Effect o energy levels in diets of constant energy: protein ratio on performance, metabolism and thyroid hormones of broilers

Abstract

The effect of dietary energy level on productive performance, plasma thyroid hormone (T4 and T3) levels and metabolic rate of broiler chicks was studied. Three energy levels were tested (2 600, 2 900, and 3 200 kcal EM/kg), at energy/protein ratios (kcal/%) of 144 and 160 from 1-28 days and from 29-52 days of age, respectively. Metabolic rate was determined at 28 and 49 days of age, at environmental temperatures of 15ºC, 25ºC, and 35ºC. The chicks fed at the highest energy level had the highest rate of gain when protein and other nutrients were suitably adjusted according to the energy level of the diet. Metabolic rate was dependent on the ambient temperature and age of the birds, but was not affected by the energy level of the diet. No correlation was observed between concentration of plasma thyroid hormones and dietary energy level.