Persistence of residues in milk following antibiotic treatment of dairy cattle.

A study was conducted to determine the persistence of antibiotic residues in milk beyond the recommended withdrawal period. Composite milk samples (n = 122) were collected from 58 lactating dairy cows in the university herd receiving antibiotic treatments for any reason but only when administered as a single drug. Samples were obtained 72 h posttreatment and sampling continued every 24 h until result for antibiotic residue was negative by the Bacillus stearothermophilus disc assay. The antibiotic (n = 7) administered accounted for significant variation in drug persistence; however, route of administration, case number (for cows treated for more than one episode), number of days treated, animal's body weight, lactation number, and daily milk production did not affect drug persistence. Chi-square analysis indicated that 21% of milk samples were positive for residues beyond the recommended withholding period. Milk samples from cows treated with cephapirin and penicillin were the only samples that exceeded recommended withdrawal times. Often doses administered exceeded label directions.