Cows burp and fart more than they need to, says a Swiss firm that has developed a feed that cuts methane emissions from livestock - helping to combat global warming into the bargain.

A single cow produces the equivalent of about three tonnes of carbon dioxide per year, says Agolin, based in Biere near Lausanne. Co-founder and managing director Kurt Schaller told Reuters its specialist feed can reduce that by 10 percent.

"We sell our mixture for around 1 million cows per year... There are 25-28 million cows in the European Union, so it is a big percentage," he said. "That represents 300,000 tonnes of CO2 reduction today."

The United Nations says livestock farming alone is responsible for up to 18 percent of the greenhouse gases that contribute to global warming.

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Nearly a quarter of that comes from bovine flatulence.

"The burps for the cows - it is more burps than farts - contribute about ... 4 percent of (all) greenhouse gas emissions," said Anne Mottet, livestock development officer at the U.N. Food and Agriculture Organization.

Agolin says field trials have shown its product also helps increase milk yield and feed efficiency, which are benefits that farmers can more easily measure.

"A farmer does not get a cent if he reduces his methane production," said technical director Beatrice Zweifel. "That is starting to change a little bit, however, for example in California, where farmers must reduce their methane production by 40 percent."