As agriculture comes under growing pressure to reduce its contribution to climate change, livestock researchers are turning their attention to the digestion of cows and sheep. Projects are under way around the world to reduce the vast volumes of the greenhouse gas (GHG) methane that these animals belch out.
Molecule for molecule, methane (CH₄) is a far more potent greenhouse gas than carbon dioxide (CO₂). How much more powerful is a matter of debate among climate scientists, who have different ways of calculating the warming potential of each gas — depending on assumptions made about how long they last in the atmosphere.
Making reasonable estimates, John Lynch, a University of Oxford scientist studying the climate impact of agriculture, reckons each methane molecule has a warming potential 80-100 times greater than carbon dioxide over a 20-year period. That figure falls closer to 30 times greater when taking a 100-year view, because methane does not persist as long in the atmosphere as carbon dioxide.
According to the International Energy Agency, 30 per cent of the global temperature rise since the industrial revolution is attributable to this atmospheric methane, with agriculture responsible for more than one-third of it. Estimates of total GHG emissions attributable to livestock range from 10 to 20 per cent, making this a worthwhile target in the fight against global warming.
Emissions occur because ruminants, such as cattle and sheep, have complex digestive systems that include a rumen, or pre-stomach — a living fermentation vat in which trillions of microbes break down the animals’ food, particularly cellulose and plant fibres, into usable sources of energy and nutrients.
In recent years, people have become more aware of the medical significance of the human microbiome — the beneficial bacteria that populate our guts. But ruminant animals host hundreds of times more microorganisms than us. As many as a quadrillion (a million billion) live within a typical dairy cow, including bacteria, archaea, protozoa and fungi, says Liam Sinclair, professor of animal science at Harper Adams University in the UK.
Methane is the inevitable byproduct of the rumen’s microbial activity, explains Sinclair, though the amount can be reduced by altering the animals’ diet and, in the longer term, by breeding strains whose genetics make them generate less of the gas.
“If you upset the rumen too much by changing the feed, you can have a negative effect,” he says, “but a 30 per cent reduction in methane is OK and you can cut it by 70 per cent or more in the short term.” Feed additives that have been shown to reduce methane include seaweed and the synthetic chemical 3-Nitrooxypropanol (3NOP), which inhibits a methane-generating enzyme in the rumen’s microbes.
However, as Sarah Morgan, a lecturer on sheep and beef production at Harper Adams University, points out, “for pasture-based livestock such as sheep, introducing a feed additive presents logistical challenges”.
Other approaches are being explored. “Breeding programmes could cut methane production by 1.5 per cent a year and achieve a reduction of 20-30 per cent by 2050,” notes Sinclair. “We are not talking about genetically modifying animals but about using genetics to identify animals that are likely to produce less methane.”
It is, though, “quite a complex trait to select for”, warns Morgan. “It’s not like doing a simple genetic test.”
But there is support for new projects and studies. New Zealand, whose economy is particularly dependent on livestock farming, is a world leader in research aimed at reducing agricultural methane emissions. The country’s Pastoral Greenhouse Gas Research Consortium has invested more than NZ$90mn ($54mn) in the endeavour since 2003.
Elsewhere, projects are under way in several countries. This summer, the UK government announced £2.9mn funding for research into breeding low-methane sheep, with a wide range of partners from the public and private sectors. The initial phase of this Breed for CH4ange programme focuses on data gathering, with plans to measure methane emissions from 13,500 sheep in 45 flocks.
A key piece of equipment is the portable accumulation chamber, says Annie Williams, business development manager at the Centre for Innovation Excellence in Livestock (CIEL), one of the project participants. The PAC is a sheep-sized chamber in which the animal is kept for an hour or so while air samples are analysed for methane concentration as it breathes.
80-100Multiple of impact of methane on global warming over 20 years compared with carbon dioxide
“The PAC is fairly straightforward to use, but it is a really expensive piece of kit which is never going to be viable on a commercial farm,” says Williams. “We’ll generate lots of data which we can analyse to create proxies for methane emissions that can be applied on a commercial scale.”
Morgan, who is principal investigator for the project at Harper Adams University, adds: “We aim to see how much variation there is between individual sheep in their methane output. Work in New Zealand has shown a difference of up to 20 per cent between low- and high-emitting animals.”
In the US, Dipti Pitta, associate professor of ruminant nutrition at the University of Pennsylvania, is comparing low and high methane emitters among dairy cows. A recent study showed that low emitters produced 22 per cent less of the gas than high emitters in the same herd and had microbiomes that converted food more efficiently into energy and protein, she says. Crucially, however, their milk output showed no significant difference.
By combining strategies of selectively breeding animals that host more efficient microbiomes and feeding methane inhibitors to them, Pitta is sure the global warming impact of livestock farming could be cut substantially — without reducing the amount of milk and meat produced.