After service, spermatozoa rapidly ascend the female’s genital tract through the cervix and uterus to the oviducts. Contractions of the muscles in the tract aid this movement.

Fertilization occurs in the upper part of the oviduct when the egg is penetrated by one sperm. The external membrane of the ova immediately becomes impenetrable. Fertilization is complete when the nuclei of the sperm and the egg fuse.

At first, the embryo lies free within the uterus and can be moved about by contractions of the uterus wall, so that in animals which have litters, the embryos become distributed through both horns of the uterus. After a period, the developing membranes become attached to the wall of the uterus. This stage is called implantation.


The pregnancy or gestation period varies with species (and, to a lesser degree, with the breed).

The growth of the foetus is much more rapid in the last third of pregnancy and consequently, growth of the foetus is less affected by nutrition of the mother during the first half of pregnancy. It is greatly affected later on.


Signs of parturition are as follows:

  • The abdomen enlarges;
    • Mammary glands enlarge and begin to secrete a milky substance a few days before parturition;
    • The vulva swells and usually discharges a thick mucous;
    • Relaxation of the abdominal wall with a sinking in of the flanks and a dropping of the belly;
    • The animal is restless, seeking to be on its own;
    • The animal frequently gets up and down; and
    • Attempts to urinate frequently.

The big muscles which make up the wall of the uterus contract, and this forces the placenta, and the young animal together with its surrounding tissues and fluid, against the cervix; the neck of the womb. This pressure causes the cervix to open up gradually so that the foetus (the young animal) can pass into the vagina.

The placental sac, the bag which encloses the foetus, now bursts and the uterus contracts more strongly, contracting and relaxing in waves with a short pause after each contraction. The abdominal muscles also contract and help to force the foetus out of the vagina, and so out of the body of the mother.

Normally the bag which has contained the young animal, and which is called the afterbirth, becomes detached from the uterus and falls away shortly after the calf, lamb or foal has been born. It should be delivered within 24 hours of the birth of a calf, otherwise infection sets in and the afterbirth starts to rot while still inside the cow. This condition requires veterinary treatment.

From the start of labour, the contractions of the uterus and abdominal muscles, a calf should be born within 8 hours and a lamb within 1 – 2 hours. If birth has not taken place within this time period, it means that something is wrong and the animal will have to be helped.

The normal presentation of the calf is shown in the diagram below. Notice that the two front legs are pushed forward and the head is lying between them. The first parts of the calf to appear at the entrance of the vagina are the two front feet.

Figure 2: The Cow’s Reproductive System


One of the main causes of difficulty at birth is the abnormal presentation of the calf, lamb or foal. Three examples of this are given below.

Figure 3: Abnormal Presentation

In this case the front legs are in the correct position but the head has fallen back and cannot pass through the vagina. By putting his hand into the vagina, the herdsman can pull the head into the correct position for the birth to take place.

Figure 4: Abnormal Presentation

In this presentation the calf is lying on its back with the head at the wrong end of the uterus and the back feet inside the vagina. The correct procedure here is to turn the calf right side up and pull it out by the back feet.

Figure 5: Abnormal Presentation

This type of presentation is called a breech birth, and can be dealt with by pushing the calf back down into the uterus, straightening out the back feet, and using them to pull the calf out of the vagina.

There are a number of other abnormal presentations, the most common being a calf with one foot and the head in the normal position, and the other foot drawn back and lying inside the uterus. All these conditions can be put right by a skilled stockman. But anyone without the proper knowledge can do great damage to both the calf and the cow and may even kill both. If at all doubtful about one’s ability to assist in the birth, it is much better to call for the assistance of a qualified veterinary officer, or failing that, an experienced livestock man.

Another cause of difficult births is a foetus which is too big for the birth canal of the mother. Sometimes it is the whole foetus which is too big, and at other times it can be a calf with a very large head. Trying to pull out a calf like this by brute force can cause damage to the mother and is quite likely to kill her. This type of calf can be removed by caesarian section, whereby an incision is made in the flank of the cow and the calf is lifted out, the cut being stitched up after the operation. This method usually saves the life of the calf as well as the mother. The other method is to cut the calf into small pieces inside the cow, and pull each piece out. This is done usually when the calf is already dead inside the cow. Both these methods must be carried out by a veterinary surgeon.

A third cause of difficulty at birth arises when the calf is completely abnormal, as in the case of Siamese twins, two calves joined together, or a calf having two heads. These conditions are very rare. Trouble can happen in the case of twin calves, with one foot from each calf being presented, but this can be dealt with by someone with the necessary skills.


The mammary glands make up the udder of the cow as shown in the diagram below.

Figure 6: The Cow’s Udder

The udder of the cow is made up of four quarters, each separate and with the milk for each teat being produced by the glandular tissue of that particular quarter. In each quarter, the glandular tissue consists of alveoli, which are the structures for milk production; the alveoli are drained by ducts which unite and lead into the teat cistern (see the diagrams on next page). At the end of the teat cistern is a narrow opening called the streak canal. This streak canal is surrounded by a sphincter of circular muscle fibres which keep it closed and prevent milk from leaking out. Sometimes these muscles become too slack, or become damaged, in which case milk does leak out. Sometimes they become too tight, and the cow is difficult to milk out on that quarter. Such a cow is known as a ‘slow milker’.

The udder is attached to the abdominal wall of the cow by suspensory ligaments. In older cows these become slack and the udder hangs down making the cow difficult to milk by machine.

Figure 7: The Udder

Figure 8: The Glandular Tissue and Alveolus of the Udder

Milk is produced directly from blood, and the characteristic white colour is caused by the globules of fat suspended in the watery solution. Milk is, in fact, an emulsion, and if left to stand, the fat globules rise to the surface and form the cream.

Blood enters the udder via the mammary artery, and this branches out to form smaller vessels which then become capillaries surrounding the alveoli. The part of the blood which forms the milk passes through the capillary walls into the alveoli, and the remainder is carried away by the capillaries which join up to form the mammary vein. This is shown in the diagram above.

  • 3.       SECRETION OF MILK

The mammary glands develop in female mammals at puberty, a development that is brought about by the action of two hormones produced by the ovaries. These are oestrogen, which stimulates the growth of the ducts in the udder, and progesterone, which acts to produce the alveoli.

The amounts of these two hormones produced during pregnancy cause the udder to develop and enlarge, but the balance between the amounts of each hormone makes sure the udder develops without actually producing milk. Just before the animal gives birth, this balance is altered, and milk is produced.

Milk production, or lactation, is maintained by a number of hormones produced by the endocrine glands, including the pituitary gland, the adrenal glands, the ovaries and the thyroid gland. The hormones produced by each of these are shown in the diagram below:

Figure 9: Milk Production Process


As milk is manufactured by the cow, the alveoli, large ducts and teat cistern fill up with milk, and this is held inside the udder by the action of the sphincter muscle at the end of the teat canal. The milk is released partly by the sucking and pressing action of the calf when it suckles the mother, or by the action of the hand or milking machine, if the cow is being milked for milk production. It is partly by the action of a hormone called oxytocin. This hormone is produced by a reflex action of the cow. If the udder is stimulated, either by the nuzzling of the calf or by being wiped with a warm, damp cloth, this stimulation is carried along the central nervous system to the pituitary gland. Oxytocin is then released, carried by the blood system to the udder, and releases the milk from the alveoli. This is commonly called the ‘let down’ of milk by the cow.

As the cow’s lactation progresses there is a gradual decrease in the number of active alveoli. Milk secretion slows down and the amount of connective tissue increases. This causes a decrease in the amount of milk produced until, after about 10 months, the cow dries off and milk production ceases altogether.

The first milk produced by the mammary glands is called colostrum, and this is rich in antibodies which give the newborn animal immunity to disease.

COW  6 – 18 months  21 days average  10 – 20 hours10 – 14 hours after end of oestrus280 days15 – 24 months
SOW  5 – 7 months  21 days average1 – 3 days18 – 36 hours after onset of oestrus116 days7 – 9 months
  EWE4 – 12 months17 days average  1 – 2 daysJust before end of oestrus  150 days  15 – 18 months