Abstract
A variety of nutritional management strategies that reduce enteric methane (CH4) production are discussed. Strategies such as increasing the level of grain in the diet, inclusion of lipids and supplementation with ionophores (>24 ppm) are most likely to be implemented by farmers because there is a high probability that they reduce CH4 emissions in addition to improving production efficiency. Improved pasture management, replacing grass silage with maize silage and using legumes hold some promise for CH4 mitigation but as yet their impact is not sufficiently documented. Several new strategies including dietary supplementation with saponins and tannins, selection of yeast cultures and use of fibre-digesting enzymes may mitigate CH4, but these still require extensive research. Most of the studies on reductions in CH4 from ruminants due to diet management are short-term and focussed only on changes in enteric emissions. Future research must examine long-term sustainability of reductions in CH4 production and impacts on the entire farm greenhouse gas budget.
Generated Summary
This review article, published in the Australian Journal of Experimental Agriculture, examines nutritional management strategies for reducing enteric methane (CH4) emissions from ruminant livestock. It explores various approaches, including modifying the diet to alter the production of CH4 in the rumen, the primary site of CH4 production in ruminants. The study reviews current and potential strategies, such as increasing concentrate levels, adding lipids to the diet, using ionophores, and incorporating plant secondary compounds. The review also identifies areas where further research is needed to optimize these strategies and assess their long-term sustainability and impact on overall greenhouse gas emissions. The methods involve a literature review and synthesis of existing in vivo data. The scope includes assessing the commercial availability and near-market readiness of each CH4 abatement strategy.
Key Findings & Statistics
- Ruminant livestock globally produce ~80 million tonnes of methane (CH4) annually.
- CH4 accounts for ~28% of anthropomorphic emissions.
- In diets that contain >90% concentrate, the CH4 conversion rate as a percentage of gross energy intake is Ym = 3% vs. 6.5% in other diets (IPCC 2006).
- Adding lipids to the diet can reduce CH4 emissions by decreasing ruminal organic matter fermentation, the activity of methanogens and protozoal numbers.
- Reductions in CH4 of ≥40% are possible with high levels of lipid supplementation, but reductions of 10-25% are more likely in commercial practice.
- Over a broad range of conditions, CH4 (g/kg DMI) was calculated to be reduced by 5.6% with each 1% addition of supplemental fat.
- In some studies with coconut oil, CH4 was reduced by 63.8% with 7% added oil.
- In some studies with myristic acid, CH4 was reduced by 58.3% with 5% added fat.
- The level of added fat explained 67% of the reduction in CH4 emissions relative to control treatments.
- Monensin (33 mg/kg) lowered CH4 emissions in beef cattle by up to 30%, with short-term decreases in CH4 of up to 30% being reported when 33 ppm of monensin was included in high or low forage diets.
- Higher doses (24-35 ppm) reduced CH4 production (g/day by 4-10% and g/kg DMI by 3-8%) in beef cattle and dairy cows.
- In a recent in vitro study, an enzyme candidate increased fiber degradation of corn silage by 58%, with 28% less CH4 produced per unit of fiber degraded.
- In a recent study with dairy cows, supplementing a corn silage-based diet with an enzyme additive reduced CH4 production (g/g DMI) by 8.8% (P=0.13).
- Total farm GHG emissions increased as forage proportion increased (1.26, 1.38 and 1.62 kg CO2 equivalent per kg of milk).
- Forage legumes, are often explained by the presence of condensed tannins (CT), lower fibre content, higher DMI and faster rate of passage from the rumen.
Other Important Findings
- Increasing the level of concentrate in the diet reduces the proportion of dietary energy converted to CH4.
- Replacing grass silage with maize silage holds some promise for CH4 mitigation.
- Increased dietary concentrate may sometimes increase total net emissions due to increased grain production, processing, and transportation.
- Adding lipids to the diet can be effective.
- The effectiveness of adding lipids depends on many factors including level of supplementation, fat source, fatty acid profile, form in which the fat is administered, and the type of diet.
- Refined oils that are high in MCFA (medium-chain fatty acids) such as coconut oil, palm kernel oil, high-laurate canola oil, or pure myristic acid are particularly effective in reducing CH4.
- Ionophores such as monensin are antimicrobials that can increase the acetate-to-propionate ratio of volatile fatty acids (VFA) in rumen fluid and decrease ruminal protozoal numbers.
- The CH4-suppressing effect of plants rich in saponins seems to be particularly related to their anti-protozoal effects.
- Yeast products may reduce CH4 output while promoting rumen fermentation and fiber digestion.
- Enzymes can improve ruminal fiber digestion.
- Improvements in pasture quality may reduce CH4 output if stocking densities remain static.
Limitations Noted in the Document
- The review primarily focuses on in vivo data.
- The review focuses mainly on the in vivo data available for each CH4 abatement strategy.
- The review focuses mainly on the in vivo data available for each CH4 abatement strategy and indentifies future research needs and opportunities.
- Most studies on CH4 reductions are short-term and focused on changes in enteric emissions.
- Assessments of individual systems are needed to confirm the effects of increased concentrate feeding.
- The studies that evaluate the effects of fat supplementation on CH4 have not yet tested whether the reductions in CH4 are sustained over entire lactations or finishing periods.
- The inhibitory effects of ionophores on methanogenesis may not persist over time.
- There is a need for animal studies that directly compare grass silage with maize and other cereal silages to quantify the reduction in CH4.
- Comparisons need to be done on entire farm budgets to account for changes in GHG emissions other than enteric CH4.
- Many CT-containing legumes can be cultivated in several geographical regions, but they frequently lack the agronomic rigor of more traditional forage sources.
Conclusion
The review emphasizes that while several nutritional management strategies show promise in reducing enteric methane (CH4) emissions from ruminants, their adoption by farmers is complex and dependent on various factors. The most likely strategies for near-term adoption include those that improve feed efficiency, such as increasing grain use, lipid supplementation, and ionophore use. However, the study highlights that an increase in concentrate feeding may also increase total net emissions due to its associated environmental impacts. The review suggests that future research should focus on evaluating the long-term sustainability of these mitigation strategies and their impact on the overall farm greenhouse gas budget. Additionally, the study notes that the impact of nutritional strategies on CH4 emissions can be influenced by factors such as feed quality, forage species, and the use of plant secondary compounds, with some plant sources containing tannins and saponins showing promise. The study also highlights that while yeast and enzyme feed additives could play a future role in mitigating CH4, considerable research and development are needed before commercial applications. The research indicates that some strategies reduce emissions while increasing farm profitability, but this is often market-dependent and regionally variable. Ultimately, the review concludes that a comprehensive, whole-farm approach is necessary, and simultaneous application of multiple dietary measures may have non-additive effects.