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

There are several CH4 (methane) inhibitors on the market with promising but variable results. Supplementation of CH4 inhibitors (e.g., 3-nytrooxypropanol; 3-NOP), tanniferous forages, electron sinks (e.g., nitrate), oils and fats, and oilseeds decreased daily CH4 emission by 21% on average (ranging 12%–35%) across ruminant species, but the results may not be representative of the differences which occur in dairy cattle nutrition. For example, analyses indicated that the efficacy of 3-NOP and nitrate in mitigating enteric CH4 emission interacted with the dietary nutrient profile, feed additive dose, DMI, BW, and OM digestibility. Furthermore, all modifiers (i.e., Asparagopsis spp., essential oils, saponin, tannins, microalgae, biochar, and bacterial and fungal direct-fed microbials [DFM]) were found in previous studies as having medium or low agreement and limited evidence to classify them as having low to high CH4 reductions. It does appear if the efficacy of anti-methanogenic feed additives may be affected by dose, dietary nutrient composition, and total supplementation period, as well as the type and mode of action of each fed additive. Thus, the objective of the study cited was to quantify the CH₄-mitigating potential of a selection of feed additives in dairy cattle, and to characterize the influence of dose, dietary nutrient composition, and supplementation period on their efficacy using a meta-analytical approach.

Data from 218 studies with dairy cattle published between 1963 to 2022 were reviewed. Individual CH4 mitigation strategies selected for the analysis were algae (Asparagopsis spp.), 3-nitrooxypropanol, nitrate, lipids, plant secondary compounds, and direct-fed microbials (DFM). Variables evaluated were daily CH4 emission (g/d), CH4 yield (g CH4/kg DMI), and CH4 intensity (g CH4/kg milk yield [MY] and ECM). Relative mean differences between treatments and control means reported in the studies were calculated and used in the statistical analysis. The robust variance estimation method was used to analyze the effects of CH4 mitigation methods. 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.

Asparagopsis spp. effectively decreased daily CH4 emission, CH4 yield, and CH4 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 CH4 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 fibre (i.e., F:C, NDF, and ADF), whereas increasing dietary starch concentration increased the efficacy of 3-nitrooxypropanol at mitigating enteric CH4 emission. Nitrate supplementation decreased CH4 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 CH4yield and CH4 intensity was positively associated with dose, and efficacy of nitrate at mitigating CH4 yield was positively associated with dietary starch concentration. Lipid supplementation decreased CH4 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 CH4 emission, compared with control. Condensed tannins and plant-derived bioactive compounds decreased CH4 yield by 11.3 ± 2.9% and 5.7 ± 2.5%, respectively, but oregano did not affect enteric CH4 emission metrics in the current meta-analysis. Direct-fed microbials were not effective in mitigating enteric CH4 emission variables. Data were however too limited to determine the effects of dietary nutrients and duration of supplementation on efficacy of Asparagopsis spp., plant secondary compounds and DFM.

In conclusion, supplementation of the diet with Asparagopsis spp., 3-nitrooxypropanol, nitrate, and lipids were the most effective strategies for decreasing enteric CH4 emission in dairy cattle. Variability in the efficacy of most CH4 mitigation strategies can be partially explained by differences in treatment dose, dietary nutrient composition, and supplementation period.