Residues from long-acting antimicrobial preparations in injection sites in cattle

To investigate tissue residues of two long-acting oxytetracycline (OTC) preparations in cattle.

A randomised drug residue trial.

Two hundred and forty beef cattle in 24 groups of ten.

Two blind-coded 200 mg/mL OTC preparations were used in five treatment regimens of various combinations of injection sites (from one to five) and administrations (one or two). Five cattle from each group were slaughtered at 21, 30 and 60 days after injection and the injection site, urine, kidney and diaphragm muscle analysed for residues.

The OTC concentration exceeded the maximum residue limit in kidney in animals slaughtered 21 days after treatment, which is the prescribed withholding period. Concentration at the injection site was much greater than the maximum residue limit 30 days post-treatment, but not 45 days post-treatment. The residue was smaller when OTC had been injected in multiple sites. There was no difference between the two OTC preparations.

A review of the maximum injection volume, site of injection and the withholding period is needed for long-acting OTC formulations.