Tissue disposition and depletion of penicillin G after oral administrationwith milk in unweaned dairy calves

Objective-To determine tissue depletion of penicillin G in calves after ora l 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 c ontrols. After I 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 eutha natized in groups of 3 each 4, 6.5, 9.5, and 13 hours after the final feedi ng. Concentrations of penicillin G were determined in tissues, blood, and u rine 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 wer e 13 ng/ml, 92 ng/g, and 142 ng/g, respectively. Thirteen carcasses had vio lative drug residues; 12 had violative residues in the liver only, and 1 ha d violative residues in the liver and kidney. A 21-hour withdrawal period w as estimated. Conclusions and Clinical Relevance-Liver had the highest concentration of p enicillin G and was most likely to have violative residues. Feeding calves milk containing penicillin G has the potential to cause violative drug resi dues in tissues. It is recommended to observe an appropriate withdrawal tim e prior to slaughter if calves are fed milk from cows treated with penicill in G.