Antibiotic Growth Promotion: Effects of Zinc Bacitracin and Oxytetracycline on the Digestive, Circulatory, and Excretory Systems of New Hampshire Cockerels

High (500 g./ton [907.2 kg.] of feed) and low (10 g./ton of feed) levels of zinc bacitracin (B) or oxytetracycline (OT) were incorporated into the diet fed specific-pathogen-free New Hampshire cockerels. Weights of the liver, heart, and kidneys were recorded, and weights and lengths of segments of the digestive system were recorded for birds about 2, 4, 6 and 8 weeks of age, with the following results: Crop weight relative to live weight was increased by feeding bacitracin at either level. Gizzard weight was increased on diets supplemented with high levels of B or OT. A diet containing 500 g. OT/ton of feed induced a significant increase in size of the proventriculus (at 2 and 8 weeks) and decreased weight of the liver (at 4 and 8 weeks). Erratic growth patterns were noted for the other visceral organs, and occasional significant differences were observed among the various diets, e.g., at 2 weeks, heart and kidney weights were lowest in birds fed diets containing high and low levels of bacitracin, respectively. Shapes (weight/10 units length) of the small intestine and cecum were altered significantly by both antibiotics. Cecum shape changed from thickest at 2 weeks to thinnest at 8 weeks in birds fed a high level of either antibiotic.