Galactopoietic effect of milking in lactating Holstein cows: Role of physiological doses of oxytocin

The aim of the study was to delineate the components of the galactopoietic effect of milking in lactating cows. After individual determination of oxytocin physiological injection doses, eight Holstein cows were assigned to treatments over a 5-10-day period: two daily milkings (TD) as control, four daily milkings to measure the effect of additional milking, TD + two oxytocin injections to measure the oxytocin effect without milking, TD + two milkings after Atosiban (oxytocin receptor-blocking agent) injections to measure the effect of milking alone (i.e., without the oxytocin effect), TD + two udder emptyings via cannula and oxytocin injection to measure the effect of milking with a limited systemic hormonal discharge other than oxytocin. Daily milk yields, milk composition and plasmatic oxytocin and prolactin concentrations were measured. Additional milking increases milk production by 8% (P < 0.05) and total milk constituents by 6%, confirming its galactopoietic effect. Injections of physiological doses of oxytocin induce an intermediate galactopoietic effect between the twice and four daily milkings, but only when accompanied by milk removal. This shows that the prevention of alveolar milk stasis is essential to measure the galactopoietic effect. The limitation of systemic hormonal release, other than oxytocin, during additional milking increases milk production by only 4.4% (not significant), suggesting a limited role of systemic hormones for the expression of the galactopoietic effect of milking compared to oxytocin. An additional role of oxytocin on mammary epithelium, coexpressed with alveoli emptying, cannot be excluded.