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What’s your carbon footprint? Jerry Alford looks at the thorny issue of greenhouse gas emissions from livestock and ways that you can reduce your carbon footprint There has been a lot of comment recently about greenhouse gases and carbon footprints, and in particular the poor performance of ruminants. There is no doubt that this is a one-sided view of something more complex, and we need to acknowledge the ability of cattle and sheep to produce highquality products we can utilise, such as Three GHGs in particular result from livestock in the UK: carbon dioxide, methane and nitrous oxide. Carbon footprint calculations can be complex, so a figure of carbon dioxide equivalence is used (CO2e per unit). All of the inputs and outputs have to be included, including transport and production of inputs, such as fertilisers. It’s also important to note Strategies to reduce the carbon footprint of ruminants are tricky as we often end up substituting one gas for another the units being compared, for example kg CO2 per tonne or hectare, per kg wool, milk and meat, from a low-quality food source that we cannot – grass. Livestock also play an important role in maintaining biodiversity and provide an essential conservation tool for habitat management. Although there are many more sources of emissions than farming, it is an area that we need to be aware of and, where possible, take action. Greenhouse gases A carbon footprint is defined as the total greenhouse gas (GHG) emissions caused by an individual, event or product. These greenhouse gases are associated with global warming. Greenhouse gases differ in their global warming capacity – for example, methane is a far more potent GHG than carbon dioxide – so each gas is given a greenhouse gas potential value. live weight gain or per head. Therefore, standard protocols are being developed for more accurate comparisons. Additionally, we need to be aware that other gases are involved, for example, ammonia is not a GHG, but is a source of nitrous oxide. Carbon sequestration This term is commonly used, and is often seen as a benefit of grazing cattle. It refers to the build-up of carbon in the soil. It can be influenced by the system but it also depends on the starting point. Arable land converted to long-term grass will sequester organic matter, but long-term grazing may already be at a point where losses are balanced by inputs. Biologically, carbon can be added to the soil in two ways. Firstly, through physically adding materials such as straw, composts, roots and uneaten or trampled vegetation. Secondly, through photosynthesis, in particular carbohydrate exudates from plants roots that feed soil life. Soil bacteria and fungi release carbon dioxide and nitrous oxide, so there is always a process of uptake and release. Tackling your carbon footprint  Ruminant emissions Livestock contribute to greenhouse gas emissions in two ways, through rumen fermentation and their manure. We cannot stop ruminants producing methane, and although there is some research (including an Innovative Farmers trial on biochar), there is currently little evidence that anything we feed can do so. Losses from manure can be offset when it is applied to grass, as it contributes to increased grassland yield and improved storage of soil carbon (see below). Strategies to reduce the carbon footprint of ruminants are tricky as we often end up substituting one gas for another. For example, adding cereals to the diet reduces methane production, but increases nitrous oxide and carbon dioxide emissions, because of the fertiliser and cultivations involved in cereal production, while silage and hay-making are sources of emissions too.  Manure is a source of methane, particularly from stores where methane is released due to anaerobic digestion. Dung 44 Organic Farming Spring 2019
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LIVESTOCK  UK greenhouse gas emissions from agriculture source: Defra 2017 UK GREENHOUSE GAS EMISSIONS GHG Source Greenhouse gas potential % UK GHG % of agricultural production Carbon dioxide (CO2) Emitted when carbonbased materials, such oil, wood or straw, are burnt or decay 1 81 Methane (CH4) Fermentative digestion from ruminants, released from manure and slurry 25 11 12 56 Nitrous oxide (N2O) Microbial activity in soils, such as denitrification and losses from N-fertiliser 310 4 31 By trampling grass back into the ground, this mob-grazed herd helps to build up soil organic matter S H U T T E R S T O C K and urine in the field release less methane because the breakdown is aerobic and the nitrates and methane are taken up by soil bacteria and recycled in readily available forms. Where manures are spread, they are better applied to growing crops. Care needs to be taken with slurries as they contain more volatile nutrients. Shallow injection will reduce emissions by half, while recent government rules on ammonia emissions recommend trailing shoe or dribble bar applications to reduce wastage of resources. Extending grazing, where weather permits, helps to reduce manure emissions.  Soil organic matter The best way to reduce a farm’s carbon footprint is to increase soil organic matter. Organic arable rotations have been shown to increase SOM by using grass leys, animal manures and covercrops (Scientific Reports volume 9, article number:1702) and similar practices can be applied to livestock systems. Rothamsted research has shown that animal performance on individual fields was positively associated with soil organic carbon levels. Grasses take in carbon dioxide and convert it to sugars that promote growth. Since 80 per cent of a plant’s dry matter lies below ground, bigger plants mean more roots. Cutting or grazing the grass results in roots decaying, which leads to an increase in SOM. This will be broken down over time by soil bacteria and fungi, releasing carbon dioxide. The process is continuous, with short-term carbon sequestration followed by longer-term release. However, overgrazing leads to reduced rooting because there is no chance for reserves to build up. The same occurs if the grass is regularly mown. Rotational and mob grazing control the availability of grass and prevent overgrazing, and in the case of mob grazing, trample grass into the soil.  Nitrogen-fixing bacteria in the soil and in the root nodules of legumes take atmospheric nitrogen and convert it to a form that can be used by plants. In a trial of conventionally raised cattle at North Wyke, there were lower GHG emissions/kg live weight gain on a white-clover-based pasture, due mostly to reduced nitrogen use. It’s likely that similar results would be seen on organic systems (Journal of Cleaner Production, volume 171, 10 January 2018, p1672–1680 https://bit.ly/2HZLfM2). Legume silages are also beneficial because they have lower fibre levels and fewer ruminant gases are produced during their digestion. The use of diverse leys, particularly those with big deep-rooting species, will also benefit soil organic matter as well as harvest nutrients from deeper in the soil, but they may require different grazing management systems.  Overwintering cover forage crops These crops not only protect the soil but also keep nutrients in the rooting zone, preventing them from being leached while increasing organic matter. Keeping cultivations to a minimum and as shallow as possible is beneficial. Direct drilling reseeds and stitching clover into existing underperforming pastures can also be a carbon-friendly way of improving field performance.  Agroforestry Silvopasture and hedges help to sequester carbon, partly through the provision of shelter from weather and the supply of tree forage, which leads to higher productivity and a lower carbon footprint and also through their extensive root systems that harvest nutrients from deep in the soil. Jerry Alford is an Arable and Soils Advisor at the Soil Association. He can be contacted on 07917 671738 or at jalford@soilassociation.org FIND OUT MORE  For more information and to measure your own carbon footprint go to https:// www.farmcarbontoolkit.org.uk A heap of cattle dung is a source of methane 45 Organic Farming Spring 2019

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What’s your carbon footprint?

Jerry Alford looks at the thorny issue of greenhouse gas emissions from livestock and ways that you can reduce your carbon footprint

There has been a lot of comment recently about greenhouse gases and carbon footprints, and in particular the poor performance of ruminants. There is no doubt that this is a one-sided view of something more complex, and we need to acknowledge the ability of cattle and sheep to produce highquality products we can utilise, such as

Three GHGs in particular result from livestock in the UK: carbon dioxide, methane and nitrous oxide. Carbon footprint calculations can be complex, so a figure of carbon dioxide equivalence is used (CO2e per unit). All of the inputs and outputs have to be included, including transport and production of inputs, such as fertilisers. It’s also important to note

Strategies to reduce the carbon footprint of ruminants are tricky as we often end up substituting one gas for another the units being compared, for example kg CO2 per tonne or hectare, per kg wool, milk and meat, from a low-quality food source that we cannot – grass. Livestock also play an important role in maintaining biodiversity and provide an essential conservation tool for habitat management. Although there are many more sources of emissions than farming, it is an area that we need to be aware of and, where possible, take action.

Greenhouse gases A carbon footprint is defined as the total greenhouse gas (GHG) emissions caused by an individual, event or product. These greenhouse gases are associated with global warming. Greenhouse gases differ in their global warming capacity – for example, methane is a far more potent GHG than carbon dioxide – so each gas is given a greenhouse gas potential value.

live weight gain or per head. Therefore, standard protocols are being developed for more accurate comparisons. Additionally, we need to be aware that other gases are involved, for example, ammonia is not a GHG, but is a source of nitrous oxide.

Carbon sequestration This term is commonly used, and is often seen as a benefit of grazing cattle. It refers to the build-up of carbon in the soil. It can be influenced by the system but it also depends on the starting point. Arable land converted to long-term grass will sequester organic matter, but long-term grazing may already be at a point where losses are balanced by inputs.

Biologically, carbon can be added to the soil in two ways. Firstly, through physically adding materials such as straw, composts,

roots and uneaten or trampled vegetation. Secondly, through photosynthesis, in particular carbohydrate exudates from plants roots that feed soil life. Soil bacteria and fungi release carbon dioxide and nitrous oxide, so there is always a process of uptake and release.

Tackling your carbon footprint  Ruminant emissions Livestock contribute to greenhouse gas emissions in two ways, through rumen fermentation and their manure. We cannot stop ruminants producing methane, and although there is some research (including an Innovative Farmers trial on biochar), there is currently little evidence that anything we feed can do so. Losses from manure can be offset when it is applied to grass, as it contributes to increased grassland yield and improved storage of soil carbon (see below). Strategies to reduce the carbon footprint of ruminants are tricky as we often end up substituting one gas for another. For example, adding cereals to the diet reduces methane production, but increases nitrous oxide and carbon dioxide emissions, because of the fertiliser and cultivations involved in cereal production, while silage and hay-making are sources of emissions too.  Manure is a source of methane, particularly from stores where methane is released due to anaerobic digestion. Dung

44 Organic Farming Spring 2019

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