ANSC 438 Home / Beginning / Milk Composition / Mammary Structure
Mammary Development / Mother & Neonate / Lactation / Mastitis

Mastitis Case Studies

Milking Machine


Dairy cattle are milked by machine. Milking machine design and function is critical for rapid and efficient removal of milk without damage to the teat or gland and with minimal risk for transmitting pathogenic microorganisms that might cause mastitis. The only way for mastitis-causing organisms to enter the teat is through the streak canal. This is the same route by which milk leaves the gland, therefore, anytime that the streak canal is opened to remove milk the gland is susceptible to potential intramammary infection.

Much of the recommended process of milking relates directly or indirectly to control and prevention of mastitis. The physical and functional interface between the teat and the machine are carefully designed to minimize the chances of intramammary infection occurring as a result of milking. Although the milking machine may only account for 5-10% of mastitis cases, proper milking machine function must be maintained over time. Incorrect vacuum or pulsator settings or worn teat cup liners all can enhance the role of the milking machine in contributing to intramammary infection.

The machine includes teat cups that contact the cow’s teats and remove the milk, a claw where milk pools as it is removed from the four teats, vacuum tubes that provide vacuum to the teat cups and a milk tube that removes milk away from the claw, a source of vacuum for the machine, and a pulsator that regulates the on-off cycle of the vacuum. Many milking machines today have an automatic take-off (ATO or detacher) device that removes the machine from the cow when milking is completed. In addition, many milking machine systems are linked to a computer system that both regulates the machine and generates data about the cow and its milk as milking is occurring. The description below is an overview of the milking machine components.

Milking machine parts on a cow.
Milking machine on a cow indicating machine parts. Note the chain leading from the claw to the left side of the image. This chain is part of the ATO of this sytem.

Most cows have four functional teats. Therefore, milking machines are designed with four teat cups. These are composed of an inner rubber liner and an outer shell, usually made of metal. The rubber liner is relatively thin in the section that sits inside of the shell, while the liner’s tube below the shell is thicker rubber.

Milking machine with 2 liners removed.
Claw with 2 liners removed.
Rubber liner and shell.
Milking machine with two of the teat cup liners removed from the claw.
Closer view of the claw with two liners removed.
Rubber liner and the stainless steel shell.

If you consider this arrangement for a moment, you will realize that the teat cup assembly results in two chambers (see below): A) one inside of the liner and B) one between the metal shell and the outside of the liner. A vacuum is pulled in both chambers; the vacuum in chamber A is continuous, while the vacuum in chamber B alternates between atmospheric pressure and the vacuum.

Diagram of milk phase.
arrows.
Diagram of rest phase.
Milk phase of the pulsation cycle.
Rest phase of the pulsation cycle.

When the teat cup is applied to the teat, the end of the inside chamber is filled by the teat. During the milk phase the vacuum applied inside the liner is constant and keeps a constant negative pressure at the end of the teat, drawing milk (in yellow) from the gland. The vacuum applied to chamber B, between the shell and the thinner walled part of the liner, keeps the liner from collapsing under the vacuum. During the rest phase, the vacuum inside chamber B is monetarily off. Air (in green) enters chamber B instantly reaches atmospheric pressure, colapsing the rubber liner around the teat end, massaging the teat and maintaining blood flow. The lower part of chamber A maintains its vacuum (lower part of right diagram), while the upper part around the teat momentarily loses vacuum. This alternating vacuum-atmospheric pressure in chamber B is controlled by a pulsator.

However, if the constant vacuum were left onto the teat end for an extended period, blood and lymph would accumulate in the end of the teat, causing trauma to the teat. This would be like attaching a vacuum hose to the end of your finger. The area exposed to the hose would turn red with accumulated blood. To prevent teat-end trauma, This alternating vacuum-atmospheric pressure, referred to as pulsation, is important for maintaining teat end health.


A proper pulsation rate, that is the number of cycles of vacuum on - vacuum off (in Chamber B above), or milk phase – rest phase cycles, usually is about 45-60 per minute. The ratio of time that the machine is in milk phase vs rest phase should be between 50/50 to 60/40 (pulsation ratio). In some systems, pulsation ratios are slightly different for teat cups milking the fore quarters vs the rear quarters. This is done because typically the rear quarters are larger and contain more milk than fore quarters. Therefore, rear quarters usually take slightly longer to milk out compared with fore quarters. The adjusted pulsation to rear vs. fore can account for this difference so that the rear quarters milk out faster and all quarters are properly milked out when the automatic take-offs detach the machine.


Teat-end vacuum should be stable and should be approximately 11-12 inches of mercury. Teat end vacuum fluctuations can occur because of several things. Improper vacuum, either because of incorrect vacuum settings, flooded milk lines (see below for high lines), or uneven milkout of quarters, can cause air to leak in between the teat and the rubber liner of the teat cup. This often results in a sucking sound called liner slips. It is not unusual to hear a sucking or squawking noise in any milking operation, but if they are too frequent, it is a sign of something improper in the milking system. The concern about liner slips (liner squawks) with respect to mastitis is that air entering one teat cup can forcefully blow milk from the claw up the other liners and droplet so milk may enter the other teats. This also may occur if the teat cup slips off of one of the teats, or the person putting the machine on the cow allows too much air to be sucked into a teat cup while milk is being removed from the other quarters. If a milk droplet entering another teat is contaminated with a bacterium, then transmission of mastitis-causing pathogens may occur.


After the milk leaves the teat it collects in the collecting bowl of the claw (image to the right). The milk then is drawn from the claw, through the milk hose to the milk line by the same vacuum as inside the liner (same as Chamber A above).

Machine on a cow.

If a milk line is above the cow (high line), then the milk in the milk hose must flow against gravity. If too much milk is flowing through the milk hose and the vacuum is blocked off, the vacuum inside the milk hose/claw/liners can be momentarily reduced (vacuum fluctuations), causing liner slippage on the teats or even for the machine to fall off the cow. If the milk line is lower than the cow (low line), as in a parlor where the people milking the cows are in a pit below the level of the cows, then milk flows away from the claw with gravity. These low line systems result in overall more efficient milk removal.

Milking parlor showing low milk line.
Checking slope of low milk line.
Low milk line in a parlor. Note milk hose goes down from the claw hanging under the udder to the milk meter. The milk meter is attached to the low milk line (larger of the pipes in this image).
Checking the slope of a low milk line.

Many milking systems today are equipped with an automatic detacher (called an automatic take-off or ATO; see resources on the Milking Process). The milking system detects flow rate of milk coming from the gland. When that flow rate drops to a specified level, the vacuum is turned off and a mechanical arm or chain retracts and pulls the machine from the cow's udder. This prevents overmilking of the cow's udder that often happens when humans have to make the decision of when to take the milker off.


Milk in the milk line flows to a pump that pumps it to the milk tank, usually housed in a separate room from where the milking is occurring.

Pump.
Milk bulk tank.
Pump that pumps milk into the bulk tank.
Bulk tank.

Milking systems today often are controlled by computer systems that record production information, sometimes indicators of mastitis, and other cow information. Milk yield is determined by a milk flow meter.

Computer control.
Milk meter.
Computer control for milking machine.
Milk flow meter measures milk yield.

Obviously, because the milking machine comes into contact with the cow’s teats or milk in the claw can impact up on the teat ends, cleanliness of the machine and hygiene during the milking process are critical for successful control of mastitis during the milking process. After milking, the machine is thoroughly cleaned with hot water, soap, acid and germicide solutions.


Mastitis Case Studies
Mastitis Resources