Oxytocin release and milk removal in ruminants

Before milking, less than 20% of the milk yielded by dairy cows is stored within the cistern, where it is immediately available for removal. Most of the milk is available for the milking machine only after milk ejection, which occurs in response to tactile teat stimulation and oxytocin release. For complete milk removal, milk ejection is necessary throughout the entire milking process. The continuation of stimulatory effect of the milking machine until the end of milking is, therefore, essential. Premilking teat stimulation causes induction of alveolar milk ejection before the start of milking. Thus, bimodal milk flow curves (i.e., interruption of milk flow after removal of the cisternal milk) are avoided. Continual ejection of milk is dependent on the presence of elevated oxytocin concentrations during the entire milking. Any interruption of the milk ejection process can disturb milk removal. Disruption of milk removal can be caused by peripheral inhibition of oxytocin effects on the mammary gland or by inhibition of oxytocin release by the central nervous system. Peripheral inhibition is induced by elevated concentrations of catecholamines through stimulation of alpha-adrenergic receptors in the mammary gland, likely via changes in ductal resistance. Inhibition of oxytocin release by the central nervous system has been observed in primiparous cows immediately after parturition, during peak estrus, and during milking in unfamiliar surroundings; concentrations of beta-endorphin and cortisol are elevated in this situation. However, the role of endogenous opioid peptides in the inhibition of oxytocin release in cows remains unclear. In conclusion, during machine-milking, the physiological requirements of the cows need to be considered, and, most importantly, stressors must be minimized.