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Rethinking the Stink About Antibiotics in Manure

By   /  July 11, 2016  /  2 Comments

New research on how antibiotics degrade in manure tells us we’ve been measuring incorrectly and that different antibiotics provide different results.

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Thanks to the Soil Science Society of America and Susan Fisk for this great article.

Antibiotics and manure. You probably don’t think of them at the same time. But across North America, manure and antibiotics often share the same pile.

Antibiotics ward off diseases. Low levels of antibiotics are also often fed to livestock to improve their growth. However, much of the antibiotics fed to livestock goes to waste–literally.

Photo showing animals that received antimicrobials in their diet and antimicrobial-containing manure accumulation on the floor of the pen.

Photo showing animals that received antimicrobials in their diet and antimicrobial-containing manure accumulation on the floor of the pen.

“Often, 90% or more of the antibiotic is excreted, according to previous studies,” says Francis Zvomuya. “Manure containing these is then used as fertilizer on crops.” Zvomuya is a researcher at the University of Manitoba in Winnipeg.

And that’s not the end. Once applied with manure to crops, those antibiotics are in the natural environment, mixing with air, soil, and water. This can lead to antibiotic-resistant bacteria, a public health concern across the world.

One way to prevent such a chain reaction is to make sure antibiotics are broken down before they reach the field, for example, via composting or stockpiling. The question is, “How quickly are these antibiotics breaking down during such handling?” says Zvomuya.

Traditionally, researchers have tried to answer this question by adding antibiotics to antibiotic-free manure. They then use the manure as fertilizer or compost it before applying to crops and test how long it takes the antibiotics to degrade. The process is cost-effective and efficient. However, it doesn’t accurately represent how antibiotics get to the manure pile in the real world.

To find the real scoop, researchers fed combinations of antibiotics to steers. Then they collected their droppings. A second group of steers was not fed antibiotics. Researchers added the same antibiotic combinations to these steers’ antibiotic-free manure. Then it was a matter of time: Which antibiotics would degrade faster during composting?

Results were mixed. For some antibiotics, the excreted antibiotics degraded more quickly. Other antibiotics degraded faster when added directly to manure.

That result is to be expected, says Inoka D. Amarakoon, first author of the study. Amarakoon is a Ph.D. student at the University of Manitoba. When antibiotics pass through the digestive system of animals, they undergo different chemical and biological changes. “That can affect how quickly they degrade once they are excreted,” says Amarakoon.

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,” says Amarakoon.

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%. Some results were as high as 99%. “This shows that composting manure before using it as fertilizer can reduce the spreading of antibiotics to the environment, thus reducing the risk of antibiotic resistant bacteria,” says Zvomuya.

Zvomuya recommends real-world accuracy over convenience. “We need to exercise caution while looking at results from studies in which antibiotics are added to excreted manure,” he says.

Read more about this study in the Journal of Environmental Quality.

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About the author

editor and contributor

Rachel's interest in sustainable agriculture and grazing has deep roots in the soil. She's been following that passion around the world, working on an ancient Nabatean farm in the Negev, and with farmers in West Africa's Niger. After returning to the US, Rachel received her M.S. and Ph.D. in agronomy and soil science from the University of Maryland. For her doctoral research, Rachel spent 3 years working with Maryland dairy farmers using management intensive grazing. She then began her work with grass farmers, a source of joy and a journey of discovery.

2 Comments

  1. Graham says:

    This may have already posted but I don’t see it.
    This article asserts the concerns about antibiotic resistance from consistent feeding of low dose antibiotics to farm animals are based in field applications of manure. I was under the impression that of just as much or greater concern is that those populations of rapidly reproducing bacteria directly leave the farm via air / wind, via humans, via birds and other wildlife, via waterways and discharge, via clothing and vehicles, etc. Are these factors a concern? Why are people allowed to apply low dose antibiotics over time to non-human animals, but not humans? Why do we not apply similar standards of care and concern for the general population? If living conditions and diets are poor enough to require consistent feeding of antibiotics then the problem is clearly human management and it imperils the animal, the nutritional quality of meat, the lives of workers and surrounding communities of humans and non-humans as well as waterways and all down steam and downwind and down the food chain.

    Another question: is the “composting” of manure in these studies composting (pile coming to heat, turned, etc) or is it digested in a manure lagoon or similar structure?

    Thank you!

    • Kathy Voth says:

      Hi Graham,
      I think the authors are on the same page as you noting that “Once applied with manure to crops, those antibiotics are in the natural environment, mixing with air, soil, and water. This can lead to antibiotic-resistant bacteria, a public health concern across the world.” Low-dosing animals with antibiotics has been a way of increasing weight gain and growth and wasn’t necessarily done to offset poor living conditions. Because of the issue of antibiotic resistance over the past couple years the FDA has been working on getting farmers to reduce their use and to use them only for treating illness. Some large companies are now advertising that they have given up the daily use of antibiotics. So things are changing.

      You can read more about the study by clicking on the link at the end of the article. I took a quick look and it seems that the manure was composted not digested in a manure lagoon. The study article contains all the details on how they did that.

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