You may have heard that leaving milk in the udder leads to more mastitis, affects milk quality and lowers milk production.

For many years, we thought removing all the milk at every milking was the right thing to do.

The positive results of strategies such as maximum milking time or changing automatic cup remover settings to remove cups at a higher milk flow rate has been confronting to these views.

But many farmers have discovered that strategies to reduce cow milking times and increase milking efficiency are also good for udder health.

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What does the research show us?

In New Zealand, trials in the 2010s reported on the effects of maximum milking time or raising the automatic cup remover thresholds, for cows milked twice daily and with low somatic cell counts.

Results showed no increase in mastitis or decrease in milk production, even when more than 0.7 litres of milk was left in the udder in some cases.

"Many farmers have discovered that strategies to reduce cow milking times and increase milking efficiency are also good for udder health."

Similar results were observed in Australia, where researchers concluded that incomplete milking (in their case leaving behind 0.5 litres) had no effect on somatic cell counts even for cows with mild, subclinical mastitis.

That means cows with a higher somatic cell count shouldn't prevent farmers from using strategies to improve their milking efficiency.

A DairyNZ animation shows what happens to milk retained in the udder when maximum milking time is applied. Check it out at dairynz.co.nz/maxt-herringbone and dairynz.co.nz/maxt-rotary.

What about gross under-milking?

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Two international studies have explored the effects of gross under-milking for high-yielding cows, milked twice daily.

In France, researchers found little impact on milk volume or somatic cell counts even if 30 per cent of the milk volume was left in the udder at a single milking.

But when comparing udders with both fully milked glands and ones with 25 per cent of the milk left behind at each milking, US researchers found some depression in milk production, and a doubling of the somatic cell count over a six-week period.

Both studies led to much greater volumes of milk retained in the udder after milking compared with maximum milking time; neither reported increases in the risk of mastitis.