Oxytocin and milk removal: two important sources of variation in milk production and milk quality during and between milkings.

This review describes the effects of milking (routine and management) on milk yield and milk quality on dairy ruminants and examines the physiological effects of milking on the synthesis and secretion of milk. During milking, differences in the composition of milk as a result of milk ejection reflex are observed: the cisternal milk, immediately available, contains little fat, then milk ejection provokes active transport of high-fat content alveolar milk, into the cisternal compartment. Milking frequency has the capacity to affect milk production too. So, an increase in milking frequency augments milk yield whereas a decrease in milking frequency decreases milk production, with effects on milk composition. The milk ejection reflex is mediated by oxytocin, which induces myoepithelial cell contraction. Nevertheless, other actions of oxytocin may exist, such as a direct effect on proliferation and differentiation of myoepithelial cells and on secretory processes in the mammary epithelial cells.

Effect of calf suckling on oxytocin, prolactin, growth hormone and milk yield in crossbred Gir x Holstein cows during milking.

This study evaluated the effect of different milking managements on oxytocin, prolactin and growth hormone release in Gir x Holstein cows. Six cows were suckled by their calves, before and after milking (SM group); another six cows were submitted to exclusive milking (M group). High OT levels were observed during suckling of the SM group, however both groups had similar levels of OT during milking. The SM group presented PRL levels significantly higher than the M group, however only during suckling. For GH levels, the SM group showed higher levels than the M group, however this difference was significant only during the first suckling, cleaning of the teats and at the beginning of machine milking. Despite these results, SM cows produced more milk (milking plus suckling) than M cows, however milk obtained by milking was similar for both groups.

Plasma levels of cortisol and oxytocin, and uterine activity after cervical artificial insemination in the ewe.

The objective was to compare in the ewe the effects of easy and difficult procedures for artificial insemination (AI) (as related to rapid or poor accessibility of the cervix, respectively) on plasma cortisol (CORT) and oxytocin (OT), and uterine motility. All AI were simulated using a catheter empty of semen to study genital and environmental stimuli only. In experiment 1, 40 ewes were sampled after Al, and whether it was an easy or difficult procedure was reported for each animal. While CORT concentrations rose to a similar amount in all ewes, whatever the Al procedure, a significant OT response occurred after a difficult procedure only (n = 18) (17.4 +/- 1.7 versus 12.7 +/- 0.7 pg x mL(-1) before Al, p < 0.05). In experiment 2, uterine activity was monitored in 4 ewes using an implantable telemetric transmitter equipped with an intrauterine pressure catheter. An increased uterine activity occurred during 2 +/- 1 min after an easy Al (n = 5), whereas the evoked activity lasted for 15 +/- 4 min after a difficult Al (p < 0.001, n = 7). A similar long-lasting response occurred after OT administration (100 mIU, i.v.). We concluded that the increase in uterine motility after a difficult Al resulted from a reflex release of OT, and not to a "stress" effect.