LONDON (Reuters) - Cows belching and breaking wind cause methane pollution but British scientists say they have developed a diet to make pastures smell like roses -- almost.

"In some experiments we get a 70 percent decrease (in methane emissions), which is quite staggering," biochemist John Wallace told Reuters in a telephone interview.

Wallace, leader of the microbial biochemistry group at the Rowett Research Institute in Aberdeen, said the secret to sweeter-smelling cows is a food additive based on fumaric acid, a naturally occurring chemical essential to respiration of animal and vegetable tissues.

A 12-month commercial and scientific evaluation of the additive has just begun, but he said if it proves successful it could be a boon to cutting down on greenhouse gas emissions.

"In total around 14 percent of global methane comes from the guts of farm animals. It is worth doing something about," Wallace said. Other big sources of methane are landfills, coalmines, rice paddies and bogs.

Scientists in Australia and New Zealand have also been working to develop similar products amid growing concern about greenhouse gas emissions from cattle and sheep.

In New Zealand the government in 2003 proposed a flatulence tax, with methane emitted by farm animals responsible for more than half the country's greenhouse gases. The plan was ultimately withdrawn after widespread protests.

"We've had more success than they (scientists in Australia and New Zealand) have. Everyone has been trying different methods. We just got lucky," Wallace said.