Dietary effects of soy isoflavones on growth and carcass traits of commercial broilers

Three experiments (EXP) were conducted to determine the effect of soy isoflavones (ISF) on growth and carcass traits of commercial broilers. The EXP were conducted simultaneously and a common control was used. In each EXP, treatments were replicated five times with five chicks each. Average initial and final BW were 102 and 2,890 g, 102 and 2,657 g, and 102 and 2,803 g for EXP 1,2, and 3, respectively, and the EXP were conducted from 9 to 52 d posthatching. In EXP 1, the effects of graded levels of supplemental ISF above those typically found in a corn-soybean meal (C-SBM) diet were studied. The treatments were 1) C-SBM, 2) C-SBM + ISF levels two times those in C-SBM (2x), and 3) C-SBM + ISF levels five times those in C-SBM (5x). The 2x and 5x levels of ISF decreased gain:feed (linear, P < 0.04) but increased (P < 0.04) breast weight compared with the C-SBM diet. Other performance and carcass traits were not affected (P > 0.05) by treatment. In EXP 2, the effects of low ISF levels in a C-soy protein concentrate (C-SPC) diet were studied. The treatments were 1) C-SBM, 2) C-SPC (low ISF), and 3) C-SPC + ISF (ISF levels equal to those in C-SBM). Average daily gain (ADG) and average daily feed intake (ADFI) were decreased (16 and 9%, respectively; P < 0.01) in chicks fed the C-SPC diets, regardless of ISF level. Gain:feed of chicks fed the C-SPC + ISF diet was decreased 9% (P < 0.02) compared with chicks fed the C-SBM diet, and gain:feed of chicks fed C-SPC was intermediate between the two. Carcass traits were not affected (P > 0.05) by treatment. In EXP 3, the effects of low ISF levels in a low CP diet were studied. The treatments were 1) C-SBM, 2) low CP (17,14, and 12% in the starter, growing, and finishing diets, respectively) with supplemental crystalline amino acids (low CP-AA), and 3) low CP-AA + ISF (ISF levels equal to C-SBM). Daily gain and gain:feed were decreased from 7 to 9% (P < 0.01) in chicks fed the low CP-AA and low CP-AA + ISF diet relative to those fed the C-SBM diet. Abdominal fat pad percentage was increased (P < 0.01) in chicks fed the low CP-AA diets compared with those fed the C-SBM diet. Dietary ISF can affect ADG and ADFI and may affect carcass traits in some instances.