If Ogden Nash were writing this poem today, he might want to add a line or two. In the new millennium, both ends of our bovine friends are under scrutiny — but for very different reasons. While governments are trying to figure out how to control cow burping, a significant factor in global warming, cow manure is gaining popularity as one of Earth's greenest sources of electricity.

Many United States farmers already know the meaning of "cow power." They collect the methane given off by fermenting cow manure and use it to generate electricity. The procedure is relatively simple: manure is stored in huge tanks — anaerobic digesters — which are deprived of oxygen and kept at temperatures of 100°F. The conditions are designed to let anaerobic bacteria thrive and do the work of breaking the manure down. The large volume of "biogas" released — which contains about 90% methane — is piped to an engine which burns the gas and uses the heat energy to generate electricity. The leftover manure is compressed; fluid is drained away and used as fertilizer; and the solids are dried out and used as bedding for the herd and compost.

The method offers a neat solution to the manure waste problem. America's 100 million cattle produce their fair share of manure — on Tinedale Farm, in Milwaukee, the 1800 Holsteins produce about 48,000 pounds per day, much of which is processed to generate electricity. By using manure in this way, farmers are transforming problematic waste into new, useable commodities: electricity, compost, and fertilizer.

According to Environomics, a company that manufactures manure-digesters, 32 farms in the United States are using the digesters for electricity-generation. The technology has not been more widely adopted because the systems are expensive to install, costing from $200,000 to $1,000,000 each, depending on the size of the herd. To encourage farmers to generate their own electricity, the state of California's Energy Commission is making $10 million in funding available to support farmers' initiatives. It is currently reviewing about 30 applications for grants and plans to install several digesters by this summer.

A Bad Case of Gas
While manure-derived methane is proving very useful, the methane cows burp is causing problems. Methane is a greenhouse gas and, in the atmosphere, contributes to global warming. Cows burp an abundant supply of it every day — about 280 liters per animal (in other words, the average cow could fill 140 two-liter soda bottles with gas daily). Unfortunately, burped methane is more difficult to collect, with the result that about six million metric tons of it float blissfully up into the atmosphere every year. And that's just from herds in the United States. (Worldwide, ruminant livestock — including cattle, sheep, goats, and buffalo — produces about 80 million metric tons of methane per year, accounting for 22% of anthropogenic methane emissions.)

Methane is second only to carbon dioxide in the list of greenhouse gases. According to the Intergovernmental Panel on Climate Change, it's 21 times better at trapping heat in the atmosphere than CO₂ (a fact that can be attributed to the larger size of CH₄ molecules). The six million tons of methane that North American cows burp annually are equivalent to 36 million tons of carbon dioxide.

Why do cows burp so much methane? As with the anaerobic digester, the answer lies with bacteria. Billions of bacteria are busy at work in the cow's rumen (the first of the four chambers in its stomach), breaking down grass and hay in a process known as enteric fermentation. The bacteria — which live symbiotically in the cow's gut — are essential to its digestive process. One of the anaerobic bacteria produces large quantities of methane as a byproduct, which the cow gets rid of by burping.

Physics: The activities referred to in this section require Logal Express. Get a free trial subscription.

1. Electricity
The Haubenschilds' dairy farm in Princeton, Minnesota, is widely recognized as an exemplary "green farm." They have been using a manure-digester to collect methane for electricity generation since 1998.

• Students can see how methane is collected from the manure-digester at the Environmental Protection Agency's Web site.

Once the mostly methane biogas is collected, it is piped to an engine. Methane is a hydrocarbon fuel which the engine burns, producing heat. This thermal energy is used to heat a liquid which expands and builds up pressure, turning the turbine of a generator. The generator converts the kinetic energy of the rotating turbine into electrical energy.

• The Haubenschilds' generator produces 20,000 kilowatts of power a week. What is a kilowatt? In the Electric Power activity, from Middle School Science Gateways, students learn that power is the product of voltage multiplied by current. They calculate power, kilowatt-hours, and the cost of using various electrical appliances.

2. Gas Laws
The average cow burps 280 liters of methane per day. Because of the large quantity of gas generated, burping performs a vital biological function for the cow. You can ask students to consider what might happen if a cow were unable to rid itself of the excess gas. (There are various urban legends about exploding cows, but none that we could corroborate.) This provides an unusual context for thinking about Boyle's Law.

• Students may look at the Boyle's Law activity from Middle School Science Gateways to study the relationship between volume and pressure in a gas.

Earth science: Begin by asking students if they have heard anything about cows harming the environment. You may get some interesting answers: for example, it's a common misconception that cow flatulence is damaging the ozone layer. Two misconceptions are rolled into one here. First, the depletion of the ozone layer is being confused with the greenhouse effect when in reality the two phenomena are separate. Second, cow flatulence is being blamed, when it's actually a much less significant problem than cow eructation (burping).

The methane cows burp does not damage the ozone layer. Instead, it helps build that canopy of gases in the atmosphere that traps heat, causing global warming. Cows can accept no responsibility for ozone depletion.

For a separate lesson on global warming, with special attention to politics and the Kyoto Treaty, refer to "Bush Cool on Global Warming," a Teaching the News article from last month.

• The generator at Haubenschilds' dairy farm in Princeton, Minnesota, produces 20,000 kilowatts of power a week. The farm itself uses 9,000 kilowatts and the remainder is piped to a power supplier that serves the local community. Given that the average household uses 140 kilowatts per week, how many homes does the excess power from the Haubenschilds' farm serve?
• Is the Haubenschilds' electricity-generation model a scalable one? Could it be used to supply the electricity needs of entire cities, or would it better serve local, rural communities? When researching the answer, students can read the CNN.com article, "From each cow, a burst of — electricity?" to find out about plans to build a 3- to 4-megawatt electrical power plant fueled with biogas in Washington state. Students should also realize that the average coal-fired power plant supplies 300 megawatts.
• Given that the Haubenschilds' dairy farm — which processes manure from 500 cattle — generates 20,000 kilowatts each week, how many cattle would be needed to generate the 68,000 megawatts that New York City uses during a warm (84 °F) week?

Earth science students: Ask students to consider the following questions:

Renewable versus non-renewable energy:

• Burning manure-derived methane to generate electricity is held up as a very environmentally-sensitive method. What are the main advantages of the method? What are the disadvantages, if any? In answering these questions, students may examine other renewable energy sources (solar, wind, wave).

Extending the Problem
Life without burps: It's important for cows to burp, so what can be done to mitigate their methane emissions? Students will find answers in "Ruminant Livestock and the Global Environment" from the Environmental Protection Agency. By managing cattle's diet (with better grazing management and dietary supplements) and giving them antibiotics (to dampen bacterial activity in the gut), cows will burp significantly less gas. Are these practical measures? Students should realize that a large portion of the world's herds are in third world countries, where economic conditions rule out such measures.

Flatulence tax?: Students will probably be amused to learn that there has been talk of a "flatulence tax" in some countries. (It's a misnomer — "eructation tax" would be more accurate.) Ask students if they consider this a plausible means of controlling cow emissions. What else might governments do to get farmers to take action? Students can read this press release from New Zealand's Green Party about the party's opposition to so-called "flatulence tax."

Methane in Lake Kivu: Large quantities of methane and carbon dioxide are dissolved at the bottom Lake Kivu which lies on the border of Congo and Rwanda, Africa. In January of this year, nearby Mount Nyiragongo erupted. If lava were to flow into Lake Kivu, the heating effect would cause the methane and carbon dioxide to be released, with potentially deadly results. Students can read more about this unusual and dangerous natural phenomenon in "In Goma, lava flow brings second threat," an article from WFAA.com (the Web site of Dallas/Fort Worth's Channel 8). Students can also read BBC.com's full description of the Nyiragongo eruption, which displaced thousands of people.