Ranchers giving their cattle antibiotics may be doing more than creating drug-resistant microbes: They could be boosting greenhouse gas emissions as well, a new study suggests. Researchers looked at the effect of a 3-day treatment of tetracycline, a commonly used antibiotic, on the amount of methane generated within the manure of cattle. Over the course of the experiment, emissions of planet-warming methane from the dung of antibiotic-dosed cows were, on average, 80% higher than those from the manure of untreated cattle, the team reports online today in the Proceedings of the Royal Society B. The boost may be due to a relative increase in methane-producing microorganisms called archaea in the digestive systems of treated cattle due to the suppression of antibiotic-susceptible bacteria, the team suggests. The findings are the first to note increased greenhouse gas emissions due to antibiotic use in cattle; a recent study suggests that methane emissions from cud-chewing livestock worldwide, including cows, account for about 4% of the greenhouse gas emissions related to human activity. Because methane emissions from a cow’s manure are typically lower than those released from its belching, future studies should look at the effect of antibiotics on that source of the greenhouse gas, too, the researchers suggest.