Effects of plant essential oils on milk yield and methane emissions in dairy cows: a meta-analysis

Enteric methane emissions from ruminants are a major agricultural source of greenhouse gases and represent a loss of dietary energy, creating linked environmental and production-efficiency challenges. Plant essential oils (EOs) have been investigated as natural feed additives, but findings in dairy cattle are heterogeneous. A focused synthesis is needed to examine milk yield together with methane emission, methane yield, methane intensity, and potential sources of between-study variation. We conducted a systematic review and meta-analysis of EO supplementation in dairy-cow production systems. Studies published through July 2025 were retrieved from PubMed, Scopus, and Web of Science, and 31 eligible reports were included as documented in the source manuscript. Random-effects models showed no statistically significant pooled effect on milk yield (standardized mean difference [SMD] = 0.47; 95% confidence interval [CI], −0.29 to 1.22; P = 0.225), methane emission (SMD = −0.87; 95% CI, −1.85 to 0.12; P = 0.083), methane yield (SMD = −0.40; 95% CI, −1.55 to 0.75; P = 0.498), or methane intensity (SMD = −0.79; 95% CI, −1.66 to 0.08; P = 0.075). Heterogeneity was substantial for all outcomes (I² = 85.76%–95.2%). Subgroup and meta-regression analyses were exploratory. Some point-estimate patterns were observed for EO formulation, supplementation at 2–6 g/cow/day, feeding for 20–60 days, and methane measurement method; however, subgroup sizes were small and between-group differences were generally not statistically significant. These patterns are hypothesis-generating and do not establish an optimal formulation, dose, or feeding duration. EOs may warrant further evaluation as a methane-mitigation option that does not clearly compromise milk yield, but confirmatory, adequately powered trials with standardized methods are required.