Effects of propolis and stocking density on growth performance, nutrient digestibility, and immune system of heat-stressed broilers

Two experiments were conducted to study the effects of propolis (bee glue; BG) supplementation and stocking density on growth performance, ileal nutrient digestibility, gut morphology, and immune response of broilers exposed to heat stress. In Experiment I, a total of 300 1-day-old male chicks (Ross 308) were assigned to six treatments, with five replications of 10 birds each. The birds were fed diet containing no additive (CON) or diet supplemented with different levels of BG (1, 2, 3, 4, and 5 g/kg of feed). Broilers-fed BG at 4 g/kg had higher average daily gain (ADG) (linear and quadratic, p < .05) and average daily feed intake (ADFI) (linear, p < .01) than those fed the CON diet, but BG supplementation (5 g/kg of feed) reduced (quadratic and linear, p < .01) levels of triglycerides, total cholesterol and low-density lipoprotein (LDL). Inclusion of BG (4 and 5 g/kg of feed) increased villus height (VH) and villus height-to-crypt depth ratio (VH/CD) in the jejunum (linear and quadratic; p < .01), and plasma IgM concentration (linear; p = .021). In Experiment II, 1-day-old Ross 308 male broiler chickens (n = 240) were allocated to four experimental groups for 42 days. The experimental treatments consisted of a 2 × 2 factorial arrangement with two levels of BG (0 or 4 g/kg of diet) and two levels of stocking density (low stocking density; LSD, 10 birds/m2 or medium stocking density; MSD, 14 birds/m2). From Day 22 on the birds were either kept at thermoneutral zone (22 °C) or subjected to cyclic heat stress by exposing them daily to 33 °C for 10 h (from 08.00 to 18.00) and 22 °C from 18.00 to 08.00. The results showed that ADG, ADFI, and jejunum VH were decreased (p < .05) more in the birds stocked at MSD than in those housed at LSD. In the birds stocked at MSD, dietary BG supplementation increased (p < .05) the nitrogen digestibility, AME, blood IgG and heterophil:lymphocyte ratio (H:L). In conclusion, the optimum inclusion level of BG in broiler diets is 4 g/kg, and feeding BG had positive effects on immune system and stress indicators of heat-stressed broilers housed at a MSD