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Methane from rice and livestock, and how to cut it

Why farming is the largest human source of methane, how rice fields and ruminants make it, and the practices and additives that cut it, with numbers.

Current topic7 min readUpdated
On this page
  1. Why methane matters
  2. Methane from flooded rice
  3. Methane from ruminant livestock
  4. Methane from manure
  5. Measuring and accounting
  6. Key points
  7. What the exams ask
  8. Questions people ask
  9. Sources
  10. Related on Laisha
  11. Discussion

Methane is the second most important greenhouse gas driven by human activity, after carbon dioxide, and agriculture is its largest single human source: around 40 per cent of the total, mostly from ruminant livestock and flooded rice. Because methane is powerful but short-lived, cutting it is one of the fastest ways to slow warming in the coming decades.

That has made agricultural methane a central topic in climate policy, in livestock and rice research, and in essays on agriculture and climate change. This article explains where it comes from, how much it matters, and what actually reduces it.

Why methane matters

  • Powerful: over a hundred years, a tonne of methane warms the climate about 27 to 30 times as much as a tonne of carbon dioxide (the IPCC's 100-year global warming potential); over twenty years, about 80 times.
  • Short-lived: methane lasts about a decade in the atmosphere, compared with centuries for much of the carbon dioxide emitted. Cut emissions, and the warming effect falls within years.
  • Rising: atmospheric methane concentrations have risen sharply since the mid-2000s, with agriculture, wetlands and fossil fuels all contributing.

More than 150 countries have joined the Global Methane Pledge (2021), aiming to cut methane emissions 30 per cent below 2020 levels by 2030, though most of the pledged cuts are in fossil fuels and waste rather than farming.

Where agricultural methane comes from and how it is cut: methane from flooded rice is made by soil archaea when oxygen runs out and is reduced by alternate wetting and drying, mid-season drainage and better straw management; methane from ruminants is made in the rumen and reduced by better feed, higher productivity, feed additives and breeding; methane from manure is reduced by anaerobic digesters and covered, cooler storage Flooded rice floodwater soil without oxygen: methanogenic archaea CH₄ through plants and bubbles Cut it with • alternate wetting and drying • mid-season drainage • straw removed, composted or incorporated off-season • direct seeding, varieties watch: draining can raise nitrous oxide Ruminant animals rumen fermentation CH₄ belched Cut it with • better feed quality • healthier, more productive animals: fewer per litre or kg • feed additives (e.g. 3-NOP) • breeding low-emitting animals the largest agricultural source of methane Manure liquid stores lagoons, slurry pits CH₄ and N₂O Cut it with • anaerobic digesters: capture gas as energy • covered, cooler, shorter storage • composting, solid handling pasture dung emits little methane; slurry emits more
The three agricultural sources of methane and the main ways to reduce each. Livestock digestion is the largest; flooded rice comes second; manure stores add more where slurry systems are common.Diagram: Laisha, original.

Methane from flooded rice

When a rice field is flooded, oxygen in the soil is used up within days. In this oxygen-free soil, organic matter (straw, roots, manure) is broken down by a chain of microbes ending with methanogenic archaea, which produce methane. Most of it escapes to the air through the rice plants' own air channels (aerenchyma), the rest by bubbling and diffusion. Rice cultivation accounts for roughly a tenth of human methane emissions, most of it in Asia.

Emissions are higher with continuous flooding, fresh organic matter (straw incorporated just before flooding, green manure), warm temperatures and long flooded periods.

Reducing it

  • Alternate wetting and drying (AWD): letting the field water level fall below the soil surface before re-irrigating. In the widely promoted "safe AWD", the farmer watches water in a perforated tube sunk into the soil and irrigates when it drops to about 15 cm below the surface, keeping fields flooded around flowering. AWD typically cuts methane substantially, often by a third to a half or more, and saves irrigation water, usually without lowering yield.
  • Mid-season drainage: one or two drainage periods during the season.
  • Straw management: removing straw for other uses, composting it, or incorporating it well before flooding instead of just before.
  • Direct-seeded rice and shorter-duration varieties, which shorten the flooded period; varieties also differ in emissions.

The trade-off: drying the soil can increase nitrous oxide emissions, another greenhouse gas; in most studies the methane saving outweighs the increase, but fertiliser timing matters.

Worked example

Estimating the methane saving from alternate wetting and drying

The IPCC's 2019 inventory guidelines give a default daily emission factor for continuously flooded rice of about 1.19 kg of methane per hectare per day, and a scaling factor of about 0.55 for fields with multiple drainage periods (such as AWD). Take a 100-day flooded season.

  • Continuous flooding: 1.19 × 100 = 119 kg CH4 per hectare.
  • AWD: 119 × 0.55 = 65 kg CH4 per hectare.
  • Saving: about 54 kg CH4 per hectare per season, or 54 × 27 ≈ 1.4 t CO2e per hectare using a 100-year GWP of 27.

On a million hectares, that is about 1.4 million tonnes of CO2e a season. Real emissions vary widely with soil, climate, organic inputs and how well AWD is practised; default factors are for inventories, not for predicting one field.

Methane from ruminant livestock

Cattle, buffalo, sheep and goats produce methane in the rumen, the fermentation chamber where microbes digest fibre. Methanogenic archaea use the hydrogen released by fermentation to make methane, which the animal belches. Enteric fermentation is the largest agricultural source of methane, roughly a quarter to a third of all human methane emissions, with cattle responsible for most of it.

Methane is also lost feed energy: typically 2 to 12 per cent of the gross energy an animal eats. Reducing it can improve efficiency.

Reducing it

  • Productivity: healthier, better-fed, more productive animals produce more milk or meat per unit of methane. Improving feed quality, animal health, fertility and herd structure lowers emission intensity (methane per kilogram of product), often with economic benefit; whether total emissions fall depends on herd size.
  • Diet: more digestible forages, better pasture management, and in some systems more concentrates or added fats, reduce methane per unit of feed.
  • Feed additives: the synthetic compound 3-nitrooxypropanol (3-NOP) inhibits a key enzyme of methanogenesis and, in many trials with dairy and beef cattle on mixed rations, cut enteric methane by around 20 to 35 per cent; it has been approved in a number of countries. The red seaweed Asparagopsis, whose active compound is bromoform, has given larger reductions in trials, with open questions about safety, residues, supply and persistence of the effect. Delivering additives to grazing animals remains a challenge.
  • Breeding: methane output is partly heritable, and some countries have begun including it in breeding indices.
  • Vaccines and rumen microbiome approaches are under research.
Worked example

A feed additive on a dairy farm

A dairy cow emitting about 158 kg of methane a year (the figure worked out in our article on rumen digestion) is fed an additive that reduces enteric methane by 30 per cent.

  • Methane avoided: 158 × 0.30 ≈ 47 kg CH4 per cow per year.
  • In CO2 equivalent (GWP 27): 47 × 27 ≈ 1.3 t CO2e per cow per year.

For a herd of 200 cows, about 250 t CO2e a year, before the emissions of manufacturing and transporting the additive. Who pays for it, through milk premiums, carbon payments or regulation, decides whether it is used.

Methane from manure

Manure stored as liquid slurry in pits and lagoons creates oxygen-free conditions and emits methane, especially when warm and stored for long periods; dung dropped on pasture emits little methane. Options include anaerobic digesters that capture the methane as biogas for energy, covering stores (with flaring or use of the gas), shorter and cooler storage, solid-manure handling and composting.

Measuring and accounting

Countries report agricultural methane in their national greenhouse gas inventories using IPCC methods: default emission factors (Tier 1) or country-specific factors based on animal numbers, weight, feed and production (Tier 2), or detailed models and measurements (Tier 3). Farm-level measurement uses respiration chambers, sulphur hexafluoride tracers, breath analysers ("sniffers") in milking robots, and for rice, closed chambers on the field. Because inventories built on default factors cannot show the effect of better practice, countries and companies moving to Tier 2 and Tier 3 methods can recognise improvements that Tier 1 would miss.

A debate continues over metrics: because methane is short-lived, a constant rate of emissions keeps adding roughly constant warming, while a falling rate can cool. Alternative metrics such as GWP* express this difference; critics worry they could be used to excuse current high emitters. Exams may ask for the GWP values and the reasoning.

Key points

  • Agriculture causes about 40 per cent of human methane emissions: ruminants (largest), rice, and manure.
  • Methane's 100-year GWP is about 27–30; its 20-year GWP about 80; it lasts about a decade.
  • Rice methane comes from methanogenic archaea in flooded, oxygen-free soil; alternate wetting and drying, mid-season drainage and straw management reduce it, with a possible rise in nitrous oxide.
  • Ruminant methane comes from rumen fermentation; productivity, diet, additives such as 3-NOP, and breeding reduce it.
  • Manure methane comes from liquid stores; digesters and covered, cooler storage reduce it.
  • Emission intensity and absolute emissions both matter; Tier 2 and 3 inventories recognise practice changes.

What the exams ask

  • Sustainability, agronomy and animal science courses: sources of agricultural methane, the microbial processes in rice soils and the rumen, and mitigation options.
  • Research-fellowship and postgraduate entrances: alternate wetting and drying, feed additives and their modes of action, emission factors and IPCC tiers, and methane–nitrous oxide trade-offs.
  • Civil-service and policy examinations: the Global Methane Pledge, methane's short-lived climate role, and policies for rice and livestock emissions.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

Does alternate wetting and drying reduce rice yields?

When done safely, keeping fields flooded around flowering and re-irrigating when water drops to about 15 cm below the surface, studies across Asia generally find no significant yield loss, along with water savings. Poorly timed drying can reduce yield.

Is grass-fed beef lower in methane?

Not necessarily per kilogram. Animals on poor-quality forage often grow slowly and emit more methane per kilogram of meat. Well-managed grazing can store soil carbon, but whether that offsets methane depends strongly on the system.

Should people eat less meat to cut methane?

Reducing high consumption of ruminant meat and dairy is one of the most effective ways to lower food-system emissions in high-consuming countries. In many low-income regions, livestock are vital for nutrition and livelihoods, and the priority is improving productivity and efficiency.

Sources

  1. UNEP and Climate and Clean Air Coalition. Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions. United Nations Environment Programme, 2021.
  2. IPCC. 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 4, Chapters 5 (rice) and 10 (livestock).
  3. IPCC. Climate Change 2021: The Physical Science Basis, Chapter 7, Table 7.15 (global warming potentials).
  4. Lampayan, R. M. et al. "Adoption and economics of alternate wetting and drying water management for irrigated lowland rice." Field Crops Research 170, 95–108 (2015).
  5. Hristov, A. N. et al. "An inhibitor persistently decreased enteric methane emission from dairy cows with no negative effect on milk production." PNAS 112, 10663–10668 (2015).
  6. FAO. Pathways towards lower emissions: a global assessment of the greenhouse gas emissions and mitigation options from livestock agrifood systems. FAO, 2023.

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Discussion

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