Oxytetracycline in bovine plasma, milk, and urine after intrauterine administration.

Six healthy lactating cows with normal but varying stages of estrous cycles were treated with chelated or nonchelated oxytetracycline by intrauterine infusion, 4 mg/kg body weight. The chelated drug was a mixture of special metal halides and oxytetracycline while the nonchelated form was a commercial oxytetracycline solution suitable for intramuscular or intravenous administration. Oxytetracycline concentrations in plasma, milk, and urine were determined at timed intervals following treatment. Both forms of the drug appeared in the plasma, but the chelated form was a small amount. The nonchelated oxytetracycline also appeared in the milk. However, no oxytetracycline was in milk of cows treated with chelated oxytetracycline. Both forms of the drug were excreted in the urine. Milk should be withheld from market 24 and 0 h after administration of nonchelated and chelated forms.