The effect of dietary genistein on performance, intestinal morphology, caecal Lactobacillus spp. count, and tibia composition in broiler chickens after 21 and 37 days of feeding was investigated. A total of 360 Cobb 500 broiler chickens (21 days old) were randomly allocated to five treatments with six replicates of 12 birds each. They were fed a basal diet (C) or a basal diet supplemented with 200 (T1), 400 (T2), 600 (T3) and 800 (T4) mg genistein/kg of feed. Genistein supplementation did not affect feed intake, but improved bodyweight, weight gain and feed conversion ratio (FCR) after 21 days, while 600 mg/kg led to a significant increase in FCR after 37 days of feeding. Plasma triglyceride level decreased with dietary genistein after 21 days, while increases were found in T3 and T4 groups after prolonged supplementation. Significantly improved duodenal and jejunal villus length and width, crypt depth and villus/crypt ratio were observed after the first and the second finishing periods, respectively, while adverse effects were found in the ileum for both periods. At 42 days old, greater spleen and heart weights were measured in broilers fed diets with 800 mg/kg than in other broiler groups. The shorter genistein supplementation period (21 days) of 200 and 400 mg/kg had a positive effect on tibia wet weight, ash and calcium (Ca) content, while 37 days of the higher genistein doses administered to the T2, T3 and T4 birds significantly increased caecal lactic acid bacteria (LAB) counts. Thus, recommended doses should not exceed 400 mg/kg.
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