When working in a milking parlor, it doesn’t take very long to learn how critical sufficient udder and teat stimulation is to milk letdown. If that doesn’t happen, what occurs instead can be described as bimodal milk letdown.

“The bimodal letdown happens when we don’t stimulate [the cow] long enough and we don’t wait long enough for the oxytocin to arrive from the posterior pituitary after we have stimulated her teats, simulating the calf nursing her,” explained Roger Thomson on the November 17 Hoard’s Dairyman DairyLivestream.

He continued on to describe the two “modes” of bimodal milking. “When we violate the physiology of the cow, we end up getting a little milk that comes out initially — that’s the pre-milk in the cisternal part of the udder — then no flow, and then we get the oxytocin-released glandular milk,” said the veterinarian who has done extensive research on the topic with his team at Michigan State University. He noted that in cattle, 80% to 90% of milk is stored in the glandular part of the udder and requires oxytocin to be let down.

The no-flow period is what is extended when the milking process is rushed. The larger herds and larger parlors that the industry has seen in recent years are not problems in this regard on their own, Thomson said. “But what we do have is an urgency or need to milk more cows faster and faster. What we end up compromising will be steps to prep the cow: the stimulation and the latency, or lag time, until we attach the unit,” he cautioned.

A delay in milk letdown has significant influences on udder health as well as milk production. Vacuum pressure on the teat when no milk is being ejected can damage teat ends, and with incomplete letdown, cows return to the barn with milk remaining in their udder. This can open the door to teat and mammary gland infections.

Thomson also explained that the longer milk letdown is delayed, the more milk is lost in that milking instance since it is not released from the gland. In fact, they found that a one-minute delay in letdown resulted in a reduction in milk production of 7 pounds per day.

Bimodal milking is much more damaging than just wasting time in the parlor, he concluded.

To watch the recording of the November 17 DairyLivestream, go to the link above. The program recording is also available as an audio-only podcast on Spotify, Google Podcasts, Apple Podcasts, and downloadable from the Hoard’s Dairyman website.

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