From sheptonvet.com

Published Papers
Influence of Milking Machines on Mastitis
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Oct 10, 2006, 22:13

Milking machines can influence mastitis and bactoscan or TBC. They are used more than other pieces of equipment on the dairy farm and yet their function, maintenance and effect on mastitis is poorly understood by many dairy farmers. As a result of this, they are often neglected despite the fact that they are responsible for generating the majority of dairy farmer’s income.

It is important that veterinary surgeons involved in mastitis investigation have at least a basic understanding of the milking machine and it’s effect on mastitis. It is very useful to be able to help pinpoint possible problem areas so that the machine can either be included or excluded as a contributory factor. An understanding and involvement with the milking machine allows the veterinary surgeon to provide a ‘complete’ mastitis service that does not rely solely on the help and opinions of milking machine dealers and manufacturers.

This article summarises the principles of machine milking and discusses the various ways in which the machine can increase the likelihood of mastitis. Some basic investigation methods and tests to check milking function are also described.

Basic principles of machine milking

A full description of the function of the milking machine has been published previously (In Practice, Vol 15, No 1 pages 12 to 17).

Milk-out occurs by applying reduced pressure (vacuum) to the teat-end which causes the teat canal to open letting milk flow. The oxytocin induced let-down reflex assists this process, increasing the pressure within the udder.

A constant pre-set vacuum level (as specified by the manufacturer) should be maintained throughout milking. The vacuum pump extracts more air than is necessary to operate the milking system. This overproduction can be measured and is called the vacuum reserve. Adequate vacuum reserve is required to ensure that the vacuum level at the teat-end remains stable whenever atmospheric air enters the system, such as when units are attached or fall off etc. The vacuum regulator (or controller) maintains stability this by leaking atmospheric air into the system as and when required during milking in order to maintain a fixed vacuum level.

Pulsation is responsible for ensuring adequate blood circulation of the teat. Pulsation is achieved by alternating atmospheric air and vacuum in the pulsation chamber (the space between the liner and the teat cup shell) causes the liner to open and close. When the liner is open, milk flows away from the udder. When the liner is closed, milk flow ceases and blood is able to circulate around the teat. Blood flow around the teat during milking is essential to ensure that the teat does not become congested which may impair milk removal.

The milking machine can have an effect on the incidence of mastitis in one of four ways.

1. Formite

Mastitis organisms may be physically transmitted from cow to cow or from one quarter to others in the same cow. Cow to cow spread can occur through contaminated milk remaining on the liner during milking (Plate 1) and quarter to quarter spread may occur through backflow of milk up the liner. Backflow occurs if there is flooding at the clawpiece where milk is not swiftly removed from the cluster. Large capacity clawpieces help to limit this effect by removing milk rapidly from the short milk tubes.

Staphylococcus aureus infection can be spread to the next six to eight cows milked through a contaminated liner (Grindal & Bramley). This does not necessarily result in infection, as these organisms may not invade the teat canal and should be killed through post milking teat disinfection. The risk of this occurring is increased if the liners are worn as bacteria adhere more easily to their roughened surface (Plate 2).

Mycoplasma infections can also be spread from cow to cow via the liner. While this is not a common pathogen in the UK, it is prevalent in hot desert areas of the world. Affected animals are generally culled, as treatment is ineffective. For this reason, many milking installations have an automatic backflushing unit that disinfects the milking unit between cows. (Plate 3).

Liners are the only piece of the milking machine that comes into direct contact with the cow. They are made from complex rubber or silicone material and have a limited useful life. The majority of rubber liners are expected to last for 2,500 milkings or six months, whichever comes first. Dairy detergents, especially the halogens chlorine and iodine, will denature rubber after six months of washing. Rubber liners lose their elasticity after 2,500 milkings due to the physical action of opening and closing.

Silicone liners have a much longer life of 1,500 hours of use equivalent to about 10,000 milkings but are more expensive. Silicone is far more supple and gentle and softer than rubber. These liners are pre-collapsed when new and their life expectancy can be measured using special testing equipment.

2.Damage to the teat

The teat canal is the primary defence mechanism in preventing new intramammary infections. Milking cows through a faulty machine that damages the end of the teat end will increase the risk of new infections. These may occur due to the impaired ability of the teat canal to prevent bacteria entering the udder.

Damage to the teat skin, especially cuts and chaps, provides an ideal environment for the growth of mastitis organisms such as Staphylococcus aureus and Streptococcus dysgalactiae. The efficacy of post milking teat disinfection may be reduced on damaged teats, as the solution is unable to penetrate damaged skin. For these reasons, quarters with teat lesions are often associated with an increased incidence of clinical and subclinical infection.

One of the commonest and most significant forms of damage is hyperkeratosis of the teat end. This can be characterised as a smooth or roughened ring of dry creamy white tissue surrounding the teat sphincter (Plate 4). Keratin acts as a type of blotting paper mopping up many bacteria present or trying to penetrate the teat canal. Any damage to the teat canal is likely to increase the new infection rate.

The level of hyperkeratosis can be scored using a scale of 0 to 5 (0 being a normal teat and 5 showing severe hyperkeratosis). The target figure is to have an average score of 1.0 or less. Scores of 4 and 5 are likely to lead to an increased risk of mastitis. Table 1 shows that quarters with high scores of hyperkeratosis have higher somatic cell counts and therefore higher levels of subclinical infection (Lewis).

Some degree of hyperkeratosis in milking cows is a normal finding, and mild degrees of the condition may have little effect on the new infection rate. The presence of severe levels, however, suggests a milking machine problem that needs urgent attention. In a Dutch trial, Heijenhuis et al found that cows with clinical mastitis had a higher level of hyperkeratosis compared to healthy cows if clinical mastitis occurred after the first month and before the sixth month in lactation.

Hyperkeratosis can be caused by problems due to worn liners where the liners open slowly or not fully, pulsation, rough removal of the cluster while under vacuum, and overmilking cows in plants with poor pulsation possibly compounded by high vacuum levels.

3. Impact forces or RPGs (Reverse pressure gradients)

Impact forces result in milk particles being propelled up the short milk tube or claw piece against the teat end. This may occur when there is a pressure difference between the teat end and the cluster. Impact forces are most frequently caused by liner slip (Plate 5) where the liner slips down the teat allowing air to suck in between the side of the teat and the top of the liner. This results in a difference in vacuum levels between the teat-end and claw piece and need only occur for milliseconds to create impact forces.

During milking, a pressure difference of 4 kPa is required to penetrate the teat canal compared to 15 kPa between milkings. The dynamic tracing in Figure 1 shows liner slip causing a pressure differential of over 20 kPa.

Table 2 shows that large irregular vacuum fluctuations result in a large increase in the new infection rate compared to a plant with good vacuum stability (Nyhan). Irregular vacuum fluctuations may be due to inadequate vacuum reserve, faulty equipment or a poorly designed plant.

With impact forces, milk particles may be driven up against the teat at speeds of up to 40 miles per hour. This force is such that penetration of the canal may occur. If these droplets are contaminated with bacteria, then infection may follow.

Some of the more common causes of liner slip are listed in Table 3. The majority of liner slips result in ‘squawking’ of air as it enters through the top of the liner. They tend to occur towards the end of milking and so pose two dangers. Firstly, there is a little resistance at the teat end because the canal is at its most open phase. Secondly, if bacteria penetrate, because there is little milk left to remove, it is more likely that they will remain in the udder until the next milking. This will allow time for multiplication and possibly infection to develop.

Impact forces combined with poor premilking teat preparation can result in a high incidence of environmental mastitis. This risk will be increased when cows with dirty teats are washed but not dried, water contaminated with environmental bacteria collects around the top of the liner as shown in Plate 6. If the cause of the liner slip is identified and resolved, and pre-milking teat preparation improved, the reduction in clinical mastitis can be immediate and very significant.

4. Colonisation of the teat canal

During milkout, ‘shear’ forces of milk flow through the teat canal, strip off excess keratin and in so doing remove attached bacteria. Keratin acts as a type of blotting paper trapping bacteria that are trying to invade the udder.

If there is poor pulsation and milk flow rates are reduced, the shear forces may be less able to remove this excess keratin. This may lead to a build up in bacterial colonisation of the teat canal. These bacteria can then continue to grow up into the teat sinus and result in a new infection. C. bovis is a commensal of the teat canal and if found in large numbers either suggests a problem with post milking teat disinfection or the milking machine.

5. Over and undermilking

There have been suggestions that overmilking may lead to increased levels of mastitis. It is known that over milking can create poor teat conditions, which in turn can pose a risk to an increased likelihood of intramammary infection.

Table 4 shows how over milking can affect teat condition (Hillerton, Pankey & Pankey 1999). This shows that the longer cows are over milked, the greater the level of teat congestion. Overmilking increased the level of teat orifice erosion in several experiments. Dodd et al (1950) found that the new infection rate increased almost four fold after cows were overmilked by five minutes. Gleeson et al found that overmilking of teats for 5 minutes over a four month period caused teat sinus injury and loss of teat keratin.

Undermilking may affect mastitis, in particular Strep agalactiae infections where bacteria will remain and increase in numbers between milkings.

Evaluation of the milking machine

The milking machine, like any other mechanical piece of equipment, needs to be serviced and maintained regularly. A static or dry test should be carried out every six months with a dynamic test check at least annually. A static test is the equivalent to the MOT in a car, it tells you how well the plant works when it is not milking cows. The dynamic test is like the road test, and tests the system under load to see what happens when cows are being milked. Testing should conform to British and/or International Standards with a written report of results.

Simple machine checks

Few veterinary surgeons are likely to get involved in detailed testing of the milking machine as this is time consuming, requires training and uses sophisticated equipment that is expensive. There are a few simple checks that can be carried out without the need for any equipment that are useful to help to identify possible problem areas.

These checks are intended to complement any existing maintenance or testing of the plant, not to replace them. Two types of evaluation may be carried out ~ visual observations and measurement of machine performance.

a) Teat scoring

Teat condition should be evaluated immediately after the milking unit is removed. Teats should be examined for any signs of hyperkeratosis, congestion, petechial haemmorrhages, and damage to the teat skin, oedema, ‘wedging’, etc. Teat conditions, such as hyperkeratosis, congestion and oedema, are all machine induced and so these findings are very significant.

The presence and severity of any findings should be recorded for every teat that is examined. Ideally, the entire milking herd should be examined. Excellent lighting is essential as are a free pair of hands to feel and observe teat condition. A hands free miner’s light is a very useful way to carry out this procedure (Plate 7).

The role of the veterinary surgeon should be one where these problems are identified, quantified and the findings passed on to the milking machine dealer or company to resolve. After correction of the problem, the veterinary surgeon is able to repeat this examination and to check on progress. Teat scoring is also very useful for comparison before and after a new machine has been installed, or after a major upgrade of the plant.

b) Pulsation

The best way to evaluate pulsation is to examine teats after milking. If there is no congestion or oedema then the pulsation is likely to be acceptable. Congestion and oedema is easily identified after milking (Plate 8)

Other tests are useful. Place your thumbs and those of the milker into a cluster, apply the vacuum and feel the action of the liner. Does it open and close fully? Is it comfortable? If you get a throbbing feeling it indicates that the circulation of blood around your thumbs is impaired. This may also be found when examining teats after milking. Remember that the teats of the cow are far more sensitive than your thumb!

c) Vacuum reserve

Read the vacuum level from the farm gauge. Leak air into the milking system through one milking unit for five seconds; record any fall in vacuum level, and time how long it takes to return to the normal working vacuum level (this is called the vacuum recovery time and should not be more than 3 seconds). If the vacuum level drops when air leaks in through one milking unit, then this suggests the vacuum reserve is inadequate. One unit is equivalent to the milker attaching one unit to a cow.

Repeat this test leaking air in through two units. In any plant, if the vacuum level drops by more than 2 kPa with two units leaking in air, then this is also unacceptable, as this is equivalent to one unit being attached and one unit being kicked off a cow.

AS a rule of thumb, you should be able to leak in air through one milking unit for every five in the plant without the vacuum level falling by more than 2 kPa. So, for a plant with 20 milking units, you should be able to leak air in through four units with the vacuum level dropping by no more than 2 kPa.

d) Controller or Regulator function

Leak air in through a few milking units to drop the plant vacuum level by 4 to 5 kPa. Listen to the regulator (Plate 9). If it is still leaking air into the system, then it is faulty and needs to be cleaned, serviced or replaced. Once you stop air leaking into the system, watch the vacuum gauge and ensure that it does not ‘overshoot’, again the sign of a faulty controller.

A faulty regulator will not be able to maintain stable vacuum levels in the milking system. These tests are no foolproof but help to identify possible problems with the milking machine.

e) Liners

As you are checking liner movement, feel the condition of the inside of the liner. Is it rough or smooth? Work out how many milkings they have carried out. Remove a liner from the teat cup shell. Is it round or collapsed? Split it open, the inside surface may surprise you (Plate 10).

Summary

The milking machine can act as a vector, produce teat damage, create impact forces and result in an increased colonisation of the teat canal. All of these may affect the incidence of both clinical and subclinical mastitis. It is important that the veterinary surgeon involved in mastitis work has an understanding of the effect of the machine and also is able to help identify possible problem areas.