Dairy cows emit large amounts of the greenhouse gas methane due to microbial fermentation in their stomachs, which poses an environmental problem. It also decreases the cows’ growth efficiency, as some energy from feed is lost as methane. The supplement 3-nitrooxypropanol (3-NOP) can help reduce methane emission by inhibiting a methane-forming enzyme, but 3-NOP’s effects on the microbiome in the rumen (the stomach compartment where fermentation occurs) haven’t been investigated. It’s also unclear why hydrogen gas (H₂) accumulates less than expected when methane production is blocked by 3-NOP. To learn more, researchers recently characterized the rumen microbes in 3-NOP-supplemented dairy cows. 3-NOP reduced the abundance of Methanobrevibacter species, which make methane from carbon dioxide. To a lesser extent, it also reduced the abundance of Methanosphaera species, which make methane from methanol. Fluctuations in rumen H₂ concentrations in 3-NOP-treated cows were accompanied by changes in the expression of [FeFe]-hydrogenase enzymes, which regulated H₂ production. In addition, genes encoding butyrate-pathway enzymes were upregulated after 3-NOP supplementation, supporting removal of excess H₂. Although further research is needed, this study clarifies the effects of 3-NOP on the rumen microbiome, helping to characterize the mechanism of this effective “methane mitigator” in dairy cows.