Isoflavones are phytoestrogens which have estrogen-like activity in animals and humans. Because of this characteristic influence, the aim of this investigation was to examine the effect of genistein and daidzein on testicular weight and testosterone secretion in ISA Brown roosters. Genistein and daidzein as non-steroid phytoestrogens were added in large amounts to the feed of roosters. Soy isoflavones were added to rooster diets in amounts of 0, 300, 600, 1200 and 1800 mg/kg of feed. At the end of the 37th week no significant differences were found between the groups in testicular weight (P>0.05) and testosterone secretion (P>0.05) measured in the blood of birds. Blood testosterone concentration was significantly higher in younger (20-week-old) roosters than in 37-weekold roosters (P<0.01). Genistein and daidzein present in the feed did not influence the weight of testes and no significant differences were found. Further studies are required to investigate the effect of isoflavones on other androgenic characteristics.
