Effects of dose, dietary nutrient composition, and supplementation period on the efficacy of methane mitigation strategies in dairy cows: A meta-analysis.

The objective of this meta-analysis was to quantify the potential of CH-mitigating strategies in dairy cattle when accounting for the effects of treatment dose, dietary nutrient composition, and supplementation period. Data from 218 studies with dairy cattle published between 1963 to 2022 were reviewed. Individual CH mitigation strategies selected for the analysis were algae (Asparagopsis spp.), 3-nitrooxypropanol, nitrate, lipids, plant secondary compounds, and direct-fed microbials (DFM). Response variables evaluated were daily CH emission (g/d), CH yield (g CH/kg DMI), and CH intensity (g CH/kg milk yield [MY] and ECM). Relative mean difference between treatment and control means reported in the studies were calculated and used in the statistical analysis. Robust variance estimation method was used to analyze the effects of CH mitigation strategies. Dose, forage-to-concentrate ratio (F:C), dietary concentrations of CP, ether extract (EE), NDF, ADF, and starch, and supplementation period were used as continuous explanatory variables. Data for algae supplementation were limited and responses to studied species were contrasting but, overall, Asparagopsis spp. effectively decreased daily CH emission, CH yield, and CH intensities by 29.8 ± 4.6%, 23.0 ± 5.3%, 34.0 ± 4.3%, and 22.6 ± 7.3%, respectively. Supplementation of 3-nitrooxypropanol decreased daily CH emission, yield, and intensity (per kg MY and ECM) by 28.2 ± 3.6%, 28.7 ± 2.8%, 29.2 ± 3.1%, and 31.8 ± 2.8%, respectively, compared with control. Decreasing dietary fiber (i.e., F:C, NDF, and ADF), whereas increasing dietary starch concentration increased the efficacy of 3-nitrooxypropanol at mitigating enteric CH emission. Nitrate supplementation decreased CH emission, yield, and intensity (per kg ECM) by 18.5% ± 1.9%, 17.6 ± 1.6%, and 13.0 ± 0.2%, respectively, compared with control. Efficacy of nitrate at mitigating enteric CH yield and CH intensity was positively associated with dose, and efficacy of nitrate at mitigating CH yield was positively associated with dietary starch concentration. Lipid supplementation decreased CH emission, yield, and intensities by up to 14.8 ± 2.3%, respectively, compared with control. Efficacy of lipids supplementation was positively associated with dietary EE, starch, and supplementation period, but negatively associated with dietary ADF concentration. Free oil supplementation tended to increase lipid efficacy by 31% at decreasing CH emission, compared with control. Condensed tannins and plant-derived bioactive compounds decreased CH yield by 11.3 ± 2.9% and 5.7 ± 2.5%, respectively, but oregano did not affect enteric CH emission metrics in the current meta-analysis. Direct-fed microbials were not effective in mitigating enteric CH emission variables. Data were limited to determine the effects of dietary nutrients and duration of supplementation on efficacy of Asparagopsis spp., plant secondary compounds and DFM. Overall, supplementation of the diet with Asparagopsis spp., 3-nitrooxypropanol, nitrate, and lipids were the most effective strategies for decreasing enteric CH emission in dairy cattle. Variability in the efficacy of most CH mitigation strategies can be partially explained by differences in treatment dose, dietary nutrient composition, and supplementation period.