8. Green Package / Climate Change
9.1 CCI 602: Mitigation of ruminant methane: determination of effects of diet and specific methanogen populations on methane emissions
| Programme Title: | Mitigation of ruminant methane: determination of effects of diet and specific methanogen populations on methane emissions |
| Programme Leader: | Dr Keith Joblin |
| Institution: | AgResearch, Grasslands Research Centre |
Summary
This programme aimed to obtain information on methane emissions and methane-producing microbes (methanogens) in natural variants of sheep emitting methane at different levels to those of normal sheep and to examine effects of diet. Measurements of methane emissions from selected sheep confirmed the high- and low-methane emission status of the two sets of sheep. Our experiments with a new DNA method developed to quantify methanogens in rumen contents indicated that both sets of sheep had very similar levels of methanogens. One methanogen species, Methanosarcina barkeri was found to be very low in all sheep and below the level of detection. Methane emissions were lower when sheep grazed high tannin pasture. PCR analysis of DNA extracts suggested that the population densities of methanogens belonging to the Methanobacteriales were low in sheep grazing the high tannin pasture.
Background
New Zealand's major greenhouse gas, methane, arises mainly from grazing ruminants. This `ruminant methane' is produced in the rumen by methane-producing microbes (methanogens) during forage digestion. In previous studies, we have shown that methane emission levels can vary widely between animals in a flock of grazing sheep. The reasons for differences in emissions from apparently similar animals are not known but an understanding of the factors involved may lead to natural ways for lowering methane emissions. We need to know whether differences in emission levels reflect changes to ruminal methanogen populations. The goal of this study was to obtain information on methane emissions and on ruminal methanogen populations in high- and low-methane sheep and to investigate effects of died on methane emissions.
Approach & Outcomes
Seven sheep, previously identified as either high- or low-methane emitters from a study on 20 sheep, were grazed together on ryegrass/clover pasture in February 2000 and methane emissions measured using the SF6 tracer technique. Methane emissions were measured daily over a three-day period. Samples of rumen contents were removed on one day and frozen prior to DNA extraction and analysis.
| Sheep Ranking | CH4 emission (g/day) |
DM intake (g/d) |
DM digestibility (%) |
CH4 /DMI (mg/g) |
| Low | 27.3 | 2517 | 83.1 | 10.8 |
| High | 36.4 | 2350 | 83.1 | 15.5 |
The results show that there was considerable difference between methane emissions from the high- and low-emitters both in absolute terms (33 percent) and in terms of methane emission per unit of dry matter intake (43 percent). Preliminary analysis of total methanogen populations in rumen samples using MPN-PCR procedures based on specific 16S rRNA primers revealed that ruminal digesta contained inhibitors of PCR and that the true levels of methanogens were underestimated by this analytical method. Confirmation of this was obtained from a study comparing methanogen levels derived from MPN-PCR with levels independently enumerated.
To obtain a more precise method for measuring methanogens in ruminal digesta, specific competitive-PCR analyses were developed and tested for total archaea, total eubacteria and for methanogens belonging to the Methanosarcinales and Methanobacteriales families. When applied to DNA from digesta samples, these analyses revealed that both high and low-methane sheep contained mean total methanogen population densities greater than 109 per ml of rumen fluid. This is an agreement with previous levels of 109 per ml found in grazing sheep using culture-based enumeration. Our data suggest that the ratio of methanogens to total eubacteria may be higher in high-methane sheep than in low-methane sheep. Further analysis is required to confirm this and to obtain more information on sub-populations of methanogens in these animals. In both high- and low-methane sheep, methanogens belonging to the Methanosarcinales were below the level of detection showing that Methanosarcina barkeri was not a major contributor to their methane emissions.
In an effort to determine effects from different diets on methane emissions from sheep, the SF6 tracer technique was used to measure methane emissions from six sheep grazing two different pasture types. Methane emissions were measured in the sheep grazing a high tannin (Lotus corniculatus) pasture in December 1999, and the same sheep grazing ryegrass/white clover pasture in February 2000. Methane was measured on 3 consecutive days.
| Pasture Type | CH4 emission (g/day) |
DM intake (g/d) |
DM digestibility (%) |
CH4 /DMI (mg/g) |
| Ryegrass | 29.4 |
2621 |
83 |
11.1 |
| Lotus | 22.1 |
1572 |
68 |
14.1 |
The results, presented as the mean, indicate that the grazing sheep produced more (33 percent) methane from ryegrass pasture in absolute terms than from the high tannin pasture. On the basis of dry matter intake, 21 percent less methane was produced from the ryegrass pasture. The latter result, contrasting with results from previous experiments, is likely to be an artefact from inaccurate measurement of intake. Analysis of DNA from the rumen samples showed that the mean population density of methanogens in sheep grazing the high tannin pasture was very similar to that in sheep grazing ryegrass-clover at 1-2 x 109 per ml rumen fluid. Moreover, the ratio of total archaea to total eubacteria in rumen fluid samples from the sheep also was similar on both pasture types. Data from specific analyses for methanogens belonging to the Methanobacteriales family suggest that these methanogens may be significantly lower in sheep grazing the high-tannin pasture. This needs to be confirmed because of possible interference in PCR assays from tannins.
Publications
The development of PCR methods for quantifying specific methanogen populations and results on methane emissions from grazing sheep are being prepared for publication.
Contact for Enquiries
Farm Monitoring Programme Manager
Monitoring and Evaluation
MAF Policy
PO Box 2526
Wellington
NEW ZEALAND
Phone: +64 4 894 0623
Fax: +64 4 894 0741
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