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Livestock and greenhouse gases
Global warming or global climate change have recently become a major concern. There is increasing evidence that global temperatures are rising (0.3°C to 0.6°C over the last century). This is caused by so-called greenhouse-gases trapping radiant heat reflected from the Earth's surface before it is released into space. Global warming has a double effect on agriculture. Agriculture, including livestock, contributes to global warming and, in turn, it is directly affected by the resulting changing climatic patterns. This specific livestock-environment interaction offers, however, considerable scope for mitigating adverse effects on the environment with improved technology, and the development of win-win scenarios. The challenge is to identify these win-win scenarios and to develop corresponding sets of action.
The greenhouse gases include carbon dioxide (CO2), methane (CH4), ozone (O3), nitrous oxide (N2O) and other trace gases (Bouwman, 1995). Their potential to global warming is greatly different, as shown in Table 5.7, which shows that methane is about 20 times more aggressive and nitrous oxide even 300 times more damaging that carbon dioxide.
Carbon dioxide emissions. Burning biomass is the main agricultural source of CO2 emission. Destruction of global forests produces about 1 to 2 billion tons of CO2 per year. Burning of savanna vegetation, sometimes initiated by traditional herders to get high quality new grass shoots during the dry season, but also practised by hunters and croppers to clear the land or chase the game, is another important contribution to CO2 emissions.. Although exact estimates are lacking, one estimate (Menault, 1993) puts the annual emission of the savannas at 18 percent of the global agricultural emissions of CO2. Of this, the African continent would produce 43 percent whereas Asia and South America combined produce 39 percent. However, this release of CO2 is usually reabsorbed the next season by the new growth . In addition, increased atmospheric CO2 levels (from 250 to 350 ppm over the last 150 years), mainly as a result of the industrial and automotive revolution, present a specific environmental challenge in that they favour especially the so-called C3 herbs and trees over grasses (Mayeux, 1993). These contribute to bush encroachment and reduce biodiversity in the grassland areas. The increased CO2 level will probably contribute to changes in the structure of the livestock industry. Eckert et al., (1995) estimate that the shift from C4 to C3 plants would result, in the USA, in a shift of livestock production to the north because rangelands in the southern states would become less productive. Mixed species grazing can, however, slow down or possibly halt this shift in vegetation.
|Table 5.7 Global warming potential (GWP) and other properties of CO2, CH4, and N2O.|
|Gas||Concentration||Annual increase||Lifetime (years)||Relative absorption capacity *||GWP|
|CO2||355 ppmv||1.8 ppmv||120||1||1|
|CH4||1.72 ppmv||10-13 ppbv||12-17||58||24.5q**|
|N2O||310 ppbv||0.8 ppbv||120||206||320|
|ppmv = parts per
million by volume ppbv = parts per billion by volume
* per unit mass change from present concentrations, relative to CO2
GWP Global Warming Potential following addition of 1kg of each gas, relative to CO2 for a 100 year time horizon
** Including the direct effect of CH4 and indirect effects due to the production of tropospheric ozone and stratospheric water vapour.
|Source: Bouwman, 1995.|
Methane emissions. The global average methane concentration is about 1.7 ppmv and is increasing at about 0.8 percent per year (World Bank, 1992). It is a result of human activities such as animal production and manure management, rice cultivation, production and distribution of oil and gas (pipelines) and coal mining and landfills. As shown, livestock and manure management contribute about 16 percent of total annual production of 550 million tons. Increasing methane emissions affect human and ecological health and, because methane has a high capacity to absorb infrared radiation, it is an aggressive greenhouse gas.
|Figure 5.4 Sources of methane emission.|
|Source: USEPA, 1995.|
Methane emission by livestock is a direct result of the capacity of ruminants to utilize large amounts of fibrous grasses, which cannot be used as human food. Ruminant livestock, such as cattle, sheep and goats have a large anaerobic digestion vessel the rumen, which contains a large microbial fauna, which ferment and digest roughages. This digestion results in relatively large methane emissions per unit of feed energy consumed. Pigs and poultry cannot digest these fibrous feeds and have therefore relatively low emissions. Thus, ruminant animals, when fed low quality feed, have higher methane emissions per unit of product than better fed animals, although the latter - as has been shown before - often leave other heavy environmental loads in soil and water pollution.
|Figure 5.5 Methane production by system.|
|Source: USEPA, 1995.|
Methane production is determined by two animal production parameters. First, ruminant animals with low levels of productivity use a large fraction of their feed intake for maintenance and, consequently, the emissions are spread over a relatively small output, resulting in a high level of emission per unit of product. More productive animals emit less methane per unit of product. Second, feed quality has an important impact on the level of methane emissions. Very low quality feeds, such as straw and poor forages of sub-humid savannas, have low levels of digestibility, and therefore higher emissions per unit of feed intake.
Low productivity and poor feed quality are characteristic for most of the land-based production systems in arid regions, and to an even greater degree in the humid tropics and sub-tropics, where emissions per unit of product are therefore comparatively high. Grazing and mixed systems in the tropics and sub-tropics are thus the main contributors to high methane emission levels. Generally, temperate and highland zones have the best quality grazing and other forage resources for ruminants, and therefore lower emission levels. In irrigated areas, fodder is usually of better quality resulting in lower emissions. Annually, livestock produce a total of 86.6 million tons methane of which more than 80 percent (74.5 million tons) comes from digestive fermentation.
|Figure 5.6 Methane production by species.|
|Source: USEPA, 1995.|
Stored liquid manure, produces the remaining 13.1 million tons. Liquid manure management facilities are most commonly used where there is a large concentration of animals at a single facility, such as large dairy or pig farms. Manure that is handled in dry form, spread on fields, dried for fuel, or deposited by grazing animals produces much less methane. Thus, intensive mixed farming and industrial production systems are the main contributors to methane emissions from manure. Because of their large ruminant populations, USA and OECD countries, and the rainfed mixed farming systems and the grazing systems, have the highest methane emissions.
Nitrous oxide emissions. Nitrous oxide is another greenhouse gas contributing to global warming. Total N2O emissions have been estimated by Bouwman (1995) at 13.6 TG N2O per year, which exceeds the stratospheric loss of 10.5 TG N2O per year by an atmospheric increase of 3.1 TG N2O per year. Animal manure contributes about 1.0 TG N2O per year to total emissions. Indirectly, livestock is associated with N2O emissions from grasslands and, through their concentrate feed requirements, with emissions from arable land and N-fertilizer use.
Carbon dioxide. In discussing carbon dioxide a clear distinction should be made between temporary and permanent emissions. Many CO2 emissions related to livestock production are part of a normal ecological cycle, with CO2 being released at the end of a growing season, but immediately recaptured again in the next growing season. The emissions from savanna burning fall into this category. Most temperate grasslands therefore have also a neutral balance. Livestock-induced deforestation in grazing systems, driven by road construction, land speculation and inappropriate incentives (Chapter 2), and fossil fuel use in the industrial system, driven by increased demand (Chapter 4) are thus the main sources of permanent carbon release.
Methane. Methane emissions from livestock have been stagnant, despite a growing livestock population. The reasons for this stagnation are twofold. First, increases in productivity lowers emission levels per animal and per unit of product. Advances in feed resources and nutrition, and breed development are contributing factors. Second, monogastric production is growing at a much faster pace than ruminant production. About 80 percent of the total growth of the livestock sector is attributable to pigs and poultry, which emit comparatively small amounts of methane. The growth in meat production is therefore not accompanied by a proportional rise in methane emissions. While methane emission from digestion processes in the rumen is stable, methane emission from manure management is likely to grow fast as production units grow in size and productivity in many countries. This implies that there will be an increased reliance on confined animal production systems and on liquid-based manure management systems which have higher emissions per head and per unit of product.
Response: Technology and policy options
Technologies. Most technologies that reduce greenhouse gas emission are typical examples of win-win situations. They are being used because they make economic sense and their benefit is an added benefit.
Savanna improvement, as well as forestation, can lead to substantial permanent CO2 sequestration. Recent findings by CIAT scientists (Fisher, et al., 1995) show that, by introducing improved pastures (Andropogon) with a much larger root system in the Colombian savannas can fix up to 50 tons of CO2 per hectare, which is comparable with the 50 to 100 tons per hectare that forest plantations can store. Increased production thus leads also to lower CO2 levels, and provides a win-win situation.
To reduce methane emissions, improved breeding, veterinary care, better feeds and improved feeding technologies are the main weapons, although these technologies are usually not employed for their environmental benefits but for increasing profitability. Improvements in animal productivity have two beneficial effects. They direct a greater proportion of feed energy to the production of useful products (milk, meat, draft power, offspring), and thus reduce methane emissions per unit of product. In addition, they lead to reductions in the herd size required to meet a given production level.
More specifically, improvements in nutrition through mechanical and chemical feed processing, or through strategic supplementation, can drastically reduce methane emissions. Chopping, and alkali and urea treatment of low digestible straws, improve digestibility. When fed to ruminants, these options may decrease methane emissions in the order of 10 to 25 percent in normally poorly fed animals. Strategic supplementation is another option. Critical nutrients, such as nitrogen and essential minerals can be provided, through molasses/urea multi-nutrient blocks and bypass protein techniques (Box 5.9). Substantial increases in animal productivity can be achieved in tropical areas with chronic feed constraints. Here, methane emissions can be reduced by 25 to 75 percent per unit or product. Productivity may also be enhanced through the use of production enhancing agents such as bovine somatrotropin and anabolic steroids, for which emission reductions of 5 to 15 percent have been demonstrated.
|Box 5.9 Reducing methane emissions from digestive fermentation through strategic supplementation in South Asian countries.|
|For efficient digestion, the rumen requires a diet that contains
essential nutrients for the fermentative micro-organisms. Lack of these
nutrients lowers animal productivity and raises methane emissions per unit
of product. For animals on low quality feed, the primary limitation to
efficient digestion is the concentration of ammonia in the rumen. Supplying
ammonia can therefore greatly enhance digestive efficiency and utilisation
of available feed energy. Ammonia can be supplied by urea, chicken manure or
soluble protein that degrades in the rumen. Urea is broken down in the rumen
to form ammonia, and adding urea to the diet has been the most effective
method of boosting rumen ammonia levels.
Urea and other supplemental nutrients are mixed with molasses to make it palatable to livestock. In addition, molasses provides the energy needed to realize the improved microbial growth that can result from enhanced ammonia levels. These Multi-Nutrient Blocks have been used in many countries including India, Pakistan, Indonesia and Bangladesh, Habib et al. (1991), Hendratno et al. (1991) and Leng (1991). Typical results have been: milk yield increases of 20 to 30 percent; growth rate increase of 80 to 200 percent and increased reproductive efficiency. Based on these results, methane emissions per unit product went down by up to 40 percent. Bouwman et al., (1992), estimated that strategic supplementation of dairy animals will reduce methane emissions by 25 percent while increasing milk production by 35 percent
There are promising techniques for recovering methane from manure, thereby not only reducing emissions but providing a source of energy, for example for generating on-farm electricity. Large, confined animal operations make such techniques profitable. The slurry effluent can be used as animal feed, as aquaculture supplements, and as crop fertilizer. In addition, managed anaerobic decomposition reduces the environmental and human health problems often associated with manure management. The controlled bacterial decomposition of the volatile solids in manure reduces the potential for contamination from run-off, significantly reduces pathogen levels, removes most noxious odours and retains the organic nitrogen content of the manure.
The primary approaches for recovering this methane include the following:
|Figure 5.10 Gas recovery from covered lagoons.|
|In California (USA) a 1,400 sow farrow-to-finish pig farm has successfully demonstrated the covered lagoon technology for over 10 years. Royal Farms has a 4,000 m2 primary lagoon covered with industrial fabric. A small gas pump draws the biogas trapped under the cover through a series of perforated pipes. Gas recovery currently ranges from about 1,400 m3/day in winter to about 2,000 m3/day in summer. The gas is used to operate two generators with a combined capacity of 175 kW. The costs of installing the entire system, including the engines, was about US$ 220,000 in the early 1980s. Annual operating costs are about US$ 8,000. Annual energy savings, including electricity and heating, are about US$ 84,000. Overall, the system paid for itself in four years.|
|Source: USEPA, 1995.|
Policies. The identification of specific policies to reduce greenhouse gas emissions are complex. Ideally, they are part of an overall development process driven by the need to improve productivity. Technologies which enhance productivity, improve root biomass for carbon sequestration and nutrition, and for recycling methane for energy, are likely to be the most effective and should be promoted through education and training. While some of these technologies are profitable and improve the environment at the same time, others, such as urea treatment, are not always adopted by farmers because of high input costs, which are often not off-set by increases in productivity alone.
An important issue is how to reward the adoption of techniques which reduce greenhouse gases. If the additional benefit of greenhouse gas reductions could be captured by farmers, adoption rates could be much higher. For example, while valuation mechanisms for CO2 are still in an early stage of development, Brown et al., (1993), estimating the damage from a doubling of atmospheric CO2 in terms of global warming at 1 percent of Gross World Product, arrive at a price of US$ 10 per ton of CO2 trapped. This would mean that the above mentioned improvement of South American savannas with Andropogon could be valued at US$ 500 per hectare. International arrangements are required beyond those of the Global Environment Facility (GEF) to achieve such transfer mechanisms.
A number of donor agencies are implementing projects, where reduction of methane emission is an explicit objective and its degree of achievement has an impact on project approval. Policies may be directed to establish regulations to limit, or ban, lagoon manure storage, or to impose methane recovery. In some countries governments favour the establishment of facilities to recover methane through direct subsidies on the construction of such facilities (California, Denmark). However, the most important determinants for the adoption of these technologies are the price and availability of energy, which largely influence the profitability of these systems. For large-scale facilities, energy requirements for heating, for example, may be high and the high costs of commercial energy may induce investment into methane recovery, whereas for small-scale operations it is often lack of infrastructure and energy supply that make its establishment profitable, as in, for example, remote areas with no electricity. The use of methane as a regenerative source of agriculture is thus determined by the larger picture of fossil energy supply and national policies in that regard. Some OECD countries have started favouring the use of regenerative energy vis-à-vis fossil sources and create corresponding price incentives.
The emission of greenhouse gases is subject to the UN Framework Convention on Climate Change. The agreement, now signed by more than 150 countries, became legally binding in March 1994, and many signatories have voluntarily agreed to stabilize their carbon dioxide emissions at 1990 levels by 2000. The agreement is targeted at the other trace gases with which livestock production can be associated, i.e. methane and nitrous oxide. However, to minimize climate change from greenhouse gases, much more specific international agreements are required and efforts must be made to respect them.
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