Methane is also generated in animal manure, because some microbes live and in fact thrive in animal manure, especially if it is stored in anaerobic ponds.
In New Zealand, only a relatively small amount (about 3 per cent) of total livestock methane emissions come from manure because our animals mostly graze on pastures.
This means their manure is mostly deposited on grass, which doesn't provide conditions as good for methane-generating microbes.
Livestock produce not only methane but also another greenhouse gas called nitrous oxide.
This is also generated by microbes, but in this case the microbes live in soil.
Those microbes break down the urine and dung deposited on pastures, which makes some of the nitrogen contained in animal excreta available for plant growth (hence animal manure is a good fertiliser).
But as part of this process, a fraction of the nitrogen contained in animal excreta gets converted into nitrous oxide, which then gets released into the atmosphere.
Nitrogen fertilisers also cause nitrous oxide emissions through essentially the same process.
Finally livestock also cause indirect emissions through the carbon dioxide emissions associated with energy production and use on and off-farm, mainly transport fuels, cool storage, and the energy used to produce fertilisers or to grow feed off-farm.
Historically, clearing New Zealand's forests to make space for pastures also released large amounts of carbon dioxide into the atmosphere, but this process has now mostly stopped and is reversing in some places, as low-quality and erosion-prone land is reverted back into scrub and in some places, forests.
Overall, about three quarters of total direct on-farm livestock emissions in New Zealand are estimated to be in the form of methane, and about one quarter in the form of nitrous oxide.
Emissions of carbon dioxide on and off-farm related to livestock production amount to roughly 10 per cent of total emissions (dependent on farm system; around 15 per cent for dairy farms and less than 10 per cent for sheep).
Globally, what percentage of the world's methane output can be attributed to New Zealand agriculture - and how does our methane output rate in comparison to that of other countries?
This is difficult to say precisely, because estimates of methane emissions in other parts of the world are rather uncertain.
A global database that takes a consistent approach across countries estimates that New Zealand produces about 1.1 per cent of the world's total livestock-related methane emissions.
This share is considerably higher than New Zealand's share of the global human population (0.06 per cent), reflecting the fact that New Zealand is a major exporter of ruminant livestock products.
If emissions from paddy rice fields are included, New Zealand's share of global total agricultural methane emissions is about 0.8 per cent.
Other methane sources come from discharges from gas and coal mines (most methane-generating microbes depend on conditions that exclude fresh air) and processing and distribution of fossil fuels, and landfills and other forms of waste management.
Once those other sources are included, New Zealand's share drops to about 0.3-0.4 per cent of the world's total human-caused methane emissions.
The "emissions intensity" of New Zealand's agricultural production has declined since 1990. Can you explain what has brought this about?
The term "emissions intensity" refers to the amount of greenhouse gas emissions per unit of product, such as the amount of emissions per kilogram of milk solids or per kg of beef or lamb produced.
Since 1990, the on-farm emissions intensity of milk, beef and lamb production declined by about 20 per cent.
The main reason for this is that farmers have increased the productivity per animal; for example, more milk produced per cow and heavier lambs and cattle at time of slaughter.
This means that a greater fraction of the grass that animals eat is used to generate the product we want (milk or meat) and less to simply keep the animals alive.
As a result, emissions per animal have increased, but emissions per kg of milk or kilogram of meat produced by the animals have reduced.
The ways in which this productivity increase has been achieved are manifold. One part is improved animal selection and breeding, meaning that animals are simply performing better with the same inputs.
But improved pasture management, increased and more targeted use of fertiliser, improved herd management and animal health care have all contributed to this.
The benefits of this can be clearly seen in the case of lamb production: farmers have increased their lambing percentage (more live lambs born per ewe) and lambs are growing faster and are heavier at slaughter on average (in part because farmers have taken stock of very poor-quality low-yielding pastures).
This has allowed farmers to drop total sheep numbers substantially - from 57 million in 1990 to under 30 million today - with almost no drop in the national amount of lamb meat produced.
However, we've seen overall agricultural emissions rise since 2008. Why?
The simple reason is that overall production across all livestock sectors has grown faster than emissions intensity has dropped.
Since 1990, absolute emissions from sheep have dropped by about 35 per cent, emissions from beef are about the same today as they were in 1990 (while beef production has increased), but emissions from dairy have more than doubled (while milk production has almost tripled).
As a result of the overall production increase, total emissions from agriculture have increased by about 14 per cent between 1990 and 2013, even though emissions intensity has fallen by about 20 per cent.
How was the New Zealand Agricultural Greenhouse Gas Research Centre conceived, and why?
The idea for a national research centre on agricultural greenhouse gases was first floated by the former Minister Simon Upton, who is now the Environment Director at the Organisation for Economic Collaboration and Development (OECD) in Paris.
The idea was picked up by the National Party in its election manifesto in 2008 and put into place after the election by the end of 2009.
The New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC) works closely with and extends work by the Pastoral Greenhouse Gas Research Consortium (PGgRc), an industry-government joint venture that has worked since 2003 to develop ways of reducing methane emissions from livestock.
The NZAGRC is fully funded by the Government and supports research reducing methane and nitrous oxide and enhancing soil carbon in New Zealand farm systems.
Our work complements research supported by the Ministry for Primary Industries under its Sustainable Land Management and Climate Change initiative and work to improve the national greenhouse gas emissions inventory.
The NZAGRC also leads New Zealand's scientific inputs to the Global Research Alliance on Agricultural Greenhouse Gases, which is a voluntary initiative between governments to collaborate in efforts to reduce the emissions intensity of food production while enhancing food security.
Can you provide an overview of what it has achieved to date, in terms of finding ways to reduce our agricultural emissions output?
Much of the initial work of the NZAGRC and PGgRc was focused on better understanding the ways in which microbes and animals interact to produce greenhouse gases and to identify specific intervention options to reduce these emissions. More recently, this has moved closer to solutions and on-farm application. Some of the key successes include:
• We demonstrated that animals differ naturally in the amount of methane they produce (without any apparent reduction in productivity), and that this difference is heritable. Hence this can now be exploited through targeted breeding.
• We identified several animal-safe compounds that can selectively knock out the microbes that generate methane in the rumen. So far, we have only done short-term animal trials and more work is needed to ensure compounds remain effective in the long term and leave no residues. Nonetheless, this is a significant step that opens the door for discussion with commercial partners via the PGgRc.
• We showed that some supplementary feeds, for example forage rape and fodder beet (if given in high amounts), can reduce methane emissions for the same amount of animal production. The challenge is that the total amount of these supplementary feeds consumed by animals in the New Zealand farm system is quite low, and it may not be cost-effective to greatly increase their use; we also need to make sure their increased use does not inadvertently increase emissions of other greenhouse gases or loses soil carbon.
• We demonstrated in field trials that different plants can significantly influence nitrous oxide emissions from urine deposited onto pastures. This opens the door to identify and use natural compounds that can help reduce nitrous oxide emissions and potentially also reduce nitrate leaching.
• We have made steady progress towards developing a vaccine that would selectively block methane generating microbes in the rumen; we demonstrated ways of generating antibodies and how to deliver them into the rumen. But the rumen is a complex environment, and we have not yet managed to achieve an actual change in methane emissions via this route.
• We have clarified trends in soil carbon, and demonstrated that high diversity swards may lead to increases in soil carbon.
NZAGRC and PGgRc have also done work to demonstrate how and why emissions intensity has reduced in the different livestock systems, and what options exist to continue and perhaps even accelerate these reductions within New Zealand's farm systems.
Some more detail of the NZAGRC's work in 2014/15 can be found in the annual highlights documents accessible from our website: http://www.nzagrc.org.nz/annualreport.html.
An overview of current and potential future options to reduce emissions can be found here.
What have been the key challenges facing scientists in this space?
The main challenge is that both the rumen and agricultural soils are highly complex biological systems that have evolved over millions of years.
Reducing emissions while allowing agricultural production to increase is not a matter of simply flicking a switch or implementing a new gadget, as one can now for example switch from a petrol to an electric car and still get just as efficiently from A to B.
So the research into solutions that are appropriate for New Zealand farm systems involves a lot of fundamental science work.
Some of it is very technical, for example when it comes to understanding the genetic make-up of methanogens to identify how we can suppress them without impairing the overall function of the rumen.
At the same time, any solution needs to be able to be implemented within our New Zealand farm systems.
We have fewer options for emissions reductions in New Zealand than some other countries because our animals spend most of their time on pastures, which means they are not as intensively managed as in some other parts of the world.
Where animals are housed and fed a pre-prepared mix of feeds, we can change the feed mix to reduce emissions; in New Zealand, any solution has to work on a predominantly grass-based diet.
In September, scientists here announced they'd pinpointed the microbial differences in the rumens of sheep with high or low methane emissions. What potential does this have?
This research ties two important areas together.
We knew already that some sheep naturally emit less methane than others even though they are as productive, and that this is a heritable trait.
This seems to be related to the size of the rumen and how quickly material flows through it. What we know now is that these different rumens seem to host different microbial communities: in smaller rumens where material passes through faster, slightly different groups of microbes are involved in the fermentation and methane generation process than in larger rumens with slower passage rates.
Knowing this link between the animal and the microbial community it supports can help speed up the process of selecting naturally low-emitting animals and using them in breeding programmes.
It can also help design complementary interventions - for example, via inhibitors or supplementary feeds - that focus specifically on those microbes that predominantly reside in the rumen of high-emitting animals.
At the same time, a global team of scientists led by researchers at AgResearch in New Zealand announced that they had analysed the microbial communities from a large variety of different ruminants eating very different diets (cattle, sheep, goats, buffalo, even camels and an elephant).
They found that even though some microbes were more strongly associated with certain hosts and some with certain diets, similar bacteria and methanogens dominated in nearly all rumens across the world and only a few microbial species appeared to be responsible for all the methane produced by ruminants everywhere.
This discovery, which was funded by the New Zealand Government in support of the Global Research Alliance on Agricultural Greenhouse Gases, is important because it means that a locally developed solution to reduce methane emissions is likely to have global applicability.
Despite any future scientific advances, do you feel we can only get so far in limiting the methane output from stock? Is there a wall, so to speak, beyond which the only answer might be reducing stock rates?
It is unlikely that we will ever get to zero methane emissions from livestock, but we could achieve quite large emissions reductions of 30 per cent or more from the successful development of inhibitors and vaccines that target methanogens.
If you add to this the potential reductions from breeding for naturally lower emitting animals, supplementary feeds that reduce methane and nitrous oxide emissions, further development of nitrification inhibitors that reduce nitrate leaching and nitrous oxide production, and ways of increasing soil carbon, then the potential to reduce overall emissions and increase sinks from agriculture is quite high.
The main challenge is that most of those technologies and practices still lie in the future, and we cannot yet say when any of those options will be commercially available, at what cost or benefit to farmers, and how effective they will be.
It is worth keeping in mind that reducing stocking rates does not necessarily mean reducing food production.
Sheep farmers have already demonstrated that they were able to substantially reduce stock numbers without reducing the total national production of lamb meat, simply by becoming more and more productive and efficient.
So in my personal view there is no hard wall that we are likely to hit, but it will take continued and long-term effort to keep climbing this wall (or, if you like, dismantle the wall brick by brick).
It also comes down to where we see the balance between efforts to increase livestock production further and the economic benefits this provides, and alternative land uses that would help us reduce our absolute greenhouse gas emissions as part of global efforts to reduce the very serious risks and damages from unmitigated climate change.
