Livestock are important sources of methane. The United States Environmental Protection Agency calculated that livestock, especially ruminants such as cattle and sheep, account for approximately one-third of global anthropogenic emissions of methane (US-EPA, 2006). The methane is produced primarily through the process of enteric fermentation and released through the process of eructation (Crutzen, 1995). In addition, N2O emissions are generated by livestock through secretion of nitrogen through the urine and faeces.
The processes to reduce the CH4 and N2O emissions from livestock fall into three general categories:
1) Improved feeding practices
2) The use of specific agents or dietary additives
and 3) longer management changes and animal breeding
The technical mitigation potential per region for all management practices (i.e. restoration of degraded lands; cropland management; grazing land management/pasture management; management of agricultural lands) and all affected GHGs is shown in Figure 1. Clearly, mitigation through the different management practices can substantially contribute to global climate change mitigation efforts.
Ruminant animals have a unique digestive system. Ruminants possess a rumen, or large fore-stomach, in which microbial fermentation breaks down coarse plant material for digestion. Nonruminant domesticated animals (e.g., swine, horses, mules) also produce CH4 emissions through enteric fermentation, although this microbial fermentation occurs in the large intestine, where the capacity to produce CH4 is lower (USEPA, 2005b). Enteric fermentation enables ruminants to eat plant materials, but also produces methane, a potent greenhouse gas that contributes to global climate change (Pew Center on Global Climate Change (PCGCC), no date). During digestion, microbes present in an animal’s digestive system ferment food consumed by the animal. This microbial fermentation process is referred to as enteric fermentation and produces CH4 as a by-product, which can be exhaled or eructated by the animal. The amount of CH4 produced and excreted by an animal depends primarily on the animal’s digestive system and the amount and type of feed it consumes.
As can be seen in Figure 2, enteric fermentation is a large source of methane emissions. The Figure shows the methane emissions in the United States over time. Within the U.S., one of the largest methane emission sources is the ruminant digestion process, enteric fermentation. In fact, with an emission of approximately 140. 8 Tg CO2eq in 2008, enteric fermentation accounts for about 2 % of the total U.S. emissions in 2008 (EPA, 2010)
In Figure 3, the enteric fermentation emissions within the U.S. are displayed per livestock type. In the U.S., beef cattle are by far the largest contributor of methane emissions, accounting for 72 percent in 2008. Dairy cattle accounted for 23 % in 2008, with the remainder of emissions arising from horses, sheep, swine and goats (EPA, 2010)
Clearly, livestock are important sources of methane. In fact, the United States Environmental Protection Agency calculated that livestock, especially ruminants such as cattle and sheep, account for approximately one-third of global anthropogenic emissions of methane (US-EPA, 2006). Enteric CH4 emissions from livestock are estimated to be the second largest source of global agricultural non-CO2 (IPCC, 2007).In addition, N2O emissions are generated by livestock through secretion of nitrogen through the urine and faeces.
As such, it is important to address methane emissions from livestock. The processes to reduce the CH4 and N2O emissions from livestock fall into three general categories:
1) Improved feeding practices
2) The use of specific agents or dietary additives
and 3) longer management changes and animal breeding
Improved feeding practices
An animal’s feed quality and feed intake affect CH4 emissions. In general, lower feed quality or higher feed intake lead to higher CH4 emissions. Feed intake is positively related to animal size, growth rate, and production (e.g., milk production, wool growth, pregnancy, or work). Therefore, feed intake varies among animal types, as well as among different management practices for individual animal types.
Because CH4 emissions represent an economic loss to the farmer—where feed is converted to CH4 rather than to product output—viable mitigation options can entail feed efficiency improvements to reduce CH4 emissions per unit of beef or milk (IPCC, 2007). However, these mitigation options can actually increase CH4 per animal.
Through replacing forages with the feeding of more concentrates, methane emissions can be reduced (Lovett et al., 2003). While the concentrates may increase the daily methane emissions per animal, the emissions per kg of feed intake and per kg of product are reduced. The extent of reduced emissions per kg of feed intake or per kg of product decreases as production increases (IPCC, 2007). Feeding concentrates' benefits depend on whether the number of animals can be reduced or whether slaughter age can be reduced. In addition, it is important to consider how the practice affects land use, the nitrogen content in the manure and the emissions from transporting and producing the concentrates in the first place (Lovett el al., 2006; IPCC, 2007).
Other practices to reduce methane emissions are available. Adding certain oils or oilseeds to the diet, improving pasture quality and optimizing protein intake to reduce nitrogen excretion and N2O emissions are examples (IPCC, 2007).
Video 1 shows an experiment with the aim of reducing methane emissions from dairy cows.
Specific agents and dietary additives
Adding agents and/or dietary additives can also reduce methane emissions. Most of these agents or additives aim at suppressing methanogenesis which is the chemical process that creates methane. A wide range of these agents or additives have been proposed to reduce methane emissions:
(This information is taken from the IPCC 2007 report)
a) Ionophores are antibiotics that can reduce methane emissions, but their effect may be transitory and they have been banned in the European Union.
b) Halogenated compounds inhibit the growth of methanogenic bacteria (the bacteria that produce the methane), but their effects can also be transitory and they can have side-effects such as reduced intake.
c) Adding plant compounds can also reduce methane emissions. Examples of such plant compounds are condensed tannins, saponins, or essential oils. However, adding such compounds may have the negative side-effect of reduced digestibility of the diet.
d) Probiotics have shown to result in small and insignificant effects. However, selecting strains specifically for methane reducing ability might be able to improve results.
e) Propionate precursurs such as fumarate or malate reduce methane formation. These precursors reduce methane by acting as alternative hydrogen acceptors. However, this option is expensive due to the requirement for high doses of precursors to elicit effect.
f) Hormonal growth implants do not specifically reduce methane emissions in itself, but by improving animal performance, they can reduce emissions per kg of animal product.
g) Vaccines against methanogenic bacteria are being developed but are not yet available commercially.
Longer-term management changes and animal breeding
See also: Genetically modified rumen bacteria  
Productivity increases through better management and breeding practices often reduces methane emissions per kg of animal product (Boadi et al, 2004). However, directly selecting cattle for reduces methane production is still impractical due to difficulties in accurately measuring methane emissions (IPCC, 2007).
Through improved efficiency, meat-producing animals reach slaugther weight at a younger age. Therefore, lifetime emissions are reduced (Lovett and O'Mara, 2002). However, emissions over the whole system may not always decrease as the result of such practices. For example, intensive selection for higher yield in dairy cattle may reduce fertility. The reduced fertility requires more replacement heifers in the herd which increases whole system emissions (Lovett et al., 2006).
Figure 4 summarizes the techniques available for methane mitigation in livestock management.
The technical mitigation potential per region for all management practices (i.e. restoration of degraded lands; cropland management; grazing land management/pasture management; management of agricultural lands) and all affected GHGs is shown in Figure 5. Clearly, mitigation through the different management practices can substantially contribute to global climate change mitigation efforts.
Figure 6 shows the mitigation potential divided over the different management practices. While the Figure shows that livestock management practices is not the management practice with the highest mitigation potential, it does show that there is still considerable mitigation potential.
As can be seen in Figure 7, the socio-economic development and environmental protection contributions are as of yet not clearly quantified. The IPCC (2007) shows that there is a economic benefit of improved feeding practices, as effiency is increased in the livestock management system. However, the other aspects are uncertain and require more investigation.
In addition to the additional investigation required into the social development and environmental protection consequences, there is also the need for additional investigation into health consequences for both the livestock as well as the health consequences related to the consumption of the livestock products.
The role of alternative strategies changes across the range of prices for carbon as can be seen from Figure 8. At low prices, dominant strategies are those consistent with existing production such as changes in tillage, fertilizer application, livestock diet formulation, and manure management. Higher prices elicit land-use changes that displace existing production, such as biofuels, and allow for use of costly animal feed-based mitigation options. A practice effective in reducing emissions at one site may be less effective or even counterproductive elsewhere. Consequently, there is no universally applicable list of mitigation practices; practices need to be evaluated for individual agricultural systems based on climate, edaphic, social setting, and historical patterns of land use and management.
While manure management from livestock is an actively used sector for CDM projects, the reduction of methane emissions from livestock management itself is currently not incorporated into the CDM.
For more information on the CDM baseline establishment and latest news see: http://cdm.unfccc.int/ 
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- Mitigation of greenhouse gas emissions in livestock production - A review of technical options for non-CO 2 emissions  (pdf). Food and Agriculture Organisation of the United Nations (FAO), 2013.