Australian scientists have said they are hoping to breed sheep that burp less as part of efforts to tackle climate change.
The scientists have been trying to identify a genetic link that causes some sheep to belch less than others.
Burping is a far greater cause of emissions in sheep than flatulence, they say.
About 16% of Australia's greenhouse emissions come from agriculture, says the department of climate change.
Australia's Sheep Cooperative Research Council says 66% of agricultural emissions are released as methane from the gut of livestock.
"Ninety per cent of the methane that sheep and cattle and goats produce comes from the rumen, and that's burped out," John Goopy from the New South Wales Department of Industry and Investment told ABC.
"Not much goes behind - that's horses."
The scientists in New South Wales have been conducting experiments in specially designed pens where they measure how much gas sheep emit by burping.
They have found, from tests on 200 sheep so far, that the more they eat, the more they belch.
But even taking that into account, there appear to be "significant differences" between individual animals, Mr Goopy said.
The scientists' goal in the long term is to breed sheep that produce less methane, which produces many times more global warming than carbon dioxide.
"We're looking for natural variations so we'll steer the population that way, " said Roger Hegarty, from the Sheep Cooperative Research Council.
Australian aims to breed 'green' sheep that burp less
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