a comparison of the effect of dark period, gamma-amino butyric acid and melatonin supplements on reducing heat stress in broiler chicken

background: searching for factors that reduce the adverse effect of heat stress (hs) on broiler performance has particular importance. objectives: the present study was designed to investigate the influence of dietary melatonin, gamma-amino butyric acid (gaba) and dark period on growth performance, carcass characteristics and blood parameters of broiler under heat stress condition. methods: a total of 143 one-day-old chickens (broiler line arian) were used in an unbalanced completely randomized form with 4 treatment, 13 experimental units and 11 birds per experimental unit. the treatment consists of 40 mg of melatonin/kg diet from the age of 30 to 40 days, 100 mg (gaba)/kg diet from the age of 10 to 40 days, dark of period from 10 to 40 days of age and control. all treatments were kept a circular hs (40±1°c) for 6 h from 35 to 40 post hatch. weekly growth performance of chickens was determined and blood samples were taken, on days 10, 30 and 40 of age for measurement of glucose, cholesterol, triglyceride, hdl and ldl parameters. on days 35 and 38 of age t3, thiobar butyric acid reacting substances (tbars) and corticosterone parameters were measured. data were analyzed by using of glm procedure of sas. results: the diet supplementation of gaba increased feed intake in the second (p