Ghasemi et al., / Acta Advances in Agricultural Sciences 02 (2014), 10:64-74 64 ARTICLE INFO ABSTRACT Article history: Received: July 24, 2014 Revised: August 20, 2014 Accepted: September 03, 2014 Available online October 23, 2014 This study was conducted to evaluate the effects of a dietary organic acid blend on performance, carcass characteristics, intestinal morphology and antibody- mediated immune response to infectious bursal disease (IBD) and infectious bronchitis (IB) vaccines. A total of 180 one-day-old male broiler chickens (Ross 308) were allocated to 12 pens, then, pens were assigned to 1 control and 2 experimental treatments with 4 replicate pens per treatment having 15 chickens per pen. The birds were provided a standard basal diet or basal diet supplemented with 0.2 or 0.4% organic acid blend (OAB) supplementation. Body weight of chickens was measured at 1, 10, 24 and 35 days, feed intake was measured for different periods and feed conversion ratio (FCR) was calculated accordingly. On days 28 and 35, blood samples were taken for measuring antibody titers. On day 35, 2 chickens from each replicate were also killed and 1 segment of intestine (middle of jejunum) was dissected for morphological evaluation. Supplementing 0.2 % OAB increased (P<0.05) body weight of chickens at 25 days of age, but final body weight was not affected. Broilers fed 0.2 % OAB diet showed the most efficient FCR from 1 to 35 days of age (P<0.05). Diets supplemented with 0.4 % OAB caused increase (P<0.05) in the weight of lymphoid organs. However, other internal organs weight and carcass characteristics were not influenced (P>0.05) by treatments. Feeding 0.2 % OAB diet also increased (P<0.05) the villus height and villus height:crypt length ratio in the jejunum of chickens compared with control diet. Primary antibody titers against IBD and IB vaccines were higher (P<0.05) in broilers fed 0.4 % OAB compared with control group. In conclusion, dietary supplementation of 0.2 % OAB could improve FCR by improving intestinal morphology. Furthermore, OAB at level of 0.4 % positively influenced antibody-mediated immunity. © 2014 AAAS Journal. All rights reserved. Keywords: Organic acid Performance Intestinal morphology Immunity Broilers * Corresponding Author; E. Mail: h-ghasemi@araku.ac.ir; haghasemi89@gmail.com Tel: +98 86 32762087 ivestock production, especially the poultry sector, has been growing rapidly and is very important for worldwide food security. As poultry industry developed, disease outbreaks and physiological L Advances in Applied Agricultural Sciences Volume 02 (2014), Issue 10, 64-74 Journal homepage: www.aaasjournal.org Research Article Effects of dietary organic acid blend supplementation on performance, intestinal morphology and antibody-mediated immunity in broiler chickens Hossein Ali Ghasemi 1 *, Hossein Akhavan-Salamat 2 , Iman Hajkhodadadi 1 , Amir Hossein Khaltabadi-Farahani 1 1 Department of Animal Science, Faculty of Agriculture and Natural Resources, Arak University, 38156-8-8349 Arak, Iran. 2 Department of Animal Science, Islamic Azad University, Khoy Branch, Khoy, Iran.