New research being done on farms in Vermont in the USA indicates that the artificial diet fed to cattle since the second world war is responsible for a large part of the methane emitted by agriculture. The experiment introduced plants like alfalfa and flax seed into the cattle’s diet mimicing the natural grasses that cows ate earlier last century and replacing corn and soya bean feed. Milk production remained steady while methane emissions dropped by 18 per cent. The first of 15 farmers to try the experiment, Guy Choiniere, said that the cows are healthier, their coats are shinier and their breath is sweeter. Methane is a greenhouse gas, twenty times as effective as carbon dioxide at trapping the sun’s heat. Feedlot cattle produce well over 100kilograms of methane each year as well as consuming 700litres of water for every litre of milk they produce.

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The program was initiated by Stonyfield Farm, the yogurt manufacturer, at the Vermont farms that supply it with organic milk. Mr. Choiniere, a third-generation dairy herder who went organic in 2003, said he had sensed that the outcome would be good even before he got the results.

“They are healthier,” he said of his cows. “Their coats are shinier, and the breath is sweet.”

Sweetening cow breath is a matter of some urgency, climate scientists say. Cows have digestive bacteria in their stomachs that cause them to belch methane, the second-most-significant heat-trapping emission associated with global warming after carbon dioxide. Although it is far less common in the atmosphere than carbon dioxide, it has 20 times the heat-trapping ability.

Frank Mitloehner, a University of California, Davis, professor who places cows in air-tight tent enclosures and measures what he calls their “eruptions,” says the average cow expels — through burps mostly, but some flatulence — 200 to 400 pounds of methane a year.

More broadly, with worldwide production of milk and beef expected to double in the next 30 years, the United Nations has called livestock one of the most serious near-term threats to the global climate. In a 2006 report that looked at the environmental impact of cows worldwide, including forest-clearing activity to create pasture land, it estimated that cows might be more dangerous to Earth’s atmosphere than trucks and cars combined.

In the United States, where average milk production per cow has more than quadrupled since the 1950s, fewer cows are needed per gallon of milk, so the total emissions of heat-trapping gas for the American dairy industry are relatively low per gallon compared with those in less industrialized countries.

Dairy Management Inc., the promotion and research arm of the American dairy industry, says it accounts for just 2 percent of the country’s emissions of heat-trapping gases, most of it from the cows’ methane.

Still, Erin Fitzgerald, director of social and environmental consulting for Dairy Management, says the industry wants to avert the possibility that customers will equate dairies with, say, coal plants. It has started a “cow of the future” program, looking for ways to reduce total industry emissions by 25 percent by the end of the next decade.

William R. Wailes, the head of the department of animal science at Colorado State University who is working on the cow of the future, says scientists are looking at everything from genetics — cows that naturally belch less — to adjusting the bacteria in the cow’s stomach.

For the short run, Professor Wailes said, changes in feed have been the most promising.

Stonyfield Farm, which started as a money-raising arm for a nonprofit organic dairy school and still has a progressive bent, has been working on the problem longer than most.

Nancy Hirshberg, Stonyfield’s vice president for natural resources, commissioned a full assessment of her company’s impact on climate change in 1999 that extended to emissions by some of its suppliers.

“I was shocked when I got the report,” Ms. Hirshberg said, “because it said our No. 1 impact is milk production. Not burning fossil fuels for transportation or packaging, but milk production. We were floored.”

From that moment on, Ms. Hirshberg began looking for a way to have the cows emit less methane.

A potential solution was offered by Groupe Danone, the French makers of Dannon yogurt and Evian bottled water, which bought a majority stake in Stonyfield Farm in 2003. Scientists working with Groupe Danone had been studying why their cows were healthier and produced more milk in the spring. The answer, the scientists determined, was that spring grasses are high in Omega-3 fatty acids, which may help the cow’s digestive tract operate smoothly.

Corn and soy, the feed that, thanks to postwar government aid, became dominant in the dairy industry, has a completely different type of fatty acid structure.