Farm level GHG emission estimates methodology

More detailed methodology and quality information about data collected in the FBS and its headline measure of income, namely Farm Business Income (FBI), is available in Scottish farm business income: annual estimates: methodology.

The ‘Scottish agriculture greenhouse gas emissions and nitrogen use’ publication reports on the following headline metrics of GHG emissions:

• Whole-farm GHG emissions per hectare (absolute gross emissions in tCO2e/ha)
• Enterprise GHG emission intensity (absolute gross emissions in CO2e per unit of product)

Absolute gross emissions are emissions from a unit area of agricultural land (hectare) adjusted for forage quality. This accounts for large differences in rough grazing area and quality between farms.

Emission intensity is the carbon footprint of a unit of product or output up to the farm gate. Enterprise emission intensities report the carbon footprint of the primary product of beef, sheep, milk and cereals enterprises. Enterprise results replace whole-farm emission intensity results from the 2023-24 publication onwards.

For beef and sheep enterprises, emission intensities are calculated per kg of carcass deadweight (kg dwt). Milk emission intensities for dairy enterprises are calculated per kg of fat and protein corrected milk (FPC milk). Cereals emission intensities are calculated for total barley, wheat, oats and minor cereals enterprises (tonne crop).

Results for beef and sheep enterprises are based on assumptions about typical average weights of animals. This can lead to some higher or lower estimates where the sale weights of animals are higher or lower than average.

Cereal emission intensities are based on the grouping of different cereal enterprises. Due to changes in the type of crop grown each year, some variation between yearly estimates may be attributable to changes in cereals composition.

Organic farms are included in farm level emission estimates. Organic farms tend to have different manure management, animal rearing, and fertiliser practices compared with conventional farms. This results in differences between typical emission profiles of organic and conventional farms. Therefore, some variation observed in average absolute emission and emission intensity estimates may be attributable to organic farms in the sample. Organic farms make up around 5% of the Farm Business Survey sample.

Farms may be excluded from the sample based on uniqueness of activity and weighting. For example, farms may be excluded if an activity is very different from other farms in the sample, and if the weighting means this activity would be overrepresented in the results. While weighting aims to improve the representativeness of results, it is recognised that the weighting method has limitations when results are strongly impacted by management decisions.

Data collected through the Farm Business Survey provide a consistent and high quality input data set about farm activities. GHG emissions are then estimated using a carbon footprint calculator tool. They do not account for any sequestration by woodland areas or soil. Transport of product or livestock off farm is not included.

Different results may be obtained using different carbon calculators or different methods, for example using an inventory approach.

The Agricultural Resource Efficiency Calculator (Agrecalc) is used. This is a farm carbon footprint tool developed by the consulting division of Scotland’s Rural College (SRUC). It can estimate greenhouse gas (GHG) emissions from agriculture and identify their main sources. It can also be used by farms to benchmark key performance indicators and simulate mitigation measures and what-if scenario for planning purposes.

The three main greenhouse gases produced in agriculture are estimated by Agrecalc:

• carbon dioxide, produced by burning fossil fuels
• methane, a natural by-product of animal digestion
• nitrous oxide, released from soils following the application of nitrogen fertiliser (manufactured and organic) and soil disturbance.

Total emissions are presented in carbon dioxide equivalent (CO2e) units. This takes into account the different effects that the different gases have on climate change, known as their global warming potential (GWP). Over a 100-year period, methane’s GWP is considered to be 28 times stronger than that of carbon dioxide, while nitrous oxide’s GWP is considered to be 265 times stronger than carbon dioxide.

Agrecalc is based on the life cycle assessment (LCA) framework for estimating emissions from products and processes. The LCA accounts for emissions up to when product leaves the farm. It uses the latest Intergovernmental Panel on Climate Change (IPCC) Tier I and Tier II guidelines as well as national figures from the UK National Greenhouse Gas Inventory in its calculations. It is PAS2050:2011 certified.

The IPCC methods for greenhouse gas reporting are split into three tiers of increasing complexity and specificity. Tier I reporting standards use default figures published by the IPCC which provide a general estimate of greenhouse gas emissions, but likely miss important sources of variance. Tier II reporting standards are slightly more specific than Tier I, as they use national research to generate country-specific emission factors. Finally, Tier III reporting uses process-based models to predict emissions with the greatest accuracy and system-specificity.

To balance model performance and data requirements, Agrecalc makes use of higher Tier methods for large emissions sources. Default methods are used where higher Tier methods would increase data requirements beyond what is generally available on a farm. They are also used for smaller emission sources, and for emission sources where more research is needed to improve the resolution of emission factors.

In the next sections, a description of how IPCC methodology is used in each enterprise is provided.

Beef and dairy

The beef and dairy models are based on IPCC Tier II guidelines. This Tier II calculation includes detailed modelling of the energy requirements of beef and dairy herds based on activity levels, growth rates, life stages, gender, and climate. Enteric methane emissions and emissions from manure deposited on grazing lands are also calculated using IPCC Tier II methods published in the most recent guidance.

Sheep

The sheep model is also based on the latest IPCC Tier II guidelines. This Tier II calculation includes detailed modelling of the energy requirements of sheep flocks based on activity levels, growth rates, life stages, gender, and climate. Like the beef and dairy models, this model estimates emissions from enteric fermentation and manure deposited on grazing lands using IPCC Tier II methodology.

Pigs

The pig model goes beyond IPCC reporting standards, incorporating a detailed energy balance model for pigs published by Food and Agriculture Organisation (FAO). This expansion upon standard methods allows the model to use Tier II reporting standards for pigs, improving the detail of the pig model outputs beyond that provided by standard greenhouse gas reporting for pigs.

Poultry

The poultry model also goes beyond IPCC reporting standards, incorporating a detailed energy balance model for poultry published by FAO. The outputs of this model are in line with IPCC Tier II methods. This expansion upon standard methods allows the model to estimate emissions at a similar level of detail to our beef and dairy models.

Manure Management

The manure management model estimates emissions related to storage and treatment of manures. The model uses Tier II methods to calculate methane emissions from manure, which uses the dietary characteristics of livestock to calculate methane emissions. For liquid storage systems, the model directly interacts with climate data to estimate methane emissions, in line with IPCC Tier II guidelines. The model also calculates nitrous oxide emissions from manures using Tier II methods, which incorporate information about the nitrogen content of livestock diets.

Arable Enterprises and Improved Grassland

The arable farming model is based on IPCC Tier I and Tier II factors for cropland management. This includes Tier II factors for direct nitrous oxide emissions from organic and inorganic fertilisers, derived from the UK National Inventory. Tier I emission factors are utilised for indirect nitrous oxide emissions related to volatisation and leaching. Nitrous oxide emissions from crop residues are also calculated using IPCC Tier I methods.

Embedded Resource Emissions

The carbon footprint model draws on various external databases to estimate emissions from imported feed, fuel, electricity, and fertiliser inputs. For emissions related to energy use, the tool uses figures published by DESNZ. For the embedded emissions of fertilisers, the tool uses Fertilisers Europe (2018) database. For emissions related to imported feed rations, the tool uses the Global Feed LCA Institute (GFLI) database.