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; Bar-Peled et al. 1995). In addition, exogenous OT in supraphysiological doses improves udder evacuation and enhances milk production (Nostrand et al. 1991; Ballou et al. 1993; Bruckmaier et al. 1994; Knight, 1994). It has been shown that OT concentrations must be elevated throughout the entire milking process to allow complete milk removal (Bruckmaier et al. 1994). It is possible that the importance of OT concentrations beyond the premilking threshold might change towards the end of milk removal. The quarter distribution and the quarter milking time are often very unequal (Wellnitz et al. 1999). Therefore, in most cows individual quarters are overmilked while milk is still removed from others. The goal of this study was to test the hypothesis that OT release depends on the intensity of the stimulus and that a stimulus of low intensity is more effective at the start of milking than at the end of milking. Therefore the stimulatory effect of attached teat cups without pulsation before and after milking was tested. In addition, we tested whether a vibration stimulation of one single teat, without prolongation of the total milking time, towards the end of milking was able to induce additional OT release to improve udder evacuation.