A comprehensive study on enrofloxacin residue depletion in five chicken breeds.

The widespread use of enrofloxacin (ENR) in poultry farming raises public-health concerns due to residue persistence, particularly in indigenous chickens where standard withdrawal times (WTs) may be inadequate. This study characterized depletion kinetics of ENR and its metabolite ciprofloxacin (CIP) across five commercially relevant breeds in China. We developed and validated a sensitive LC-MS/MS method for simultaneous quantification of ENR and CIP in four edible tissues. After oral administration at 10 mg/kg body weight (bw) once daily for 5 days, tissues from AA broilers, WOD168, Jinling Partridge, Xueshan, and Jinling black-bone chickens were sampled at six time points through day 14. Results showed pronounced breed-dependent differences in elimination. White-feathered lines (AA and WOD168) cleared residues rapidly, with concentrations at or below maximum residue limits (MRLs) within 3 days. In contrast, yellow-feathered breeds showed markedly prolonged persistence, especially in skin with fat. Jinling black-bone chickens exhibited the most extreme retention, with residues detectable in all tissues at day 14 and skin with fat reaching 898.96 μg/kg, a value nearly nine-fold the MRL. WTs were estimated with WT 1.4 using the EMA 95/95 approach under Chinese MRLs, with residues expressed as ENR plus CIP. Overall WTs were 2.30 to 2.64 days for white-feathered lines and 8.16 to 39.74 days for yellow-feathered breeds, with skin with fat consistently the limiting tissue. These findings provide a scientific basis for breed-specific withdrawal guidance and identify skin with fat as a critical monitoring target to ensure poultry food safety.