Growth Response of Chicks to Antibiotics from 1950 to 1961

IT HAS BEEN REPORTED that continuous feeding of antibiotics to chicks for several years in the same environment resulted in a decrease in the growth response obtained from the antibiotics (Waibel et al., 1954; Libby and Schaible, 1955). Some laboratories, however, indicated that the feeding of a new antibiotic gave an improved response where one had no longer been attainable with an “old” antibiotic (McGinnis et al., 1959; Wiese and Peterson, 1959). The work reported here summarizes the results of antibiotic feeding at this station for the past ten years. Procaine penicillin was fed throughout the 10 year period, therefore the results obtained with it are summarized separately. The responses from chlortetracycline were summarized with those from oxytetracycline as there were insufficient trials for either to warrant recording the results separately. Henceforth, these will be referred to only as tetracyclines. Fewer data are presented for zinc bacitracin because it was …