Milk Let-down - an efficient routine

Milk let-down and 'lag time'

The process of milk 'let-down' in the cow is of particular interest as the timing of let-down can be used to form an efficient routine to milk cows as quickly and efficiently as possible while minimising any teat damage that can be caused by 'overmilking' - when there is a high vacuum but little milk flow - and also by acknowledging that the time immediately following milking is crucial to controlling bacterial entry into the teat as the teat sphincter takes time to close post-milking.

Milk let-down is controlled by unconditioned factors, most notably the response to tactile stimuli provided by a calf rubbing the udder or teat when suckling, or a similar stimulus provided by the milker when foremilking the quarter or otherwise preparing it for being milked. Other, conditioned factors, such as the psychological stimuli provided by the sounds, smells and routine the cow experiences at or around milking time also contribute to milk let-down.

These stimuli result in the release of the hormone oxytocin from the cow's pituitary gland in the brain into the bloodstream, where it travels to the udder and causes several important processes to occur.

Oxytocin release causes the mass of interconnecting blood vessels at the base of the teat to fill with blood, making the teat more erect and allowing milk to enter it from higher in the udder and pass through the teat.  Oxytocin also encourages muscles throughout the udder to act to release milk. Most importantly, the muscle cells around the milk-producing alveoli deep in the udder contract and force the milk into the various ducts in the udder, down into the udder cistern and then into the teat cistern, ready for the milk to be removed by the suckling calf or the milking equipment.

This is why during milking, for efficient let-down cows should be subjected to minimal stress, as this can cause the release of the hormone adrenalin (as a response to stress) which can counter the effect of oxytocin.

The average time between beginning to prepare the cow for milking and the resultant let-down of milk is in the order of 60 to 90 seconds. During the period between milkings, an amount of milk will have already collected in the udder and teat cisterns, and will be released almost immediately upon attachment of the milking equipment. There then follows a period known as lag time, whereupon the oxytocin released into the bloodstream causes the release of milk deep in the udder. If the time between the first stimulus of the udder by foremilking or wiping the teats occurs approximately 60 seconds after beginning the process, the release of milk from higher in the udder will be practically continuous with the first release of milk stored in the teat and udder cisterns.

 

Where a longer or shorter lag time occurs, the milk flow can become bimodal; there is effectively a gap where overmilking can occur, even at this early stage of the milking process. Here, the high vacuum from the milking machine combined with a low or nonexistent flow of milk can cause significant damage to the teat end, making the cow more susceptible to mastitis, and likely also to lengthen the milking time significantly.

During milking, the teat lengthens while the teat canal opens up and becomes shorter, to allow faster removal of the milk from the cistern structures above it. Following milking, the overall teat length shortens, the teat canal lengthens and the teat sphincter begins to close, as the folds of skin around the opening close around one another, creating a tight seal, and the lipidised film around the sphincter stops a column of milk forming through which bacterial entry could occur. A waxy keratin seal begins to form in the teat canal to protect against bacterial entry after milking.

However, the sphincter muscle can take in the order of 20 to 30 minutes to close, and it is during this time that the risk of bacterial entry is greatly increased. This is why post-dip treatments play an important role, and also why cows should not be permitted to lie down for a 30 minute period post-milking.

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