FACTORS AFFECTING INDUCTION OF MILK LET DOWN IN COWS

Physical stimulation of the teats, either by the calf’s suckling or the milkers hands, excite receptors from which nerve impulses are sending to the posterior pituitary gland causing secretion of the hormone oxytocin. The hormone is transported via the blood to the mammary gland. Because both hormones and nerve impulses are involved in the milk ejection reflex, it is called a neurohormonal reflex. Oxytocin stimulates the contraction of the alveoli and small ducts thereby emptying the milk into the larger ducts and the cistern. Hereafter the milk can be evacuated from the udder.

The contraction of the alveoli may, to some extent, be enhanced by tactile stimuli of the udder (massaging, squeezing) the so called tap reflex. When calves suckle, they butt at the udder in order increase milk secretion. Manual massage of the udder during milking imitates this reflex.

Like cattle, buffaloes can get used to different stimuli. It is clear that also in buffaloes, oxytocin release is triggered by visual or audible stimuli, such as the sight of the milker, the noise of the vacuum pump or when entering the milking parlour. The animal becomes conditioned to let-down milk and has thus developed a conditioned reflex. (An unconditioned reflex is the suckling of the calf.) By letting the animals get accustomed to a strict routine, time of let-down is shortened. In cattle, it has been demonstrated that feeding concentrate during milking improves time of let-down. It has yet to be shown in buffaloes.

The milk let down reflex stimulates milk flow from the alveoli in the udder into the teat canal. It has to occur before a cow will milk out freely. A cow which has ‘let down’ prior to cup attachment /hand milking will milk out faster, and in many cases, more completely, as long as the let down does not occur too long before cup attachment / hand milking as this can slow the milking process. The let down reflex can be inhibited resulting in slow or interrupted milking of cows.

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/harsh handling of udders by fist,thumb or hand 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.

Maximise production .Cows that let down just prior to cup attachment/ hand milking will milk out more completely thus maximising production. Improve milking efficiency .

An understanding of how milk let down occurs will help milkers manage the milking process in a way which encourages it.

1.1 Milk let down

Milk is initially secreted into small sacs within the mammary gland called alveoli, from which it must be ejected for consumption or harvesting. Mammary alveoli are surrounded by smooth muscle (myoepithelial) cells which are a prominent target cell for oxytocin. Oxytocin stimulates contraction of myoepithelial cells, causing milk to be ejected into the ducts and cisterns above the teat. Oxytocin is released after the cow receives an appropriate stimulus, this can be visual, aural or physical, and should be predictable and consistent at every milking. Handling/massage of the teats for at least 15 seconds is a strong stimulus, but cows can also learn to let down through the association of the dairy environment to the milking process. The pressure of milk being forced into the ducts/cistern and down towards the teat causes the teat to swell with milk and become ‘plump’. It takes 60 – 90 seconds for teats to become plump after let down has been initiated. Cows with well-filled udders require a shorter period of stimulation to elicit a milk let down response than cows with less-filled udders. The action of oxytocin is essential for emptying of the udder during milking. As much as 80% of a cow’s milk is unavailable if this oxytocin release is insufficient or does not occur. Its let down action lasts for about 5 minutes and is strongest for the first 3 minutes of milking. It is important to get the cups attached / hand milking quickly after let down has started to make full use of the increased udder pressure that occurs.

1.2 Factors which inhibit milk let down

Slow milking time and incomplete milking out are indicators of interrupted milk let down. Agitated, fearful or stressed cows produce the hormone adrenaline which counteracts the activity of oxytocin and inhibits let down. It may take 20-30 minutes for the effects of adrenaline to subside. Pain during the milking process will affect milk let down. Ensure the gentle handling of udder in case of hand milking or milking machinery is not hurting cows.

1.3 Indications of a problem

If any of the following are occurring then you may have issues that you need to address:

There is no milk flow when the cluster has been attached for greater than 2 minutes in late lactation, sooner in early lactation.

The milk flow is ending when there is still a significant amount of milk in the udder. The flow can be interrupted part way through milking. In this case the milk flow at the start of milking is residual milk left in the cistern from the end of the previous milking and not milk which has been let down at this milking.

Damage to the teat end (hyperkeratosis)

The oxytocin concentration in the blood normally is increased within 1 to 2 min. after udder stimulation, but the amount released is declining during milking.

The timing of oxytocin release relative to milk removal is an important factor affecting milk ejection.

The sensitivity of the neuroendocrine reflex seems to decline as lactation progresses. Peak oxytocin seems to come later after mammary stimulation as lactation progresses. Peak oxytocin occurs at 1 min. at 1-2 weeks of lactation , 2 min. at 5-6 weeks, and 15-16 weeks. Maximum oxytocin concentration during milking also declines as lactation progresses. Extra-tactile stimuli often can release oxytocin before milking, but the occurrence of this declines as lactation progresses.

The dry or nonlactating period may serve to restore the sensitivity of the neuroendocrine reflex. Nonlactating cows will release oxytocin in response to udder stimulation. But, virgin heifers do not respond substantially to udder stimulation. Apparently maximum oxytocin release in response to udder stimulation occurs only if the mammary gland is lactating or has lactated.

Maximal prolactin release from the pituitary in response to tactile stimulation of the udder depends on the presence of a fully developed mammary gland.

It is estimated that the bovine pituitary has about 800 micrograms of oxytocin. This is about 40X what is in the blood under resting conditions. Only about 1/3 of pituitary oxytocin is released at a milking.

How much oxytocin is needed to elicit milk ejection? Peak oxytocin is about 11to 65 microunits/ml serum; 40 liters of blood in a cow = about 0.4 to 2.6 IU. Normally you inject 10 IU to cause milk letdown, but as little as 0.02 IU into the jugular can result in milk ejection .

Oxytocin receptors on myoepithelial cells can respond to very low levels of oxytocin.

Oxytocin has a short half-life in the blood = 0.55 to 3.6 min. This means that the removal of milk by machine or by nursing must be closely timed with stimulation of the teats.

Cpws/Buffaloes are sensitive to changes in the environment. They may withhold the milk if they are uncomfortable with the situation. If the animals are stressed, scared or in pain, the hormone adrenaline is secreted. This hormone causes constriction of the blood vessels, thereby hindering the supply of sufficient amount of oxytocin to the udder. Adrenaline also directly acts on the myoepithelial cells in the alveoli by blocking the oxytocin receptors. The inhibition if milk let-down will result in the leaving of milk in the secretory parts of the udder. Continuos exposure of stress to the buffaloes will affect the milk production negatively. Change of milker or milking routine, application of wrong milking technique or milking machines in bad conditions are some reasons for the buffaloes to with hold the milk.

Benefit of restricted suckling by calves-—

Cows and their calves interact in a variety of ways according to management, influencing saleable and consumed milk fractions, milk fat content:

• The absence of calf stimulation at milking reduces saleable milk yield and fat content.

• Calf presence increases saleable milk yield and fat content when it occurs during milking, but not before. This stimulus is not sufficient to allow the complete removal of milk from the udder.

• Suckling before milking until let down is observed increases saleable milk yield and fat content, but this stimulus does not continue during milking and milk is not completely removed.

• Milk suckled before milking has a low fat content. So the procedure should slightly increase saleable milk fat, but at levels not detected in these studies. The effect is reduced if the period since the previous suckling is short.

• In cows milked twice daily at different intervals, response to calf stimulation increases as the period from the previous suckling is reduced.

1. Introduction

Efficient milk harvesting requires the cows, people and facilities to be interacting smoothly. This Quick Note discusses how an understanding of cow behaviour can make the milk harvesting process quicker and easier through improved milk let-down.

2. Interpretation and relevance to Indian conditions

When a cow’s behavioural and sensory characteristics are understood, handling becomes easier and the milking process can take advantage of the natural milk ejection reflex. Cows that enter the shed calmly and voluntarily are more likely to have had an effective milk let-down by the time the teatcups are attached.

3. Relationship to CowTime goals

A poor understanding of cow behaviour and milk let-down causes a number of problems for both dairy farmers and dairy cows. Fearful cows are more difficult to handle, increasing the amount of handling required during milking and the likelihood of lameness or injury. This increases the time required, and the difficulty of the milking task. Stressed or agitated cows suffer a decrease in milk yield, quality and welfare.

4. Features of cow behaviour that are important for milking

Sensory characteristics

Cattle greatly rely on sight and have wide-angle (almost 360o) panoramic vision. Although they can see depth reasonably well, they may need to stop and lower their heads to judge the depth of steps or gutters. Cattle have a strong tendency to move from dimly-lit areas to more brightly-lit areas, but will not approach very bright light. Shadows and sparkling reflections will cause baulking.

Research has shown that loud noise disturbs cows, but sounds of people yelling or whistling are more stressful than sounds of clanging gates. Cattle will avoid places containing urine from stressed animals.

Cows need to maintain social space and their orientation. This, combined with their following instinct, makes it

important to allow enough space for cows to follow one another and to maintain visual contact with the leader.

Learning —–

Cows have good short-term and long-term memories and this can be used to develop handling routines. Studies show that cows can learn and adapt to new situations. This can be achieved by initially allowing the animals to move at their own pace and to minimise any fearful experiences along their route. If a cow’s routine is constantly changed (such as order of entry or position in the dairy) a cow is not able to learn her routine. This will heighten their sensitivity to fearful experiences and can sensitise the animals to be fearful to the people or facilities. After an unsettling experience it is a good idea to reward the cow with a scratch or a feed.

Fear —–

Fear is a very powerful emotional state that can be triggered by unfamiliar sights or sounds, heights, danger signals from other cows in the herd and previously remembered experiences. Fearful animals are the most difficult to handle. Cows that are already fearful of the environment and the handler are likely to show exaggerated responses to handling.

Fear of Humans

Cows learn to fear humans if handled badly and they also associate this bad handling with the place where it occurred, for example the milking dairy.

Fear of Novelty or Unfamiliarity

Because cows are initially afraid of sudden changes in their routine, such as lighting, floor surfaces or levels and

fence or wall types, it is important to keep these features as consistent as possible to reduce the fear factor. If

animals do become fearful in these situations, try to allow them some time to familiarise themselves with the

environment

Fear of Evolutionary Threats

Cows have an in-built sense of fear to avoid life-threatening situations and can interpret some relatively common situations as threats. These include a fear of heights, sudden movements, threatening or aggressive actions, prolonged eye contact and large or towering objects. These threats can be minimised through good dairy design and thoughtful handling practices.

Learned Fear

Previous bad experiences are remembered and a similar situation or location may initiate a fearful response.

Milk let-down —-

Milk let-down (ejection) is the natural process used by the cow to help remove milk from the udder. This process is brought about by the release of the hormone oxytocin from a gland at the base of the brain, following a suitable stimulus being received by the cow.

The stimulus may be visual, heard or felt and should be predictable and consistent at every milking. Most importantly it should not induce fear in cows. It is commonly thought that genetic selection has ensured that most commercial cows will let-down freely without physical contact with the teats. Handling of the teats however, is a strong stimulus for let-down with research showing that at least 15 seconds of massage per cow is required to effect an appreciable change in milking characteristics.

After release, oxytocin travels through the blood stream and has a direct effect on small muscle cells that surround the milk-producing cells in the udder. Oxytocin causes these muscle cells to contract and squeeze the milk into the milk ducts and so towards the teat. The pressure of the milk being forced into the ducts and down towards the teat causes the teat to swell with milk and become ‘plump’ with milk. It takes between 60 and 90 seconds for the teats to become plump with milk after let-down has been initiated.

Effect of fear on milk let-down —–

Although the release of oxytocin is a ‘reflex’ action, the mental state of the cow impacts on its success. Relaxed cows have an uninterrupted let-down of milk. Agitated, fearful or stressed cows may have a disrupted oxytocin release. Furthermore, adrenaline (a hormone released by fearful cows) directly blocks the action of oxytocin on the udder for 20-30 minutes.

5. Potential challenges with implementation —–

Cows take time to change their behaviour if they have learned to respond in a certain way to a particular situation. Likewise, it will take some time before cows feel comfortable again after the bad influence has been removed from their environment. The time required will depend on the degree of fear that the bad influence induced. An initial change in behaviour may take 3-4 weeks, with further improvements expected over time. A positive environment must be maintained consistently to maintain the benefits.

“Music Makes More Milk”

Stress can inhibit the release of oxytocin — a hormone key to the milk-releasing process. So the happiness of cows is very much on the minds of farmers. In our Indian civilization , we know that during ancient time Lord Krishna used to flute basuri while milking cows. Now the world scientist has proved that melody sound makes more milk in cows.

The California dairy industry even declared “Happy cheese comes from happy cows” as part of their Real California Milk Campaign. Dairy farmers have become experts in cow comfort, from barn design to climate control engineering to keep cows as content as possible. But not all the attempts to sooth cows are quite so high tech. It may sounds silly, but some farmers swear by playing relaxing tunes for their herd for maximum milk results. In 2001, there was a minor stir at the University of Leicester in England when a pair of psychologists showed that slow music played at a large dairy farm increased the cows’ milk production by 3 percent as compared to fast music which had no effect. Despite the fact that a 3 percent increase in milk production could mean significant dollars to a large dairy, scientific literature is mostly mute on the subject of bovine music preferences. music can be beneficial to the well-being of the cows, but it must be consistent and calming, “If the music volume is kept constant and the style of music is consistent, and everything else in that parlor is well managed and maintained, music can have a positive effect on milk let down.” In 2012, the BC (British Columbia) Dairy Association put the theory of music’s potential effects on milk production to the test with their “Music Makes More Milk” contest. Inviting the public to create and submit songs that cows would “vote” on based on milk production, this campaign was the brainchild of the association’s advertising agency in an effort to connect Canada’s families to the dairy industry. Studies conducted around the globe showed that music can have a calming effect on cows. Through music, people could not only understand the dairy industry better, they could actually impact the farm’s output by composing a song to make cows happy and produce more milk.” Many dairies in the U.S. play either country or Spanish-language music in parlors. This choice is highly dependent on milking employee preferences. From a practical standpoint, perhaps the use of music in the milking parlor is just an easy way to drown out mechanical noise and provide the animals with a consistent environmental element. Music is a calming sound to the cows They get used to that. If they’re calm and content, then obviously milk production is going to be better.

Scientists have known for years that the milk production of cows goes up when the animals listen to music. Farmers have known this too. But deciding exactly what songs to play remains an imprecise science.

Minimise all factors which can create stress or discomfort for cows during milking such as, for example: Inconsistent milking routines. Harsh handling of udder in case of milking by hand. Bad pipework design or maintenance which hurts cows. Poor machine set up or maintenance. Poor animal handling techniques such as hitting cows or twisting tails, aggressive yelling or very loud music. Animal health problems e.g. mastitis, lameness. Stray voltage (although this is uncommon).

Consider:

Carrying out a teat preparation routine for cows which are slow to let down (e.g. teat washing and drying or massage) as it is a strong stimulus for let down.

Action points –

Make sure there is nothing about the milking routine which could scare or stress cows and inhibit oxytocin release. Encourage a relaxed and consistent milking routine.

Reference-On Request