Composted manure is a good slow release fertilizer with the added benefit of breaking down common antibiotics.
It is estimated about 1.6 million kilograms of antimicrobials are used every year in Canadian livestock. Thirty to 90 percent of the products may be excreted as original compounds or byproducts called metabolites.
Various research projects have shown composting manure reduced the viability of pathogens and antimicrobial residues in the soil.
A multi-year project at Agriculture Canada’s Lethbridge Research Centre tested raw manure, stockpiled manure and compost to see which antibiotics were present, how quickly they degraded and if potential antimicrobial resistant genes appeared in environmental bacteria after these products were spread on cropland.
The overall conclusion was composted manure reduced the concentration of the compounds in the products and any potential environmental impact.
“The dose makes the poison. That very much holds here as well,” said Srinivas Sura, an analytical chemist and toxicologist who was part of the project. He is now based at Morden, Man.
Feedlot cattle were fed the broad spectrum antibiotics chlortetracycline, tylosin and sulfamethazine at the recommended rates.
These products can be broken down through a chemical reaction in the animal’s digestive system but can end up in manure.
Studies found chlortetracycline had a half life of 15 to 20 days in the compost, which means 50 percent of the concentration decreased in 15 to 20 days.
Sulfamethazine was about 32 days and tylosin was 30 days.
If these were applied directly to the land, the compounds could last 121 days, he said.
The conclusion was dissipation of antimicrobial-resistant genes was higher in composted than stockpiled manure. The rate of antimicrobial dissipation was also dependent on composting conditions like temperature and moisture levels.
This work resulted in a number of published papers that also looked at the spread of antimicrobial resistance in soil and water runoff as well as other potential affects.
Earlier research has found surface runoff can carry veterinary antimicrobials, nutrients, steroid hormones and antimicrobial-resistant bacteria along with resistant genes they may harbour.
The Lethbridge studies showed these compounds in raw manure can move in the soil to water.
“If you have a rain immediately, the runoff will collect all that chemical in the manure and end up in surface water,” he said.
However, intensive livestock operations have catch basins to capture runoff and this minimizes the transport of unwanted compounds to the environment.
Stockpiled manure was also evaluated. They found that different locations on the pile showed varying levels of antibiotics and other chemicals. In some parts of the pile, antibiotics degraded quite quickly.
Comparatively, compost piles are turned regularly and material is shifted around in a uniform fashion. Chunks are broken down, moisture is evenly distributed and microbes are able to do their work in breaking down organic matter.