Enayat Rahmatnejad

Abstract The present study was conducted to evaluate the effects of synbiotic on the performance, quality traits of eggs, intestinal microbial population, and intestinal morphology of laying hens under chronic heat stress and estimating its optimal point of synbiotic supplementation using broken line modeling. The experiment was conducted using a completely randomized design that included four treatments, four replications, and nine birds each. The results indicated that the addition of synbiotic increased the egg-laying percentage, egg mass, and egg weight during the 42-44 week and 44–46-week periods(p<0.05). Likewise, the egg mass and egg laying percentage in all synbiotic- receiving groups were higher compared to the control group throughout the entire period. The lowest and highest egg mass values in the control group and the 0.5-gram additive group were 45.88 and 49.6, respectively, throughout the entire period. The number of Salmonella in the 0 .5-, 1-, and 1.5-gram synbiotic -receiving groups decreased (p<0.05). The optimal synbiotic was estimated through linear and quadratic broken line modeling to ascertain the percentage of egg laying, which corresponded to 0.93 and 1.05 grams per kilogram, respectively. Overall, the use of synbiotic improved performance along with the reduction of intestinal salmonella population, and the amount of 1 gram per kilogram is appropriate for heat stress conditions.