The main source of agricultural methane is a process called enteric fermentation, a normal part of digestion in ruminant animals such as cows, sheep, goats, buffaloes and camels. Ruminants have a compartment in their stomach called a rumen where food is broken down through fermentation. This process produces methane gas, which the animals then belch out into the atmosphere. Enteric fermentation accounts for 18 percent of all anthropogenic methane emissions.
If enteric fermentation is a natural part of ruminant digestion, then what’s the solution? One strategy is to adjust the feed mix. The harder a food is to digest, the more time it spends in the rumen and the more methane that is produced. Some supplements can help as well. A recent study in Wales found that garlic may cut methane emissions.
The other major source of methane from farm animals is manure – particularly when stored in uncovered tanks or lagoons. Cow paddies sprinkled across a field of grass are exposed to the air where they quickly dry out. In tanks and lagoons (used because feedlots contain so many animals), lack of oxygen increases the amount of methane that’s emitted.
One way to handle the problem is through anaerobic digestion of manure stored in closed vessels. The captured methane can be used to generate heat and electricity. The 7,000+ dairy and swine operations in the U.S. could generate enough electricity to power nearly 600,000 homes annually, preventing the release of 1.3 million tons of methane into the atmosphere (see EPA report [PDF]). As an added benefit, the digested manure can be used as a fertilizer. (Raw manure also is used as fertilizer, but has a much stronger odor.) Since producing synthetic fertilizer burns a lot of fossil fuel energy, fertilizing with manure is a way to reduce greenhouse gas emissions as well as save money.
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