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.