Much of the mastitis research in the last fifty years has been directed toward trying to understand the role of the milking machine in the causation of this disease. Out of this work has emerged three main mechanisms of infection that are machine associated. These three mechanisms are: 1) Transfer of contagious organisms on milk contact surfaces from one cow to the next. 2) Damage to teat end health and 3) Teat end impacts with bacteria laden milk droplets due to pressure fronts or vacuum fluctuation in the unit. This discussion concerns itself with role of the liner shield in protecting the teat from the consequences of vacuum fluctuation.

Vacuum fluctuation has been analyzed to be composed of two principle components: cyclic and irregular fluctuations. Cyclic vacuum fluctuations are due to the movement of the liner wall in the normal pulsation cycle and therefore cannot be completely eliminated in the conventional milking machine. Use of larger capacity claws, alternating pulsation, claw venting, larger capacity claws stem and long milk tube drainage are all design elements that have reduced the magnitude of cyclic fluctuations.

Irregular vacuum fluctuations are caused by unplanned events such as pipeline flooding, a malfunctioning vacuum controller, a milking unit fall off, liner slipping, and removal of inflations without first shutting off vacuum to the claw. System design changes (adequately sized pipelines and vacuum pumps), attention to vacuum controller function (controller design sophistication and maintenance), as well as emphasis on improved milker technique, all have helped reduce irregular vacuum fluctuation. Liner slipping is now probably the most common source of problems in this area even on newer installations.

Experimental evidence points to the conclusion that vacuum fluctuations by themselves are not dangerous to the cow unless they cause aerosols of milk droplets with bacteria to be thrown at and impact the teat orifice. These teat end impacts can force bacteria backward through the teat orifice leading to infection of the gland. It has been shown that these impacts are more likely to occur when there is a combination of cyclic and irregular vacuum fluctuation at the end of milking. An example of this would be to have a liner slip on a front teat at the same time the liner in a rear teat is going into the opening phase at the end of milk flow. Indeed it is suspected now that most of the hazard of over milking is due to increased opportunity for teat impacts to occur during this vulnerable period.

In the late seventies, the National Institutes for Research in Dairying, in England evaluated various design elements to protect the teat from impacts. They looked at liner venting, quarter milking clusters, and liner shielding and measured their effect on reducing the frequency of teat end impacts.

Liner Shield
This work indicated that shields prevented penetration of the teat canal 70% - 90% over controls. They showed that shields prevented transfer of bacteria within the cluster to the protected liner about 50% of the time as did short milk tube air venting. Together, the shield plus the short milk tube air venting reduced bacterial transfer over 90%. This compared to 100% prevention for quarter milking claws.

The relative importance of teat end impacts under commercial conditions in the U.S. is not known, but it is reasonable to assume that its relative importance as compared to other mechanisms for causation of mastitis varies form farm to farm. In field studies in Australia and England there was a 10% to 50% reduction in the new infection rate in commercial herds depending on the quality of the installation. Currently there are no means by which one can determine that benefit of shielding the liners on any particular herd, other than to put them in and measure the reduction in the new infection rate.

One final side benefit of the shield that many users of backflush can appreciate is the solution to the problem of the "pipe-streaming" effect. Many people using black rubber liners have not seen this phenomenon but it is quite apparent in the clear funnel bottom liners. When back flushing the unit, sometimes the sanitizing solution enters the bottom of the funnel and pipe-streams part way down the liner before the solution hits the liner wall. At the mouthpart of liner it looks like the solution is coming out on all surfaces, but the base of the liner may not have been touched. The shield can improve the effectiveness of the backflushing procedure by directing the sanitizer toward the liner wall starting right at the base of the liner.

The liner shield is a clear molded device made of FDA milk contact approved polysulfone. It easily inserts into the funnel portion of the Silicone Plastics™ SP-6000. This device was designed to redirect air movement through the funnel, without restricting the flow of milk toward the claw. The proper installation position for the shield is with the elevated dome portion pointing up toward the teat.

In the SP-6000, the shield is free to move about inside the funnel cavity and indeed observing the movement of the shield under milking condition gives dramatic testimony both to the profound forces in the liner chamber and hence the need for the shield itself. The free movement of the shield causes no adverse effects and allows movement of milk around its edge during milking and of wash water during washing. Problems with washing have not been observed nor are they expected. Occasionally the shield may come to rest on a slant within the funnel. This does not result in any impairment of function provided the funnel is fully seated on the liner cartridge and the shield is in the proper upright position. The shield can easily return to its horizontal position when this happens. There is sufficient clearance between the edge of the shield and the funnel wall so that even if the shield has settled in front of the funnel air vent hole, it will not result in obstruction of the liner vent.

When the milk is exiting the teat it may hit the dome portion of the shield and be directed toward the funnel wall above the shield. Some of this milk will pass by the edge of the shield and run evenly down the funnel wall below the shield. This can result in a sort of a "white out" effect on the funnel wall that may be confused with liner flooding. Liner flooding is a very common event in unvented liners but will only occur in a vented liner if due to a plugged air vent. It is easy for experienced SP-6000 users to appreciate the difference in appearance between the "white out" effect due to the shield doing its "redirection" function and the "flooding" of the liner chamber due to a plugged air vent. When on e observes the shield "floating" up and down with each pulsation cycle one can assume that the liner is indeed flooded and the vent needs clearing.

Liner shielding is a simple, permanent and inexpensive design change that greatly improves the safety of the milking machine, and all producers should seriously consider taking advantage of this important tool in mastitis prevention.

Udder Health Systems, Inc.
6401 Old Guide Rd
Bellingham, WA 98226
Lab 360-398-1360


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