| For this research, two hypotheses were proposed. If violative drug residues could occur in edible tissues of calves due to feeding milk containing beta-lactam antibiotics, then beta-lactam antibiotics should be detectable in urine, serum, and tissue following the feeding of milk containing the drugs. Furthermore, if this practice could cause violative drug residues, an appropriate withdrawal time prior to market could be established.; An initial study was performed wherein calves were fed milk replacer spiked with penicillin G or amoxicillin at different concentrations. Penicillin G and amoxicillin were detected in both serum and urine shortly after consuming the beta-lactam antibiotics in milk replacer. Penicillin G and amoxicillin were detected in urine for up to 24 hours. Concentrations of penicillin G above the FDA established tolerance level (0.05 mug/g) were measured in liver tissue. The concentrations in the kidney approached, but did not exceed, the tolerance level.; A study was performed to investigate age-related disposition kinetics and bioavailability of penicillin G when administered intravenously or orally with milk. Calves, 1 and 5 weeks of age, received sodium penicillin G orally and intravenously or were given procaine penicillin G orally. The bioavailability of penicillin G was 10.2 and 7.4% at 1 and 5 weeks of age, respectively. Age dependent disposition was delineated; the median terminal half-lives for oral administration were 2.1 and 1.6 hours at 1 and 5 weeks of age, respectively.; Finally, a study was undertaken to investigate drug residue depletion in tissues of dairy calves fed milk replacer containing procaine penicillin G. Calves were fed milk replacer containing 3.33 mug/ml (ppm) of penicillin G at 12% body weight. Tissue, blood, and urine samples were obtained serially between 4 and 13 hours after the final feeding of milk containing penicillin G. Samples were analyzed using high performance liquid chromatography and microbial inhibition tests. Using the US Food and Drug Administration protocol for establishing a withdrawal period, a withdrawal time of 21 hours was determined for the dosage used. |
