ORIGINAL ARTICLE Antioxidant enzyme activities, plasma hormone levels and serum metabolites of finishing broiler chickens reared under high ambient temperature and fed lemon and orange peel extracts and Curcuma xanthorrhiza essential oil A. Akbarian 1,2 , A. Golian 1 , H. Kermanshahi 1 , S. De Smet 2 and J. Michiels 2,3 1 Centre of Excellence in the Animal Science Department, Ferdowsi University of Mashhad, Mashhad, Iran 2 Laboratory for Animal Nutrition and Animal Product Quality, Department of Animal Production, Ghent University, Melle, Belgium, and 3 Department of Applied Biosciences, Ghent University, Ghent, Belgium Summary The negative effects of high ambient temperature during some months of the year on poultry production have been of great concern in many countries. Dietary modifications are among the most practical ways to alleviate the effects of high temperature. Possible effects of dietary supplementation with 200 or 400 mg/kg feed of lemon peel extract (LPE), orange peel extract (OPE) and Curcuma xanthorrhiza essential oil (CXEO) under hot conditions (34 °C with 50% relative humidity for 5 h daily starting from day 28 until day 38 of age) on blood antioxidant enzyme activities, biochemical parameters and antibody titres of broiler chickens were investigated. All extracts are rich in phenolic compounds and highly available. Compared to control, supplementation with OPE at 400 mg/kg and CXEO significantly increased erythrocyte glutathione peroxidase and superoxide dismutase activ- ity, plasma growth hormone concentrations and serum phosphorus, total protein and chloride concentrations and decreased serum low-density lipoprotein and cholesterol concentrations in chickens at 38 days of age. Regarding antibody titres, CXEO supplementation at 400 mg/kg caused a significant increase in bronchitis anti- body titres. Supplementation with LPE and OPE gave more inconsistent results. Most interesting, 400 mg/kg LPE significantly increased 3,5,3 0 -triiodothyronine and GH concentration as compared to the control. In conclusion, the herbal extracts tested in this study, in particular CXEO at 400 mg/kg, may relieve some of the changes in blood composition induced by increased ambient temperatures. Keywords antibody titre, antioxidant status, heat stress, hormones, plant extracts Correspondence J. Michiels, Department of Applied Biosciences, Ghent University, Valentin Vaerwyckweg 1, 9000 Ghent, Belgium. Tel: +32 2488863; Fax: +32 92424279; E-mail: joris.michiels@ugent.be Received: 10 May 2013; accepted: 28 February 2014 Introduction Chronic heat and high humidity, occurring in temper- ate countries as well as in the tropical world, exert del- eterious effects on performance and mortality of broiler chickens. In south of Iran, the environmental temperature during the greater part of the year remains well beyond the upper limit of the bird’s ther- moneutral zone, and the adverse effects of high tem- perature can make poultry production difficult and uneconomical (Ahmad et al., 2006). It has been well documented that exposing broiler chickens to contin- uously high temperature, especially during the fin- isher phase, leads to chronic heat stress (Sahin et al., 2003; Ahmad et al., 2006). The metabolic changes induced in chickens by chronic heat exposure include impairment in endocrine functions. Decreased serum concentrations of T 3 and T 4 , important growth pro- moters in animals and suppressed immune system function during high ambient temperature have been reported (Yahav and Mcmurty, 2001). Heat stress increases plasma corticosterone, glucose and choles- terol concentrations, reduces serum protein concen- trations, depletes potassium and other minerals in the body, increases lipid peroxidation products in blood and tissues and alters the delicate electrolyte balance in the body. Alteration in electrolyte balance due to increased temperature is probably an indirect result of panting or hemodilution following increased water consumption. Importantly, high ambient temperature Journal of Animal Physiology and Animal Nutrition © 2014 Blackwell Verlag GmbH 1 DOI: 10.1111/jpn.12188