Antibiotic Residues in Milk Following Antimicrobial Therapy During Lactation

Milk was collected at 24 h intervals through 120 h after treatment from lactating dairy cows treated with antibiotics. Antibiotic residues were determined qualitatively by microbiological assays utilizing Bacillus stearothermophilus . Intrauterine infusion of antibiotics resulted in the lowest percentage of milk samples positive for residues. A high percentage of samples was positive for residues 24 and 48 h after intramuscular injection of antibiotics; however, most samples were negative by 72 h after treatment. Intramammary therapy resulted in a high proportion of samples positive for residues at 24 and 48 h after treatment, and some samples were positive 96 to 120 h after treatment. Samples from treated quarters were often positive when corresponding composite milk samples were negative. Treatment with more than one antibiotic by multiple routes resulted in the highest percentage of samples positive for residues for the longest time. Any variation of antibiotic dosage, duration of treatment, or use of multiple antibiotics should alert dairy producers of the probability of antibiotics being excreted in milk beyond recommended withdrawal times.