Compressive load applied by the teatcup liner to the bovine teat.

The magnitude of the cyclic load applied to the teat apex by the collapsed liner during milking was estimated by measuring the pressure required to cause retrograde flow of milk into the teat canal from a manometer attached to the external teat orifice. The study was extended by observation of the milk flow cycle within a transparent teatcup assembly and by pressure recordings within the teat canal using a catheter-tip transducer. The source of the force compressing the teat is the small airspace, within the collapsed liner, directly below the teat apex. The total force generated is determined mainly by the size of this airspace and the pressure difference (PD) acting across the opposing liner walls. When the PD reached 80-90% of the liner vacuum, the load was just sufficient to occlude the teat canal. Further increase in PD provided the compressive load capable of offsetting vascular congestion of the teat apex. Increasing liner tension increased the load applied. The narrow range of compressive loads applied by conventional liners (5-12 kPa above atmospheric pressure) may be sufficient to offset congestion and oedema in the teat. Loads greater than the mean arterial pressure within the teat apex might serve only to expose the tissues to non-productive compression.