OXYTETRACYCLINE RESIDUES IN MILK OF COWS
TREATED FOR RETAINED PLACENTAS

-- abstracted from P. Dinsmore, R. Stevens, M. Cattell, M. Salman, and S. Sundlof Journal of the Amer. Vet. Med. Assn. Nov. 15, 1996, Vol. 209, No. 10, p. 1753.

Oxytetracycline is an antibiotic routinely used in an extra-label manner for intrauterine therapy. Research supporting the efficacy of intrauterine antibiotics is minimal, yet the practice is common. A study was performed to establish the duration of oxytetracycline residues in milk from cows with retained fetal membranes given the antimicrobial via intrauterine infusion. The study also investigated whether the number of infusions or the presence of fever affected the duration of residues.

Fifty-four Holstein cows with retained fetal membranes on a single 1,400-cow commercial dairy were used in this study. Cows were treated once a day with 5 g of oxytetracycline by intrauterine infusion for at least 2 days, or until the membranes were expelled. Cows that became clinically ill (fever and off-feed) were also given procaine penicillin G at 20,000 IU/kg (9,000 IU/lb) of body weight, IM daily for 2 to 4 days. Milk samples were collected at 24-hour intervals during treatment, and at 12-hour intervals after the last treatment. All samples were frozen and submitted every 2 weeks for high performance liquid chromatography (HPLC) analysis for oxytetracycline.

Oxytetracycline was detected in milk of all cows during treatment, at a maximum concentration ranging from 47 to 1,800 æg/kg (ppb). Duration of oxytetracycline residues after the last infusion ranged from zero to 144 hours (6 days). Neither the number of infusions received nor development of rectal temperature greater than 39.7øC (103.5øF) affected the maximum concentration or the duration of oxytetracycline residues in milk.

Milk obtained from cows that are treated for retained fetal membranes by intrauterine infusion of oxytetracycline may contain the drug for variable lengths of time. Milk should be discarded to avoid illegal residues. Contact your veterinarian for appropriate withholding times.