The Effect of Feeding Antibiotics for Different Periods on Growth Rate, Feed Conversion and Metabolizability of Dietary Energy in Growing Chickens

Current concern regarding antibiotic-resistant bacterial strains in the environment prompted experiments to ascertain whether the feeding of antibiotics as growth stimulants could be confined to only part of the growing period of boiler chickens without significantly reducing the advantages conferred on growth rate and efficiency of feed utilization. In one experiment, 1920 broiler chicks were fed a practical type diet from hatch to 8 weeks of age. The diet was fed with and without a mixture of oleandomycin and zinc bacitracin to supply 11.0 and 27.5 mg./kg. of the respective antibiotics. Antibiotic administration was as follows: (1) control—no antibiotic, (2) antibiotic 0–4 weeks only, (3) antibiotic 4–8 weeks only, (4) antibiotic 0–8 weeks. In a second experiment a basal diet was fed to White Leghorn chicks with and without antibiotic supplementation as in experiment 1 from hatch to 3 weeks of age. At this time, half the chicks were continued on the diet that had been fed to 3 weeks and the remaining birds were transferred to the alternate diet. The metabolizable energy of the diet for chicks on each of the dietary treatments was determined during each of the three succeeding weeks. In experiment 1 stimulation to growth and improvement of feed efficiency from antibiotic treatment occurred only during the periods when the antibiotic was present in the diet. Average body weight and efficiency of feed conversion were similar when antibiotics were fed from 0–8 weeks or from 4–8 weeks only. Supplementation of the diet with antibiotic or withdrawal of antibiotic rapidly altered growth rate and feed utilization. The metabolizable energy value of the diet fed in. experiment 2 declined within one week of antibiotic withdrawal from the diet. It is concluded that with practical type broiler diets the beneficial effects of dietary antibiotics are immediate and that they do not persist following withdrawal of the antibiotic from the diet.