Persistence of oxytetracycline residues in milk after the intrauterine treatment of lactating cows for endometritis.

Milk samples were collected at one day intervals after the last dose from 31 cows that had received an intrauterine infusion of oxytetracycline once daily between one and five times. The tetrazolium chloride assay was used to determine whether there were significant residues of the antibiotic in the samples. A single treatment resulted in residues for between one and eight days, and the period tended to be longer in the cows that had received more than one dose. Of the 31 cows, six remained tetracycline-positive for more than five days after their last dose of the drug.