Oxytetracycline residue depletion in whole eggs following in-feed administration to laying hens

Oxytetracycline (OTC) is a broad-spectrum tetracycline antibiotic widely used in domestic animals. In South Korea, OTC was not approved as a veterinary medicinal product (VMP) for use in laying hens at the time of this study, although residues may occur in eggs through misuse, off-label use, or feed cross-contamination. Oxytetracycline residue depletion was evaluated in individual whole eggs to provide reference data for residue management in laying hens. Korean Hy-Line Brown laying hens were administered OTC in feed for 14 consecutive days at 402 mg/kg of feed (1×, estimated intake, 18.6 mg/kg bw/day, the feed concentration stated on the feed additive product label for chickens) or 804 mg/kg of feed (2×, 43.0 mg/kg bw/day, a conservative overexposure scenario). Eggs were collected over 35 days (14-day administration and 21-day withdrawal) and analyzed individually as whole eggs by a validated LC–MS/MS method. The coefficient of determination (R 2 ) of the matrix-matched calibration curves (3.125–100 µg/kg) ranged from 0.9882 to 0.9998. The mean recovery at the lower limit of quantification (LLOQ) level was 112.6%, and intra- and inter-assay precision were acceptable (CV, 3.27–5.78% and 2.62%, respectively). Residue concentrations peaked on day 6 and did not increase progressively thereafter, suggesting a steady state. Mean concentrations remained below the maximum residue limit (MRL value; 400 µg/kg) throughout; however, one 2 × sample (1 of 155; 0.65%) slightly exceeded the MRL value (405.5 µg/kg) during the administration period, indicating that overexposure may increase the risk of MRL non-compliance. With the WT1.4 software, an egg withdrawal period of zero days would be required for MRL compliance under the tested 1 × dosing conditions. These data suggest that, under label-based dosing, OTC residues in eggs are unlikely to exceed the MRL value, and may support residue management and safe-use recommendations for OTC in laying hens when clinically necessary.