Excessive Levels of Dietary Protein and Energy Induce Lack of Growth Promoting Effects of Clenbuterol in Broilers

The present study examined the effects of excessive dietary protein and energy on growth response to clenbuterol in broilers. The chicks were allocated into 6 groups at 14d old, and used for a $3{\times}2$ factorial experiment. Birds were fed six diets, the control diet containing 21% crude protein (CP) and 3,100 kcal of metabolizable energy ME/kg, a high protein (30% CP) or a high energy (3,500 kcal/ ME/kg) diet, with or without 1 ppm clenbuterol, for 18 d. Clenbuterol feeding markedly decreased (p < 0.05) body weight gain by 23% in the high energy group. Feed intake was also decreased (p < 0.05) by clenbuterol administration across diet treatments. Abdominal fat weight was reduced (p < 0.05) by clenbuterol only when chickens were fed the high energy diet. Clenbuterol increased (p < 0.05) leg muscle weight in the control diet group, but decreased (p < 0.05) it in the high energy group. Muscle protein concentration was increased by 11 % in leg muscle only of the birds at the high energy level. In leg muscle, clenbuterol enhanced the protein/DNA ratio by 18%, except for the high protein group. These results indicate that feeding a diet containing excessive amounts of protein and more energy than normal did not necessarily improve growth response to clenbuterol.