Food safety assessment of milk from crossbred cows supplemented on pasture with Virginiamycin

This study was performed to measure the depletion of virginiamycin residues in the milk of dairy cows for different withdrawal periods, in order to determine the food safety of this antimicrobial additive. The experiment was conducted during 38 days (1^st to 38^th experimental day), where animals were kept on pasture and were supplemented with concentrate feed. The first 14 days were the adaptation period. Milk samples were collected for 5 (five) consecutive days from 20 (twenty) cows, supplemented with 340 mg of virginiamycin/animal/day during 21 (twenty-one) consecutive days (15^th to 35^th day). Four animals were kept as a control group, and did not receive virginiamycin during the experimental period. The milk samples were kept frozen under -20°C and virginiamycin residues (virginiamycin marker residues-S1) analyses were performed by LC-MSMS, according to a previously validated method. Samples did not exhibit detectable residues of virginiamycin (limit of detection (LOD) of 0.0116ppm (mg/kg), from day -1 to day 3 of sampling (days 34^th to 38^th). To analyze the withdrawal period, the virginiamycin Maximum Residue Limits (MRL) established by the Japan Positive List (JPL) and the Australian Pesticides and Veterinary Medicines Agency (APVMA) was used; therefore, the MRL is 0.1mg/kg of milk. Finally, the administration of 340mg/animal/day of virginiamycin to dairy cows resulted in residues lower than 0.0116mg/Kg (LOD), therefore lower than the MRL of virginiamycin for milk. Based on this data, no withdrawal period is necessary after the interruption of virginiamycin treatment; as a result the withdrawal period is zero days. Thus, the milk of dairy cows treated with virginiamycin for 21 days may be considered safe for human consumption.