Antibiotic use in livestock could increase greenhouse emissions, new research suggests.

Key points: Cattle are an important source of methane, a potent greenhouse gas

Cattle are an important source of methane, a potent greenhouse gas Dung from cattle fed the antibiotic tetracycline produces more methane

Dung from cattle fed the antibiotic tetracycline produces more methane Tetracycline could also increase methane content of burps and farts from cattle

The study, published today in the Royal Society's Proceedings B, found that dung from cattle fed tetracycline produced more methane than dung from animals not fed the antibiotic.

"This is the first report that we're aware of that shows antibiotics having this increased effect on methane release," study co-author Tobin Hammer, from the University of Colorado, said.

"We think it's because of microbial competition going on inside the cattle's intestines where the balance is altered by antibiotics."

Mr Hammer explained that if this hypothesis was correct, then the antibiotic would likely have the same effect on direct gaseous emissions from the animals as well, which produce most of the methane that comes from livestock.

"If this effect also occurs for belching, I think that would be maybe cause for concern," he said.

Balance of gut microbes shifted

Livestock are a well-known source of methane, which is a more potent greenhouse gas than carbon dioxide.

The methane comes from activity of particular gut microbes called archaea, which thrive in anaerobic — or air-free — conditions.

It appears tetracycline shifts the balance of livestock gut microbes towards these methane-producing types, said Mr Hammer, who is doing a PhD in entomology and microbial ecology.

Mr Hammer and his colleagues in Finland compared the dung of cattle-fed tetracycline with that of those not fed the antibiotic, and then measured the amount of methane, nitrous oxide and carbon dioxide emitted from the animals' dung.

Analysis showed radical changes in the microbiota of the dung from cattle fed the antibiotic, and an increase in its methane emissions.

"The relative abundance of archaea that produce methane increased with antibiotic treatment, and that presumably is why we saw a strong increase in methane emissions," said Mr Hammer.

Effect of antibiotics on dung beetles

The study also looked at the impact of antibiotics on dung beetle activity.

Dung beetles play an important role in breaking down manure, recycling nutrients and reducing methane emissions by oxygenating the dung and reducing the number of anaerobic archaea, Mr Hammer said.

He and colleagues found that beetles feeding on dung from cattle-fed tetracycline also had changed microbiota.

Despite this, the beetles still reduced methane production by cattle dung — although this was not enough to offset the increase in methane due to the antibiotics.

"The good news is that antibiotics don't interfere with the beetle's beneficial suppression of methane from dung," Mr Hammer said.

"The bad news is that antibiotics still increase methane emissions, whether or not the beetles are present."

Tetracycline is one of the most widespread livestock antibiotics in use around the world.