Scientists believe that processing manure will also provide opportunities for preventing antibiotic resistance. This is evident from a joint project between the Finnish Food Authority, the Natural Resources Institute Finland (Luke), and the Finnish Environment Institute (SYKE), which monitored the migration of antibiotic residues and antibiotic-resistant intestinal bacteria from antibiotic-treated dairy cows to cowsheds and manure storage and also to the biogas process.
When an animal is treated, antibiotics are excreted in its faeces and urine, ending up in the manure storage and pasture. In Finland, however, antibiotics are used moderately in farm animals and the animals are treated only for medical purposes.
"In animal farms can, however, emerge also multi-antibiotic-resistant intestinal bacteria that end up in manure," says DVM Anna-Liisa Myllyniemi, Director of Microbiology unit at the Finnish Food Authority.
In Finland, also the manure of treated animals is used mostly unprocessed and spread as plant nutrients on the fields, which can spread antibiotic residues, resistant intestinal bacteria, and resistance genes. As a result of spreading the manure on the fields, wildlife, plants, and humans can be exposed to it. It can also end up in the food chain and especially in vegetables that are eaten as such.
The study found that cattle treated with antibiotics had high levels of antibiotics in faeces and urine during the treatment, as expected. Researchers estimate that the manure from their faeces and urine, and especially poorly miscible dry manure, may contain locally high levels of antibiotic residues.
“On the other hand, the antibiotic levels measured in the slurry manure of dairy cattle were very low in this study. The results were not surprising, since the washing waters mixed with slurry and the manure of untreated animals dilute the antibiotic levels,” says Ph.D. Pertti Koivisto, Research Professor of Chemistry unit at the Finnish Food Authority.
Processing can reduce the risk
Manure processing can potentially degrade drug compounds into a less harmful form or destroy resistant intestinal bacteria prior to the use of the manure as fertilizer.
“The biogas process studied in the project did not completely remove the antibiotics in the manure. On the other hand, the microbiological biogas process was not disturbed in the antibiotic levels measured on the farm's slurry,” says Ph.D. Sari Luostarinen, Senior Researcher at Luke.
Resistant intestinal bacteria did not appear to enrich in the process. The amount of live intestinal bacteria in the manure decreased as a result of the process, but no significant effect on the proportion of resistant intestinal bacteria was observed. However, in the biogas process, the manure or the resulting digestate can be hygienized, which could destroy the remaining resistant intestinal bacteria.
In the processing of manure, the overall impact must be assessed
"Efficient processing of manure can at the same time produce renewable energy, improve the usability of the manure as a fertilizer product, and thus provide environmental benefits over traditional manure treatment," says Master of Agriculture and Forestry Juha Grönroos, Senior Researcher at SYKE.
According to Grönroos, attention should also be paid to process emissions and the subsequent treatment of end products in order to minimize emissions from process chains. The most environmentally unfavourable are such manure processing methods that completely eliminate the nitrogen and organic matter of the manure while reducing the usefulness of phosphorus to plants. This is the case, for example, with the burning of manure, which, on the other hand, would destroy drug residues and bacteria.
“When developing manure processing, the overall impact must be assessed. Antibiotic resistance should also be taken into account,” Grönroos concludes.
Pioneering the circular economy
It is not yet known to what extent the spreading of antibiotic residues and resistant bacteria via manure is a risk to human, animal and environmental health. The lack of research data has been an obstacle to risk assessment. However, the increasing prevalence of antibiotic resistance is a serious threat to human and animal health, and the project's researchers therefore estimate that some risk management measures are recommended to be taken at this stage.
“In animal husbandry, for example, it could mean that the faeces and urine containing high levels of antibiotic residues in an animal treated with antibiotics would not be used for fertilization without processing. In addition, cost-effective methods could be sought to reduce resistant intestinal bacteria and resistance genes in manure before its use as fertilizer. At the same time, we must also continue to study the risks of the spreading of resistance and residues in Finnish conditions,” thinks Master of Agriculture and Forestry Maria Aarnio, Researcher from the Microbiology unit at the Finnish Food Authority.
"Finland could be a pioneer in the fight against antibiotic resistance also in the more efficient use of manure, as we have done with the prudent use of antibiotics in animals," Aarnio suggests.
Read the research results in the publication Antimicrobial resistance and residues on cattle farms – effects on the environment and health (NAMI) (pdf) (in Finnish, description in English).
The project was funded by The Development Fund for Agriculture and Forestry (Makera), Finnish Food Authority, Natural Resources Institute Finland, and the Finnish Environment Institute.
Head of Unit Anna-Liisa Myllyniemi, Finnish Food Authority, +358 (0) 40 028 7398
Research Professor Pertti Koivisto, Finnish Food Authority, +358 (0) 40 481 1595
Senior Researcher Sari Luostarinen, Natural Resources Institute Finland, +358 (0) 295 326 346
Researcher Maria Aarnio, Finnish Food Authority, +358 (0) 40 489 3456
Senior Researcher Juha Grönroos, Finnish Environment Institute, +358 (0) 295 251 128