Farm Animals and Methane

This is Part 1 of a three-part series on Food and Farming.

1. Increased CO2 and Food Quality
2. Farm Animals and Methane
3. "Food Mile" Complexities


The authors of today's post are Lisa Moore, Ph.D., a scientist in the Climate and Air program, and Tim Male, Ph.D., a senior ecologist in the Land, Water & Wildlife program.

For the second course in our food series, let's start with some pie – specifically, Bill's greenhouse gas pie showing the contribution of different greenhouses gases to global warming. The first chart shows that methane is the second largest contributor to global warming.

Globally, nearly half of that hefty methane slice comes from agriculture. What causes these emissions, and how can we reduce them?

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.

There are challenges with anaerobic digesters. Odors can be a problem if the digester effluent isn't stored and managed properly. Also, as this Cornell University presentation [PDF] points out, you must have enough land to absorb all that fertilizer – or a plan to export it if you don't. Anaerobic digestion can be used in concert with other manure management techniques, such as solid-liquid separation and composting.

Livestock production contributes to global warming – no question about it. But fortunately, there are steps that farmers and ranchers can take to decrease methane emissions while potentially increasing profits.

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2 Comments

  1. reconsidering
    Posted September 11, 2007 at 6:36 pm | Permalink

    This information seems to apply only to nations that raise commercial livestock, disregarding the enormous populations of natural ruminents that exist in the African continent (wildebeest, hartebeest, impala, etc.). Although accurate population estimates aren't readily available (I can only find charts indicating trends, not numbers per se), one would assume that the wild populations are far greater than commercial livestock, given that much of the US and Europe are actually populated with people. One simply doesn't find vast herds of cattle like you do in Africa–not any more, anyway, i.e., buffalo, antelope, etc.

  2. Posted September 13, 2007 at 11:09 am | Permalink

    Anthropogenic (human-caused) global warming is all about human disturbances to the background "carbon cycle", which was in equilibrium – i.e., CO2 levels were roughly stable – until humans started burning fossil fuels and forests.

    Livestock production is anthropogenic, but wild ruminants are part of the natural carbon cycle, so they do not contribute to global warming.

    Also, to get a sense of how livestock are distributed around the world, check out this interactive map.

    – Lisa and Tim

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