Antibiotics manure risk requires a rethink

Manitoba researchers say previous studies ignore the interplay of animals’ digestive systems on the drugs

A University of Man­itoba research paper may upend the way environmental scientists consider the issue of residual antibiotics in manure.

They’re a cause of concern because when they’re fed to animals, a lot of the antibiotics pass right through the animal and into the manure. Scientists have worried that could promote antibiotic resistance.

“Often, 90 per cent or more of the antibiotic is excreted, according to previous studies,” says Francis Zvomuya, a researcher at the university.

Lead author Inoka D. Amarakoon, a PhD candidate, looked at those previous studies and realized that those researchers were fortifying manure with antibiotics and then tracking their breakdown. In the real world, when they pass through the gut of an animal, they undergo chemical and biological changes.

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“That can affect how quickly they degrade once they are excreted,” said Amarakoon.

The Manitoba researchers fed combinations of antibiotics to steers and collected their droppings to compare to a control herd’s droppings that were fortified.

Results were mixed. For some antibiotics, the excreted antibiotics degraded more quickly. Other antibiotics degraded faster when added directly to manure. Amarakoon said such mixed findings were to be expected when looking at different products.

Also, compared to the antibiotics added to manure, excreted antibiotics can be arranged differently within the manure.

“That can change whether the antibiotics are even available for chemical or biological degradation,” Amarakoon said.

Zvomuya, Amarakoon, and their colleagues at Agriculture and Agri-Food Canada found that composting manure for 30 days reduced the concentrations of the antibiotics by at least 85 per cent. Some results were as high as 99 per cent.

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