The role of ruminants
28 May 2010
The role of livestock in farming systems has been at the centre of a debate in recent years, with concern about their greenhouse gas (GHS) emissions suggesting we need to reduce the amount of red meat we eat on climate change grounds. Richard Young examines the role of ruminants and argues that the use of grazing animals in mixed farming systems can have a positive impact in GHG emissions.
When cattle domestication moved out from the fertile river valleys where it first occurred and across what is now the Sahara Desert about 8,000 years ago, it was largely in response to climate change caused by changing weather systems as ice sheets retreated to the north. Wet periods with lush plant growth alternated abruptly with arid conditions in cycles sometimes lasting several hundred years. It is believed this caused a decline in wild herds of animals and triggered the move from hunter-gathering to early forms of agriculture.1
Depending on your perspective, it may appear either paradoxical or fitting that cattle and other ruminants are now widely seen as one of the causes of climate change. The issue today, of course, being the significant contribution to greenhouse-gas emissions from the estimated global population of 1.3 billion cattle.
The human–cattle symbiosis made it possible for one person to produce enough food for several people, allowing the division of labour and the development of specialist skills. Cattle transferred fertility from rough grazing to cultivatable land, they provided the power to pull ploughs and carts, and yielded a wide range of valuable products. In some regions sheep or goats played a similar, though less fundamental role. While many of today’s technological societies prosper without such an obvious link with ruminants, every step of human progress from Neolithic times until the advent of artificial nitrogenous fertilisers just a century ago was only possible because of ruminant livestock.
Since the publication of the United Nations’ Livestock’s Long Shadow report in 2006, many campaigners, scientists, policy-makers and politicians have called for cuts in the number of cattle and sheep as a way of helping the UK to meet its emissions targets.2 In the coming decades, agricultural policy will inevitably be driven by concerns about climate change, and because methane is a potent greenhouse gas this poses a potential problem for the further expansion of organic farming. Strangely, the conversion of cattle and sheep farms may be less affected than those producing organic crops. In Northern Europe, most organic food production (vegetables, soft fruit, cereals, and even chicken and pork) depends to a greater or lesser extent on the ability of ruminants to make economic use of grass-clover leys – the principal means of introducing adequate nitrogen for durable organic systems with optimum yields.
As researchers from Reading University have shown, converting Britain to organic farming would involve a significant increase in cattle and sheep, despite the fact that livestock numbers would decline on purely grass farms, due to lower stocking rates.3 There are exceptions: stockless organic farms and horticultural units where legumes are mulched and only compost derived solely from vegetation is used. However, these approaches depend on the viability of a fertility-building phase in the rotation during which the land generates neither income nor food for human consumption. In a future where food security is likely to become an increasing concern, such systems may be less viable economically because they are unlikely to continue benefiting from subsidy schemes, like set-aside, which paid farmers to take land out of production.
The case against ruminants has been compounded by a government-funded analysis at Cranfield University which claims that the carbon footprint of free-range beef and lamb is approximately four times higher than that of intensive chicken meat.4 The principal reason is that poultry produce little methane and grow incredibly quickly. In contrast, a beef animal will typically take two years to achieve slaughter weight and produces 200 litres of methane a day (up to 550 litres for a high-yielding dairy cow).
It is not just the global-warming potential of methane that causes concern; it is the fact that methane is seen as the primary target for agricultural greenhouse gas action. It persists for just 12–15 years in the atmosphere, and a reduction now would bring real benefits within a decade or so. But has adequate consideration been given to all aspects of the case against ruminants?
Globally, the soil contains more carbon than all the world’s forests and the atmosphere put together. However, its transfer from the soil accounts for one-tenth of all the carbon already added to the atmosphere as a result of human activity since 1850.5 The primary cause of this has been the reliance on artificial nitrogen fertilisers. Although higher nitrogen applications can increase organic matter above arable soil baselines in some situations, the ready availability of nitrogen has avoided the need for mixed farming, specifically the use of grass in the rotation, and as a result organic matter levels are very much lower than they would otherwise be.
When grassland is converted to continuous arable cropping, both carbon and nitrogen are lost from the soil. Most arable soils have already lost over 30 tonnes of carbon (110 tonnes of carbon dioxide) per hectare to the atmosphere. Dutch research indicates that the combined losses over one hundred years will be equivalent to 250 tonnes of carbon dioxide per hectare.6 It is widely accepted that peat soils, such as those in the East Anglian fens, are still losing substantial amounts of carbon every year.7 Scientific opinion is divided over the extent to which long-established arable soils continue to lose carbon at the present time, though it seems probable that this largely relates to the length of time since the land was last ploughed out of grass.
Unlike fossil fuels, which cannot be replenished, it is possible not only to stop the current losses from agricultural soils, but also to rebuild their carbon reserves. Organic farming systems sequester atmospheric carbon dioxide through photosynthesis and plant growth and lock a proportion of it up in the soil as humus – the vital and persistent fraction of the organic matter entering the soil every year.
A recent report by the United Nations Food and Agriculture Organisation (UNFAO) concluded: “There is scientific evidence that organic agriculture can sequester more carbon than conventional agricultural practices…”.8 Exactly how much carbon is sequestered under organic systems is of considerable significance to the organic case. The UNFAO report reviewed 11 studies and suggested a figure of between 200–400kg of carbon per hectare per year.
A more in-depth Soil Association report, however, reviewed 39 studies. This found that the average increase in Northern Europe was 28%, which equates to a net annual gain of 560kg of carbon per hectare.9 According to the Centre for Ecology and Hydrology, soil carbon levels will continue to increase on a steadily reducing annual basis for at least 100 years after the conversion of cropland to grassland.10
Intergovernmental Panel on Climate Change (IPCC) scientific advisers have recognised that soil carbon sequestration represents almost 90% of agriculture’s total greenhouse gas mitigation potential. In the UK, the Climate Change Act commits the government to achieving substantial cuts in greenhouse-gas emissions over the coming decades. However, although the Department for Environment, Food and Rural Affairs (Defra) recently recognised the importance of soil carbon sequestration in relation to both mitigation and adaptation in its strategy document, Safeguarding Our Soils: A Strategy for England 11 – and intends to commission research to find ways to reduce carbon losses from agricultural soils by 2020 – it currently has no significant practical recommendations on how this could be achieved, having finally recognised that the long-favoured option of reduced tillage will have little or no overall benefit.12
Since the Second World War, UK agricultural policies have strongly discouraged mixed farming and encouraged specialisation in either intensive livestock production or arable cropping. This was an issue which greatly concerned early Soil Association members during the 1940s.13 For mixed farming systems to reach their optimum productive and carbon sequestration potential it is necessary for approximately half the land to be under grass (and legumes like clover) and half under arable cropping at any one time. However, unless the grassland is also producing food for human consumption such systems are only economical in purely agricultural terms if they produce high-value crops.
Ruminants play an essential role in making such systems possible because only they utilise a high enough proportion of grass and forage legumes to prevent the eventual run-down of fertility in organic systems. Their role in the creation of farmyard manure is also important because it helps to build the carbon-rich humus content of the soil far more than the separate incorporation of slurry and straw. Straw-based bedding systems also produce much lower emissions of greenhouse gases than slurry-based systems.
Yet, despite this all ruminants have acquired such a bad image that the idea of reintroducing them onto the wide open fields of East Anglia in order to increase organic crop production now appears to many people the exact opposite of what we should be advocating.
The UK cattle breeding herd has already fallen by 27% since 1990,14 and it could be that the relatively low level of adoption of organic methods on arable farms in recent years has been partly as a result of the negative publicity surrounding ruminants. More so than any other European country, except perhaps Ireland, the UK is ideally suited for beef production, due to its high rainfall and large areas of hill grazing. Yet it has gone from being a net beef exporter 20 years ago to importing 300,000 tonnes (20%) of beef today. Cutting cattle numbers in the UK still further would help to meet UK targets for reducing methane emissions, but overall it would increase emissions globally and have other negative consequences. Imports would increase further, often coming from regions where cattle have a much higher carbon footprint, such parts of South America, where rain forest is being cleared for cattle ranching at a rate of several thousand hectares a day.15 Fields left vacant would be ploughed for cropping where suitable, leading to substantial emissions of carbon dioxide and nitrous oxide. Even planting trees can produce net carbon losses on soils with a significant peat content.
During the last half century, of course, an ever-increasing proportion of global cattle production has not been an integrated part of sustainable farming systems. It has changed from being in harmony with nature to being antagonistic to it. Most particularly, large numbers of cattle are now fed on grain which could be used directly by humans, instead of grass which only they and other ruminants can utilise. This is illustrated by contrasting the traditional cattle-rearing systems still widely used in many parts of the UK which depend on grazing and in some cases use no grain at all, with the factory farming beef systems advocated for many years by the Ministry of Agriculture, Fisheries and Food and the feedlots in countries where thousands of cattle can be kept together and fed on cereals and maize instead of forage.
But could we justify the return of cattle or sheep to large areas of Britain’s most productive cropland in climate change terms alone, even without the import substitution argument? Let’s consider a theoretical 500ha arable farm where no cattle or sheep have grazed since the 1940s. To convert this to an organic system all the land would need to go through a fertility-building phase, but after a few years the simplest rotation would see half the land growing crops for human consumption, and half growing grass and clover. There would, of course, be capital costs associated with this. Fences would be needed, ideally some hedges planted, water laid on to all fields for drinking and, in many cases, buildings erected or modified for livestock. In some regions, it might even require new infrastructure such as abattoirs.
We know from published research how much methane is produced annually from different classes of livestock and how much is released from straw-bedding and slurry-based manure systems.16 From the examples of existing organic farms, we can say that typical ruminant stocking densities will be no more than 0.5 livestock units (LU) per hectare over the whole farm, once through an organic conversion.
Taking the 560kg carbon per hectare per year average figure from the Soil Association’s Soil Carbon and Organic Farming report we can undertake a calculation. One tonne of carbon is equivalent to 3.66 tonnes of carbon dioxide. An adult beef animal produces 48kg of methane per year from enteric fermentation and 2.74kg from manure.17 The accepted IPCC conversion factor for contrasting the global warming potential of methane with that of carbon dioxide over a century is 21. To take account of the ongoing nature of ruminant emissions and recent research indicating that the warming potential of methane has been underestimated this figure needs to be increased, possibly to as much as 90.18
On this basis, the sequestered soil carbon from the conversion to organic is equivalent to 0.56 (560kg per ha) x 3.66 = 2.05 tonnes of carbon dioxide per hectare each year.
The methane emissions from introducing ruminants, however, are equivalent to 0.5 (stocking density) x 90 (estimated conversion factor) x 52.74 (annual methane emissions per adult beef animal) = 2.37 tonnes of carbon dioxide per hectare.
On the face of it the methane emissions are slightly greater (2.37 - 2.05 tonnes = 320kg) than the carbon sequestered. However, on organic farms there is no need for nitrogen fertiliser. In contrast, we know that the average use on arable farms is currently 147kg nitrogen per hectare per year. During the production of each tonne of nitrogen from EU factories, 6.7 tonnes of greenhouse gases (in CO2 equivalent) are released into the atmosphere. If this is included in the comparison there is an additional benefit for the ruminant-based system of 0.99 tonnes CO2 per hectare per year, giving a net advantage of 660kg of CO2 per hectare per year across the whole farm. Over recent years approximately half of all nitrogen fertiliser has been imported from countries where fertiliser factories are up to four times less efficient in energy usage, suggesting that in reality the organic advantage is even greater.
But what about the Cranfield research which indicates that intensive poultry production is responsible for much lower greenhouse gas emissions than organic beef or sheep production? A detailed analysis of this question is outside the scope of this article, but it should be noted that the researchers made no allowance for soil carbon loss in the arable fields that supply them and no allowance for the greenhouse gas emissions associated with imported feeds like soya.
There are two key weaknesses to the organic case. The first is that while the calculation is very favourable on a land area basis, the often lower yields on organic farms would reduce this when comparing on a tonnage basis. The second is that over time the carbon sequestration rates will gradually decline, while the methane emissions will continue indefinitely.
However, methane is part of the carbon cycle and ruminants on organic farms do not increase the total quantity. It has only become a problem because concentrations have risen for largely other reasons, principally our habit of sending kitchen vegetable waste to landfill sites which accounts for 41% of UK emissions, compared with 38% from ruminants. There are also a number of significant factors which cannot currently be quantified accurately. The 560kg sequestration figure relates only to the top 18cm of soil (the limit of most studies) yet a few studies which have looked at carbon levels in organically farmed soils down to 30cm have found additional carbon gains. Arable cropping, on which intensive pig and poultry production depends, is also the main cause of soil erosion in the UK. Each year, over two million tonnes of topsoil is lost from farms at an estimated cost of £45 million.19 Nitrogen fertiliser application leads to emissions of ammonia, which contributes to acid rain and is also the biggest cause of nitrous oxide emissions – the most potent of all greenhouse gases.
More so than with any other species, the development of humanity is bound up with our relationship with ruminants – and especially cattle. A careful look at how food systems function suggests to me that even the cities in which the majority of the world’s population now live and the benefits of most cultures could never have been created or sustained without them. There can be no doubt that, somewhere in the recent past, we have gone wrong. Overall, cattle now contribute to global warming and in many situations do more harm than good. In part, that is simply a result of the enormous growth in the human population, but it is also because agriculturalists have crossed natural boundaries purely in the quest for increased profits. The most obvious manifestation of this has been the bovine spongiform encephalopathy (BSE) crisis caused by the use of animal protein in the diets of herbivorous animals. Yet the simple substitution of grain for grass may prove to have far more serious consequences. As we search for a new and more sustainable way forward we would do well to remember what we owe to cattle and other ruminants, and recognise that even now they probably still hold the key to our future success.
Richard Young is Soil Association policy adviser. He is author of numerous reports, including a series of reports on The Use and Misuse of Antibiotics in UK Agriculture.
1 Hassan F A (1999) ‘Climate and cattle in North Africa: a first approximation’ in The Origins and Development of African Livestock, eds. Blench R.M. and Macdonald K.C., pp 61-86.
2 Steinfeld, H., Gerber, P., Wassenaar, T., Castel, V., Rosales, M. and de Haan, C. (2006) Livestock’s Long Shadow, Food and Agriculture Organization of the United Nations.
3 Jones, P and Crane, R. (2009) England and Wales Under Organic Agriculture: How much food could we produce?, Centre for Agricultural Strategy, Reading.
4 Williams, A.G., Audsley, E. and Sandars, D.L. (2006) ‘Determining the environmental burdens and resource use in the production of agricultural and horticultural commodities,’ Main Report, Defra Research Project IS0205. Bedford: Cranfield University and Defra.
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6 Vellinga, Th. V., van den Polo-van Dasselaar, A. and Kuikman, P. J. (2004) ‘The impacts of grassland ploughing on CO2 and N2O emissions in the Netherlands’. Nutrient Cycling in Agroecosystems vol 70, pp. 33-45. Kluwer Academic Publishers.
7 Williams, A. G., (2007) Draft data from ‘Developing and delivering environmental life cycle assessment of agricultural systems’. Defra Project IS022.
8 Müller-Lindenlauf, M., (2009) Organic Agriculture and Carbon Sequestration. Food and Agriculture Organization of the United Nations, Rome.
9 Azeez, G. (2009) Soil Carbon and Organic Farming: A review of the evidence for agriculture’s potential to combat climate change. Soil Association, Bristol. http://www.soilassociation.org/climate.aspx
10 CEH (2009) Inventory and projections of UK emissions by sources and removal by sinks due to land use change and forestry. Defra Contract GA01088
11 Defra (2009) Safeguarding Our Soils: a strategy for England. Defra www.defra.gov.uk/environment/quality/land/soil/
12 Defra (2009) Soil Strategy for England: Supporting evidence. Defra http://www.defra.gov.uk/environment/quality/land/soil/documents/evidence-paper.pdf
13 Conford, P. (2001) The Origins of the Organic Movement, Floris Printers.
14 Eblex (2009) In the Balance? The future of the English beef industry. Eblex report, Milton Keynes, UK.
15 Greenpeace International (2009) ‘Slaughtering the Amazon’, http://www.greenpeace.org/international/en/publications/reports/slaughtering-the-amazon/
16 CEH (2008), ‘Annexes of the UK Greenhouse Gas Inventory, 1990 to 2007’, Centre for Ecology and Hydrology, Edinburgh http://www.airquality.co.uk/reports/cat07/0905131425_ukghgi-90-07_Annexes_Issue2_UNFCCC_Final.pdf Accessed 12 September 2009 p 374.
17 CEH, (2008) ‘Annexes of the UK Greenhouse Gas Inventory, 1990 to 2007’, Centre for Ecology and Hydrology, Edinburgh.
18 Young, R., (2009) The Role of Livestock in Sustainable Food Systems. Soil Association, Bristol. note 19 HYPERLINK http://www.soilassociation.org/LinkClick.aspx?fileticket=qm0ueyxHQjI%3d&tabid=313" www.soilassociation.org/LinkClick.aspx?fileticket=qm0ueyxHQjI%3d&tabid=313
19 Mortimer, N.D., Cormack, P., Elsayed, M.A. and Horne, R.E. (2003) Evaluation of the comparative energy, global warming and socio-economic costs and benefits of biodiesel. Sheffield Hallam University, p28