Effect of stimulation intensity on oxytocin release before, during and after machine milking

Release of oxytocin (OT) is essential for milk ejection in dairy cows (Lefcourt & Akers, 1983; Bruckmaier & Blum, 1998). During milk ejection, alveolar milk is shifted into the cistern, which causes an increase of intracisternal pressure (Bruckmaier et al. 1994). To initiate maximum milk ejection at the start of milking, increasing OT concentration beyond a threshold level is sufficient (Schams et al. 1983). Increasing OT concentration beyond this threshold has no additional effect on intracisternal pressure, i.e., milk ejection (Bruckmaier et al. 1994). Stimulatory effects of milking by hand or by machine or by suckling are well documented (Gorewit et al. 1992; Bar-Peled et al. 1995; Tancin et al. 1995; Bruckmaier & Blum, 1996). At the start of milking, stimulatory effects of machine milking without pre-stimulation or with a manual pre-stimulation and subsequent machine milking cause the release of comparable amounts of OT (Gorewit & Gassman, 1985; Mayer et al. 1985; Bruckmaier & Blum, 1996), whereas the timing of the applied pre-stimulation is important for the shape of the milk flow curve. Should the pre-stimulation period be too short, or absent altogether, the start of the main milk flow is delayed resulting in a bimodal milk flow profile (Bruckmaier & Blum, 1996). Furthermore, the stimulation of only one teat causes an OT release similar to that caused by stimulation of all four teats (Bruckmaier et al. 2001). However, milk production is greater for hand milking or suckling than for machine milking, possibly owing to higher OT concentrations (Gorewit et al. 1992; BarPeled et al. 1995).