One of the most significant sources of methane from agriculture comes from stock manure. The decomposition of animal waste in an anaerobic (oxygen-free) environment produces methane. According to the U.S. Environmental Protection Agency (EPA), manure storage and treatment systems account for about 9 percent of total U.S. emissions and one-third of all methane emissions in the agricultural sector.
Liquid-based manure systems, such as manure ponds, anaerobic lagoons, and holding tanks, account for more than 80 percent of total methane emissions from animal wastes. Solid manure management practices (such as spreading manure across the surface of fields) produces insignificant amounts of gas, but it can lead to increased nutrient runoff, which can have a negative effect on water quality. From 1990 to 1996, emissions from manure management increased by 11 percent as farm animal populations grew and farmers expanded their use of liquid manure management.
An anaerobic digester is a container, like a covered lagoon. Methane produced by digesters, known as biogas, can be captured cost effectively and used as an energy source. Biomass recovery systems trap the gas in covered manure lagoons or other manure digesters, collect it in perforated pipes, and transmit it to an electric generator or boiler.
Farmers can use biogas to produce electricity, heat, hot water, and refrigeration for use on the farm, while at the same time controlling methane emissions and surface/groundwater contamination. Electricity can also be sold to utilities, and the digested solids—a high-quality fertilizer—can be sold to other farmers, home gardeners, or landscape designers.
The federal AgSTAR program, a joint initiative of the EPA, the U.S. Department of Energy (DOE), and the U.S. Department of Agriculture (USDA), teaches farmers how to manage manure profitably and protect the environment at the same time.
To make sure that methane recovery systems are designed, installed, maintained, and operated correctly, the USDA's Natural Resources Conservation Service and the EPA have developed conservation practice standards for methane recovery systems.
Globally, agriculture is responsible for 20 percent of greenhouse gas emissions. In the United States, the national average from agriculture is 8 percent. Agriculture emissions come primarily from methane and nitrous oxide. While these gases exist in smaller quantities in the atmosphere than CO2, they are much more potent, which makes them serious greenhouse gas contenders.
A recent report from the United Nations indicates that livestock are responsible for 18 percent of greenhouse gas emissions worldwide. The
United Nations contends that it is also going to get much worse. As living standards climb in the developing world, the demand for meat and diary products increases as well. To back this claim, they report that annual per capita meat consumption in developing countries doubled from 31 pounds (14 kg) in 1980 to 62 pounds (28 kg) in 2002, based on data compiled by the Food and Agriculture Organization of the United Nations (FAO). The report predicts that global meat production will more than double by 2050. These statistics mean that the environmental damage from ranching would have to be cut in half just to keep emissions at their current, dangerous level.
The truth is that cattle, sheep, goats, and other ruminants naturally expel methane and nitrous oxide. It is estimated that a single cow can belch out anywhere from 25 to 130 gallons of methane a day. Farmers do have some ways of reducing this negative environmental impact. Improving their cattle's diet can improve methane and nitrous oxide emissions. Another option to reduce emissions is to capture and destroy methane that may have been created in manure lagoons. The greatest opportunities so far, however, are to use farms to produce biofuels and displace the emissions from fossil fuels used by other sectors.
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