Anatomy of the udder
Inside the udder
Blood supply to the udder
Nerves in the udder
Milk secretion and ejection
Interfering with milk ejection
Milk secretion and milking practices
Frequency of milking
The milking process
Milking is the process of persuading the cow to let down its milk and allow the dairy farmer to remove it for his or her own consumption or for sale. It is therefore not entirely a natural process. The dairy farmer must manipulate the natural process so that he receives the maximum benefit. It is therefore essential that one understands the natural process in order to manipulate it.
This chapter describes the structure of the udder and teats; blood circulation to the udder; nerves which reach the udder; manufacture of milk from precursors in the blood by the udder; the process of milk let-down; the effect of different milking routines on milk production; and means of withdrawing milk from the udder.
Most of these processes or characteristics are the same whether cows live in the tropics or in the freezing climates of North America or Europe. However, there are two obvious and major differences. The first is one of quality. Milk is an excellent substrate for micro-organisms. In hot countries the temperature is ideal for the growth and multiplication of many bacteria. Some of these micro-organisms cause illnesses in humans (pathogens), and many cause the milk to spoil in a variety of ways. This additional challenge of the tropics has implications for smallholder dairy farmers.
The second factor is the breed of cow. In temperate climates, all dairy cattle are different breeds of European cattle, Bos taurus. They have been selectively bred for a variety of milk producing characteristics over hundreds of years. As heat tolerance is not required in temperate climates, these high producing breeds have not been selected for this trait or for tick resistance, both of which the Bos indicus of tropical countries have acquired. Bos indicus cattle, including, for example, the dairy breeds of the Indian subcontinent, Tharparkars, Sahiwals and Sindhis, have not been selected for dairy characteristics to the same extent as the Bos taurus dairy breeds. This is reflected in the dam's general willingness to release milk without the calf present. Bos taurus dairy breeds release milk on demand.
Where cross breeding programmes have been set up to improve heat tolerance and tick resistance of milking cows, such traits of the Bos indicus inheritance need to be taken into account.
The cow's udder is made up of four `quarters'. Each quarter is a separate milk-producing gland which secretes milk toward the teat. The udder is packed full of tissue held in place by the skin of the udder wall. The internal tissue is of two broad types. There is the fatty and connective tissue which carry and support the blood, the nerves and lymph on its way to the glandular tissue, which manufactures the milk and stores it between milkings. Size of udder is not a good guide to the amount of milk a cow will produce. A large udder may, in some cases, contain a large proportion of fatty tissue and may therefore not have the potential to produce as much milk as a smaller udder with a higher proportion of glandular tissue.
Unfortunately for dairy farmers it is not possible to tell how much glandular tissue and how much fatty tissue is inside an udder from outside observation.
The udder is a large organ often weighing over 50 kg, including milk and blood content. It therefore requires strong support. A diagrammatic depiction of the support mechanism is shown in Figure 16.1. There are two sheets of strong, flat ligaments which support the udder. One runs down the middle of the udder separating the left from the right side of the udder. This is called the `median suspensory ligament'. Around the sides of the udder is a web of ligaments, which are called the `lateral ligaments'. Both sets of ligaments are attached at one end, either to the pelvic bone or to the strong tendons of the abdominal muscles in the pelvic region. At their other ends they join for the full length of the udder. Where they join is easily seen from the outside of the udder. Most cows will have a groove which runs the length of the udder at the place where the two ligaments join. Both halves of the udder are supported by their own ligament network.
Figure 16.1. Support mechanism of the udder.
When the udder is full of milk, the teats stick out to the sides. This is because the lateral ligaments will not stretch, but the median ligament is elastic, so as the udder fills with milk and becomes heavier the central or median ligament stretches and the teats are pulled out to the side by the inelastic lateral ligaments.
The udder is completely separate from the abdominal cavity except for two narrow passages on either side, through which pass blood and lymph vessels and some nerves. These passages are called the `inguinal canals.'
As mentioned above the inside of the udder has two main types of tissue: glandular tissue, and connective and fatty tissue. It also contains a system of milk ducts and blood vessels.
A cross section of an udder from top to bottom, through the teats, is represented in Figure 16.2.
At the top the tissue would appear to be quite dense, with no obvious ducts or canals. Further down the udder the tissue would appear more like a sponge, and near the bottom ducts and reservoirs would become quite clear. Just above each teat, a substantial space is obvious. Unlike the simplified diagram the `gland cistern' is not a simple container, but is branched and convoluted into an irregularly shaped space.
Below the gland cistern is the teat, which contains the `teat cistern.' At the end of the teat is the small hole through which the milk is drawn by the calf or the dairy farmer. This hole is called the `teat canal' or the `streak canal'. The size of the streak canal and the tightness of the circular muscle (or `sphincter') which surrounds it determines whether the cow will be hard or easy to milk. The characteristics of the streak canal also have a bearing on the ease with which the bacteria which cause mastitis can enter the udder.
Figure 16.2. Cross-section of an udder at the teat.
An enlarged representation of an alveolus section is presented in Figure 16.3. This structure has been likened to a bunch of grapes, with all spaces between the stems and the grapes filled with fatty and supporting connective tissue. The large ducts branch into smaller ducts, and these branch again and again into smaller and smaller tubes, like the stems of the bunch. At the end of the smallest ducts, are lobes called `alveoli.' These alveoli are like little sacks, inside which the milk is manufactured.
Figure 16.3. Section of an alveolus cell.
The alveolar cells which form the main structure of the wall, are a single layer of cells that convert raw materials in the blood into milk. The alveoli have a network of cells surrounding them called `myoepithelial cells'. As well as these the alveoli are well supplied with a myriad of blood capillaries.
Blood supply to the udder is critical as its constituents are converted to milk. A cow producing 20 litres of milk a day has around 9000 litres of blood circulated through the udder during that 24-hour period.
Blood flows to the udder from a pair of arteries which branch off from the aorta and enter the udder on each side of the cows body by way of the inguinal canals. They branch into smaller and smaller vessels until they finish up as the capillaries surrounding the alveoli. The blood returns to the heart by way of two veins. One follows much the same path as the artery which brought the blood to the udder, that is back up through the inguinal canal and along under the spinal cord to the heart. The other vein called the `milk vein` travels under the skin in front of the udder and goes back into the body cavity near the breast bone, then to the heart.
It is sometimes thought that the size of this clearly visible milk vein will give a good indication of how much milk the cow will be capable of producing. But since quite a large proportion of the blood returning to the heart goes by way of an internal vein, this is not a reliable indicator of potential milk production.
Most of the nerves in the udder carry sensory messages from the udder to the brain. There are only a few nerves that control functions in the udder. These control blood flow through the arteries by changing the diameter of the blood vessels. While the alveoli are poorly supplied with nerves, the teats are richly endowed.
Milk secretion is the process of manufacture of milk from the raw materials in the blood by the alveolar cells and the storage of that milk in the cavity of the alveolus. This is a continuous process which only stops when the alveolar cavity is full and the pressure on the alveolar cells inhibits further secretion.
Milk ejection is the process through which milk is released from the alveoli and flows into the ducts, the gland cistern and the teat cistern, where it can be removed by the calf or the human milker.
Milk secretion is controlled by hormones, unlike some other glands such as the salivary glands. The hormones of major importance are those secreted by the anterior lobe of the pituitary gland. Prolactin and pituitary growth hormone have a direct effect on the action of the alveolar cells. The hormones which control the thyroid and the adrenal glands are also important.
Unless the cow `lets down' milk, neither the calf nor the dairy farmer can remove the full yield. Milk secretion takes place continuously. As the cavities of the alveoli fill with milk some of it will pass into the duct system and move down into the gland and teat cisterns. This milk can easily be withdrawn. All the rest of the milk is held in the cavities of the alveoli, and unless forced out, it remains there.
The `let down' or milk ejection reflex is controlled by hormones. When the calf suckles her mother, or when the milker washes the udder or starts to milk the cow, messages are taken from the nerve endings in the teats to the brain. When the message reaches the brain the rear end of the pituitary gland, the posterior lobe, is set into action. `Let down' hormone (oxytocin) is released into the blood, it reaches the udder and causes the myoepithelial cells, which surround the alveolus, to contract. Using the analogy of a bunch of grapes, the contraction of the myoepithelial cells acts as a series of hands squeezing the grapes in turn. `Let down' is an involuntary process.
Oxytocin in the blood does not last for a very long time. Most of it will have disappeared in about two minutes. Once it has gone the myoepithelial cells relax, the alveoli resume their normal shape and as they do so they draw the milk back from the ducts and cisterns into the cavity of the alveolus. From there it cannot be withdrawn until another release of oxytocin causes the milk to be again squeezed from the alveoli. Some cows cannot release oxytocin for the second time for about fifteen minutes, others can do so almost immediately. The average is around seven or eight minutes. It is therefore important that the removal of milk takes place immediately after milk ejection has occurred. If hand milking does not commence immediately after `let down', milking can turn into a long drawn out process, and if using a milking machine, it may cause injury to the udder by unnecessarily prolonging contact with the machine.
Excitement, stress, pain or fear will interrupt the `let down' process. The hormone adrenaline is released from the adrenal cortex when the cow experiences any uncomfortable situation. Adrenaline completely blocks the action of oxytocin. Therefore, it is not possible to achieve high yields of milk if the cow is milked in a situation where it is likely to be frightened, stressed, hurt or excited.
Milk secretion is a continuous process which takes place in the alveolar cells high in the udder. The rate at which milk is secreted is reasonably constant, and for most cows remains steady for at least 12 hours after milking, and for many will not change for 20 hours. However, milk production records do not show this pattern because of the carryover effect of what is called `residual milk', that is, milk that cannot be removed from the udder by normal means. Residual milk is generally around 10 to 15 per cent of the yield, and it can be 15 to 20 per cent of the milkfat. There is always some milk that was manufactured since the last milking that will be left in the udder under all practical conditions. The only way to release residual milk is by injection of the hormone oxytocin, and this is only undertaken experimentally or for veterinary reasons.
As an example, assuming the milk secretion rate for a particular cow is at a rate of one litre per hour, and that this rate does not change up to 16 hours, and the amount of residual milk is 10 per cent. What happens to the apparent rate of milk secretion when the amount that can be removed by hand is measured, when the daytime milkings are eight hours apart and overnight the milkings are 16 hours apart? Over the 16-hour interval 16 litres of milk will have been secreted and there will be 10 per cent residual milk from the previous 8 hours. The calculation below shows how the apparent rate of milk secretion works out.
The relationship between milk secretion and collection is demonstrated below:
|Milking after 16 hours|
|Total milk in the udder||= 16.8 litres|
|Milk withdrawn||= 16.8 minus 1.68 litres (10 per cent residual milk)|
|= 15.1 litres|
|Apparent secretion rate||= 15.1 divided by 16|
|= 0.98 litres per hour|
|Milking after 8 hours|
|Total milk in the udder|| = 8 litres plus 1.68 litres
(residual 10 per cent of daily total)
|= 9.68 litres|
|Milk withdrawn||= 9.68 litres minus 0.97 litres (10 per cent of 9.68)|
|= 8.71 litres|
|Apparent secretion rate||= 8.71 divided by 8|
|= 1.09 litres per hour|
This exercise shows that, without taking account of residual milk, you may be influenced to think that milking at equal intervals has an advantage over more convenient intervals for the farmer. The results of many experiments indicate that the interval used in the example of sixteen hours and then eight hours between milkings is probably the extreme. With some individual cows, it may have exceeded the limit, and production would be lost. However, for the majority of cows, a sixteen-hour milking interval will have little or no effect on milk production. Beyond that though, a significant reduction of milk yield would be expected.
It is commonly believed that substantial increases in milk yield will be obtained if cows are milked three times daily, instead of twice.
Care must be taken with these claims as some of the information on three times per day milking has come from studies of milk recording information. In this case, individual farmers have selected which cows will be milked three times per day, and it is not unreasonable to believe that farmers select the cows which are the highest producers to milk three times, while the lower producers are only milked twice per day. Differences of 15 to 40 per cent in favour of three times per day milking over twice per day have been reported. When direct experiments have been carried out the difference in favour of three times per day milking has been between five per cent and 15 per cent. Four times a day milking gives a small increase over three times per day. From the perspective of reducing labour involved in milking, there has been some interest in reducing the number of milkings. However, it appears that milking once per day for the whole lactation reduces yields by about 40 to 50 per cent. Similarly, milking thirteen times per week instead of fourteen, for the whole of the lactation has been found to reduce yields by five to 10 per cent. A much smaller reduction in yield occurs when omitting a milking per week is practised only for the last half of the lactation.
The widely held belief is that cows which are not milked completely at every milking will have reduced milk production. Experiments have shown that this is not strictly accurate. Leaving half a kilogram of milk in the udder (on top of the residual milk) will result in only a reduction in lactation yield of approximately three per cent. However if greater amounts of milk are left in the udder, the reduction in yield will be much greater.
Milking cows to produce the maximum amount of milk follows on from the principles outlined above. Understanding the milk ejection process explains how cows can be conditioned to let down their milk by the actions that make up the milking routine.
To reinforce the conditioned `let down' response the routine followed in the milking shed should be the same every milking. For example, bring the cow into the shed, put it in the stall, give it some feed, wash the udder, and start milking. It is important that the handling (washing) of the udder is the last thing in the routine before milking starts. The other parts of the routine can be in any order which suits the farmer, but they should follow the same pattern at each milking.
Difficulties in conditioning the let down response can arise with some cows, especially when they have a proportion of Bos indicus inheritance. As mentioned earlier the let down response is naturally elicited by the presence of the calf and the calf suckling. With highly selected Bos taurus dairy breeds the presence of the cow's calf is usually not necessary. However, with Bos indicus breeds and Bos indicus crossbreeds, it is sometimes necessary to have the cow's calf nearby before let down can be achieved, and with some individuals it is necessary to let the calf suckle the cow for a short time to get the let down response to occur.
Below are two examples of how milking routine can affect the time taken to milk the cow and the amount of milk produced.
A farmer from Northeast Thailand who had recently started dairying complained that it was taking more than 20 minutes to milk each cow. It was suspected that since the farmer was inexperienced at milking, there would be something wrong with his milking technique. However after visiting the farm and watching the milking process the problem became clear. The farmer was bringing all five cows into the milking shed, giving them some concentrates, washing them all and then milking them. By the time he had milked the first cow, the `let down' on the others was well passed and so he had to go through the motions of milking them, getting very little milk until the cow was ready for a second `let down'. With some of the cows, he believed that he had got all the milk there was and he stopped milking them when there was still maybe half or three quarters of the milk still left in the udder. This meant that the milk production of these cows was reduced significantly.
By merely changing the routine each cow was washed, then immediately milked, the time taken to milk each cow was markedly reduced, and the amount of milk the cows gave increased as well.
Another dairy farm also in Northeast Thailand run by two sisters achieved high levels of production. Two out of five cows were producing over thirty litres of milk each day. The cows were very well fed and the milking routine was distinctive. The cows stood in the yard and came up to be milked when they were called up by name. As each cow came into the milking stall it was fed and then washed all over, the udder being washed last. Then one sister sat on each side of the cow, and milked together while chatting constantly. The procedure would be repeated for the next cow, and so on until all five cows were milked. In this way each cow was milked very quickly, and there was no chance of the `let down' being over before the milk was all removed. This particular milking routine really suited the cows and they responded by giving excellent yields.
To remove milk from the udder it is necessary to create a pressure differential across the streak canal, so the milk is forced from the teat cistern. In other words the pressure inside the teat has to be higher than the pressure outside the teat, before the milk will flow. When a cow is being milked by hand the milker squeezes the top of the teat between thumb and forefinger thus preventing milk moving back into the gland cistern, then the teat is squeezed with the other fingers against the palm. This action effectively raises the pressure inside the teat and the milk flows through the streak canal. The stronger the milker's hands, the harder the milker can squeeze, creating a larger pressure difference, and faster milking.
With a milking machine, the pressure difference is obtained by reducing the pressure outside the streak canal, by creating a partial vacuum inside the teat cup. The squeezing of the rubber liner inside the teat cup does not simulate the milkers hand by increasing the pressure inside the teat. It merely gives the cow some periodical relief from the negative pressure on the teats. This in itself is important for the cow's comfort, but it does nothing to the mechanics of withdrawing milk from the udder. The milking machine is not a faster milker than the best hand milkers, but the machine does not get tired, as even the best hand milkers do. It is not surprising that the calf is, in fact, the fastest milker of the three. This is partly explained by the fact that the calf produces the pressure difference across the streak canal, by both methods. It sucks, producing a lower pressure outside the teat as the milking machine does, and it squeezes the teat between its tongue and the roof of its mouth, thus increasing the pressure inside the teat as the hand milker does.
Bacterial contamination of milk and infections in the udder also affect milking. The most common disease of dairy cows is mastitis. It is most often contracted at milking. Clean surroundings with no mud makes a huge difference to the number of cows which contract mastitis. As well as this a milking routine which keeps the udder clean and places a disinfection barrier around the streak canal, is essential to prevent infection occurring. The disinfection barrier is of key importance because after milking the streak canal does not close immediately and while open, the udder is vulnerable to infection.
Clean surroundings require an adequate supply of clean water. The problem of mud around the yards and shed, and a supply of clean water can often be solved by a system to catch and store the water that falls on the roofs of sheds and other buildings. A good supply of clean water is also the main requirement of producing milk that is not contaminated with bacteria which will spoil the milk. While detergents, hot water and disinfectants are important, without an adequate supply of clean water, the job of producing clean milk is almost impossible.
Theil C.C. and Dodd F.H. (n.d.). Machine Milking. Technical Bulletin 1. The National Institute for Research in Dairying, Reading, England, UK.