It’s a well-known fact that cattle and other livestock are responsible for releasing greenhouse gases like methane into the atmosphere. However, contrary to popular belief, it’s actually bovine burps, not farts, that are to blame. Methane from belching is a serious problem, accounting for 25% of total man-made emissions.
The gases themselves are produced by gut microbes that help cows to digest the tough plant material that makes up their diet. The micro-organisms produce nutrients for the cow, spewing out waste gases like methane, carbon dioxide and nitrous oxide in the process.
But these microbes are also found in cow faeces, meaning that dung continues to emit greenhouse gases long after it passes out of the cow.
Now, research published in Proceedings of the Royal Society B has found that antibiotics used in livestock may intensify this process. The study found that tetracycline, a broad spectrum antibiotic used in cattle, caused a 1.8-fold increase in methane released from cow dung.
The study also points to a potential mechanism. Methane is produced by a group of microbes known as archaea (superficially, archaea look similar to bacteria, but in fact, they’re very different). The study found that in dung pats from cows treated with antibiotics, the proportion of methane-producing archaea increased in the same way that methane levels did. The authors propose that this is because tetracycline is less effective against archaea, if at all.
“There’s good evidence that in mammalian gut environments, these methanogenic (methane-producing) archaea are competing with bacteria,” says study lead author Tobin Hammer. “When you treat a cow with an antibiotic that doesn’t [specifically] target these archaea, you shift the competitive balance, and that allows them to outgrow the bacteria.”
Tobin says that the results were something of a surprise finding, as the initial focus of the paper was to investigate something rather different – the downstream effects that antibiotics may have on dung beetles.
“There has been a lot of research on things like anti-parasite compounds, to show that those can be really harmful to dung beetles because they’re still present in the dung when they feed,” he explains. “These can impact the beetles’ positive effects on dung – for example, reincorporating dung into soil.”
Previous research suggests that dung beetles may play a role in reducing the amount of greenhouse gases released from dung pats. Many of the microbes that produce these gases are obligate anaerobes, meaning that they can’t survive in the presence of oxygen. Beetles may kill these micro-organisms by tunnelling into the dung and exposing the inside to oxygen.
In the study, the group predicted that antibiotics would shift the composition of microbes in the cow dung, and this would have downstream effects on the microbiota of the dung beetles – which is what they observed.
They also predicted that this shift could have negative consequences on beetle health in terms of their size and numbers, making the beetles less effective at reducing greenhouse gas emissions.
The study didn’t observe any of these effects, although as Tobin notes “We didn’t measure behaviour or other aspects of beetle biology that might have been affected without us seeing it.”
The relative contribution of dung to greenhouse gas emissions is less than that of belching, although it is still significant. However, it’s also possible that antibiotics like tetracycline could affect the cow microbial gut community, and so impact the composition of their belches.
“An important follow-up study will be to look at whether methane released from belching is affected,” says Tobin. “If it does, it would suggest the antibiotic effect is more important than we currently appreciate.”