Abstract
A systematic literature review of in vitro studies was performed to identify methane (CH₄) mitigation interventions with a potential to reduce CH₄ emission in vivo. Data from 277 peer-reviewed studies published between 1979 and 2018 were reviewed. Individual CH₄ mitigation interventions were classified into 14 categories of feed additives based on their type, chemical composition, and mode of action. Response variables evaluated were absolute CH₄ emission (number of treatment means comparisons = 1,325); total volatile fatty acids (n = 1,007), acetate (n = 783), propionate (n = 792), and butyrate (n = 776) concentrations; acetate to propionate ratio (n = 675); digestibility of dry matter (n = 489), organic matter (n = 277), and neutral detergent fiber (n = 177). Total gas production was used as an explanatory variable in the model for CH₄ production. Relative mean difference between treatment and control means reported in the studies was calculated and used for statistical analysis. The robust variance estimation method was used to analyze the effects of CH₄ mitigation interventions. In vitro CH₄ production was decreased by antibodies (-38.9%), chemical inhibitors (-29.2%), electron sinks (-18.9%), essential oils (-18.2%), plant extracts (-14.5%), plant inclusion (-11.7%), saponins (-14.8%), and tannins (-14.5%). Overall effects of direct-fed microbials, enzymes, macroalgae, and organic acids supplementation did not affect CH₄ production in the current meta-analysis. When considering the effects of individual mitigation interventions containing a minimum number of 4 degrees of freedom within feed additives categories, Enterococcus spp. (i.e., direct-fed microbial), nitrophenol (i.e., electron sink), and Leucaena spp. (i.e., tannins) decreased CH₄ production by 20.3%, 27.1%, and 23.5%, respectively, without extensively, or only slightly, affecting ruminal fermentation and digestibility of nutrients. It should be noted, however, that although the total number of publications (n = 277) and treatment means comparisons (n = 1,325 for CH₄ production) in the current analysis were high, data for most mitigation interventions were obtained from less than 5 observations (e.g., maximum number of observations was 4, 7, and 22 for nitrophenol, Enterococcus spp., and Leucaena spp., respectively), because of limited data available in the literature. These should be further evaluated in vitro and in vivo to determine their true potential to decrease enteric CH₄ production, yield, and intensity. Some mitigation interventions (e.g., magnesium, Heracleum spp., nitroglycerin, β-cyclodextrin, Leptospermum pattersoni, Fructulus Ligustri, Salix caprea, and Sesbania grandiflora) decreased in vitro CH₄ production by over 50% but did not have enough observations in the database. These should be more extensively investigated in vitro, and the dose effect must be considered before adoption of mitigation interventions in vivo.
Generated Summary
This meta-analysis systematically reviewed in vitro studies to identify methane (CH4) mitigation interventions that could potentially reduce CH4 emissions in vivo. The study reviewed 277 peer-reviewed publications published between 1979 and 2018, categorizing CH4 mitigation interventions into 14 categories of feed additives based on their type, chemical composition, and mode of action. Response variables such as absolute CH4 emission, total volatile fatty acids (VFA), acetate, propionate, butyrate concentrations, acetate to propionate ratio, and digestibility of dry matter, organic matter, and neutral detergent fiber were evaluated. Relative mean differences between treatment and control means were calculated and used for statistical analysis, with the robust variance estimation method used to analyze the effects of CH4 mitigation interventions. The study aimed to identify mitigation strategies that effectively decreased in vitro CH4 production without adversely affecting ruminal fermentation and nutrient digestibility, with the goal of informing future in vivo research.
Key Findings & Statistics
- The meta-analysis included data from 277 peer-reviewed studies published between 1979 and 2018.
- The interventions were classified into 14 categories of feed additives.
- Total number of treatment means comparisons for CH4 production = 1,325.
- In vitro CH4 production was decreased by:
- Antibodies (-38.9%)
- Chemical inhibitors (-29.2%)
- Electron sinks (-18.9%)
- Essential oils (-18.2%)
- Plant extracts (-14.5%)
- Plant inclusion (-11.7%)
- Saponins (-14.8%)
- Tannins (-14.5%)
- Overall effects of direct-fed microbials, enzymes, macroalgae, and organic acids supplementation did not affect CH4 production.
- When considering individual mitigation interventions with a minimum of 4 degrees of freedom:
- Enterococcus spp. decreased CH4 production by 20.3%.
- Nitrophenol decreased CH4 production by 27.1%.
- Leucaena spp. decreased CH4 production by 23.5%.
- Total VFA concentration was increased (P < 0.09) or not affected by the mitigation categories.
- Acetate concentration was decreased (P ≤ 0.08) by most mitigation categories evaluated, except for chemical elements, enzymes, and macroalgae supplementation.
- Propionate concentration was increased by most mitigation categories.
- Butyrate concentration was increased (P < 0.04) or not affected by most mitigation categories.
- Acetate to propionate ratio was decreased by chemical inhibitors (95% CI = -15.5 to -4.5%, P < 0.001), direct-fed microbials (95% CI = -13.8 to -0.7%, P = 0.03), electron sinks (95% CI = -13.9 to -3.9%, P < 0.001), organic acids (95% CI = -29.2 to -8.2%, P < 0.001), plant inclusion (95% CI = -9.4 to -2.1%, P < 0.001), and saponins (95% CI = -16.2 to -4.5%, P < 0.001).
Other Important Findings
- The study found that chemical inhibitors, on average, were the most effective in reducing CH4 production.
- Enterococcus spp. supplementation decreased CH4 production without affecting total VFA and acetate concentrations.
- Nitrate, nitroglycerin, and nitrophenol were the most effective individual interventions within the electron sinks category.
- Fructus ligustri (65.8%), tannins from Achras zapota, Dodonaea angustifolia, and Sesbania grandiflora decreased CH4 production by over 50%.
- Tannins extracted from Leucaena spp. decreased in vitro CH4 production by 23.5% without extensively affecting ruminal fermentation and nutrient digestibility.
Limitations Noted in the Document
- Data for most mitigation interventions were obtained from less than 5 observations.
- The number of treatment means comparisons for NDF digestibility was substantially lower compared with other response variables.
- In vitro studies may use higher doses than what would be practical or safe for animals.
- Toxicity levels for some feed additives have not been well established in vivo.
- The study acknowledges the potential for interactions between CH4 mitigation interventions and diet composition, animal species, and animal physiological state, which were not tested in the current analysis.
- The study’s conclusions are based on in vitro data, and the representativeness of in vivo responses needs further validation.
Conclusion
The meta-analysis of in vitro studies provides valuable insights into the potential of various feed additives to mitigate enteric CH4 emissions. The findings highlight the efficacy of chemical inhibitors, with specific individual interventions like Enterococcus spp., nitrophenol, and tannins from Leucaena spp. showing promising results. The study emphasizes the need for further in vitro and in vivo research to confirm the true potential of these interventions, especially for those with limited data. The authors note that the effect of many mitigation interventions on ruminal fermentation and nutrient digestibility is not well documented, and the data should be interpreted with caution. Moreover, the high heterogeneity and between-study variance observed across the models evaluating feed additives highlight the complexity of the subject and the need for standardized experimental conditions and thorough reporting. The study’s findings support the need for further investigations into the long-term efficacy and safety of these interventions, as well as their effects on animal performance and the environment. The results underscore that while some strategies, such as those involving chemical inhibitors, show considerable promise, the complexity of the rumen ecosystem requires a nuanced approach that considers potential interactions with diet, animal species, and physiological state. Overall, the study provides a solid foundation for future research aimed at developing effective and safe CH4 mitigation strategies for ruminant livestock systems. The study indicates that while some strategies are highly effective at reducing CH4 emissions in vitro, the translation of these effects to in vivo conditions and the potential for long-term sustainability and animal health impacts warrant thorough investigation.