Antibiotic residues in milk following bulbar subconjunctival injection of procaine penicillin G in dairy cows

Objective-To determine whether, and at what time, penicillin enters milk at a concentration that is detectable following bulbar subconjunctival inject ion in lactating dairy cows. Design-Randomized clinical trial. Animals-66 Holstein cows that were at least 2 weeks past calving and had no t been treated with antibiotics in the preceding 30 days. Procedure-Cows were randomly assigned to receive a treatment of 1 mi (300,0 00 units) procaine penicillin G by bulbar subconjunctival injection or rema in untreated, Composite milk samples were collected immediately before trea tment and 4, 10, 16, 22, 28, and 40 hours after treatment. Milk samples wer e tested by use of a commercial test for beta-lactam antibiotics. Results-Among penicillin-treated cows, the first positive lest results were observed 4 hours after treatment, and the last positive result was observe d 22 hours after treatment. The percentages of positive test results before treatment and at 4, 10, 16, 22, 28, and 40 hours after treatment were 0, 9 , 87, 42, 8, 0, and 0%, respectively. None of the untreated cows had positi ve test results for beta-lactam antibiotics at any sampling time. Conclusions and Clinical Relevance-Penicillin was detected in milk for up t o 22 hours after a single subconjunctival injection of procaine penicillin G in cows. This result should be considered when recommending milk withhold ing periods following the administration of penicillin by this route in lac tating dairy cows.