The fate of antibiotic residues in the manure of treated animals is of considerable concern because of the potential development of antibiotic-resistant bacteria in the environment and because of the effect of these residues on manure treatment systems. The objective of this study was to determine the fate, and effect of oxytetracycline (OTC) during the anaerobic digestion of manure from medicated calves. Five beef calves were medicated for 5 days with 22 mg/kg/day of OTC. Manure samples collected from calves prior to and after medication were diluted five-fold with water, loaded into replicate 1.21 anaerobic digesters and incubated at 35 degrees C. OTC levels in the manure slurry decreased from 9.8 +/- 0.1 to 4.1 +/- 0.1 mg/l in 64 days (59% removal) yielding a calculated value half-life for OTC of 56 days. Levels of the OTC epimer 4-epi-oxytetracycline increased gradually from 0.55 +/- 0.03 mg/l at the start of experiment to 1.3 +/- 0.1 mg/l on day 27 and then decreased to 0.84 +/- 0.04 mg/l on day 64. Levels of two other OTC metabolites (u-apo-oxytetracycline and P-apo-oxytetracycline) decreased or remained unchanged during the anaerobic digestion process. Cumulative biogas production was 27% lower from digesters containing manure from medicated calves relative to that from digesters containing unmedicated manure. However, the presence of OTC did not show other negative effects on process stability as there were no significant differences in biogas methane content or in reductions of volatile solids and soluble organic carbon. (c) 2006 Elsevier Ltd. All rights reserved.