As liners age, their shape, tension and surface condition changes gradually. This gradual deterioration can have subtle but significant effects on their milking characteristics. Both the internal surface and the milking performance of liners tend to deteriorate more quickly soon after they reach their designated use-by date.

As liners age and stretch, several key features of their milking performance alters. In particular:

• the average milking time per cow increases

• the frequency of liner slips increases

• the amount of milk left in the quarters of an udder when cups are removed increases

• the teat-end condition worsens

• cow behaviour worsens

Any effect on teat end condition or any increased liner slips are particularly significant for increased risk of mastitis. As a general rule of thumb, if you notice an improvement in milking performance after replacing liners, the old ones were used for too long!

When renewing liners, always change all four liners within a cluster at the same time to maintain similar mounting tension and milking characteristics between the four teatcups. Prevention is better than cure - change liners according manufacturers' recommendations to avoid the risks from using worn out liners.

Change liners at regular intervals

Teatcup liners are designed to flex and squeeze the teat during each pulsation cycle. This is essential to massage the teat and maintain its blood supply. When fitted into a correctly matched teatcup, the liner should be stretched 5–15% more than its original length.

As soon as they start work, liners begin to lose tension, absorb fat and hold bacteria. Once they have been used for too many cow milkings, the deterioration is sufficient to reduce the speed and completeness of milking, increase teat end damage, and increase spread of mastitis bacteria.

The effective life of liners is influenced by:

• the characteristics of the materials they are made from
• the conditions of storage, cleaning and use they experience
• and their exposure to sun, heat, chemicals and ozone.

Most manufacturers recommend that rubber liners are used for 2000 – 2500 cow milkings, after which they should be changed. Some manufacturers also specify a time of 4–6 months as the maximum recommended life for liners.

Liner life

Tip: Most manufacturers recommend that rubber liners are used for 2000– 2500 cow milkings, after which they should be changed.

Measure liner life in 'cow milkings'.

When liners are changed, estimate when the next 2500 cow milkings (or manufacturer's recommendation) will have occurred, and mark the date to replace liners on your calendar.

Guide to estimating number of days for 2500 cow milkings:

Number of days = 2500 x number of milking units herd size x number of milkings per day

For example:

A herd of 120 cows (herd size) milking twice per day (milkings per day) in a 12 unit swingover shed (milking units) would take 125 days (number of days):

Number of days = 2500 x 12 =30,000 120 x 2 = 240 = 125 days