Do cows pollute as much as cars?

Why do cows and other ruminants produce methane?

Methane gas is more harmful than carbon dioxide.
Methane gas is more harmful than carbon dioxide.
© Photographer: Igor Terekhov | Agency:

With the development of large-scale agriculture in the mid-20th century, farming became a big business for some companies. Farms became consolidated into large enterprises with many thousands of animals across large acreages.

Initially, grazing areas were filled with a variety of grasses and flowers that grew naturally, offering a diverse diet for cows and other ruminants. However, in order to improve the efficiency of feeding livestock, many of these pastures became reseeded with perennial ryegrass. With the aid of artificial fertilizers, perennial ryegrass grows quickly and in huge quantities. The downside is that it lacks the nutritious content of other grasses and prevents more nutritious plants from growing. One commentator called it the "fast food" of grasses [Source: Guardian Unlimited].

This simple diet allows many cows to be fed, but it inhibits digestion. A perennial ryegrass diet also results in a significant number of weak and infertile cows, which have to be killed at a young age. This is where the methane comes in. The difficult-to-digest grass ferments in the cows' stomachs, where it interacts with microbes and produces gas. The exact details of the process are still being studied, and more information may allow scientists to reduce cows' methane output.

A study at the University of Bristol compared three types of naturally grown pastures to ryegrass pasture grown with chemical fertilizers. Lambs were fed on each type of pasture. The meat from lambs fed on natural pastures had less saturated fat, more omega-3 fatty acids, more vitamin E and higher levels of conjugated linoleic acid (CLA), a "good fat" that is believed to fight cancer. The meat from these lambs was considered very high quality and scored well in flavor tests.

Because of concerns about ruminant diets, many researchers are investigating ways to alter what livestock eat and to mix the best of old cow pastures -- diverse, naturally growing, nutrient rich grasses and plants -- with the best of the new -- fast-growing and resistant to invasive species. One possibility is to increase the ability of beneficial, nutrient-rich plants and flowers to grow alongside the fast-growing grasses commonly used in pastures. Another branch of research focuses on plants that are high in tannins, which are believed to lower methane levels in ruminants and to boost milk production -- although excessively high level of tannins are harmful to a ruminant's growth.

A study by researchers in New Zealand recommends the use of plants like birdsfoot trefoil that are high in alpha-linoleic acid, which boosts CLA levels. Planting legumes and genetically engineered plants to trap airborne nitrogen will also improve nitrogen levels in the soil, which is important for rich soil and healthy plants.

Some dairy farmers use processing systems to harvest methane from cow manure. The energy is used to power the farm while excess is often sold back to the local electrical grid. Some dairy farmers use processing systems to harvest methane from cow manure. The energy is used to power the farm while excess is often sold back to the local electrical grid.
Some dairy farmers use processing systems to harvest methane from cow manure. The energy is used to power the farm while excess is often sold back to the local electrical grid.
Image courtesy Alden Pellett/Associated Press

Believers in naturally grown, mixed-species pastures say that the use of them will reduce greenhouse gases, improve animal health and meat quality and reduce the use of artificial fertilizers. Efforts like methane-reducing pills or the addition of garlic may just be stopgap measures that fail to address some of the core problems of livestock, namely ground and air pollution, cutting down of forests, the production of weak animals that later have to be culled and the use of artificial fertilizers and steroids.

Another possibility exists in trapping the methane gas and using it as energy or selling it back to the electrical grid. Some farmers already extract methane from livestock waste, but that does not solve the bigger problem of belched methane. Harnessing that methane would mean trapping it in the air, perhaps by housing cattle indoors or outfitting them with special muzzles that may inhibit eating.

For more information about ruminants, methane, global warming and related topics, check out the links below.

Related HowStuffWorks Articles

More Great Links


  • Adam, David. "Move to cut methane emissions by changing cows' diet." The Guardian. July 10, 2007.,,2122455,00.html
  • Buller, Henry. "Eating Biodiversity." University of Exeter.
  • Harvey, Graham. "Pastures new." Guardian Unlimited. July 11, 2007.
  • Lean, Geoffrey. "Are cows killing the planet?" Daily Mail. July 11, 2007.
  • McCandless, Kevin. "UK Hunting Ways to Minimize 'Greenhouse Gas' Emissions - From Cows." CNSNews. July 11, 2007.
  • Smith, Lewis. "How to stop cows burping is the new field work on climate change." The Times. July 10, 2007.
  • Woodward, S.L, Waghorn, G.C. and Laboyrie, P.G. "Condensed tannins in birdsfoot trefoil (Lotus corniculatus) reduce methane emissions from dairy cows." New Zealand Society of Animal Production.
  • "What is CLA?" Eat Wild.
  • "Ag 101, Dairy Production Glossary." U.S. Environmental Protection Agency. March 6, 2006.
  • "Garlic 'may cut cow flatulence.'" BBC News. July 10, 2007.
  • "Nitrogen-fixing trees."
  • "Plant Nutrients." Plant Nutrition. North Carolina Department of Agriculture & Consumer Services.
  • "Dictionary." Sustainable Table.
  • "Tannins: positive effects." Animal Science at Cornell University. Sept. 16, 2001.