Simple SummaryMethane is a greenhouse gas that substantially contributes to climate change. Agriculture is the largest source of methane globally, and more specifically, methane produced by ruminants during feed digestion (32% of global methane emissions). Essential oils have properties that may reduce the amount of methane produced by ruminants. This study tested the effectiveness of an essential oil product in reducing methane emissions from dairy–beef cattle. Methane emissions were measured from individual animals by confinement in respiration chambers. Results showed that although there was no difference in the mass of methane produced, the yield (grams of methane per kilogram of feed dry matter consumed) was lower in the animals receiving the essential oils. This reduced methane yield was caused by control animals consuming less feed during methane measurement periods. The same reduction in feed intake was not observed in the treatment animals. This suggests that animals supplemented with essential oils were less affected by confinement in respiration chambers than control animals.AbstractAgriculture is the largest source of methane globally, and enteric methane accounts for 32% of methane emissions globally. Dairy–beef is an increasingly important contributor to the beef industry. The objective of this study was to investigate if supplementation with a blend of essential oils (Agolin Ruminant) reduced enteric methane emissions from dairy-bred steers. Methane was measured from thirty-six Holstein Friesian steers (18 control and 18 treatment) in open-circuit respiration chambers, at three time-points relative to the introduction of Agolin Ruminant: (i) −3 (pre-additive introduction co-variate), (ii) 46 days after introduction, and (iii) 116 days after introduction. A significantly lower methane yield was observed in treated animals compared to control animals at both 46 days (p< 0.05) and 116 days (p< 0.01) after the introduction of Agolin Ruminant, although there was no difference in methane production (g/day). Control animals appeared to be more affected by isolation in respiration chambers than animals receiving Agolin Ruminant, as indicated by a significant reduction in dry matter intake by control animals in respiration chambers.