Growth, feed conversion, and carcass composition in females of four broiler crosses fed starter diets with different energy levels and energy to protein ratios.

Body weight, feed conversion, and carcass composition were studied in females of four commercial broiler crosses that differed in weight at 7 wk of age. The birds were fed starter diets for the first 3 wk containing different energy levels but the same protein level (Experiment 1) or varying energy or protein to equilibrate energy:protein ratios (Experiment 2). Experiment 1 was conducted in floor pens; Experiment 2 in batteries. All birds received a common grower diet from 4 to 7 wk of age. No significant cross by starter diet interactions were found for body weight, feed conversion, carcass weight, or protein percentages. In the floor experiment, 3- and 7-wk body weights were lower in birds receiving the highest energy starter than in those receiving lower energy starters; in the battery experiment, body weights were unaffected by energy of the starter. Feed conversion was lower in the birds receiving the high-energy starter in both experiments. Carcass weights essentially paralleled live weights in both experiments; however, percentage of protein in carcasses of 7-wk-old broilers was unaffected by starter diets. Feeding a high-energy starter for the first 3 wk posthatch resulted in higher carcass fat percentages at 7 wk of age but lower absolute and relative abdominal fat pad weights. The significant cross by starter diet interactions indicated that the fastest growing broilers responded to the high-energy starter diet by a significant reduction in abdominal fat without an overall reduction in carcass fat.