OXYTOCIN RELEASE AND MILK REMOVAL IN RUMINANTS

Before milking, less than 20% of the milk yielded by dairy cows is sto red 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 ox ytocin 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 m ilk ejection before the start of milking. Thus, bimodal milk now curve s (i.e., interruption of milk now 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 int erruption of the milk ejection process can disturb milk removal. Disru ption of milk removal can be caused by peripheral inhibition of oxytoc in effects on the mammary gland or by inhibition of oxytocin release b y the central nervous system. Peripheral inhibition is induced by elev ated concentrations of catecholamines through stimulation of alpha-adr energic receptors in the mammary gland, likely via changes in ductal r esistance. Inhibition of oxytocin release by the central nervous syste m has been observed in primiparous cows immediately after parturition, during peak estrus, and during milking in unfamiliar surroundings; co ncentrations of beta-endorphin and cortisol are elevated in this situa tion. However, the role of endogenous opioid peptides in the inhibitio n of oxytocin release in cows remains unclear. In conclusion, during m achine-milking, the physiological requirements of the cows need to be considered, and, most importantly, stressors must be minimized.