A hormone called oxytocin must be released into the cow’s bloodstream in order to start the emptying of the udder. This hormone is secreted and stored in the pituitary gland. When the cow is prepared for milking by the correct stimuli, a signal is sent to the gland, which then releases its store of oxytocin into the bloodstream.

milking_1In the primitive cow the stimulus is provided by the calf’s attempts to suck on the teat. The oxytocin is released when the cow feels the calf sucking. A modern dairy cow has no calf but is conditioned to react to other stimuli, i.e. to the sounds, smells and sensations associated with milking.

The oxytocin begins to take effect about one minute after preparation has begun and causes the muscle-like cells to compress the alveoli. This generates pressure in the udder and can be felt with the hand; it is known as the letdown reflex. The pressure forces the milk down into the teat cistern, from which it is sucked into the teat cup of a milking machine or pressed out by the fingers during hand milking.

The effect of the letdown reflex gradually fades away as the oxytocin is diluted and decomposed in the bloodstream, disappearing after 5 – 8 minutes. Milking should therefore be completed within this period of time. If the milking procedure is prolonged in an attempt to “strip” the cow, this places an unnecessary strain upon the udder; the cow becomes irritated and may become difficult to milk.

Hand milking

On many farms all over the world milking is still done by hand in the same way as it has been done for thousands of years. Cows are usually milked by the same people every day, and are quickly stimulated to let down just by hearing the familiar sounds of the preparations for milking.


Milking begins when the cow responds with the letdown reflex. The first lets of milk from the teats are rejected, as this milk often contains large amounts of bacteria. A careful, visual check of this first milk enables the milker to detect changes that may indicate that the cow is ill.

Two diagonally opposed quarters are milked at a time: one hand presses the milk out of the teat cistern, after which the pressure is relaxed to allow more milk to run down into the teat from the cistern of the udder. At the same time milk is pressed out of the other teat, so that the two teats are milked alternately. When two quarters have been stripped this way, the milker then proceeds to milk the other two until the whole udder is empty.

The milk is collected in pails and poured through a strainer, to remove coarse impurities, into a churn holding 30 – 50 litres. The churns are then chilled and stored at low temperature to await transport to the dairy. Immersion or spray chillers are normally used for cooling.

Machine milking

On medium to large dairy farms, the usual practice is to milk cows by a machine similar to that shown in figure 1.5. The milking machine sucks the milk out of the teat by vacuum. The milking equipment consists of a vacuum pump, a vacuum vessel which also serves as a milk collecting pail, teat cups connected by hoses to the vacuum vessel, and a pulsator which alternately applies vacuum and atmospheric pressure to the teat cups.

The teat cup unit consists of a teat cup containing an inner tube of rubber, called the teat cup liner. The inside of the liner, in contact with the teat, is subjected to a constant vacuum of about 0.5 bar (50% vacuum) during milking.

The pressure in the pulsation chamber (between the liner and teat cup) is regularly alternated by the pulsator between 0.5 bar during the suction phase and atmospheric pressure during the massage phase. The result is that milk is sucked from the teat cistern during the suction phase. During the massage phase the teat cup liner is pressed together to stop milk suction, allowing a period of teat massage and for new milk to run down into the teat cistern from the udder cistern. This is followed by another suction phase, and so on, as shown in figure 1.6.

Relaxation of the teat during the massage phase is necessary to avoid accumulation of blood and fluid in the teat, which is painful to the cow and will cause her to stop letting down. The pulsator alternates between the suction and massage phases 40 to 60 times a minute.

The four teat cups, attached to a manifold called the milk claw, are held on the cow’s teats by suction. During milking, suction is alternately applied to the left and right teats or, in some instances, to the front teats and rear teats. The milk is drawn from the teats to the vacuum vessel or into a vacuumised transport pipe. An automatic shut-off valve operates to prevent dirt from being drawn into the system if a teat cup should fall off during milking. After the cow has been milked, the milk pail (vacuum vessel) is taken to a milk room where it is emptied into a churn or a special milk tank for chilling.

To eliminate the heavy and time-consuming work of carrying filled pails to the milk room, a pipeline system may be installed for direct transport of the milk to the milk room by vacuum, figure 1.8. Such systems are widely employed on medium sized and large farms and allow milk to be conveyed in a closed system straight from the cow to a collecting tank in the milk room.
This is a great advantage from the bacteriological point of view. It is however important that the pipeline system is designed to prevent air leakage agitating the milk in a harmful way.

The machine milking plant is also provided with cleaning-in-place (CIP) facilities.