Counting carbon: a fresh angle on farm efficiency, and a future currency.

What is a “carbon footprint”?

A carbon footprint is the total weight of greenhouse gas emissions emitted over the life cycle of a product or service, expressed as kg carbon dioxide (CO2) equivalent (hence “carbon”), representing the contribution of that product or service to climate change. It is based on the compilation of all inputs and outputs from a production system throughout its life cycle (ISO, 14040). Carbon footprints may be undertaken according to a British Standard methodology (PAS 2050). For livestock farms, the main sources of carbon are in fact enteric methane (CH4) emissions from ruminant animals and nitrous oxide (N2O) emissions from soils. Considering a 100 year timeframe, and based on the length of time gases circulate in the atmosphere and their physical properties, one kg of CH4 will have a warming effect equivalent to 25 kg of CO2, and one kg of N2O will have a warming effect equivalent to 298 kg of CO2. Undertaking a farm footprint involves four main steps:

  1. Defining the boundary of the farm system (field areas, herd(s)/flock(s), inputs and outputs).

  2. Quantifying activity on that farm, in terms of e.g. animal numbers, annual fertiliser, diesel and electricity inputs (based on farm records).

  3. Multiplying on farm activities by relevant emission factors for CO2, CH4 and N2O, and multiplying inputs by embodied carbon factors.

  4. Multiplying emissions of CO2, CH4 and N2O by 1, 25 and 298, respectively, to calculate the total farm footprint as kg CO2e per year.

The farm footprint may then be divided by the total quantity of product(s) exported from the farm (e.g. litres of milk, kg of beef per year) to derived product footprints expressed as e.g. kg CO2e per litre milk. These footprints can be benchmarked across farms and production systems to identify relative carbon efficiencies. The figure here displays the main factors contributing to the carbon footprint of milk production in an intensive UK dairy farm; beef production has a similar profile.

Source: Styles et al. (2015). GCB Bioenergy, 7, 1034–1049.

How can a carbon footprint be reduced?

As can be seen in the above figure, enteric CH4 is the largest source of greenhouse gas emissions in milk production, and also represents a loss of over 6% of the energy consumed by cattle. Thus, improving the digestibility of feed via controlled rotational grazing, careful silage production and storage, and precise feed rationing can reduce the amount of dietary energy lost as CH4 and improve productivity and financial performance whilst reducing the carbon footprint. In general, increasing animal productivity improves feed conversion efficiency and reduces the footprint per unit of production because a higher share of energy intake goes towards production (milk or weight gain) versus maintaining basic bodily functions within the animal. Maintaining high levels of animal health is thus beneficial for footprints. However, when changing animal diets to reduce enteric CH4, care must be taken not increase upstream emissions during feed production, especially by ploughing up grassland to plant highly digestible grain crops – which can release a large amount of CO2 from soils. This is one of the possible trade-offs between carbon efficiency at the animal and farm or landscape level that we are considering the CLEANER COWS research cluster involving Bangor, Aberystwyth and Cardiff Universities.

Other important measures to reduce the carbon footprint include covered storage of slurries to reduce CH4 and ammonia losses, which also means that slurries contain more nitrogen (N) when applied to land, therefore reducing fertiliser requirements. Covered slurry tanks also prevent rainwater ingress and reduce storage capacity requirements and spreading costs. Appropriate timing and type of slurry application (e.g. early spring application with trailing shoe or injection) can significantly reduce emissions and reduce fertiliser requirements. Convenient software such as MANNER NPK can help farmers to optimise management of organic nutrients. Precise rationing of animal diets can also reduce the amount of N excreted by animals, improving the overall efficiency of N use in farm systems (which typically lose 80% of N inputs as emissions to air and water!).

Farmers may also play a role in climate change mitigation through diversification, for example using their land and resources to generate renewable energy or provide feedstock for bio-based materials (e.g. grass packaging). Carbon foot-printing can ascertain whether these forms of energy and material production are more climate-friendly than the conventional options they replace.

Free tools!

There are many free-to-use carbon footprint tools available online, including the Cool Farm Tool and the simple benchmarking Altech E-CO2 What If? Tools. Various companies offer more detailed carbon footprint services, at a price. Carbon footprint tools can provide a fresh angle on farm efficiency, highlighting “invisible” resource flows (energy and N losses), and providing a useful metric for benchmarking overall farm efficiency. In the future, as planned GHG reduction targets become tighter, regulations and carbon pricing may lead to carbon becoming a form of currency – becoming familiar with this future currency early will keep farmers one step ahead. However, some caution is required. Farms are complex systems, and carbon footprint calculators inevitably involve simplification. Some tools are more detailed and useful than others. For example, some tools are not sensitive to important management practices that affect GHG emissions. Understanding the methodology behind the tools is particularly important for farm advisors who would like to navigate the plethora of tools out there, and provide robust recommendations to farmers on carbon efficiency.      

Online course

  • How are carbon and other (nutrient, energy and water) footprints calculated?

  • What tools are available to calculate these footprints, and which tools can I rely on?

  • How do I interpret carbon footprint data?

  • Where do most GHG emissions arise, and how can carbon footprints be reduced?

  • Are there trade-offs between carbon efficiency at the animal, farm and landscape level?

A part-time MSc module on Carbon Foot-printing and Life Cycle Assessment will be delivered entirely online by Bangor University from January through to April 2018, drawing on freely available online calculators and the latest research, to provide answers to these questions and many more. 

Dr Dave Styles
Bangor University

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