Six experiments were carried out with male and female broiler chicks in a factorial arrangement to evaluate the combined effects of fat concentration or energy-to-protein (E:P) ratio in diets fed from 1 to 7 or 14 d of age, and of the E:P ratio in diets fed from 1 or 2 to 7 weeks of age on fattening and performance of the birds. 2. Diets with a wide E:P ratio fed from 1 or 2 to 7 weeks consistently and significantly increased abdominal fat pad size at 7 weeks of age. On the other hand, in 5 of the experiments, the nutritional treatments from 1 to 7 or 14 d did not significantly affect this variable in chicks fed on diets with either the narrow or the wide E:P ratio. 3. Body weights and food utilisation at 7 or 14 d were consistently and significantly improved by increasing dietary fat concentration and using diets with a narrow E:P ratio. However, at 7 weeks of age, in only two of the experiments were body weights still significantly affected by early nutrition. 4. Diets with a wide E:P ratio consistently and significantly reduced the performance of chicks up to 4 weeks of age. Nevertheless, at 7 weeks of age the performance of these chicks tended to be better (at times significantly so) in most of the experiments, despite their excess fattening.
