stimulation activities were higher in nonpregnant than in pregnant cows. Cycling cows with high persistency had lower pre-stimulation and higher peak post-stimulation activities than cycling cows with low persistency. Residual milk was lower in cycling cows with high persistency and early gestation. Both pre-stimulation and peak post-stimulation plasma oxytocic activities were depressed during the second week of estrous cycle as compared to either the first or third weeks, and residual milk yield was greater in the third week than during the previous two weeks of the estrous cycle.