The belches of millions of cows and sheep may not immediately seem like a serious problem. But for some countries they send greenhouse-gas emissions through the barnyard roof, thanks to methane-producing bacteria in the animals' guts.

Scientists at Australia's largest research body, the Commonwealth Scientific and Industrial Research Organisation (CSIRO), now hope to change this with a vaccine that targets methane-producing bacteria. In a test on 30 sheep, the team's vaccine reduced methane emissions by 7.7% (A. D. G. Wright et al. Vaccine doi:10.1016/j.vaccine.2004.03.053; 2004).

Making methane is a ruminant's way of letting off steam. As food ferments in the animal's first stomach, or rumen, hydrogen is produced and reacts with carbon to form methane, which is then exhaled. Although this accounts for less than 3% of total greenhouse-gas emissions in fossil-fuel burning countries such as Britain and the United States, it makes up nearly 40% of emissions in agricultural New Zealand (see chart).

Andre-Denis Wright a molecular biologist at CSIRO Livestock Industries in Perth, who led the study, says that the team's vaccine should cut emissions by 20% once it matches the microbes in sheep better — rumen bacteria vary from region to region and from season to season.


But critics point out that this diversity will also limit the vaccine's usefulness. “There are a huge number of microbes in the rumen that can't be cultivated. And if you can't cultivate them, you can't raise antibodies and can't make a vaccine against them,” says Athol Klieve, a rumen microbiologist from the Queensland Department of Primary Industries and Fisheries in Brisbane.

Killing off some methane-producing bacteria in the gut may also open the way for other methane-producing microbes to take their place.

Wright's team hopes to get round these problems by tailoring the vaccine to target a protein common to many methane-producing bacteria. Other groups are working on different solutions. Athol, for example, plans to introduce bacteria from the guts of kangaroos into cows in the hope that these will out-compete methane-producing microbes. The kangaroo bacteria produce acetate instead.

But the vaccine will never eliminate methane production altogether, says Roger Hegarty, a livestock researcher for the New South Wales state government's agriculture department. Complete blockage of burping could result in an unhealthy build-up of hydrogen in the animals, he says.