Farm Business Survey 2021-22: Farm level emissions and nitrogen usage: Methodology

Greenhouse gas (GHG) emission estimates methodology

The ‘Farm Business Survey: Farm level emissions and nitrogen usage’ publication reports on the following headline metrics of GHG emissions:

• Absolute gross GHG emissions (tonnes CO2 equivalent per hectare): absolute emissions measures the GHG emissions from a unit area of agriculture land (hectare).
• Emissions intensity (kg CO2 equivalent per kg of output): emissions intensity measures the carbon footprint of a unit of product up to the farm gate. It includes all types of farm product.

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.

Emissions are calculated for the whole farm and per unit (kg) of product. All mainstream agricultural enterprises are included. This includes cattle, sheep, dairy, pigs, poultry, cereals, oilseeds, potatoes, vegetables, and fruits.

Total emissions are presented in carbon dioxide equivalent (CO2-e) 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 25 times stronger than that of carbon dioxide, while nitrous oxide’s GWP is considered to be 298 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 [1] as well as national figures from the UK National Greenhouse Gas Inventory in its calculations. It is PAS2050:2011 certified.

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 Defra for UK energy use and figures published by the Greenhouse Gas Protocol (2012) for energy use in other countries. For the embedded emissions of fertilisers, the tool draws on a study by Fertilisers Europe (2018) which reported emissions related to different fertilisers by region. For emissions related to imported feed rations, the tool currently uses the Dutch Feedprint database and are in the process up updating these values to the Global Feed LCA Institute (GFLI). However, work on migrating to the GFLI database is currently undertaken to allow the users to choose from a greater variety of feed ingredients.