Liner condition and interaction with teats plays a critical role in maintaining healthy teat ends and preventing mastitis infections. Learning more about how teats and liners interact will allow us to fine-tune the balance of efficient and quick milk-out with an appropriate level of pressure which does not lead to teat end damage.
In several recent papers, Dr. Graeme Mein from Weribee, Victoria, Australia, and Dr. Doug Reinemann, University of Wisconsin-Madison looked more closely at the pressure applied to teats during the "rest-phase” of each pulsation cycle. Specifically, their papers look at the degree of liner compression applied to the teat during the d-phase or rest-phase. This has a marked influence on teat condition, cow comfort, and peak milk flow-rate.
Mein and Reinemann point out that it is critical to understand liner compression so we can control its effects on teat end condition. The pressure difference acting across the liner walls, especially over the area surrounding the small airspace directly underneath the teat, is the source of the force that compresses and massages the teat during the d-phase of pulsation.
They define liner compression (LC) as the average compressive pressure applied to the inner tissues of the teat apex by the liner during the rest phase of pulsation. This differs from the touch point pressures or other measures taken at the teat - liner interface, and gives us a more true measurement of the pressure being applied by the liner to the teat.
Why it's important that we look more closely at liner compression is because of the wide variety of results seen using the exact same pulsator setting. In their research of commercially available liners, Mein and Reinemann have seen a range in pressure applied from less than 3 kPa (about 1 inch of Hg) up to more than 20 kPa (6 inches of Hg) depending on the style and type of liner used. There are several factors which induce this variation.
Liner barrel wall thickness and material hardness will both initially cause an increase in liner compression at lower levels. For wall thickness up to around 2 millimeters or hardness measures from 35 to 50 ShoreA, this will be the case. Above those measures flexibility of the material will be reduced, leading to a reduction in liner compression because there is greater resistance to bending around the teat apex and to bending along folded edges.
A higher mounting tension of the liner in its shell will cause a larger airspace beneath the teat end and lead to greater resistance to bending around it. This also will cause a higher liner compression.
The liner's cross-sectional shape will impact liner compression as well. Triangular liners have thicker edges which will work to hold it open, resulting in a loss of liner compression. Square liners are more variable depending on how the milking equipment is designed to function.
The two milking management experts recommend targeting a liner over-pressure within the range of 8 to 12 kPa (2.4 – 3.5 inches of Hg) to achieve the main purposes of pulsation and maintain good teat end health. Values below 8 kPa (2.4 inches of Hg) may be too low to fully relieve the teat wall congestion induced by the milking vacuum during the b-phase of pulsation. This can lead to reduced milking speeds. Increasing the liner over-pressure will lead to greater peak milk flow-rate with the largest increases seen at a higher milking vacuum and longer b-phase. However, teat end condition will begin to suffer at these higher compressive pressures.
Mein, G.A. and D.J. Reinemann. 2009. Liner Compression Applied to Teats During the ‘Rest Phase' of Each Pulsation Cycle: Everything You Always Wanted to Know. Proc. NMC Annual Meeting, pp 152 – 153.
Mein, G.A. and D.J. Reinemann. 2009. Biomechanics of Milking: Teat – Liner Interactions. Proc. American Society of Agricultural and Biological Engineers, paper number 09743.
Mein, G.A., D. M. Williams and D.J. Reinemann. 2003. Effects of Milking on Teat-End Hyperkeratosis: 1. Mechanical Forces Applied by the TeatCup Liner and Responses of the Teat. Proc. NMC Annual Meeting, pp 114 – 123.