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Introduction We conducted a meta-analysis of quantitative data extracted from selected peer-reviewed papers describing in vivo studies on enteric methane emissions from dairy cattle treated with feed additives, compared with a control group. The aim was to identify feed additives that significantly reduce enteric methane emissions, expressed as g/day, g/kg dry matter intake, g/kg milk produced, and g/kg energy-corrected milk. The feed additives considered were polyunsaturated fatty acids (PUFAs), 3-nitrooxypropanol (3-NOP), essential oils (EO), and monensin. Four electronic databases (PubMed, CAB Abstracts, Web of Science, and Scopus; 2001–2024) were used to retrieve papers, following the PRISMA 2020 statement. Methods Effect sizes were calculated as log response ratio percentages and analyzed using three-level random-effects models. Heterogeneity, cluster-robust variance estimation, and leave-one-out diagnostics were applied. A total of 34 studies met the inclusion criteria. Results Of these, 19 investigated the impact of PUFAs, yielding 45 data points; seven investigated the impact of 3-NOP, yielding 23 data points; eight investigated the impact of essential oils on enteric methane emissions, yielding 12 data points; and three investigated the impact of monensin, yielding four data points. PUFAs significantly reduced enteric methane emissions across all metrics, although high heterogeneity remained (I² ≈ 86%–95%). 3-NOP exhibited the most substantial average reductions in enteric methane emissions; however, the significance of these effects varied depending on the metric and model formulation. In addition, basal crude protein significantly influenced the effectiveness of 3-NOP. The effects of essential oils were generally non-significant and dependent on formulation. Evidence for monensin was limited and descriptive only. Discussion Based on the current body of evidence, PUFAs and 3-NOP represent the most reliable nutritional strategies for mitigating enteric methane emissions in dairy cows. PUFAs supplementation has been shown to reduce methane production without measurable adverse effects on milk yield or energy-corrected milk. Similarly, 3-NOP produced the greatest average reductions in methane emissions without impairing milk production, although its efficacy may vary depending on diet composition and may decline over time. Future research should prioritize standardized dosing, harmonized measurement methodologies, and extended trial durations that simultaneously assess efficacy, dietary covariates, persistence, productivity, and cost-effectiveness.
Dalmonte et al. (Thu,) studied this question.