Methane: Abatement Technologies and Recommended Research
Enteric emissions from the grazing ruminant are responsible for about 87% of New Zealand's total methane emissions and for about 99% of the methane emissions from agriculture (Section 7.2.1). In 2000, sheep, dairy cows, beef cattle and deer were responsible for 46%, 29%, 21% and 3% of the ruminant emissions respectively. Since 1990, total ruminant emissions have risen by 7%, the percentage of this change being attributed to increases of 50%, 8% and 219% in dairy cows, beef cattle and deer and a decrease of 22% by sheep, mainly due to changes in livestock numbers. It is predicted that ruminant methane emissions will be 16% over 1990 levels by 2010 if the present trend continues.
Methane is produced in the rumen and caecum by the anaerobic microbial fermentation of pasture plant organic matter. Methane is synthesised from hydrogen and carbon dioxide at the end of the microbial digestion chain by the methanogenic archaea, a group of microorganisms that is widely distributed in nature and is also responsible for methane synthesis in manure, effluent ponds and the soil. If hydrogen is allowed to accumulate in the rumen it depresses digestion, so the archaea remove it as methane. Management of hydrogen in the rumen is the key to controlling ruminant methane emissions (Section 7.1.1.2).
It is important, for both inventory and mitigation purposes, that methane emission from grazing animals is measured as accurately as possible. To date only one technique, the sulphur hexafluoride (SF6) tracer technique, is satisfactory for free ranging animals (Section 5.1.2). Further work is also needed before the SF6 technique can be accepted as reliable: rigorous evaluation against the respiration calorimeter; confirmation of the proportion of methane that is excreted in the flatus; and evaluation of the high variability of the technique compared to chamber measurements.
Many possibilities for mitigating methane emissions have been proposed in the literature, and many of them have been evaluated experimentally. They include:
- Reducing livestock numbers. This is not an acceptable solution as a stand-alone option. However, it may be possible to reduce methane by combining improvements in animal efficiency with lower livestock numbers (Section 7.4.1).
- All animals have an obligatory maintenance requirement that results in no production, yet has an associated methane emission. The strategy must be to dilute the effects of this maintenance methane by various measures such as increasing feed intake, increasing metabolic efficiency and genetic improvement (Section 7.4.2.1).
- Manipulation of dietary composition by increasing digestibility, reducing cell wall carbohydrates, increasing starch, addition of certain lipids and increased protein can reduce methane.
· A wide range of feed additives has been proposed to reduce methane. These include alternative hydrogen acceptors (e.g. malate, fumarate), halogenated methane analogues (e.g. chloroform, bromoethanesulphonic acid), antibiotics (e.g. monensin, mevastatin), defaunating agents (e.g. manoxol, teric), probiotics, bacteriocins and naturally occurring plant compounds (e.g. condensed tannins). Problems with these compounds, such as toxicity to the microbes and the animal, short-lived effects due to microbial adaptation, volatility, expense, and failure to meet consumer acceptance have ensured that none have yet been used successfully in agriculture for reducing methane emissions. With grazing animals, other than dairy cows, a delivery system would be required to ensure regular delivery into the rumen. Delivery by breeding into pasture plants is possible, but the time needed to get viable pasture swards established under the range of New Zealand pastoral conditions should not be underestimated (Section 7.4.4).
- Immunisation of animals against methanogens has been attempted by Australian scientists. This is a good concept, but the experimental results to date have not been made public or available for scientific peer review (Section 7.4.5).
- Many suggestions have been made for manipulating the rumen microbial ecosystem to achieve methane reduction. These include targeting methanogens with microbial antibiotics, bacteriocins or phage, removing protozoa and developing alternative sinks for hydrogen such as acetogenic bacteria. Development of mitigation technologies from this type of research is well in the future because of the need to first understand the complexities of the rumen microbial ecosystem (Section 7.4.6).
- There are several nutritional and farm management strategies currently available that, if applied in a systematic manner, would be expected to reduce methane emissions (Section 8.1).
Research Priorities
In the area of methane mitigation, a review of the literature, visits to current research programmes in New Zealand, and Workshop deliberations identified a number of areas where further research could be warranted. In order of priority these are:
- A basic research programme studying the ecology of the rumen archaea should be supported. The aim should be two-fold: to identify opportunities for reducing methane synthesis; and to divert accumulated hydrogen into products that can be utilised by the animal. Core skills to do this exist (Section 7.4.6).
- The development of farm-scale modeling, resource accounting techniques and complementary on-farm testing protocols to implement both existing knowledge and technology-based developments of abatement strategies is seen as a high priority. Core skills and expertise to do this already exist: they need to be brought together and focused on this problem (Section 8.1).
- The large differences that seem to exist between animals in methane emission should be evaluated and, if proven, genetic markers should be sought to underpin a selection programme (Section 7.4.3).
- The CSIRO antimethanogenic vaccine is an interesting concept. There are plans to test the vaccine in New Zealand. Research into elucidating the mode of action of the vaccine and ensure that its efficacy is consistently greater than 20% is needed (Section 7.4.5).
- From time-to-time new feed additives/naturally occurring methane inhibitors will be identified within or external to the New Zealand programme (monensin, malate, condensed tannins). Before any research commences, the feasibility of each candidate should be evaluated with a desk study that considers criteria such as specific activity of the compound, proposed delivery vehicle, time frame and cost of development of a viable product, cost of product to the farmer, and acceptability to consumers (Section 7.4.4).
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