Risks of antibiotic residues, antibiotic resistance genes and pathogens when using pig manure as fertilizer on crop fields (VARMEST)

The results showed that high concentrations of antibiotic residues could be present in pig manure which are further diluted in the soils after fertilization. These can be detectable in the soils up until harvest. However, this is dependent of the antibiotic and soil type. The presence of antibiotic resistance genes in manure were similar for the pigs which were administered antibiotics and the ones which were not. Although the antibiotic resistance genes studied were already present in the soil environment prior to fertilization, an increase in the relative abundances of most of the antibiotic resistance genes studied was observed immediately after fertilization. This increase was followed by a gradual decline to initial levels over time. These results may indicate that the input of resistance genes through fertilization with manure is more important with regard to dissemination of resistance than the selection pressure exerted through the antibiotic residues present in the manure. Pathogens present in the manure could be detected in the soil environment directly after fertilization but not after one month. 

Finally, it was demonstrated that biological nitrogen removal from pig manure as a manure treatment process is able to slightly reduce the concentration of certain antibiotic residues and several antibiotic resistance genes and to largely reduce the amount of zoonotic bacteria in the effluent.