Livestock country inventory: United Kingdom

Overview of UK’s current Tier 2 approach

The UK reports emissions from three cattle categories. It uses a Tier 2 approach for dairy cows and beef cows, and a Tier 1 approach for all other cattle (Table 1). A Tier 1 approach is used for all other livestock. For lambs, the UK has adjusted the Tier 1 IPCC default factor to UK conditions. Total emissions from enteric fermentation, enteric fermentation from cattle and enteric fermentation from sheep, and methane and nitrous oxide emission from manure management are identified as key categories in the latest inventory (NIR 2017). NIR 2018 used a thoroughly revised, country-specific Tier 2 approach for cattle.

Table 1: Overview of Tiers used for livestock methane emissions in the UK’s national GHG inventories

*Year refers to the year of NIR submission; ** Later discontinued, then re-adopted in NIR 2018.

Enteric fermentation

Until 2018, the UK used the IPCC model to estimate enteric fermentation emissions from dairy and beef cattle. In NIR 2018, the results of commissioned research were incorporated in the inventory, which now uses a country-specific method.

(1) Approach used until 2017

Until 2018, the UK implemented the IPCC Tier 2 model for dairy and beef cows. The approach estimates daily gross energy (GE) intake on the basis of animal performance, management practices and environmental factors. GE is converted to methane using a methane conversion factor (Ym), and estimated daily emissions are multiplied by number of days to make an estimate of annual emissions per head. Activity data on population of livestock of each category are multiplied by the EF to estimate total annual emissions from enteric fermentation for that category of livestock. An innovation in the UK’s implementation of the IPCC model is the use of a country-specific method for estimating feed digestibility, which it has used since NIR 2005 (see Inventory Practice UK’s country-specific method for estimating digestibility). This innovation used a country-specific energy balance model, the use of which was expanded in the country-specific methodology adopted in 2018.

Activity data: Livestock population data is provided each year from the Department of Environment, Food and Rural Affairs (DEFRA). This data is compiled from results of the agricultural census conducted in June every year by the devolved administrations (i.e. England, Wales, Scotland and Northern Ireland), which use the same livestock sub-categories to enable summation to UK population totals.

Emissions were separately estimated for breeding dairy cows, beef cows and six other types of cattle (Table 2). For dairy cows, until 2004 the dairy herd was defined as cows and heifers in milk plus cows in calf, but not in milk. In 2005, the dairy herd definition was changed to ‘cows over two years of age with offspring’, which does not include cows in calf, but not in milk (Agriculture in the United Kingdom data sets). Until NIR 2013, ‘other cattle’ included dairy heifers, beef heifers, others>2 and others 1-2 years old. This was later expanded to 6 categories (see Table 2) to better account for the different characteristics of dairy and beef animals (NIR 2013).

Table 2: Livestock categorization in the UK’s Tier 2 approach 2013-2017

Animal performance data needed for IPCC model equations:

Dairy cows: For dairy cows, the UK used country-specific data for dairy cow live weight, milk yield, milk fat content, feed digestibility and activity (proportion of the year spent grazing), each of which varies from year to year. The estimated EF thus tracks change in management practice and animal performance on an annual basis. All other parameters used IPCC default values. See Table 3.

In early NIR submissions, the UK estimated dairy cow live weight by assuming a 1% annual increase compared to the figure for 1990. In NIR 2008, the data source and method used to estimate live weight changed to use data from a carcass weight survey adjusted for a carcass ratio of 0.48. Since the BSE crisis in the 1990s, slaughter must take place at designated facilities, and monthly surveys are undertaken of numbers animals (by sub-category) slaughtered and carcass weight (Cattle, sheep and pig slaughter). NIR 2015 applied a further evolution in data sources and method, whereby abbatoir data was linked with ear tag identification to provide a more precise estimate of carcass weight for dairy cows that had been slaughtered after their first calving (see inventory practice: estimating animal weights using carcass weight data). The carcass ratio was also updated based on a research study (Minchin et al. 2009).

Milk yield data is official data from DEFRA statistics. Annual data on fat content derives from the Rural Payments Agency responsible for administering payments related to milk supply adjusted for butterfat content, which required wholesale purchasers of milk to record butterfat content (The New Butterfat Adjustment Rules).

Earlier NIR submissions assumed digestibility (digestible energy as a percentage of GE) of 65% for dairy cows. NIR 2005 revised this estimate to 74.5%. The basis for this revision was an improved method for estimating cow energy requirements that was developed in 2004 to inform on-farm feed advice for dairy farmers (see Inventory Practice UK’s country-specific method for estimating digestibility). In brief, the new method is an energy balance approach to estimate the metabolizable energy (ME) requirement for a dairy cow. First, typical concentrate use by farmers derived from a farm survey published in 2008 is combined with the digestibility (DE as a % of GE) of concentrate feed based on the typical mix of protein and energy feed ingredients. From this, the annual ME requirement that has to be met from forage is derived. The composition of forage (i.e. fresh grass, grass silage, maize silage) is then estimated on the basis of expert opinion, taking into account the proportion of time spent at grazing by dairy cows and the amount of maize grown in the UK, and digestibility values for these forage components are taken from national feed tables. The resulting estimated digestibility of 74.5% has since been used in each annual submission but is not updated annually.

Table 3: Data sources used for Tier 2 estimate of enteric fermentation emissions from dairy cows

Note: n.a. indicates no information on data sources available

Beef cows: Initially, the UK lacked a time series of live weight data, so a constant live weight of 500 kg was assumed, and the resulting EF did not change from year to year. The calculated EF was close to the IPCC default, so initial submissions used the default value was used, but this was later replaced by the country-specific value. However, in NIR 2015, analysis of data for 2008-2012 from monthly abbatoir surveys on carcass weight data was combined with ear tag identification data to produce a more accurate estimate of carcass weight for beef cows that were slaughtered after their first calving (see inventory practice: estimating animal weights using carcass weigh data). A carcass ratio of 50% was applied to estimate live weight based on a scientific publication from a neighbouring country (Minchin et al. 2009). This analysis of abbatoir data is repeated annually to produce a time series for beef cow live weight. Other parameters, such as milk yield, milk fat content and digestibility, are assumed to be constant, so the time series of the EF now varies in relation to the estimated live weight of beef cows.

Table 4: Data sources used for Tier 2 estimate of enteric fermentation emissions from beef cows

n.a. means description of data sources not available.

(2) Country-specific approach adopted in 2018

NIR 2018 adopts a country-specific methodology for enteric fermentation emission estimates from dairy and other cattle. In brief, the main features of the revised methodology are as follows:

Dairy cattle: Before 2018, the inventory represented only 1 dairy cow production system for the country, assuming a standard diet and average milk yield. The new methodology now represents 3 production systems based on breed, with breed- and region-specific data for milk yields and diet. This enables the inventory to capture changes such as increased use of forage maize. Research has established a close relationship between dry matter intake (DMI) and methane emissions, and DMI is now estimated on the basis of metabolizable energy which is determined using UK-specific energy balance equations as published in Feed into Milk (Thomas, 2004):

𝐶𝐻₄_𝑒𝑛𝑡𝑒𝑟𝑖𝑐_𝑑𝑐 = (15.8185 × 𝐷𝑀𝐼) + 88.6002

Where:

CH₄_enteric_dc is the enteric methane emission per dairy cow, g d^-1

DMI is feed dry matter intake, kg d^-1.

Calculations are performed at a monthly resolution, with characterization of production, management and feed by dairy cow category for each month.

Other cattle: Enteric methane emissions from other cattle, including dairy sector replacements and calves, and beef cattle, are estimated using the same approach as for dairy cows but with different relationships between enteric emission and dry matter intake. For non-lactating cattle:

𝐶𝐻₄_𝑒𝑛𝑡𝑒𝑟𝑖𝑐_𝑜𝑐 = (17.5653 × 𝐷𝑀𝐼) + 45.8688

where

CH₄_enteric_oc is the enteric methane emission per animal, g d-1.

For lactating suckler cows, the equation for dairy cows is used. For beef cattle, the inventory now represents 3 production systems (‘continental’, ‘lowland native’ and ‘upland’), with 6 roles and 16 age bands in each. Monthly numbers of animals in each system are provided by the cattle tracing system.

The revised inventory shows 6%-7% lower total agricultural emissions than previously estimated, but the trend in emissions between 1990 and 2015 is very similar. One benefit of adopting more advanced approaches in the 2018 inventory is that the inventory is now capable of presenting the effects of adopting GHG mitigation practices, such as change in diet or breeds.

Manure management (Methane)

Manure management methane emissions from cattle are a key category (NIR 2017).

Approach used: IPCC approach (T2 for cattle and swine), T1 for other livestock.

Implementation of the approach: The source of activity data on livestock populations is as described above for enteric fermentation. The emission factors for manure management are calculated following IPCC Tier 2 methodology using default IPCC data for volatile solids (VS) and methane producing potential (Bo) parameters for each livestock type, except for dairy and beef cows, where a Tier 2 calculation following IPCC 2006 Equation 10.24 is used to determine VS. In calculating VS, the country-specific estimates for DE% used for enteric fermentation and the IPCC default ash content (i.e. 8%) are used. With the 2018 methodological revision, DMI is estimated using the UK-specific metabolizable energy equations, and VS is estimated on the basis of the GE of feed and feed energy content.

Initially, country-specific data on the proportion of manure managed in the different manure management systems derived from a number of sources, including commissioned research that used postal surveys of farmers (Smith et al. 2000, 2001a, 2001b), expert opinion, and other available data. Since 2012, the Farm Practices Survey (an annual representative survey of 2500 farms implemented by DEFRA) has included questions covering adoption of GHG mitigation practices, including manure and slurry management. This data is now used in the estimation of proportion of manure managed in different management systems, and enables the inventory to reflect change in farming practices over time.

Uncertainty management

Until NIR 2015, the uncertainty associated with enteric fermentation and manure management was estimated using default estimates derived from the Watt Committee (i.e. ±20% for enteric fermentation and ±30.5% for methane emissions from manure management) (Williams, 1993). NIR 2015 used results of a DEFRA-commissioned study that provided improved estimates of uncertainty associated with livestock methane and nitrous oxide emissions (Milne et al. 2014). Monte Carlo simulation was applied to propagate the uncertainty from input variables to the IPCC Tier 2 models for dairy and beef cattle through to the resulting estimated aggregate emission estimate. The disaggregated input data provided by each of the UK’s devolved administrations was used, so the analysis provided geographically disaggregated insights into the main sources of uncertainty as well as identifying the contribution of GHG sources to uncertainty in the inventory. (see Inventory practice: Assessing uncertainty in the UK’s livestock inventory).

Resources

Milne AE, et al. 2014. Analysis of uncertainties in the estimates of nitrous oxide and methane emissions in the UK’s greenhouse gas inventory for agriculture. Atmospheric Environment.

Minchin W, et al. 2009. Prediction of cull cow carcass characteristics from live weight and body condition score measured pre-slaughter. Irish Journal of Agricultural and Food Research.

Misselbrook T. 2018. New UK agriculture GHG and ammonia inventories. Presentation to National Farmer’s Union.

Smith KA, et al. 2000. A survey of the production and use of animal manures in England and Wales. I. Pig manure. Soil Use and Management.

Smith KA, et al. 2001a. A survey of the production and use of animal manures in England and Wales. II. Poultry manure. Soil Use and Management.

Smith KA, et al. 2001b. A survey of the production and use of animal manures in England and Wales. III. Cattle manures. Soil Use and Management.

Williams A. 1993. Methane Emissions, Watt Committee Report Number 28, The Watt Committee on Energy, London.

Author: Andreas Wilkes, Values for development Ltd (2019)