Induction of milk ejection and milk removal in different production systems

¹ Veterinary Physiology, Vetsuisse Faculty, University of Bern, Switzerland

^* To whom correspondence should be addressed. E-mail: rupert.bruckmaier{at}physio.unibe.ch.

	Abstract

Milk ejection is important during milking or suckling to obtain the alveolar milk fraction, which can represent more than 80% of the milk stored in the udder of dairy cows. In response to tactile teat stimulation, either manually or by the milking machine, milk ejection is induced by the release of oxytocin and resultant myoepithelial contraction. The time from the start of a tactile stimulation until the occurrence of milk ejection spans between 40 s to >2 min and increases with decreasing degree of udder filling. Therefore, cows need a longer pre-stimulation in late stages of lactation or if the milking is performed shortly after the previous milking, whereas in full udders pre-stimulation is less important. Milk ejection is disturbed under several conditions such as during milking in unfamiliar surroundings (i.e., novel milking environment) or for several weeks immediately after parturition in primiparous cows. Disturbed milk ejection is due to a reduction of or absence of oxytocin release from the pituitary. The severity of disturbed milk ejection and coping capacity towards novel milking environment is related to the cortisol release in response to ACTH (i.e., adrenal cortex activity). Therefore, susceptibility of individual cows to the inhibition of oxytocin release and milk ejection can be predicted by an ACTH challenge test. Comfortable surroundings, such as feeding in and lighting of the milking parlor, can increase the secretion of oxytocin. Overcoming the lack of oxytocin release by injection of exogenous oxytocin for an extended time results in a reduction of the mammary response to endogenous oxytocin. In different production systems it has to be verified that udder stimulation is sufficient to prevent disturbed milk ejection. Different brands of automatic milking systems induce a sufficient pre-stimulation of the udder even if a few minutes are needed for a successful onset of the teat clusters. Specific breeds used for less intense milk production may need the presence of their calves for sufficient oxytocin release during milking. In conclusion, in all milk production systems, a maximal possible reduction of stress has to be targeted and a proper udder pre-stimulation must be performed for an optimal milking in regard to the cow and farmer.

Key Words: Milk removal, Milk ejection, Oxytocin, Mammary gland, Dairy cow