Use of feed additives to reduce enteric methane emissions in dairy cattle: meta-analysis of data retrieved through a systematic review

Thomas Dalmonte^*Niccolò BarbieriAlberto PalmonariValentina IndioYari VecchioGulnara GuluzadeIlgar AbdullavevAndrea FormigoniAndrea SerrainoFelice AdinolfiAlessandra De Cesare

• Universita degli Studi di Bologna Dipartimento di Scienze Mediche Veterinarie, Ozzano dell'Emilia, Italy

We conducted a meta-analysis on quantitative data extracted from selected peer-reviewed papers describing in vivo studies on enteric methane emissions from dairy cattle treated with feed additives, compared to a control group. The aim was to identify feed additives that significantly reduce enteric methane emissions, expressed in g/day, g/kg dry matter intake, g/kg of milk produced and g/kg energy corrected milk. The feed additives considered were polyunsatured fatty acids (PUFA), 3-nitrooxypropanol (3-NOP), essential oils (EO), and Monensin. Four electronic databases (PubMed, CAB Abstracts, Web of Science, Scopus; 2001–2024) were used to retrieve papers, following the PRISMA 2020 statement. Effect sizes were calculated as log response ratios percent, analysed with 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. Of those, 19 investigated the impact of PUF, yielding to 45 data points; 7 investigated the impact of 3-NOP, yielding to 23 data points; 8 investigated the impact of essential oils in enteric methane emission, yielding to 12 data points; 3 investigated the impact of Monensin, yielding to 4 data points. PUFAs significantly reduced enteric methane emissions across all metrics, with high heterogeneity remaining (I² ≈ 86–95%). 3-NOP exhibited the most substantial average reductions in enteric methane emissions, although the significance of these effects varied depending on the metric and model formulation. Additionally, basal crude protein significantly influenced the effectiveness of 3-NOP. The effects of essential oils were generally non-significant and dependent on the formulation. The evidence for Monensin was limited and descriptive only. Based on the current body of evidences, PUFA and 3-NOP represent the most reliable nutritional strategies for mitigating enteric methane emissions in dairy cows. PUFA supplementation has been shown to reduce methane production without measurable adverse effects on milk yield or energy-corrected milk. Similarly, 3-NOP produce 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 standard dosing, harmonized measurement methodologies, and extended trial duration that simultaneously assess efficacy, diet covariates, persistence, productivity, and cost-effectiveness.