Antibiotic Growth Promotion: Effects of Zinc Bacitracin and Oxytetracycline on Live Weight and Weights of Selected Muscles of New Hampshire Cockerels1

ABSTRACT Two antibiotic feeding trials were conducted using New Hampshire cockerels. Zinc bacitracin was used at low (11 mg./kg. of feed) and at high (550 mg./kg. of feed) levels in one trial; oxytetracycline (at the same levels) was used in the other. Both levels of bacitracin caused significant increases in live weight; the high level of oxytetracycline depressed the growth rate at about two weeks of age. Under the allometric model, 1) a single equation sufficed to express the relationship between carcass weight and live weight for all six groups of birds; 2) high levels of either antibiotic led to smaller leg muscle per mass per unit body weight; 3) neither antibiotic had an effect on the growth of biceps brachii or superficial pectoral muscle up to 8 weeks of age; 4) bacitracin depressed the relative growth of the deep pectoral muscle; oxytetracycline did not