Growth and performance of broiler breeders fed bacitracin methylene disalicylate and zinc bacitracin

Day-old Cobb broiler breeder pullets were randomly allotted into 20 floor pens where they were brooded and reared to 23 wk of age. Eight pens received a control diet containing no growth promotant. The other 12 were fed 55 mg of bacitracin methylene disalicylate/kg of diet. Males were reared separately on the control diet. Pullets were full-fed for the first 8 wk of life, then placed on a skip-a-day program with breeder-recommended feed allocations. At 23 wk of age bird numbers were reduced to 26 females per pen, three males added, and diets changed to the layer regimen. Levels of 0, 27.5, 55, or 110 mg/kg of zinc bacitracin were each assigned to five pens with factorial attention to growing treatment. Layer treatments were continued for 280 days. No significant differences were associated with the feeding of bacitracin methylene disalicylate among body weight or mortality data summarized at 8 and 23 wk. Eight-week feed efficiency was also not affected. In the laying phase, 110 mg/kg of zinc bacitracin significantly improved egg production and fertility over the unsupplemented controls. Hatch of fertile eggs, overall hatchability, and the number of chicks per pen were all significantly improved by 27.5 mg/kg of zinc bacitracin. Higher drug levels supported no additional improvement. Feed efficiency, mortality rate, and final body weight were not significantly influenced by any level of supplementation. No effect of grower treatment upon subsequent layer performance was noted.