A Comparison of Several Assay Procedures to Detect Penicillin Residues in Milk ¹

Milk samples from six healthy cows that alternately received intramuscular injections and intramammary infusions of penicillin were analyzed for the presence of penicillin residues using the disc assay, cylinder plate, and Delvotest-P³ methods. Time required for complete removal of penicillin from milk of treated cows was determined with each of the three tests. The disc assay antibiotic test detected penicillin in only 75% of the milk samples in which penicillin was detected by the Delvotest-P method. The cylinder plate method detected penicillin in only 83% of the milk samples in which penicillin was detected by the Delvotest-P method. Also, the Delvotest-P method detected penicillin residues for a significantly longer (P < .01) time than either the disc assay or cylinder plate methods in milk from cows following intramuscular injections. Only once following intramuscular injection was penicillin residue found in milk beyond the manufacturer's recommended withholding time of four milkings, and that did not persist beyond the fifth milking. In milk from cows receiving intramammary infusions, the Delvotest-P method detected penicillin residues for a significantly (P < .01) longer period than the disc assay method, but not longer than the cylinder plate method. No antibiotic residue was ever detected by any of the three tests beyond the manufacturer's recommended milk withholding time of six milkings.