Management of antibiotic residues from agricultural sources: Use of composting to reduce chlortetracycline residues in beef manure from treated animals

Chlortetracycline (CTC) is one of only ten antibiotics licensed in the U.S.A. for use as growth promoters for livestock. The widespread use and persistence of CTC may contribute in development of antibiotic-resistant bacteria. The objective of this study was to determine the effect of composting on the fate of CTC residues found in manure from medicated animals. The effect of CTC residues on composting was also investigated. Five beef calves were medicated for 5 days with 22 mg/kg/day of CTC. Manure samples collected from calves prior to and after medication were mixed with straw and woodchips, and aliquots of the subsequent mixtures were treated in laboratory composters for 30 days. In addition, aliquots of the CTC-containing mixture were incubated at 25 degrees C or sterilized followed by incubation at 25 degrees C and 55 degrees C (composting temperature). The presence of CTC did not appear to affect the composting process. Concentrations of CTC/ECTC (the summed concentrations of CTC and its epimer ECTC) in the composted mixture (CM) and sterilized mixture incubated at 55 degrees C (SM55) decreased 99% and 98% (from 113 microg/g dry weight (DW) to 0.7 microg/g DW and 2.0 microg/g DW), respectively, in 30 days. In contrast, levels of CTC/ECTC in room temperature incubated (RTIM) and sterilized mixture incubated at 25 degrees C (SM25) decreased 49% and 40% (to 58 microg/g DW and 68 microg/g DW), respectively, after 30 days. Concentrations of the CTC metabolite, iso-chlortetracycline (ICTC), in CM and SM55 decreased more than 99% (from 12 microg/g DW to below quantitation limit of 0.3 microg/g DW) in 30 days. ICTC levels in RTIM and SM25 decreased 80% (to 4 microg/g DW) in 30 days. These results confirm and extend those from previous studies that show the increased loss of extractable CTC residues with increased time and incubation temperature. In addition, our results using sterile and non-sterile samples suggest that the decrease in concentrations of extractable CTC/ECTC at 25 degrees C and 55 degrees C (composting temperature) is due to abiotic processes.