Comparisons of on-farm screening tests for detection of antibiotic residues.

A study was conducted to determine the effectiveness of on-farm tests for detection of antibiotic residues in milk and urine. Composite milk samples (n = 122) were taken from 58 cows receiving antibiotic treatment with a single antibiotic by any route of administration. Samples were obtained 72 h posttreatment, and sampling continued every 24 h until all residue tests showed a negative result for antibiotics. Antibiotic residues were detected by the Bacillus stearothermophilus disc assay, the Delvotest P, and the Penzyme test. Urine of cull cows was collected and tested for residues with the Live Animal Swab Test for antibiotic residues present in slaughter carcasses. No significant difference was found between the Bacillus stearothermobilus disc assay and either on-farm test. Chi-square analysis indicated that with both the Delvotest and the Penzyme test, 17% of samples gave positive results when the disc assay showed negative readings. An inordinately high percentage (69%) of urine samples from cull cows exhibited antibiotic contamination, although all animals had completed the specified withholding periods. Fifteen of these positive results were obtained from primiparous cows that had never been treated with any antibiotic.