Does feeding cattle antibiotics contribute to global warming?

Feeding antibiotics to farm animals may be another factor in global warming, according to a new study published this week that found higher amounts of methane, a gas well known to contribute to global warming, in the manure of cattle fed the now-common drugs.

"To the best of our knowledge, this is the first report of antibiotics increasing methane emissions," the international team of researchers wrote in a study published Wednesday in the Proceedings for the Royal Society B. The antibiotics kill bacteria that compete with methane-producing microbes, they reported, and so the drugs wind up allowing those microbes flourish.

"We know that there are negative consequences of antibiotics, particularly this effect of antibiotic resistance," co-author Tobin Hammer, a doctoral student at the University of Colorado Boulder, told NBC. "But this was a pretty unexpected link between antibiotics and this other important environmental issue that we care about – greenhouse gases."

Originally, the team hoped to learn about the drugs' impact on dung beetles, who burrow in cows' manure – a habit that might help combat the methane in the manure, according to previous studies.

After feeding the antibiotic tetracycline to 10 cows in Finland, however, the researchers found that it killed a broad range of bacteria, allowing their manure to produce more methane. The microbe archaea, which can produce large quantities of methane and which helps animals digest their food, seemed to be somewhat immune to tetracycline.

"We propose that by specifically suppressing bacteria in the gut and subsequently in the dung, antibiotic treatment enables methanogens to outcompete bacteria for hydrogen, increasing their concomitant methane output," the researchers wrote. However, because tetracycline is not very widely used among livestock, more testing of other antibiotics is needed, they suggest.

The tetracycline hardly hurt the dung beetles, however. And the methane in cow pats is less than that of the cows' belches, Hammer told NBC.

Antibiotics have already been found to be responsible for making farm animals like cattle grow bigger and at faster rates, but the practice is highly criticized for health reasons, in addition to many environmental ones. 

According to the American Academy of Pediatrics, feeding healthy animals antibiotics may render the drugs ineffective when they are used to treat infections in people who eat meat: so-called superbugs that resist treatment. In fact, the Food and Drug Administration has already made attempts to limit the antibiotics given to farm animals.