EFFECT OF PRESENCE OF CALF ON MILKING-INDUCED RELEASE OF PROLACTIN AND OXYTOCIN DURING EARLY LACTATION OF DAIRY-COWS

The ability of dairy cows to release either prolactin or oxytocin in response to machine milking in a conventional milking parlor was reduced among cows maintained with their calves during the 1st wk postpartum. Presence of the calf caused no prolonged inhibition; cows evaluated 9 or 23 days after calf removal released similar quantities of prolactin and oxytocin after milking compared to controls (cows whose calves were removed within 24 h after parturition) at the same stage of lactation. Ambient temperature on the day of sampling was positively associated with the premilking serum prolactin concentration among cows housed with or without their calves (correlation coefficients were 0.52 and 0.59), but neither ambient temperature nor premilking serum prolactin concentrations were closely associated with area under milking-induced prolactin response curves (correlation coefficients from -0.15 to 0.18). Unlike basal serum prolactin concentration, ambient temperature on the day of sampling was not closely associated with premilking serum concentrations of oxytocin or quantities released to the milking stimulus (correlation coefficients from -0.19 to -0.12). These results confirm that the calf can inhibit maternal secretion of prolactin and oxytocin in the dairy cow. The capacity of the cow to respond to the milking stimulus is probably independent of premilking serum concentrations of prolactin or oxytocin. Better understanding of factors that regulate the secretion of these hormones at milking may allow development of techniques to enhance hormonal response to milking stimuli and possibly enhance milk production.