Scientists in the United States have conducted first tests to study the effects of antibiotics on cows and the level of methane they release via their dung. The scientists say there may be "unintended, cascading ecological effects" from the use of antibiotics in agriculture.

Antibiotics are used "extensively" in commercial farming, write the researchers in "Proceedings of the Royal Society B," a journal of The Royal Society.

"Yet they may have major consequences for human and environmental health," they write.

The study was conducted by an international team, spanning the US, Britain, Finland and Sweden. They were led by Tobin Hammer of the University of Colorado.

Antibiotic resistance

Their findings suggest the use of antibiotics in agriculture leads to antibiotic resistance, which is seen as a growing and urgent issue by the World Health Organisation and members of the pharmaceutical industry.

The use of antibiotics "can favor the evolution of antibiotic resistance among pathogens and the spread of antibiotic genes to surrounding environments," say the study's authors.

From dung to dung beetle

Livestock dung, write the researchers, is a source of nutrients, organic matter and microbes for ecosystems. It is also a source of pathogens and significant amounts of greenhouse gases, such as methane, nitrous oxide and carbon dioxide.

Caution: these cows are carrying greenhouse gases

They say that as "antibiotics have been shown to alter the structure and activity of mammalian gut and fecal microbiota," they may "directly or indirectly modulate" greenhouse gas emissions from livestock dung.

And this could have a consequence, say the scientists, on the ability of dung beetle to mitigate methane production from dung, with potential "impacts in some agricultural systems."

Tests and results

To test their theories, the scientists treated cattle with and without a broad spectrum antibiotic. They analyzed microbial communities in cow dung and in field-collected dung beetles. They gauged the effects of antibiotics on the size of beetles and their numbers, and they tested the greenhouse gases emitted from cow dung.

The scientists say they found "a clear effect" of antibiotic treatment on dung microbiota and that this effect persisted for more than three weeks after it had been defecated and collected by the team. The antibiotic treatment also affected dung beetle microbiota, but it did not affect dung beetle size, their reproduction or survival.

They confirm that dung beetles reduce the "flux" or output of methane from dung. Previously studies have linked this to "oxygenation" of the dung caused by the dung beetle burrowing tunnels into the pat.

But the antibiotic treatment "consistently increased methane emissions." Levels of nitrous oxide also rose, while carbon dioxide emissions are described as "similar between the treatments."

Large-scale monitoring

The findings highlight "a unique feature of antibacterial pharmaceuticals," write the scientists. "Even if not directly toxic to non-target animals, they may have a range of unanticipated effects."

They are calling for larger studies to compare the effects of dung and belching in terms of their greenhouse gas emissions, and study "the global extent and purpose of antibiotic use in livestock production."