The udder is known as an exocrine gland because milk is synthesized in specialized cells grouped in alveoli, and then is excreted outside the body through a duct system that functions like the tributaries of a river.
Milk production demands a lot of nutrients that are brought to the udder by the blood. To produce 1 kg of milk, 400 to 500 kg of blood must pass through the udder. In addition, the blood carries hormones that control udder development, milk synthesis, and the regeneration of the secretory cells between lactations (during the dry period).
The alveolus is a functional unit of production in which a single layer of milk secretory cells are grouped in a sphere with a hollow center. Capillary blood vessels and myoepithelial cells (muscle-like cells) surround the alveolus, and the secreted milk is found in the internal cavity (lumen).
The functions of the alveolus are:
Milk secretion by the secretory cells is a continuous process that involves many biochemical reactions. During milking, the rate of milk secretion is somewhat depressed, but it never stops completely. Between milkings, the accumulation of milk increases the pressure in the alveoli and slows down the rate of milk synthesis. As a result, it is recommended that high-producing cows be milked as close as possible to 12 hour intervals.
More frequent ejection of milk reduces the pressure build-up in the udder, and for this reason milking three times a day can increase milk yield by 10 to 15%.
Immediately prior to milking about 40-50 per cent of the total milk is held in the gland cisterns, sinuses and large ducts, while the other 50 per cent is held in small ducts and alveolar lumena. Between milkings approximately 40% of the milk is stored in the teat and gland cisterns and the major ducts while the remaining 60% is stored in the alveoli.
Some milk constituents move directly, without a change in form, from blood to milk; while others must be synthesized by the epithelium. Casein and lactose, for example, are found only in milk, and milk fat comprises glyceride combinations not found elsewhere in the body.
After calving, lactation continues for an extended period, in the dairy cow typically 300 days. Hormonal influences are critical to the maintenance of lactation. A few weeks after parturition the cow will begin her oestrus cycle again and show heat. She will be usually be artificially inseminated at an appropriate heat around 70-90 days after calving. The objective is to get the cow calving about once per year. Milk yield declines as pregnancy progresses. In fact the hormonal changes during pregnancy and the increasing flow of nutrients diverted towards the foetus serve to down regulate milk secretion.
At around 300 days milking is usually stopped and the cow “dried off”.
The rate of decline in milk yield following peak production is commonly known as persistency.
Persistency is calculated as the month’s milk divided by last month’s expressed as a percentage. On average, the persistency should be about 94-96% (i.e. milk yield in each month is about 95% of the previous month’s yield). If a cow’s milk production falls by 7% per month after peak, it is more persistent than a cow that loses 10% per month. A general rule of thumb is that higher peak production leads to lower persistency. As for peak yield, the persistency is dependent on feeding and is therefore, to some extent, possible to influence.
Persistency differs between cows, but a first lactating cow is normally more persistent than a second or third lactating cow.
The milk let-down reflex of cows is very easily retarded or prevented by external stimuli which disrupt the normal routine for milking.
Milk let-down can be inhibited by nervous stimuli such as rough treatment of the cow, loud, unfamiliar noise, fear, pain and irritation. Such stimuli cause the brain to release adrenalin that contrasts the action of the oxytocin by:
blocking oxytocin release from the brain
constricting blood vessels and preventing oxytocin from reaching the udder
directly counteracting the effect of oxytocin on the contraction of myoepithelial cells.
If adrenalin release occurs before the milk-ejection, the ejection will be blocked.
When adrenalin is released after milk ejection has commenced, it will result in large amounts of milk being retained in the udder with associated negative effects on milk yield.
Cows handled gently and milked carefully at regular intervals seldom suffer from this problem.
The oxytocin effect usually takes place 20 to 40 seconds after the initial stimulation (e.g. udder wiping) and lasts for only about 6 minutes. It is most important that the milk is removed while this reflex is operating. The udder should be handled no earlier than 60-90 seconds before teat cup application, otherwise the beneficial effects of good milk ejection will be lost. Once the effect of oxytocin wears off, milk that has not been removed will flow back from the udder sinuses into the ducts.
The extra pressure within the udder will reduce the synthesis of milk after the incomplete milking, and if this situation occurs repeatedly the cow will reduce her yield accordingly. In dairy cows the milk ejection reflex is probably essential for 50 to 60% of the total yield at a milking.
Pregnancy has an inhibitory effect on milk yield. Most of the reduction in milk yield occurs after the fifth month of pregnancy. By the 8th month of pregnancy, milk yield may be 20% less for that month compared with a non-pregnant cow. The inhibitory effect of pregnancy is not likely due to fetal requirement, which does not increase considerably until the last two months of pregnancy. It is believed that the increase in estrogen and progesterone levels as pregnancy progresses, inhibits milk secretion.
To assist in expulsion of milk from the udder, the myoepithelial cells are arranged in two ways:
in a starlike (stellate) fashion around the alveoli to achieve a squeezing action and to collapse it;
in a longitudinal (lengthwise) arrangement along the small ducts to effect a shortening and widening of these small tubes; this aids the expulsion of the milk from the collapsed alveolus by increasing the size of the passage through which the milk must pass.
The conditioned stimuli for milk ejection include rattling of the milk buckets, washing of udders, feeding of concentrate, approach of the milker, and application of the milking machine or massage of the udder prior to application of the teat cups. These stimuli send a signal to the paraventricular nuclei of the hypothalamus and then travel to the posterior pituitary gland and causes the release of the hormone oxytocin into the blood stream. This hormone provokes the contraction of the myoepithelial cells.
The effect of ambient temperature on milk yield is dependent upon the breed. Holsteins and the other larger breeds are more tolerant to lower temperatures, whereas the smaller breeds particularly the Jersey, are much more tolerant to high temperatures. The optimum temperature for the Holstein cow is about 10 °C. Milk production declines when the environmental temperature exceeds 27 °C. The reduction in milk yield is largely due to a drop in feed intake. High temperature affect high producing cows more than low producers and it is particularly harmful during the peak of lactation.
In this phase of milking the active cooperation of the cow is not required. Nerve endings in the skin of the udder and of the teat are stimulated by washing/wiping of the udder, the slipping-on of the teat cups and by pressure of the teat liner on the walls of the teat. The teat sphincter at the tip of the teat then opens and milk begins to pass out into the milk tube. This phase, which is activated by the nerves, begins about 5 to 10 seconds after the udder and teats have been stimulated. The nervous reflex brings about a squeezing of the ducts by the smooth muscle and the milk within the ducts is pushed into the udder cisterns. Since the dairy cow has a relatively large cistern, 40 to 50% of the total milk yield may be obtained during this passive withdrawal phase.
Residual milk and available milk
Residual milk can be defined as the amount of milk left in the udder after milking is completed. About 10-20% of total milk is left in the udder as residual milk. Residual milk can be measured by giving the cow oxytocin and one minute later, milking the cow again. Part of the residual can never be recovered by conventional milking procedures while another fraction can be collected if the amount of residual milk is large due to poor milking procedure. This portion of milk is referred to as available milk. Available milk reflects how well the cow was stimulated for milking. If the cow is not well trained for milking, the amount of available milk uncollected will be great. Large amounts of residual milk reduce daily milk production, reduce lactation production and the cow will have fewer days in milk.