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Genetic selection has led to an increase in performance and lean tissue deposition in broiler chickens which have become increasingly susceptible to disease. To reduce this susceptibility, various feed restriction programs have been proposed, albeit with highly variable results. This study investigated the effects of the duration and intensity of quantitative feed restriction in the early rearing period on the performance, blood parameters and immunity of broiler chickens. Two hundred and seventy chicks were randomly assigned to a 4 × 2 factorial arrangement to examine the effects of 2 durations (7 and 14 d) and 4 levels of feed restriction (5, 10, 15, and 20%). One control (Con) group was included. Feed intake (FI) and weight gain (WG) were recorded, and the feed conversion ratio (FCR) calculated weekly. At slaughter (42 d of age), blood was collected, and the carcass composition and characteristics of the gastrointestinal tract were assessed. In comparison with the Con, feed restrictions improved FCR (P<0.01), increased relative weight (RW) of the breast (P<0.05), and abdominal fat (P<0.01), and reduced the RW, as well as the dimension of some gut segments. Feed restriction showed limited influence on blood variables, the humoral immune response to Influenza and Newcastle disease vaccinations, and on response to injection of sheep red blood cells. Feed restrictions up to 20% for 7 or 14 d are an appropriate strategy to improve the FCR and the RW of breast, with a limited effect on blood variables, and without significant effects on the immunity of broiler chickens. key words: breast; broiler chicken; feed restriction; gut; productive traits; immunity  *Corresponding author: Prof. Alireza Seidavi, PhD, Department of Animal Science, Rasht Branch, Islamic Azad University, 41857-43999, Iran, Phone: +98 91 1331 3073; Fax: +98 13 14422 4069; E-mail: alirezaseidavi@iaurasht.ac.ir DOI: 10.24099/vet.arhiv.0130

Effect of feed restrictions on performance, blood variables and immunity of broiler chickens