In most dairying countries, there is a myriad of milking liner options to choose from; New Zealand is no exception. Not only are there decisions around the type of liner you can fit to a shell within manufacturer specifications, but there are often “after-market” liners designed to fit shells which are from a different manufacturer. Regardless of how you make the decision on liner type, there are a few simple guidelines to follow which apply to all liners: 
 
Firstly, does the liner fit the shell? This sounds like a simple principle, but mistakes are made in this area more commonly than you would think. Shells are not all the same in physical dimension (let alone weight), and the range of shell dimensions gets even larger when shells from the past four decades are compared with more modern designs. Given that they are often stainless steel, 30 to 40-year-old shells can readily be found still in use. 
 
Liners are designed to be stretched between 7-18% when mounted, with the mounting tension a design consideration of the liner geometry (shape) and the shell dimension. Hence the “marriage” of liner to shell is important to retain the design performance of the liner as intended by the liner manufacturer. If their mounting tension is under or over specification, due to an ill-fitting shell, the liner will not perform as intended. This will potentially affect both milking speed and milking gentleness and, it is easy to avoid. Use only a liner designed to fit the shell you have in your shed. 
 
Secondly, liners are not intended to last forever. In fact, they are designed to last for only a certain number of cow milkings. In most cases, synthetic rubber liners available in New Zealand have a service life of 2500 cow milkings only. Beyond this point, two things happen to a liner. They will harbour more bacteria on their worn internal surfaces, despite after milking cleaning and, they will experience a reduction in liner compression and hence milking speed. Gentleness of milking may also be compromised. 
Like a set of bald car tyres, getting more than 2,500 milkings out of a set of liners may not necessarily see the liner-teat relationship “crash”, but it will certainly increase the risk of teats becoming less healthy. This is to be avoided. Also, like worn car tyres, the cost saving in delaying the liner change is false economy, even when just viewed as a milking speed consideration. 
 
Working out how may cow milkings your liners are doing each milking session is straightforward. 
• Cows milked per AM and PM milking session = 240 
• Units in the shed = 16 
• Cows milked per unit, per milking session = 240/16 = 15 (rounded) 
• Cows milked per unit, per day = 15×2 = 30 (AM and PM milking) 
• No of days of recommended liner life = 2500/30 = 83 days (3 months approximately) 
This assumes that the recommended liner life, as stated by the manufacturer, is 2500 cow milkings. While this is typical of most liners available in New Zealand, always check the packaging for the individual liner recommendation. 
 
Stretching out the liner use beyond this limit is creating risk you can and should avoid.