agricultural production guidelines
dairying in kwazulu-natal
Dairying in KwaZulu-Natal
Dairying 6.1 1995
UDDER AND MILK SECRETION
A W Lishman
Department of Animal Science, University of Natal
The cow's udder consists of four entirely
separate quarters (Figure 1). The right and left halves are separated by a distinct
membranous wall called the medial (central) suspensory ligament (Figure 2). This ligament
is composed of strong elastic tissue that extends between the halves of the udder.
Numerous branches (lamellae) extend from this ligament into the quarters. As the udder
fills with milk it expands or stretches like a balloon, thereby making room for the milk
being stored in the udder between milkings.
The elasticity of the central ligament
results in the ungainly and uncomfortable appearance of the udder in high-yielding cows.
Not only does the udder force the cow's hind legs apart, making walking difficult, but the
middle of the udder drops further from the body than do the outer portions, thus causing
milk-engorged teats to project outwards. If the central udder support is weak, then the
udder will become pendulous and the teats will face outwards, causing not only difficult
milking, but also increasing their susceptibility to injury and contamination by dirt and
Figure 1. Sub-division of the
udder into left and right halves and front and rear quarters (Quinn, 1990)
Figure 2. Support the udder
The front and rear quarters of the udder
are divided by a very thin wall of connective tissue which is irregular in outline. This
division is difficult to see from the outside.
The outer wall of the udder contains
ligaments which blend into muscles in the hind quarters of the cow. The milk-secreting
parts of the udder are supported by web-like connective tissue which cris-crosses the
udder (see Figure 3). Some cows may have large udders, not because they are high
producers, but because of a high content of connective tissue. This allows less space for
milk-secreting tissue. Udders which shrink noticeably and become flabby after milking
contain little connective or scar tissue.
Each of the four quarters contains a
seperate mammary gland (Figure 3) which, in turn is composed of:-
Secretory tissue (alveoli)
A duct system (interconnecting tubes)
Two cisterns (storage areas)
Milk is removed from each gland by the
streak or teat canal which is 8 to 12 mm long (Figure 4). It is kept closed between
milkings by a circular sphincter muscle near the tip. This muscle is important not only in
keeping the milk in the gland, but also in preventing entry by bacteria.
The character of the sphincter is important
to the cow's productivity. If the canal is small, or if the sphincter is unusually strong,
then the cow is hard and slow to milk. At the opposite extreme (large canal, or weak
sphincter), milk will leak from the teat between milkings and the udder is then open to
invasion by mastitis-causing organisms.
Figure 3. Components of the
udder (Quinn, 1980)
This cavity of the teat, located just above
the streak canal (Figure 3), stores the milk which drains from the gland. It normally
holds 15 to 40 ml of milk, depending on the size of the teat.
The teat contains many blood vessels with
small valves that maintain blood flow when the teat is massaged (by the teat cup liner).
Lack of massage during milking causes blood and other body fluids to dam-up in the teat.
This is painful to the cow, and she will then be inclined to kick.
The gland, or udder, cistern is located
just above the teat cistern (Figure 4) and is partially separated from it by annular folds
of tissue. Although the udder cistern varies in shape and size between quarters and cows,
it stores only about 500 ml of milk.
The annular folds can become the cause of
milking problems if the teat cup of the milking machine is allowed to creep up toward the
udder. This pinches the annular fold and stops the flow of milk (Figure 5). Creeping also
causes irritation to the sensitive tissue of the udder and can be the cause of injury or
Figure 4. Components of udder
showing blood supply (Frandson, 1986)
A number of large ducts (tubes) branch off
from the gland cistern. These ducts branch and re-branch into smaller and smaller ducts
(similar to roots of a tree) and finally into the small ductules that drain each alveolus
The function of the duct system is to
collect the milk from the secretory tissue, to store part of the milk between milkings and
to transport the milk to the gland cistern.
Where the milk ducts branch-off from a
larger duct, a narrowing of the tube occurs. As the lobules (milk secreting glands) fill
up with milk, their weight causes them to sag downwards (Figure 7). This causes the narrow
branch of each duct to be pinched and close. Consequently, milk does not drain from the
udder until the cow is stimulated to let-down her milk.
The main milk-producing unit is the
alveolus. This is a microscopic structure, almost spherical in shape, the outer surface of
which is lined with a single layer of epithelial (milk-secretory) cells and has a tiny
duct to allow the milk to drain out (Figure 8). The alveolus thus resembles a grape with
its stalk. The outer layer also contains a complex of tiny, muscle-like cells called
The alveoli are connected by small ducts,
and groups of alveoli form lobules. A group of lobules, in turn, forms the main lobe of
each quarter (Figure 3). The udder of the dairy cow contains about 5 x 1012 secretory cells in the
epithelium of the alveolar tissue.
Figure 5. Teat cups that are
allowed to creep tend to pinch off the opening and stop the milk flow (Quinn, 1980)
These have a short life and are
rapidly replaced. In early lactation there is a net gain in cell numbers, but after peak
yield there is a gradual decline in total numbers. Each alveolus is surrounded by a
network of minute blood vessels (capillaries which carry the substances from which milk is
When the hollow part of the alveolus is
filled with milk, these small muscles expand to their fullest length. During milking the
muscles contract and squeeze the milk out of the alveolus.
Source of milk constituents
Although the site of milk synthesis is the
epithelial cells lining the alveoli, some milk components (vitamins, minerals, certain
proteins) are not synthesized here. They are simply filtered out from the blood, through
the cells into the milk. Lactose (milk sugar), fat (butterfat), and most of the proteins
(mainly casein) are formed in the epithelial cells from substrates carried in the blood.
More than 300 kg of blood must pass through the udder to produce 1 R of milk.
The substrates are filtered from the
blood and pass into each epithelial cell (lining the alveolus) through the plasma
membrane. When this membrane functions normally, it is extremely selective as to which
substances can pass through. However, when the udder is infected (for example, as in
mastitis), the selectivity breaks down, and higher levels of blood protein and even white
blood cells gain entry to the alveolus.
Figure 6. Constrictions at
each branch of the duct system prevent milk from leaking into the teat and lower udder as
fast as it is manufactured
Figure 7. As they fill with milk,
the lobes sag with extra weight and pinch off the ducts, thus preventing further flow of
milk into the ducts (Quinn, 1980)
Figure 8. Structure of the alveolus
showing: (a) blood capilliary supply: (b) myoepithelial cells: and (c) lumen epithelium of
secretory cells lying upon a basal membrane (Whittemore, 1980)
The processes whereby milk
constituents (fat, protein etc) are released from the epithelial cells into the
cavity (lumen) of the alveolus are most fascinating. Within the epithelial cell, milk fat
is produced as small, well-defined droplets. The droplets enlarge by merging together as
they move toward the central lumen of the alveolus. As the fat droplet emerges from the
cell it becomes totally surrounded by part of the cell wall, and an actual break or
rupture in the lining forms as the fat droplet is pinched-off (Figure 9)
Casein and lactose are also gathered
together in small packages (secretory vesicles) surrounded by a membrane. As in the case
of fats, these vesicles move towards the lumen of the alveolus. When the vesicle reaches
the membrane lining of the epithelial cell, its outer lining fuses with that of the cell.
The contents are then discharged into the alveolar cavity.
Figure 9. Schematic diagram of
a mammary secretory cell showing the components of a functionally continuous endomembrane
system (Keeanan et al., 1974)
Abbr: MFG = milk fat globule; SV =
This fusion of vesicle membrane and cell
membrane repairs the loss caused by escape of fat droplets. Each epithelial cell may
discharge fat or protein and casein 15 to 20 times between milkings.
The molecules of proteins, milk sugar,
vitamins, and minerals are uniformally distributed within the water of the milk. In
contrast, the milk fat globules float in the water after entering the lumen of the
alveolus. These fat droplets then collect as tiny clusters which cannot easily pass
through the tiny ducts draining the alveoli. However, the water and its contents move
freely along the tubes and gradually fill the cisterns of the quarters. Some of the fat is
even retained within the secretory cell and will escape only when the pressure within the
udder drops during milking. Consequently the first batch of milk removed from the udder
contains only 1 to 2% fat while that coming out of the alveoli may reach 24% fat.
The rate at which epithelial cells
manufacture milk is high for the first 10 hours after milking. It then starts to decrease
at a rapid rate, and if the cow is not milked it will cease about 35 hours after the last
milking. The milk secretory process stops when the pressure inside the alveolus becomes
prohibitive. This probably occurs because the pressure constricts the blood vessels, and
stops the flow to the milk secreting cells.
Lower-producing cows are LESS adversely
affected by long intervals between milkings because lower-producing cows have less
secretory tissue in the udder. Consequently, the increase in udder pressure (between
milkings) per unit milk is greater in younger and lower-producing cows. Short milking
intervals are thus important for young cows. Incomplete milking (e.g. leaving 2
kg milk in the udder at each milking) causes a permanent depression in milk production.
Frequency of milking
The rate at which milk is secreted is
important in determining how much extra milk will be produced when cows are milked more
often than twice daily. On the average, production will be increased by 20% when milking 3
times per day as compared with twice daily. Milking 4 times per day yields a further 5 to
The additional yield occurs because :-
- the milk secreting cells operate at full
capacity for longer,
- of better feeding and management often
associated with 3 times per day milking.
Decreasing the number of milkings per day
causes a marked reduction in milk yield. Skipping one milking a week reduces total yield
by 5 to 10%, while milking once daily cuts the yield by half in first calvers, and by 40%
in older cows.
A vital part of lactation is of course the
process by which the milk is removed. This includes :-
- passive withdrawal from the cisterns and
ducts of the udder
- ejection of milk from the alveolar lumen.
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. This is called implosion.
- 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.
These myoepithelial cells have no nerve
connection and are controlled by hormones.
In contrast to the myoepithelial cells
within the udder, smooth muscle occurs around :
This smooth muscle is controlled by nerves
which pass from the spinal column through the glandular tissues terminating at the muscles
of the teat sphincter.
In this phase of milking the active
co-operation 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.
Whilst passive withdrawal of milk in the
cisterns is taking place, nerve impulses pass from the teat to the brain in response to
stimulation from attaching the milking machine (Figure 10). Other messages picked up via
the cow's nose and eyes are also transmitted to the brain by nerves. This information
causes the brain to release the hormone oxytocin into the bloodstream. Eventually the
oxytocin reaches the udder where it causes the myoepithelial cells around the alveoli to
contract. Upon contraction of the myoepithelial cells, the alveoli collapse and milk is
squeezed out into the small ducts (Figure 11). These ducts shorten and widen, and the milk
rushes through into the gland cistern. Only alveolar and ductal milk is expelled by the
action of oxytocin on the myoepithelium. There is no contraction of large ducts or
Figure 10. The milk ejection reflex
This milk ejection reflex is pre-programmed
(i.e. conditioned) by the brain. In other words, the brain interprets certain
stimuli to mean that milking is about to commence. Accordingly, when the brain receives
the correct message it automatically proceeds to trigger the release of oxytocin. 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.
An effective milk ejection reflex is needed
to achieve fast, efficient milking. It is vital that the cow is able to recognize the
signal from which she can anticipate milking to commence within a relatively fixed time,
in other words, the signal must be clear and should always occur at a set time before the
machine starts milking.
The milk ejection reflex usually takes
place 20 to 40 seconds after the initial stimulation (e.g. udder wiping) and lasts
for only about 6 minutes (Figure 12). It is most important that the milk is removed while
this reflex is operating. The udder should be handled no earlier than 30 seconds before
teat cup application, otherwise the beneficial effects of good milk ejection will be lost.
In many situations, application of the teat cups themselves is THE stimulation for milk
ejection. There will, of course, be 30 to 60 seconds lag between applying the machine and
contraction of the alveoli. However, passive withdrawal will be occurring during this
interval. This kind of lag may be particularly beneficial when milking high-producing cows
since the reduction of pressure within the cisterns (by passive withdrawal) will aid flow
from the alveoli. Once the effect of oxytocin wears off, milk that has not been removed
will flow back from the udder sinuses into the ducts and eventually into the alveoli. It
can then be removed only after oxytocin has again been released, which will probably occur
only at the next milking. The extra pressure within the udder will reduce the synthesis of
milk after the incomplete milking, and ifthis 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.
It is thus vital to capitalize rapidly and
effectively on the cow which has been properly stimulated and to remove all the milk as
soon as it becomes available. Effective emptying of the udder is thus an essential
component of successful parlour routine.
Figure 11. Milk ejection - the
contraction of the myoepithelial cells surrounding the alveolus forces milk out of the
lumen into the ducts (Schmidt, 1971)
Inhibition of milk let-down
The milk let-down reflex of cows is very
easily retarded or prevented by external stimuli which disrupt the NORMAL routine for
milking. It is often said that a cow "holds up" her milk. This is NOT possible
since the cow has no voluntary control over the let-down process, and she cannot thus put
the process into REVERSE. However, let-down can be prevented, or terminated, by nervous
stimuli such as rough treatment of the cow, loud, unfamiliar noise, pain and irritation.
Such stimuli cause the brain to release adrenalin. This hormone works against 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.
Figure 12. Pattern of milk
flow from the udder (Whittemore, 1980)
If adrenalin release occurs before the
milk-ejection stimulus, the ejection will be virtually completely 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. Milking should be a pleasant experience
for the cow in order to capitalise fully on milk ejection.
FRANDSON, F.R., 1986. Anatomy and
physiology of farm animals. Lea & Febiger. Philadelphia
KEEANAN, T.W., MORRE, J.T. & HUANG,
C.M., 1974. Membranes of the mammary gland. In: Lactation. Vol. II. Eds.
B.L. Larson & V.R. Smith. Academic Press. New York.
QUINN, T., 1980. Dairy farm management. Van
Nostrand Reinhold. New York.
WHITTEMORE, C.T., 1980. Lactation of the
dairy cow. Longman. New York.
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