It may be bad manners, but it's also necessary: Every 40 seconds or so, a cow burps. Scientists are now scrambling to make them burp less – not to make more polite cows, but a cooler planet.

As cows digest their food (up to 150 pounds of grass, hay, and silage per day, along with 20 pounds of concentrated feed), myriad microorganisms – bacteria, protozoa, fungi, and archaea – busily break down the fibers and other nutrients in their rumens. In the process, hydrogen and carbon dioxide are released. The archaea (a kind of bacteria) transform the two gases into methane (CH4), up to 100 gallons of it per cow per day, and the cows get rid of it mainly by burping.

How could a burp matter? But it does.

Odorless, colorless methane – the primary of natural gas – is a powerful greenhouse agent. According to the US Environmental Protection Agency, pound for pound methane is about 21 times more effective at warming Earth's atmosphere than carbon dioxide is. Globally, ruminant livestock – including cattle, goats, and buffaloes – produce about 80 million metric tons of methane a year, accounting for about 28 percent of man-made methane emissions annually.

Recently, researchers from the Japanese National Institute of Livestock and Grassland Science in Tsukuba calculated the environmental impact of a serving of beef and published the result in The New Scientist. According to them, the production of one kilogram of beef (2.2 pounds) results in the emission of greenhouse gases with a warming potential equivalent to 80 pounds of carbon dioxide. In other words: Serving steak to your family is the greenhouse-gas equivalent of driving 155 miles.

What's a cow to do?

As the demand for meat steadily rises globally, the question is how can we keep cows from burping.

British researchers have begun a $1.5 million government research program to propose ways to change cows' diets in order to reduce methane production by feeding them grasses with higher levels of sugar, which facilitate digestion. "These grasses present a better balance of nutrients to the microbial population in the rumen and are used more efficiently," says Prof. Mike Theodorou, head of the UK's Science Development at the Institute of Grassland and Environmental Research in Aberystwyth. "In doing so, more of the ingested carbon and nitrogen will be converted to meat, milk, hide, and wool."

The scientists are investigating how such plants can be bred to contain even higher sugar content and to grow more abundantly on pastureland.

Other researchers suggest adding certain plant ingredients to livestock menus. Michael Kreuzer, head of the Swiss Ruminant Nutrition Group in the Institute of Animal Science of ETH Zurich, proposes adding extra fat (from coconuts, crushed flaxseed, or sunflower seed), as well as extracts rich in tannins and saponins (already available in powdered form).

Unlike antibiotics, these plant-based additives seem to have no secondary effects on milk or meat. And "measurements showed that by using them it's possible to reduce the emission of methane up to 20 percent," Professor Kreuzer says. In order to control the cows' exhalations, he puts them in respiration chambers for 48 hours, feeds them, and measures the concentration of methane from the chamber every few minutes.

The animals aren't always amused. They are not used to eating more fats. Moreover, saponins taste soapy, and tannins are bitter. For the cow, tannin is like drinking a cup of black tea, Kreuzer says: "The more you put into their feed, the merrier it is, but the more awful it tastes."

Furthermore, controlled feeding is not possible everywhere – especially not in Canada, Brazil, Australia, Argentina, and India, where huge herds of animals roam free on huge grazing lands. To reduce their burping, Prof. Winfried Drochner of the Institute of Animal Nutrition at the German University of Hohenheim proposes administering a bun-size pill filled with methane-reducing substances that would dissolve in a cow's gut over a period of months.

It would be a bitter pill for cows to swallow. But at least it wouldn't give them bad breath, as another proposal might do. Prof. Jamie Newbold, at the Institute of Rural Sciences at the University of Wales, is experimenting with a feed laced with garlic. Initial results show that the extract reduces the amount of gas produced by up to 50 percent, as the garlic directly attacks the methane-producing organisms in the cow's digestive tract. But the garlic cure could not only cause halitosis, but also odd-tasting milk and meat. Kreuzer notes that in Switzerland there's a law against feeding dairy cows garlic and onions.

Kangaroos may hold the key

One strategy that Australian scientists have embarked upon suggests not only changing the cattle's diet, but also part of their intestinal tract by making them more like a kangaroo.

Australia's heraldic animals don't emit methane from fermentation. The microbes in their stomachs – while performing functions similar to those in the digestive tracts of cows – are markedly different. A kangaroo's digestive microbes produce acetate (C2H3O2), which aids digestion.

Athol Klieve and his team from the Department of Primary Industries and Fisheries in Queensland isolated 211 bacteria from the eastern gray kangaroo's gut and screened them to determine which are best at digesting native pasture. The most promising ones could be grown in a laboratory and introduced into cattle.

If this approach succeeds, one day there could be less methane in the air – and a little bit of kangaroo in every cow.