Experiments were carried out to study the changes in the values of feed efficiency (FE), total protein efficiency (TPE) and metabolizable energy efficiency (MEE) of broiler chicks fed during four age periods (14-21, 21-28, 28-35 and 35-42 days). Five experimental diets were similar in composition but different in the irradiation dose (0, 5, 10, 25 or 50 kGy) applied to the meat-bone meal content of each diet. The results indicated that feeding of broiler chicks on irradiated meat-bone meal (5-50 kGy) had no significant (P > 0.05) effects on FE, TPE, MEE and body weight gain during the four experimental age periods. The average values of the FE (feed consumption/weight gain), TPE (weight gain/total protein consumption) and MEE (metabolizable energy consumption in MJ/kg gain) were 1.96, 2.59 and 23.13, respectively. The values of FE and MEE increased while TPE decreased significantly (P < 0.05) with age. The pooled FE value was 1.60 during the 14-21 days of age and increased to 2.66 from 35-42 days of age. However, the TPE value was 3.01 during the 14-21-day age period and decreased to 1.82 over the 35-42-day age period. The energy consumption value to produce one kg of body weight increased during the 35-42-day period by 7.71-12.72 MJ/kg gain in comparison with that from the other experimental age periods. Weight gain rate decreased significantly (P < 0.05) during the 35-42-day period (66 g/week/bird) when compared with the rate observed during the 14 through 35-day age period. (C) 2003 Published by Elsevier Ltd.