|
Ancient people were already aware of the importance and need to stimulate the milk ejection reflex. On paintings in caves the effect of vaginal stimulation for milk ejection was described as well as the importance to have the calf close to the cow during milking. How important is the stimulation of the milk ejection reflex during milking in our modern dairy cows? Is it a phenomenon, the importance of which, has disappeared through genetic improvements? In order to answer those questions the biology behind milk letdown will be discussed.
During milking and suckling nerve receptors in the skin of the teat
that are sensitive to pressure are activated. This mechanical
stimulation causes impulse transmission to the pituitary gland in the brain whereby the hormone oxytocin is released. The hormone is transported to the udder via the blood. In the picture below a schematic figure of milk ejection reflex is presented.
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).
In the mammary gland the hormone causes the myoepithelial cells surrounding the alveoli to contract whereby the milk is pressed out to the milk ducts and milk cisterns, demonstrated in the picture below.
A contracted alveolus.
The time it takes from beginning of teat stimulation to milk letdown is about 30-60 seconds, but varies from cow to cow and also within the cow depending on the stage of lactation. It has been postulated in former times that the oxytocin secretion is momentary and the release just happens once during milking. Recent research has, however, indicated that oxytocin is released during the whole milking procedure.
The milk ejection reflex, including oxytocin secretion, can be
stimulated in many ways such as application of different tactile teat stimulation, by the presence, sight or hearing of the calf and by feeding concentrate in relation to milking, demonstrated in the picture below.
The most efficient stimulation of the teats for milk let down is performed by the calf. An optimal milking technique should therefore imitate the suckling. The suckling done by the calf includes prestimulation, milk intake and post stimulation.
Back to top
Prestimulation
Prestimulation is in general the procedure performed before attaching of the teat-cups, including drawing control milk, cleaning and drying the teats, massaging the teats and udder. During the pre stimulation the receptors in the teats are stimulated and the milk ejection reflex is activated.
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.
This results in milk letdown where oxytocin already has started to act on the myoepithelial cells when the milking machine is attached to the udder. A shorter milking time, a higher milk flow and in some cases a more efficient milk removal is the benefit of a proper pre-stimulation. Since the milk ejection is already activated when the milking machine is attached to the udder, the milk flow curve very seldom is bimodal, demonstrated in the pictures below.
Effect of 1 min premilking manual stimulation on average oxytocin profile (red line) and milk flow (blue line) during milking. Milking began immediately at time 0; arrows show start time of stripping (Adapted from Mayer et al J. of Endocrinol, 103:355, 1984).
This means that there is no delay in flow between milk coming from the cisterns and milk coming from alveolar area, which improves teat treatment. The prestimulation can be performed either manually or by the machine. However, so far we haven´t found a mechanical variant as efficient as the hand.
Post-stimulation
Post stimulation can be looked upon as the procedure where manipulations of teats and udder take place after the milk flow generally has decreased or stopped. Machine or hand stripping are examples. In experiments, performed on dairy cows, it has been found that machine stripping or "extra" post stimulation (the last minute of milking was shifted from machine milking to hand milking) resulted in four to five percent higher milk production. In other species it was found that continued suckling after milk intake stimulates a higher milk production. The effect on milk production due to post-stimulation might be explained by activation of local regulatory mechanisms within the udder. These mechanisms might influence the emptying of the udder and also the capacity of the milk secreting cells.
Back to top
Stimulation during milking
How the tactile teat stimulation on the teat during milking is performed is of great importance. It has been demonstrated that milking related release of the hormones oxytocin and prolactin is influenced by the tactile stimulation of the teats. In experiments where hand milking was compared to machine milking (hand milking is the milking procedure which imitates the calf suckling very well) it was found that the milking related releases of these hormones were both higher and prolonged during hand milking compared to machine milking. This effect can partly influence the milk production capacity.
During teat stimulation also local regulatory mechanisms of nervous origin in the mammary gland can be activated. A very good example demonstrating the importance of local mechanism in the mammary gland is the lactating kangaroo. She can feed two young (joeys) of different ages from two adjacent mammary glands with milk adapted to the special demands for each young. The bigger young has his (her) own teat producing milk related to his (her) demands, while the smaller young has his (her) own teat producing milk related to his (her) special demands, despite that both glands are exposed to the same hormonal and nutritional environment. It is only through the different suckling application the composition as well as the amount of milk to be produced in the specific glands can be regulated to the different demands from the young.
It has also been indicated in dairy cows that activation of local mechanisms is of importance for both milk production capacity and milk composition. In experiments it was found that hand milking resulted in a higher milk production and milk with a higher fat content compared to machine milking, demonstrated in the picture below.
Daily yield of fat in machine milked (blue line) and hand milked (red line) front teats during three periods; A- when both front teats were machine milked. B- when one front was hand milked and the other was machine milked, C- when both front teats were machine milked again (four cows) (Adapted from Svennersten et al J. Dairy Sci. 73:971, 1990).
The physiological mechanism behind these results is not yet fully
elucidated. There are indications of the existence of local nervous
reflexes in the udder, influencing mechanisms whereby the capacity in the milk secreting cell can be increased. Indeed, already during the fifties and sixties Russian scientists were aware of this phenomenon.
Another very interesting finding in monogastric animals and ruminants is that suckling/milking also might activate hormones in the stomach. However, what does this mean and what is the importance of this? Imagine a cow producing 100 kg milk per day - a very high milk production capacity. Giving this high amount of nutrients and calories is not possible unless the animal is consuming large amounts of nutrients. During the suckling/milking procedure the hormones in the stomach are activated, demonstrated in the picture below. Some of these hormones which are activated during milking influence the feed intake behaviour and some of the hormones exert a growth stimulating effect of the gastric mucosa. It is therefore possible that the milking related release of these hormones is involved in the adaptation of the gastrointestinal tract during pregnancy and lactation in order to facilitate the enhanced food intake which is of vital importance. Having in mind that milking might influence feed intake indirectly points to the importance of appropriate cow management around milking.
Both milking and feed intake activates gut hormones.
Consequently, the mammary gland is an organ that is controlled by hormones and supported by nutrients in order to produce milk.
However, the mammary gland is an organ that also controls its own function as well as it controls other organs in the body. Some of these control systems are activated through the milking/suckling process, which points to the high importance of how the machine is stimulating or acting on the teat during milking.
Back to top
Why efficient milk removal?
Efficient milk removal is one important point regarding milking technique and milking routines. There are many reasons for this. Efficient milk removal results in a higher milk yield. The composition of the milk is influenced, in particular the fat content. When the farmer is paid according to the fat content of the milk it is significant to empty the udder as completely as possible, since the last portion of the milk has the highest fat content, demonstrated in the picture below.
From our own studies fat contents as high as 15-20% have been
observed in the strip milk.
A typical curve showing the rise in fat percentage of consecutive fractions taken during a normal milking and at subsequent milkings after injections of oxytocin to remove residual milk (Adapted from J. Johnasson, Acta Agric Scandinavia. 2:82, 1952).
An important finding that supports the idea behind an efficient milk
removal is the observation that the milk contains a protein which acts with a negative feed back control on the milk secreting cells. The inhibitor is synthesized in relation to the milk. As it works directly on the milk secreting cell it is important to empty the alveoli as completely as possible. In experiments performed on lactating goats one udder half was emptied completely while the other one was unmilked. The emptied gland was immediately filled up with sucrose solution and in this gland the milk secretion continued despite the high udder pressure. Conversely, in the unmilked udder quarter the milk secretion was inhibited. This observation strengthened the hypothesis about inhibitor substances present in the milk, which suppress milk secretion. From udder health point of view an efficient milk removal is important. However, the ambition to empty the udder as completely as possible doesn´t mean that we can allow over milking of the teats, which indeed can cause bad teat treatment and result in mastitis.
Milking intervals
There is a big variation in milking intervals between milk producing
countries. In most countries 8-16 hours milking interval is common
practice due to the labour situation. On larger farms 12-12 hours
milking interval is often practiced. Twelve hour's interval is the most optimal milking interval with twice daily milking. Milk production (kg milk) increases a couple of percentage points with equal intervals compared to unequal milking intervals.
What is the mechanism behind this phenomenon? Milk secretion starts to decline 10 hours after the previous milking, while udder pressure is increasing. 35 hours after previous milking the milk
secreting process has stopped, demonstrated in the picture below.
The rise in intramammary pressure and the fall in milk secretion rate with lengthening milking intervals (Adapted from Hamann & Dodd, in Machine milking and lactation, ed Bramley et al, 1992).
From the discussion above, obviously udder pressure alone is not the only factor regulating milk secretion rate, but also the discussed inhibitor mechanisms. Consequently, to optimise milk production the length of milking intervals has to be taken into consideration.
Back to top
Milking twice a day has long been the common practice in industrial countries, mostly due to the labour situation for farmers. However, in some countries where labour was rather inexpensive, more frequent milking was practiced. During the last decade focus has been put on milking more frequently again, in particular in high yielding herds. The benefits of more frequent milking are many.
Changing from milking twice a day to three times a day increases
milk production markedly. Published data show increases from 5-25% more milk per day. In addition lactation becomes more persistent and prolonged. The reason why milk production increases with a more frequent milking could be a more frequent exposure of hormones stimulating milk secretion to the mammary gland. However, as mentioned above the milk contains an inhibitor with negative feed back control on milk secretion. A more frequent removal of this inhibitor therefore results in a higher production. An interesting finding in this respect is that cows with a small udder cistern are more sensitive to the frequency of milking. The smaller the cistern the greater the effect of frequent milk removal on milk production and the bigger the cistern the less the response to frequent milking.
Frequent milking has both long term and short term effects. The short term effect is an increased milk production due to enhanced activity in the milk secreting cells, while the long term effect is
increased production due to increased number of milk secreting cells. The latter indicates that it is possible to influence the number of milk secreting cells during an established lactation, which is of importance to the milk producing capacity, demonstrated in the picture below.
Short, medium and long term effects of 3 x daily milking on lactation (Adapted from Hamann & Dodd, Machine milking and lactation, ed Bramley et al, 1992).
Udder health has been reported to be improved with more frequent milking. However, it is worth noticing that teats have more sores, chaps and lesions with more frequent milking. On the other hand, the rates of new infections have decreased and the level of somatic cells in the milk show a tendency to decrease. More frequent milking leads to a more frequent rinse out of bacteria from the gland, which partly can explain the observations with an improved udder health, demonstrated in the picture below.
Teat end quality scores per group (group 2 milked twice a day, group 3 three times a day and group 4 four times a day) and lactation period. (From Ipema & Benders, In Proc. Int. Symp. on Prospects for Automatic Milking, 1992).
Feed consumption seems to be influenced by frequent milking. It has in some experiments been reported that increased milk production by 10-15% is followed by an increased feed consumption by only 3-5%, demonstrated in the table below.
|
Milking frequence |
|
| _________________________________________________ |
|
II |
III |
IV |
| Milk yield |
100 % |
114% |
115% |
|
Dry matter intake |
100% |
103% |
104% |
| _________________________________________________ |
Increase in dry matter intake (%) and milk production (%) when cows were milked three and four times a day compared to twice a day (Adapted from Ipema & Benders, In Proc. Int. Symp. on Prospects for Automatic Milking, 1992).
How is this possible? It has been observed that the more frequently milked animals are drawing on their body reserves to a greater extent than twice daily milked animals. It is also possible that their metabolism is more efficient due to a more frequent activation of endocrine systems related to metabolism. Indeed, it has been indicated that gastrointestinal hormones are activated during milking and it has been demonstrated in monogastric animals that the milking related hormone oxytocin is involved in the maternal metabolism.
Maybe the most important benefit of more frequent milking is improved animal welfare. It has been observed that most high yielders will not lie down during the last few hours before milking. Moreover, many high yielders are producing up to 60 kg milk per day and are milked twice a day with 8-16 hour milking intervals. This means that those cows are yielding nearly 40 kg milk during morning milking. Cows with such high amounts of milk in the mammary gland must be exposed to a tremendously high udder pressure which undoubtly will cause discomfort. Indeed it has been observed that high yielding cows want to be milked more frequently than twice or three times a day when they have their own choice.
In summary, increased milking frequency affects the high producing dairy cow in a positive way with respect to production, health and animal welfare according to current observations. A more frequent milking than twice daily is better adapted to cows normal behaviour and need, since the calf is suckling as frequently as 4-7 times per day.
Back to top
Milking routines
It doesn´t matter how well the genetic potential and feeding of the
high yielding dairy cow has been taken into consideration in order to achieve optimal milk production if milking routines and milking
equipment are not optimal. The milking routine has to be performed in a consistent way. The tube alignment has to be correct and also the installation of the milking equipment is important.
An optimal milking routine includes different steps such as: fore milking, cleaning teats and udder, manual prestimulation and teat dipping when needed after milking. A proper manual prestimulation of the teats facilitates milk ejection. The importance of pre-stimulation has been discussed previously.
How will an optimal pretreatment be performed? Experimentally the importance of a strict milking routine has been demonstrated from a production point of view. Lactation production was studied where cows were exposed to different milking routines. A strict routine (cleaning and drying teats and udder for around 30 seconds and application of milking machine within 60 seconds from start of stimulation) resulted in an increased lactation production of about 450 kg. The control treatment was a routine with a short drying time and variable waiting time, demonstrated in the picture below.
Effect of pretreatment on milk production, second and third lactation cow. Cows were exposed to a standard milking routine consisting of 31+/-9s premilking stimulation and 1.22+/-0,25 min interval and a control routine consisting of 17+/-5s premilking stimulation and 3.06+/-1,56 min interval (From M.D. Rasmussen, J Dairy Sci. 73:3472, 1990).
The teats and udder have to be cleaned and dried for the sake of good milk quality. Contamination from manure bacteria, and bacteria spores should be minimised. The teats should be cleaned by a separate towel for each individual cow in order to prevent transmission of mastitis pathogens between cows. If possible each corner of the towel should be used for separate teats to prevent transmission of pathogens within the cow.
When creating a good milking routine it sometimes helps to start
using routines which will create positive emotions for the cow. During the seventies scientists demonstrated that feeding during milking resulted in a more efficient udder emptying, higher peak flow and a tendency towards increased production. The observation resulted in a recommendation to feed concentrates in the parlour in some countries.
But what was the mechanism behind this observation and is it worthwhile to continue feeding concentrate in the parlour? Interestingly, it has been found that feeding during milking both prolonged and increased the milking related release of the hormone oxytocin, demonstrated in the picture below. From a production point of view it was further indicated that milking and feeding simultaneously increased milk flow, decreased milking time and showed a tendency to increase milk production. However, more research work is going on to evaluate when it is most optimal to feed cows around milking. Will it be just before, during or after?
Daily plasma oxytocin values (pM), when cows were: milked and fed together on four consecutive days and milked and fed separately on four consecutive days (Adapted from Svennersten et al Acta Physil Scand, 153:309, 1995).
The milk ejection reflex might be inhibited as well. There are different types of inhibition, centrally in the brain or locally within
the udder. What then can cause inhibition of the reflex? Rough treatment of the cows by the milker, discomfort during milking caused by the milking machine, unfamiliar surroundings and inconsequent management are several examples.
In order to stimulate the milk ejection reflex and not to inhibit it, it is
very important to treat the cows in the most proper way both during and before milking. Milking represents a very complex sequence of conditioning processes. The very first cues indicating approaching milking (noise of milking machine being turned on, release of cows from stanchions, etc) starts the complex series of sychophysiological processes that prepare the cow for milk let down. If this process is disturbed in one way or another milk ejection can be inhibited. Therefore the recommendation will be consistent timing of routines, such as udder wash, teat cup application and synchronization of other routines such as feeding or preparing of bedding. The events should take place in a regular order every day.
Back to top
Teat treatment/mastitis
The prerequisite to produce milk in an economical way is to have a relatively high yield with high quality, which means high production from healthy animals not suffering from any kind of disease in the mammary gland. Mastitis is the most common and costly disease in dairy herds. In many cases the farmer is only aware of the clinical cases, demonstrated in the picture below.
Clinical cases (A) and subclinical cases (B) of mastitis (Adapted from Nelson Philpot, Mastitis Management, 1978).
It has been reported that clinical mastitis rates are generally 20-100 cases/100 cows per year. Subclinical infection levels are 5-35% of quarters infected by a major pathogen bacteria. The clinical mastitis is rather easy to detect for the farmer. The symptoms are clotting and discolouration of the milk, and the gland becomes hard, red or swollen and in severe cases the cow has fever and loss of appetite. The subclinical mastitis can be harder to detect, since both the milk and udder can appear rather normal, while the somatic cells in the milk increase, demonstrated in the picture below.
©
Diagnoses of mastitis: A-clinical, B-subclinical.
Back to top
What is mastitis? Mastitis is an inflammation in the mammary gland
which can be caused by bacterial infections or trauma. When bacteria are growing, they release metabolites and toxins that stimulate the defence mechanisms in the cow. The inflammation response leads to a migration of white blood cells from the peripheral circulation into the udder. The cell count of the milk increases from normally 100 000 cells/ml or less per udder quarter up to several million per ml. The increased cell count is accompanied by an activation of several milk enzymes, demonstrated in the picture below.
©
The inflammatory process involved in mastitis (Adapted from A.J. Bramley. In Machine milking and lactation, ed Bramley et al, 1992).
The pathological consequences of mastitis are tissue damage and
alteration of secretory function. This leads to reduced milk yields and changes in milk composition. A correct estimation of the loss in milk yield is hard to do, since non-infected udder quarters tend to compensate for the decrease in yield of the infected quarter. The mechanism behind the regulation of this compensation is still unknown. Regarding the altered milk composition, fat and lactose levels are dropping, while total protein levels change only slightly serum proteins are increasing and casein are decreasing leading to deteriorated cheese making quality. The milk concentrations of ions are increasing resulting in an increased milk conductivity, demonstrated in the picture below.
The relationship between somatic cell count in milk, the quantities of the components in milk and milk production (From Giesecke et al, In Practical mastitis control in dairy herd, 1994).
How is the milking machine influencing mastitis? The milking machine can facilitate the transmission of pathogens between cows and between udder quarters. The action of the milking machine can also cause bacteria to move from the exterior of the teat into the teat sinus, caused by excessive vacuum fluctuations. Furthermore, vacuum fluctuations in the claw can cause milk moving between teat cups. The teat end can be affected by the machine resulting in teat lesions, which could be colonized by bacteria. Too high vacuum levels, overmilking and inadequate pulsation (insufficient or too short massage phase) are factors that might contribute to damage of the teats. These examples demonstrate the importance of how the milking machine is manufactured but also how the farmer is using it.
Back to top
When manufacturing milking machines, it is very important to verify that a new milking machine is not influencing the teat negatively. A method for measuring teat treatment has been developed where the changes in thickness of the teat after milking is compared to the thickness before milking. If the milking is performed in a proper way, from a teat treatment point of view it seems likely that the teat thickness after milking is not altered compared to before milking, demonstrated in the picture below.
However, teat lesions or abnormalities on the teats, which are not
related to machine milking are not unusual. It is therefore important to have in mind that several factors in the environment can influence the teat and udder health.
Development of teat end thickness values (Cutimeter) before and immediately after milking in relation to different milking methods and trial periods (Hamann and Stanitzke, Milchwissenschaft, 45:632 - 637, 1990).
What can the farmer do to avoid mastitis? Generally it can be said that a good hygienic milking routine is the prerequisite. Premilking preparation where the teats are washed with individual towels containing disinfectant is recommended. Mastitic cows or cows with increased somatic cell counts should be milked at the end of milking and teat dipping should be practiced when needed. Keeping a high hygienic level on the environment is also very important factor which means keep the cows clean and keep a good hygiene of the bedding. Last but not least, the milking machine must be regularly checked, tested and serviced, such as vacuum levels, pulsation rate/ratio, and replacement of rubber parts.
A successful animal husbandry must respect the basic biological
requirements of the animals. This means that we need to have knowledge about animal requirements for space, air supply, thermal protection, nutrition, behaviour etc. Regarding behaviour it is important to have an understanding of the cow's ability to detect events in her surroundings, memorize the specific significance and act accordingly. As an example, efficient management requires good knowledge about learning abilities of dairy cows, in particular when it comes to milking parlours and robotic milking systems.
The different ways and importance of learning can be exemplified as follows. When cows are introduced to a new area of a barn, it is useful to let the animals get accustomed to this area by their own choice, without forcing by the personnel. To find out that a new area is safe usually requires 2-4 visits. When heifers are to be introduced to the milking parlour the time for them to learn entering the parlour can be reduced if they are given the opportunity to freely investigate the parlour before the beginning of their lactation.
When it comes to milking it is important to have in mind that the motivation for this cannot be expected to be of such predictable nature and such strength as to those for drinking and food intake.
Furthermore, the animals within a herd do not act independently of each other, but prefer to act as a coordinated social unit. Usually they rest and eat together. When developing the milking parlour environment this knowledge is of vital significance.
The interaction between the milker and the dairy cow is also a crucial factor in the efficiency of dairy management. Besides the behaviour of the herdsman, interactions such as "hand and arm interaction" and "vocal interaction" between the person and animal are significant when establishing a confiding interaction. It is important for the animals to perceive a positive and safe interaction with the herdsman. Indeed, animals handled aggressively are more fearful of humans and fear is often created by uncertainty.
The herdsman's behaviour could have direct economical consequences. Indeed, experimental results demonstrate that dairy cows treated with pleasant handling milk more per year. The herdsman's handling in the parlour during milking (touching of the cows through patting and stroking) can encourage the cows to more easily enter the parlour and exhibit less stress and thereby less inhibition of milk letdown. Also, the use of the voice influences the cow's production. It has been observed that in the higher yielding herds the herdsman talked to the cows far more often than in the lower yielding herds. Talking "with" instead of talking "to" the animals was associated with high yielding cows.
In many herds today it is not unusual for animals to show different types of more or less abnormal behaviour, such as tongue rolling and bar biting. This could be a result of too little activity for the animals and possibilities to practice their normal behaviour such as search for food. Cows on pasture don't show oral stereotypes. Feeding more frequently, offering the cows more roughage, and maybe increasing the frequency of milking are examples to reduce these problems.
In conclusion, knowledge about animal behaviour, social interactions and animals psychological requirements are undoubtedly of great importance for high and efficient milk production. Increased knowledge in this area is needed to improve cow traffic to and from the milk-ing parlour and feeding station, teaching the cows to enter the milking robot etc. A better understanding of the animal will undoubtedly influence animal health, animal welfare and production.
Back to top
|
