EFFECTS OF DIETARY THYROXINE ON GROWTH PERFORMANCE AND CARCASS QUALITY OF BROILERS FED DIFFERENT LEVELS OF DIETARY PROTEIN

To investigate the effects of crude protein and thyroxine on growth performance, nutrient utilizability, carcass composition, the content of total fat and cholesterol in leg muscle, breast muscle and liver, and caloric efficiency in broiler chicks. The experiment involved 3 levels of dietary crude protein (1-3 weeks: 20, 23, 26%; 4-6 weeks: 17, 20, 23%) and 3 levels of thyroxine (0.0, 1.5, 3.0 mg/kg). In the starting period (1-3 weeks), body weight gain of chicks fed diets containing 26% crude protein and 1.5 mg/kg thyroxine was higher than any other groups, and among thyroxine levels, 3.0 mg/kg thyroxine groups were lower. The best feed efficiency was obtained at 26% crude protein with no thyroxine supplemented or 1.5 mg/kg thyroxine supplemented groups. In the finishing period (4-6 weeks) the highest body weight gain was obtained at 23% crude protein with no thyroxine supplemented group. Feed intake of 17% crude protein with 1.5 mg/kg thyroxine supplemented group was higher than those of the other groups. It was found that the utilizability of crude protein in the starting period, showed the best utilizability at 20% crude protein with 1.5 mg/kg thyroxine group. Increasing crude protein level from 17 to 23%, utilizability of crude fat was decreased. The carcass composition was significantly (p<0.05) influenced by crude protein and thyroxine. Increasing thyroxine level from 0.0 to 3.0 mg/kg, crude protein content was increased whereas, crude fat content was decreased. Chicks fed diet containing 1.5 mg/kg thyroxine showed the lowest total fat content in liver tissue. In breast muscle, it was significantly (p<0.05) affected by crude protein and thyroxine. Present data revealed that the cholesterol content was increased for the chicks fed 3.0 mg/kg thyroxine. It the caloric efficiency, chicks fed a diet containing 20% crude protein with no thyroxine supplementation showed the highest caloric efficiency and the lowest efficiency was from 23% crude protein group with 1.5 mg/kg thyroxine. From this study it may be concluded that crude fat content of carcass could be successfully reduced by dietary supplementation of thyroxine, whereas crude protein content was increased.