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Sources of GHG emissions by livestock

The bulk of GHG emissions by livestock originate from four main categories of activities: enteric fermentation, manure management, feed production and energy consumption.

Methane (CH4) emissions from enteric fermentation.
Ruminant animals (cattle, buffalo, sheep and goat) produce methane as part of their digestive process. In their rumen (stomach), microbial fermentation breaks down carbohydrates into simple molecules that can be digested by the animals. Methane is a by-product of this process. Highly fibrous feed rations cause higher CH4 emissions per unit of energy ingested. Non-ruminant species, such as pigs, do also produce methane but amounts are much lower by comparison (enteric fermentation from cattle, buffalo, small ruminants and pig, but not from poultry, is included in this assessment).

Methane (CH4) and nitrous oxide (N2O) emissions from manure management.
Manure contains two chemical components that can lead to greenhouse gas emissions during storage and processing: organic matter that can be converted in methane and nitrogen that leads to nitrous oxide emissions. Methane is released from the anaerobic decomposition of organic material.This occurs mostly when manure is managed in liquid form, such as in deep lagoons or holding tanks. During storage and processing, nitrogen is mostly released in the atmosphere as ammonia (NH3) that can be later transformed into N2O (indirect emissions).

Carbon dioxide (CO2) and nitrous oxide (N2O) emissions from feed production, processing and transport.
Carbon dioxide emissions originate from the expansion of feed crops and pasture into natural habitats, which causes the oxidation of C in soil and vegetation. They also originate from the use of fossil fuel to manufacture fertilizer, and process and transport feed. The emissions of N2O come from the use of fertilizers (organic or synthetic) for feed production and from the direct deposition of manure on pasture or during the management and application of manure on crop fields. Direct or indirect N2O emissions can vary greatly according to temperature and humidity at the time of application and their quantification is thus subject to high uncertainty.

Carbon dioxide (CO2) emissions from energy consumption.
Energy consumption occurs along the entire livestock supply chains producing CO2 emissions. At feed production level, it mostly relates to the production of fertilizers and to the use of machinery for crop management, harvesting, processing and transportation. Energy is also consumed on the animal production site, either directly through mechanized operations, or indirectly for the construction of buildings and of equipment. Finally, processing and transportation of animal commodities involve further energy use.

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