Low-carbon farming can both curb climate change and boost food output in developing nations and so must be rewarded under a global climate deal due in December, the UN's food agency said.

Steps to cut carbon emissions on farms in developing countries could also boost yields where food is shortest, the Food and Agriculture Organisation (FAO) said in a report .

More than one billion people are undernourished now and the world will have to feed an additional three billion by 2050, many in areas expected to be worst afflicted by climate change, experts say.

Certain farm practices can tackle both problems, for example conserving over-grazed pastures and caring for soils, but they involve up-front costs.

"A key part of the problem is a lack of financing," said Leslie Lipper, FAO economist and co-author of its report Food Security And Agricultural Mitigation in Developing Countries", published on the sidelines of UN climate talks in Barcelona.

"If adopted by farmers, many of these practices make them better off, but in the short run they may face reduced income," Mr Lipper said, using the example of removing cattle to allow grasslands to recover.

Agriculture has barely been mentioned in this week's Barcelona talks.

Farms accounts for 10-12 per cent of global greenhouse gas emissions directly, not including their contribution to deforestation, according to a UN panel of climate scientists.

An FAO study this year put the extra farm investment needed to boost food yields at $210 billion between now and 2050.

Some of the funding for low-carbon practices could come from carbon markets, whereby rich nations pay for cuts in developing countries to offset against their own emissions.

Low-carbon farming in developing nations could raise up to $30 billion annually through such carbon finance, the study said.