WASHINGTON, July 2, 2014 – With recent reports from both the Intergovernmental Panel on Climate Change (IPCC) and the U.S. National Climate Assessment assigning some responsibility to agriculture for “unprecedented levels” of greenhouse gas (GHG) emissions, it is not surprising that research is building on the contributions that livestock make to the increase in GHGs. While the IPCC says carbon dioxide, methane and nitrous oxide emissions have “increased to levels unprecedented,” EPA says the atmospheric concentration of methane, a GHG some 28 times more potent than carbon dioxide, has been steadily growing since the 18th century and has now increased by 50 percent compared to pre-industrial levels, exceeding 1,800 parts per billion.
While the volume of carbon dioxide emissions far exceeds that of methane, the latter has a nearly 20 percent greater impact on climate change than carbon emissions, EPA says.
Ruminant livestock are the single largest source of methane emissions, with EPA attributing at least 20 percent of them to cattle, sheep and other ruminants, making them the single largest source of methane emissions. However, not all ruminants are equal when it comes to greenhouse gas emissions, say researchers with the DOE’s Joint Genome Institute (JGI), who add that the amount of methane produced varies substantially across individual animals of the same ruminant species.
In a country like New Zealand, where sheep outnumber people 7 to 1, the increase in methane is a big deal, the researchers say. JGI scientists deployed high throughput DNA sequencing and specialized analysis techniques to explore the contents of the rumens of sheep in collaboration with New Zealand’s AgResearch Limited to see what role ruminant “microbiomes” (the microbes living in the rumen) play in this process.
“We wanted to understand why some sheep produce a lot and some produce little methane,” said DOE JGI Director Eddy Rubin. The study, which was published online by Genome Research, shows that it is purely the microbiota responsible for the difference, he said. In other words, it is how the animals’ digestive tracts react with the microbes. Furthermore, the scientists say, breeding can change the reaction.
The team measured the methane yields from a cohort of 22 sheep, and from this group, they selected four sheep with the lowest methane emissions, four sheep with the highest emissions and two sheep with intermediate emission levels. Rumen metagenome DNA samples collected on two occasions from the 10 sheep were sequenced at the DOE JGI, generating 50 billion bases of data each. In sheep with low methane emissions, they found elevated levels of one particular species of methanogen, while sheep with high methane emissions had elevated levels of another group of methanogens.
Rubin says that it is not so much the actual composition of the microbiome that determines emission which conventional wisdom would suggest but mostly the regulation of the first step in gene expression within the existing microbes that makes the difference.
“This is a concept that is relatively new in metagenomic studies,” Rubin said, noting that the team’s findings suggest new targets for mitigating methane emissions at the microbiome level.
Screening and breeding for low-methane producing sheep is still underway, and importantly, scientists say, low-methane lines then need to be tested for stability of the trait, as well as the absence of any impacts on fertility or meat or wool production.
“There needs to be an incentive for farmers to incorporate low methane animals into their flocks,” such as achieving better performance with the low methane animals or being able to claim carbon credits, Attwood said. “If everything went well, you could expect introduction of the low methane trait to begin in three years, and for there to be slow but incremental changes to the sheep industry in subsequent years.”
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