Response of antibiotics and resistance genes to high-intensity and low-intensity manure management

This purpose of thid study was to determine the response of antibiotics and antibiotic resistance genes (ARG) to manure management. A pilor field study was conducted using horse manure containing no antibiotics, into which chlorretracycline (CTC), tylosin (TYL), and monensin (MON) were spiked and compared to unspiked controls. Subsequently, a large-scale field study was conducted comparing manure from a dairy with minimal use of antibiotics and a feedlot with regular subtherapeutic use of antibiotics. The manures were subjected tp high-intensity management (HIM) (amending watering, and turning) and low-intensity management (LIM) (no amending, watering, or turning) and were monitereed for antibiotic concentrations and levels of tetracycline ARG [tet(W) and tet(O)] using quantitative real-time polymerase chain reaction. All three antibiotics in the pilot study disipated more rapidly in HIM manure, with half-lives ranging from 4 to 15 d, compared to LIM manure, with half-lives ranging from 8 to 30 d. Levels of tet(W) were significantly higher after 141 d of treatment, but levels of tet(O) were significantly lower in all treatments. In the large-scale study, the feedlot manure had higher initial concentrations than the dairy manure of tetracycline (TC), oxytetracycline (OTC), and CTC as well as tet(W) and tet(O). Tetracycline and OTC dissipated more rapidly in HIM manure, with half-lives ranging from 7 to 31 d. After 6 mo of treatment, tet(W) and tet(O) decreased significantly in feedlot manure, whereas dairy manure required only 4 mo of treatment for similar results.