What specific visible indicator during milking identifies excessive vacuum fluctuation at the teat end, and what is its immediate negative impact on udder health?

The specific visible indicator during milking that identifies excessive vacuum fluctuation at the teat end is called liner slip. Liner slip occurs when the teat cup liner, the soft rubber sleeve inside the milking cluster, repeatedly loses and regains its airtight seal around the teat, allowing air to rapidly enter the teat cup at the teat end. This is visibly observed as the teat cup moving up and down the teat, often accompanied by a distinct "squawking" or "hissing" sound as air rushes in, and sometimes visible froth or bubbles around the base of the teat inside the liner. The immediate negative impact on udder health is a significantly increased risk of new intramammary infections, which is the entry of mastitis-causing bacteria into the udder. This happens because the sudden influx of air from liner slip creates a rapid pressure fluctuation, effectively an "impact" or reverse pressure gradient, that forces milk droplets and any bacteria present on the teat skin or within the cluster back up into the teat canal and potentially into the udder cistern, bypassing the teat's natural defense mechanisms.