It seems the ability of a vaccine to knock out their methane-producing microbes depends on where the sheep graze.
This is because the gut of sheep in different areas can be colonised with different microbes, not all of which will succumb to the effects of the vaccine.
A team led by CSIRO molecular biologist Dr Andre-Denis Wright reports on a trial of the vaccine in 30 Queensland sheep in the current issue of the journal Vaccine.
Wright said one-fifth of the world's methane emissions came from farm animals belching and exhaling.
"The whole purpose of having the vaccine is two-fold: to reduce methane emissions in the atmosphere and also to return some of that lost energy back to the animal," he said.
Methane-producing microbes, or methanogens, are opportunistic organisms, said Wright, and provide no benefit to the animals they colonise. So, they prevent animals from getting all the energy they could from their food.
So for the past few years, the researchers have been developing an anti-methanogen vaccine.
In this recent trial, Queensland sheep immunised against methane-producing microbes in their rumen produced almost 8% less methane than those who weren't immunised.
"It is lower than what we expected," Wright told ABC Science Online.
He said the lower than hoped for results were due to the vaccine not being optimised for the sheep that received it.
Location, location, location
The researchers began their work in Western Australia (WA), where they used molecular biological techniques to test which methane producing microbes were in the rumen of sheep there.
They found three strains were responsible for 85% of the methane production, so developed a vaccine to knock these out.
They then needed to test the effectiveness of the vaccine in sheep placed in special respiration chambers, which measure the air going in and out.
At the time, such equipment was only available in Rockhampton, Queensland, so the researchers tried the WA-developed vaccine on Queensland sheep.
"It wasn't until after we got our end-result, which was lower than we had hoped, that we did the same molecular survey of the Queensland sheep and discovered that the population of methane-producing organisms in those sheep were totally different than those in WA," Wright said.
"The other 83% were represented by totally new species never been found before," said Wright. "The diversity of methanogens is much greater than we expected."
Wright said the next step would be to develop a second vaccine targeted at the specific methanogens in Queensland sheep.
At the same time, his team has built its own respiration chambers in WA where they are testing the effectiveness of the first vaccine.
Wright said since the team got nearly an 8% reduction in methane by targeting less than 20% of the methanogens in Queensland sheep, they expect the level of methane abatement to be higher in future trials, although he would not indicate how much higher.
He said the vaccine could also be optimised for use in Australia's 26 million cattle.
Current funding for the anti-methanogen vaccine comes from CSIRO. But the researchers hope to get funding from industries that could benefit from more efficient feeding, leading to increased cattle weight and wool production.