Objective: To determine tissue depletion of penicillin G in calves after oral ingestion with milk replacer and estimate a withdrawal period.
Design: Longitudinal controlled trial.
Animals: 26 Holstein calves.
Procedure: Once daily, 24 calves were fed milk replacer containing procaine penicillin G (0.68 mg/kg [0.31 mg/lb] of body weight); 2 calves served as controls. After 1 feeding, 12 calves were euthanatized in groups of 3 each 4, 6.5, 9.5, and 13 hours after feeding. After 14 days, 12 calves were euthanatized in groups of 3 each 4, 6.5, 9.5, and 13 hours after the final feeding. Concentrations of penicillin G were determined in tissues, blood, and urine by use of high-performance liquid chromatography.
Results: Penicillin G was not detected in muscle samples of treated calves. The highest concentrations of penicillin G in plasma, kidney, and liver were 13 ng/ml, 92 ng/g, and 142 ng/g, respectively. Thirteen carcasses had violative drug residues; 12 had violative residues in the liver only, and 1 had violative residues in the liver and kidney. A 21-hour withdrawal period was estimated.
Conclusions and clinical relevance: Liver had the highest concentration of penicillin G and was most likely to have violative residues. Feeding calves milk containing penicillin G has the potential to cause violative drug residues in tissues. It is recommended to observe an appropriate withdrawal time prior to slaughter if calves are fed milk from cows treated with penicillin G.