What are enteric emissions?

Enteric methane emissions are a form of greenhouse gas (GHG) released during the digestive process of certain livestock, particularly ruminants like cattle and sheep. These animals have a specialized stomach system that includes the rumen, where microbial fermentation breaks down fibrous plant material. A by-product of this fermentation process is methane (CH₄), which is primarily released through belching.

Methane is a potent greenhouse gas, with a global warming potential approximately 28 times greater than carbon dioxide (CO₂) over a 100-year period. As such, enteric methane represents a significant portion of agricultural emissions, especially in systems with high ruminant populations.

Emissions by Animal Type

The carbon calculator differentiates animal types based on their digestive systems and methane output:

  • Cattle and sheep: These high-emitting ruminants are given three calculation options, ranging from animal defaults to assumed UK average diets, to more detailed methods based on actual feed intake. This reflects both the significance of their emissions and the availability of detailed data, allowing users greater nuance in an area of high impact.
  • Other livestock (e.g. pigs, horses, deer, llamas, goats): These animals emit much lower levels of enteric methane. The calculator applies default emission factors for these species, with no advanced calculation options.
  • Poultry: As monogastric animals, poultry are assumed to produce negligible or zero enteric methane and are excluded from enteric methane calculations.

Options for calculation enteric methane

The carbon calculator offers three different approaches to estimating enteric methane emissions. These options vary in complexity and data requirements. Only Option 1 is available for all livestock types, while Options 2 and 3 apply only to cattle and sheep.

Option 1: Default Emission Factors (Tier 1)

This approach uses a fixed methane emission factor per head of livestock, with values based on the UK Greenhouse Gas Inventory (Tier 1 methodology). It's suitable when only livestock numbers are known. These default emission factors...

  • Use species- and class-specific data to create UK average emission factors
  • Reflect average UK feeding and management practices
  • Require no user input beyond livestock numbers

Equation 1.a

Enteric CH₄ emissions = (EF × head) × 28 / 1000

  • EF = Emission factor (kg CH₄/head/year)
  • head = Number of animals
  • 28 = CH₄ to CO₂e conversion
  • 1000 = Kg to tonnes

Option 2: Dry Matter Intake-Based (Tier 3 UK GHG Inventory)

Available only for cattle and sheep, this method uses the UK inventory’s Tier 3 approach, which estimates daily enteric methane emissions based on a calculated relationship between dry matter intake (DMI) and daily enteric emissions.
This relationship is calculated using a UK-specific energy balance that reflects the average diet of UK animals. All users need to do is enter the average DMI for the animals, the system is then assumed based on animal type.

For cattle, the assumed system includes:
  • Foraged grass, grass silage, maize silage, and whole crop silage during the grazing season
  • A balanced mix of concentrates when housed
  • An average seasonal housing pattern combining data from year-round housing, winter housing, summer overnight housing, and extended grazing systems
  • Different DMI-emission relationships are used for dairy cattle and other cattle to reflect variations in energy demand and quality of feed intake (i.e. protein content).
For sheep, the model assumes:
  • Animals are mostly grass-fed,
  • Ewes and rams supplemented with silage during winter
  • Lambs receiving silage, roots, and an average creep feed ration during the early growth period
  • And a general assumption of outdoor grazing with 42 days of indoor housing during lambing

This enteric emissions calculation approach gives more tailored results based on UK production systems whilst only requiring the average dry matter intake from the user.

Equation 1.b

Enteric CH₄ emissions = (m × DMI + c) × 365 × head × 28 / 1,000,000

  • m, c = UK constants (g CH₄/kg DMI)
  • DMI = Daily dry matter intake (kg/day)
  • 365 = Days/year
  • 1,000,000 = Grams to tonnes

Option 3: Specific Diet-Based Calculation (IPCC/FAO Tier 3)

This most detailed method uses feed-specific methane conversion factors to estimate emissions. It’s based on the IPCC Tier 2 framework adapted for UK systems.
Also limited to cattle and sheep, this method calculates enteric methane emissions based on the energy content and methane-producing capacity of the specific feed types consumed.
In this method each feed type (e.g., grass silage, cereal grains, legumes) has an associated methane conversion factor calculated, representing how much methane is produced per kilo of the feed consumed. The total emissions are determined by:

  • The amount of each feed type consumed
  • The digestibility and energy availability of those feeds
  • The methane conversion efficiency of the animal (Ym value)

This method allows the most detailed and system-specific assessment of enteric methane, ideal for users with good knowledge of feeding regimes or those modelling alternative or innovative feeding systems.

Option 3: Diet and Intake-Based

Enteric CH₄ emissions = (GEi × (Ym / 100) × 365) / 55.65 × head × 28 / 1000

  • GEi = Gross energy intake (GE × DMI)
  • Ym = 9.75 − 0.05 × (DE / GE × 100)
  • GE = Gross energy of feed (MJ/kg DM)
  • DE = Digestible energy (MJ/kg DM)
  • 55.65 = Energy in methane (MJ/kg CH₄)

Background on Feed Digestibility and Energy Data

Accurate estimation of enteric methane emissions requires detailed knowledge of the energy content and digestibility of livestock feed. This information helps determine how much energy animals extract from their feed, and how much is lost as methane during digestion.

Gross Energy (GE):

The total energy content of the feed, measured in megajoules per kilogram of dry matter (MJ/kg DM). Different feed types (e.g., grass, silage, concentrates) have varying GE values.

Digestible Energy (DE):

The portion of gross energy that animals can actually digest and metabolize. This varies by feed type and affects methane conversion rates.

D-Value vs DE:

D-value, digestibility percentage, and DE are related but not the same:

  • D-value measures the digestibility of the feed—the extent to which the feed is absorbed by the animal. It is expressed as a percentage of the feed that can be digested and typically decreases with more starchy feeds.
  • DE (Digestible Energy) estimates the actual energy available to the animal from the feed, measured in MJ/kg DM (or Mcal/kg DM).

We can use D-value to calculate gross energy (GE) from digestible energy (DE) with the formula: GE = DE / D-value.

Methane Conversion Factor (Ym):

The percentage of gross energy converted into methane during digestion. Ym is also known as digestibility and is influenced by the feed’s digestibility—less digestible feeds generally produce more methane.

In the calculator, these values are derived from UK-specific feed data and are used in Option 3 calculations to estimate enteric methane emissions based on precise feeding systems. This approach allows users to account for variations in diet quality and composition, improving emission estimates over simpler default methods.

For more information on typical diets, D-values, digestibility, and feed data used in these calculations, please see our livestock diet help guide.