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A study of the growth-promoting effects of penicillin, oxytetracycline, lincomycin, bambermycin , and tylan was made over a 2-year period involving 2030 broiler chicks in 11 experiments. The laboratory used for this study has been subjected to the continuous use of low level dietary antibiotics for over 30 years. Results show that growth promotion by penicillin, oxytetracycline, and lincomycin were still significant (P less than .01). The effect of tylan was also significant (P less than .05). The antibiotics tended to promote better growth effects in chicks from young breeder hens. Significant growth improvement by antibiotics was observed in nutritionally adequate diets regardless of the presence or absence of soybean meal and excesses of certain vitamins and minerals.

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