Bimodal milking is a result of delayed milk ejection (DME). In such cases, milk in the gland cistern empties into the claw. But, since milk has not been released from the alveoli, milk flow diminishes greatly until the effects of stimulation by the liners causes oxytocin release and then release of alveolar milk.
    When this happens, milk flow rapidly increases and proceeds as regular and normal. We have known about bimodal milking for many years. Measuring bimodal milking, or DME, have been difficult for most of us. A relatively new tool, the VaDia digital recorder, now makes measurement much easier. Since we can now measure DME, we can manage it. A couple of recent studies from Michigan have helped us better understand what causes DME and how DME affects the cow.
    In the first study (Moore-Foster et. al. 2019), milk flow dynamics were recorded for 3,824 milkings on 64 herds.
    Average SCC was 157,000, and average milk production was 83 pounds. Average stimulation time during the first pre-milking pass was eight seconds, and the average total stimulation time was 14.2 seconds. The average prep-lag time (first touch to attachment) was 103 seconds.
    DME was defined as more than 30 seconds from start of milking until the beginning of flow. DME occurred in an average of 25% of the cows in each herd, with a range of 0% to 75%.
    Decreasing the stimulation time was the most significant variable affecting DME. Herd size was the next most significant variable, with larger herds having greater proportions of cows with DME. Previous studies have shown stimulation time and prep-lag time to affect DME. Most experts suggest somewhere between 60-120 seconds as appropriate prep-lag times, and total stimulation time of greater than 10 or 12 seconds.
    Proper stimulation and prep-lag times may not be as critical for cows with greater udder fill, such as early lactation cows. Prep-lag time was not a significant factor affecting DME in this study, but the authors’ concluded that few cows in the study were observed with very short prep-lag times. Larger herds may have more emphasis on parlor throughput than smaller farms; this was the explanation for the observation of greater proportions of cows with DME in large herds.
    An interesting finding was that DME did not increase a cow’s total milking time. This is counterintuitive because one would expect milking to take longer if the flow is delayed at the beginning. Indeed, many consultants have assumed that DME increases milking time based on this reasoning. Thus, this study suggested that perhaps yield was reduced in cows with DME.
    In the second, follow up study (Erskine, 2019), milk production and milk vacuum dynamics were tracked in one herd for 10 days to determine if DME resulted in reduced milk production. Of the cows in the herd, 46% exhibited bimodal milk flow. In cows where milk let down was delayed for 60 seconds, daily milk production was reduced by seven pounds. This is an important finding because it means delayed milk ejection significantly decreases milk production and revenue on a farm. If one uses a goal of less than 20% DME, a herd with 75% DME would be losing seven pounds of production per day on 50% of their cows or would see a total reduction in milk production by 3.5 pounds per cow per day for all cows. Increasing stimulation time would not increase unit on time, though it could decrease parlor throughput if adding stimulation time significantly extended the time from opening the parlor gate until all units are off. Of course, we also know that DME results in a greater percentage of milking time in low flow which can be harmful to teat ends and ultimately may increase incidence of clinical and subclinical mastitis.
    So, why is bimodal milk let down important? Because it happens when milk ejection is delayed, and because delayed milk letdown results in significantly less milk. For herds with a significant percentage of cows exhibiting DME, a simple way to increase milk production and total revenue is to increase pre-milking stimulation time. Teat end health may also improve by reducing DME. Testing for bimodal letdown and DME is simple and can be done by monitoring one milking with a couple of VaDia testers in most cases. Ask your veterinarian for details.
    Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minn. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at with comments or questions.