Lack of Growth Response of Eight-week-old Broilers to Certain Antibiotics

Two experiments with day-old broiler chicks designed to study the efficacy of chlortetracycline, bacitracin, oxytetracycline and penicillin in growth promotion were conducted. The first experiment included five consecutive trials. In two trials, duplicate pens of 50 males per treatment were used, and in the other three trials, duplicate pens of 50 straight-run chicks were used. A typical broiler ration was used as a control. The four dietary treatments (mg./kg.) were: chlortetracycline, 25; bacitracin, 5; oxytetracycline, 25; penicillin, 5. The second experiment consisted of two trials with ten groups of 50 straight-run chicks per treatment. Five of these groups were fed a diet containing animal protein (as in the first experiment) and the other five groups were given a diet containing all-plant protein. The antibiotic levels were 0 and 50 mg. of chlortetracycline, bacitracin, oxytetracycline, or penicillin per kilogram of diet. All trials in both experiments were terminated after 8 weeks of dietary treatments. In the first experiment and in the second experiment with diets containing animal protein, antibiotics had no growth stimulatory effect. Chlortetracycline and penicillin did, however, stimulate a significant growth response when added to an all-plant protein diet. In general, heavier body weights were obtained from antibiotics with diets containing animal protein than with all-plant protein diets.