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%, the researchers reported online yesterday in the Proceedings of the National Academy of Sciences. The decline in emissions occurred during the first 2 weeks of the trials and persisted throughout the test, the researchers say. The additive didn’t seem to affect the cows’ appetite, nor did it affect either the production or the composition of the cows’ milk. And because 3NOP boosted the digestibility of nutrients in the feed, the cows that received the supplement actually gained more weight than cows that received none. Larger tests will be needed to see if detrimental effects crop up over the long term, the researchers admit, but in the meantime 3NOP is a promising methane-cutting additive. In previous short-term tests, the chemical has trimmed methane emissions in sheep and beef cattle as well as in nonlactating dairy cows.