sciencehabit writes: A simple supplement to a cow's feed could substantially decrease a major source of methane, a planet-warming greenhouse gas, a new study suggests. Each year worldwide, the methane produced by cud-chewing livestock warms Earth's climate by the same amount as 2.1 billion tons of carbon dioxide, a little more than 4% of the greenhouse gas emissions related to human activity. That makes cows tempting targets for methane reduction efforts. In a new study, researchers added the chemical 3-nitrooxypropanol, also known as 3NOP, to the corn-and-alfalfa-based feed of 84 milk-producing Holsteins and monitored their methane production for 12 weeks—the largest and longest such trial of its type in lactating cows, the scientists say. For cows whose feed included 3NOP, methane emissions dropped, on average, by 30%.