New York Times: Chewing her cud on a recent sunny morning, Libby, a 1,400-pound Holstein, paused to do her part in the battle against global warming, emitting a fragrant burp. Libby, age 6, and the 74 other dairy cows on Guy Choiniere’s farm here are at the heart of an experiment to determine whether a change in diet will help them belch less methane, a potent heat-trapping gas that has been linked to climate change. Since January, cows at 15 farms across Vermont have had their grain feed …