As bellow, the illustration of Milk ejection reflex. Stimulation of the teats (1), caused nerve impulse transmission via the spinal cord (2) to the pituitary gland (3), where oxytocin is released and thereafter is transported to the udder via the blood (4).

    Milk ejection can be stimulated in different ways, such as tactile stimulation of teats, sight or hearing of the calf, noise from the milking machine and in some cases by feeding concentrate.

According to the lecture of Dairy Science

Therefore, I suggest you observe the all factors above, maybe you will find the reason(s).

I am always concerned about Calcium ( with possibly low Phosphorus) particularly soon after calving. If I see milk leaking a bit more than normal often I am heading to grab a bottle of calcium.Blood sampling prior to Calcium administration the affected cows and similar groups of cows ( BS 6+ cows) will help check this.

This is the source:-

http://extension.psu.edu/animals/health/diseases/mastitis/mastitis/nutrition-immunity-and-mastitis

Here is a quote from it.

"Indirect Effects - Metabolic Disease

Not only can dietary nutrients have a direct impact on immune function and susceptibility to mastitis, but they can indirectly increase cow susceptibility to mastitis through their impact on periparturient metabolic diseases. All essential nutrients can induce one or more metabolic diseases when either deficient or in excess in the transition diet (Van Saun, 1991). Hypocalcemia (milk fever) has been shown to slow the closure of the teat sphincter. Cows with milk fever are 8.1 times more likely to have mastitis and 9 times more likely to have a coliform mastitis event as a result (Curtis et al., 1983). Mastitis was also associated with ketosis and retained placenta (Oltenacu and Ekesbo, 1994, Emanuelson et al., 1993). Cows with fatty infiltration of the liver have been shown to be slower in clearing E. coli from their mammary gland (Hill et al., 1985).

Role of Dry Period and Periparturient Nutrition

From a disease perspective, the cow is susceptible to infection when any factors exist that cause the teat sphincter to remain open. Physiological conditions at dry off and calving create this situation making the cow at highest risk for becoming infected at these times.

At dry off – milk flow is acutely terminated resulting in building pressure within the gland which frequently results in milk leaking from the gland. Milk is a good culture media for bacteria that may enter the teat end and move into the gland duct system.

At calving – cows are initiating milk flow, and their immune system is frequently compromised. The teat end is often slow to close following milking due to Ca deficiency."

Secondly I would be worried about unregulated vacuum of the milking machine (.e.g not serviced for a long time) sucking too hard on the teats which will slow the teat contraction post milking. So need to check the machines are operating correctly.

thirdly, could be many more causes , good idea to take a full history, "When did this start? what changes is it associated with ? "etc etc. A good history may give you some very good clues

1. Teat sphincter always take sometime to constrict the teat after milking and moreover  100% evacuation of milk is not done during milking. So in these conditions the remnant milk will be effluxed out during resting.

2. When the udder is filled with milk, the intramammary pressure will be at peak. During these times, if animals rests, then the pressure will be increased further and force the sphincter to be relaxed. 

I´m totally agree with Richard Arthur it would be necessary to review the differential pressure of the milking equipment, the pulsations of milking equipment and the milking routine lactation curve of that or those animals showing that drip

Hi Oswaldo and others colleague.

There are 3 reasons for milk leaking:

1) leaky teat sphincter (according to intramammary pressure (IMP) and thus milk production and milking interval). The recent selection for milk production alko selected on milking easiness that included an indirect information for breeder about milking speed. Thus teat sphincter resistance decrease regularly and for some high selected breeds as Hoslstein cows, the problem arrive more and more frequently.

2) Milk ejection between milking. It is possible but scarse excepted close to milking time where you can see conditioned reflex of animals when their walk to the milking parlor park, when they ear vacuum pum, noise in parlor, voice of milker and so on..  When udder is full and young arrive near the mother, it is frequent to seen milk dropping out the teats dur to this milk ejection conditioned to close suckling and indirect stimulation by odor, seeing and earing the young.

Therefore for milked cows, a nice paper of Rovai M1, Kollmann MT, Bruckmaier RM.(, J Dairy Sci. 2007 Feb;90(2):682-90. Incontinentia lactis: physiology and anatomy conducive to milk leakage in dairy cows) demonstrate that frequent leaky tats are not associated with oxytocin release and may be not due to milk ejection but probably to increase in Intra-mammary pressure due to movement of legs or during lying down of animals. A possibility of relaxing teat muscle is also evoked and you have to know that there is an adrenergic tonus of thus muscle that could be relaxed when milking is close after a time of contraction to retain milk in udder.

3) abnormal rise of IMP due to longer milking interval (once daily milking, suppression of one milking per week)

best regards

As alluded to above the cause of milk evacuation while at rest comes back to intra-mammary pressure and teat sphincter tone. If the individual cow is leaking milk directly (in a stream, not just occasional drips) after milking that would indicate that she wasn't milked out appropriately and you should investigate the milking system parameters. Also, it will be important to talk to the milker's about milking procedure as the cow/s could be one/s that kicks off the milking unit (or is a slow milker) and if the worker's are frustrated by this, or if there are time constraints she may be let out before she has let down all her milk. If the animal is losing milk several hours prior to milking she may be a high producer and the increased intra-mammary pressure from laying down will overcome teat sphincter tone. This could be influenced by the sounds associated with others in the group being milked at the time, depending on proximity to the milking area (be it a tie stall with mobile or line milking units, or a parlor system). For an individual animal I would start by ruling out a disease state for an individual, and check with milker's to see how she does in the parlor. If it is a group problem definitely start with the milking system and with milker protocols.

The ceiling of the sphincter takes some time to contract in the post milking. Also occurs in cows when esficter loses contraction capacity with anatomical and physiological Cow change. The hyperkeratosis it is a normal physiological response of the skin of the teats relative to aggressive factors, and these changes may happen abruptly, or will the long term. The occurrence of hyperkeratosis in herds allows greater opportunity for entry of microorganisms into the teat canal, which negatively influences the quality of the milk and udder health.

If there is delay in milking time the engorged teats may leak the milk while resting. Sometimes the teat sphincter may be damaged and or incomplete milking of the animal may be the cause. The pressure on udder while the animal is sitting may cause leakage from the teats due to the aforementioned reasons. 

Teat sphincter incompetence due to poormilking procedure and pressurein the udder due to longmilkingintervals couldbethemajor culprits.

because they feel an impulse that stimulate the liberation of oxytocin.

Article Behavior affected by routine oxytocin injection in crossbred...

Regards and Congratulations.

Kindly go through the following PDF attachment.

@

Oswaldo Espinoza

So in these conditions the remnant milk will be effluxed out during resting. When the udder is filled with milk, the intramammary pressure will be at peak. During these times, if animals rests, then the pressure will be increased further and force the sphincter to be relaxed.

https://www.ncbi.nlm.nih.gov/m/pubmed/14672178/

Am working in dairy farm. I always observe the oozing of milk from cows udder in the early morning before the milkers start their milking activity. I associate it with loose of sphincter muscle due to over accumulation of milk in the udder which overcome withstanding limit of smooth muscle.

In dry cows or even milking cows, Short teats, inverted or loose teat sphincters, protruding teat canal have been associated with leaking teats. In multiparous cows the case is commonly observed in cows with history of teat canal blockage where animal health attendants have attempted to open through insertion of teat cannulae. In some cases these teat canulae are left in place for some time and the end result is lose of tone of sphincter muscles. In other cases where the teat meatus is small, application of force on insertion of teat cannula leads to tear of sphincter and resultant is spraying effect of milk on expression. Such teats also tend to leak. All in all leaking teats predispose affected cows to intramammary infectious.

Milk leak in cows is probably due to impairs in sphincter functionality which is considered as a cause of mastitis. Factors which lead to that are high peak milk flow rate, short teats, teat canal protrusion, inverted teat ends, and early lactation increased the risk of milk leakage in multiparous cows

It may be due to an impaired teat sphincter function, or maybe due to  no milking, or maybe because of pressure on the udder.