It may be that during milking, the bacteria, trapped in the keratin, are flushed out by the rapid movement of milk through the canal before they can pass completely through and cause new infections. When milking is stopped the flushing action no longer occurs and so whatever bacteria are there may be able to pass through and enter the gland. Based on this logic, 2x or 3x milking may actually help reduce mastitis by flushing out bacteria. A nother major management practice change, occurring at dry off, is that teat dipping after each milking and possibly prior to each milking, is terminated. Teat dipping with quality products is a major factor in preventing new infections during lactation.
Effective dips kill most bacteria on the teat end and reduce the exposure and threat.
At dry off it is no longer in use, therefore teat end bacteria can grow, uncontrolled, and this increases the possibility of new infections. O ther factors may also play a role. The dry gland appears to be quite resistant to infection. Partly it is due to the keratin plug protecting the gland from invading bacteria. It also appears that the gland develops internal systems that limit certain bacteria from growing.