Effects of oral and injectable tetracyclines on bacterial drug resistance in feedlot cattle.

Enteric bacteria isolated from feedlot heifers treated with tetracyclines (TET) were examined for resistance to TET and ampicillin. The effects of feeding (45 days) therapeutic and subtherapeutic quantities of chlortetracycline (CTC) and the injection of therapeutic doses of oxytetracycline (3 daily doses) were compared. Performance data from the 45-day trial did not identify an antibiotic effect on average daily gain, although cattle fed subtherapeutic quantities of CTC showed improved feed efficiency. Bacterial resistance to TET was increased in the heifers given the therapeutic and subtherapeutic quantities of CTC in the ration. However, an increase in resistance to ampicillin did not occur. Oxytetracycline injections had a short-term effect, increasing the number of resistant organisms only during the week of drug administration.