An Australian scientist launched what he called a "reef and beef" study into whether feeding cows seaweed would reduce their flatulent carbon emissions, in a move that could help save the Great Barrier Reef.

Tony Parker, from James Cook University, said cattle produced up to 20 per cent of global man-made methane emissions, and the problem was largely linked to their diet.

At least 50 per cent of these cows lived in developing nations, many of which were in the tropics, where the quality of pasture tended to deteriorate in the winter, increasing emissions, Mr Parker said.

"Seaweed, algae and other sea grasses have been proven to be much more digestible than land grass because they have less cellulose and more starch," added Parker's research partner Rocky de Nys.

"A better diet for cattle, then, will encourage better digestion and thus lead to a decrease in methane emissions."

Methane gas from livestock accounts for about 12 per cent of Australia's annual greenhouse emissions, with flatulence from 120 million sheep, cows and goats comprising its third-largest source of damaging gases.

The average beef cow expels the equivalent of around 1,500 kilograms of carbon per year.

The scientists said that using seaweed as cattle fodder could also have wider benefits for the environment, by providing coastal farmers with a way to clean waterways that flowed into the Great Barrier Reef.

Seaweed could be used to clear nitrogen and phosphorus from farming water, but few farmers adopted the method because they were left with "a huge biomass that they don't know what to do with," Mr De Nys said.

He said those nutrients were partly responsible for the breakdown of aquatic ecosystems within the iconic Barrier Reef, which authorities warned this week faced significant threats from climate change and farming runoff.