Secretion of milk

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Secretion of milk

Milk is secreted in the cow’s udder – a hemispherical organ divided into right and left halves by a crease. Each half is divided into quarters by a shallower transverse crease. Each quarter has one teat with its own separate mammary gland, which makes it theoretically possible to get four different qualities from the same cow. A sectional view of the udder is shown in Figure 1.1. 

The udder is composed of glandular tissue which contains milk-producing cells. It is encased in muscular tissue, which gives cohesion to the body of the udder and protects it against injury from knocks and blows.

The glandular tissue contains a very large number (about 2 billion) of tiny bladders called alveoli. The actual milk-producing cells are located on the inner walls of the alveoli, which occur in groups of between 8 and 120. Capillaries leading from the alveoli converge into progressively larger milk ducts which lead to a cavity above the teat. This cavity, known as the cistern of the udder, can hold up to 30 % of the total milk in the udder.

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The cistern of the udder has an extension reaching down into the teat; this is called the teat cistern. At the end of the teat there is a channel 1 – 1.5 cm long. Between milkings the channel is closed by a sphincter muscle which prevents milk from leaking out and bacteria from entering the udder.

The whole udder is laced with blood and lymph vessels. These bring nutrient-rich blood from the heart to the udder, where it is distributed by capillaries surrounding the alveoli. In this way the milk-producing cells are furnished with the necessary nutrients for the secretion of milk. “Spent” blood is carried away by the capillaries to veins and returned to the heart. The flow of blood through the udder amounts to 90000 litres a day. It takes between 800 and 900 litres of blood to make one litre of milk.

As the alveoli secrete milk, their internal pressure rises. If the cow is not milked, secretion of milk stops when the pressure reaches a certain limit. Increase of pressure forces a small quantity of milk out into the larger ducts and down into the cistern. Most of the milk in the udder, however, is contained in the alveoli and the fine capillaries in the alveolar area. These capillaries are so fine that milk cannot flow through them of its own accord. It must be pressed out of the alveoli and through the capillaries into the larger ducts. Muscle-like cells surrounding each alveolus perform this duty during milking, see figure 1.2.

The lactation cycle

Secretion of milk in the cow’s udder begins shortly before calving, so that the calf can begin to feed almost immediately after birth. The cow then continues to give milk for about 300 days. This period is known as lactation.

One to two months after calving the cow can be serviced again. During the lactation period milk production decreases, and after approx. 300 days it may have dropped to some 15 – 25 % of its peak volume. At this stage milking is discontinued to give the cow a non-lactating period of up to 60 days prior to calving again. With the birth of the calf, a new lactation cycle begins. The first milk the cow produces after calving is called colostrum. It differs greatly form normal milk in composition and properties. See further in
chapter 2.

A cow is normally productive for five years. Milk production is somewhat lower during the first lactation period.