Effects of Sex, Breed, Dietary Energy Level, Energy Source, and Calorie:Protein Ratio on Performance and Energy Utilization by Broiler Chicks

Two experiments were conducted with a total of 900 1 to 3-week old chicks fed semi-purified, low-energy diets. Based on differences in rate of gain, body fat content, and energy retention observed in Experiment 1, chicks utilized energy from carbohydrate (dextrose) an average of 13% more efficiently than energy from fat (corn oil). Furthermore, the data suggested that males were more adversely affected than females by substituting fat for carbohydrate. Differences in body fat content were not correlated with total energy consumption adjusted for rate of gain but appeared to be a direct result of differences in efficiency of energy retention. In Experiment 2, it was found that the heavy-type broiler (Hubbard) was 15% more efficient at retaining ingested energy than the light-type broiler (New Hampshire). Also, body fat content of males increased as dietary energy level increased. This occurred when dietary crude protein increased in direct proportion with dietary energy (constant calorie:protein ratio) and when dietary energy level was increased in isonitrogenous diets (variable calorie:protein ratio). Body fat content of females, however, was unaffected when dietary energy concentration was varied in diets in which the calorie:protein ratio was maintained constant, but it did increase with increasing dietary energy levels in isonitrogenous diets. This sex difference was more pronounced in the heavy breed.