_1736050570 20250105041610000 Rovedar Daryoushbabazadeh@gmail.com Rovedar Journal of World’s Poultry Science J. World's Poult. Sci. 2980-7999 12 15 2024 3 4 Shabnaz Aktar https://orcid.org/0009-0008-2143-1973 Mst. Aesha Most. Rukhsana Akter https://orcid.org/0009-0002-5240-1917 KM Mozaffor Hossain https://orcid.org/0000-0001-5506-7905 Afia Khatun https://orcid.org/0000-0003-0509-4483 SM Kamruzzaman Md. Royhan Gofur https://orcid.org/0000-0002-8778-2417 Introduction: The use of antibiotics in poultry feed is becoming more prohibited globally, it is important to consider the potential of natural feed additives as antibiotic substitutes to support the development and operation of chickens' immune systems. The study aimed to assess the effect of probiotic and synbiotic supplementation in broiler feed on the post-hatch development of broiler spleen. Materials and methods: A total of 270-day-old broiler chickens of both sexes were randomly assigned to three dietary treatments each consisting of three replicates and each replicates had 30 chickens. The dietary treatments included the control group receiving a basal diet, the second group receiving a basal diet supplemented with probiotic (Pro.B® with 0.5 gm/Kg feed), and the third group receiving a basal diet supplemented with synbiotic (SynBac® with 1 gm/4-liter water). Probiotic and synbiotic supplementation in feed was started from day seven. Samples (n=15) were collected on the first day and every week until five weeks of age, and the biometry and histomorphometry parameters of the spleen in different groups were studied. Results: The biometrical values of spleen were higher at every age group in both probiotic- and synbiotic-treated broiler chickens compared to the control broilers, but significantly higher biometrical values were observed only in synbiotic-treated broiler chickens at five weeks of age compared to other groups. Regarding histomorphometry parameters, capsule thickness was insignificantly lower until day 28 and significantly lower at day 35 in both probiotic and synbiotic-treated broiler chickens compared to the control group. Moreover, the length and width of lymphatic nodules were higher in both probiotic and synbiotic-treated broiler chickens compared to the control broiler chickens. Remarkably, no significant differences were observed in the biometrical and histomorphometric parameters of the spleen between the probiotic and synbiotic-treated broiler chickens. Conclusion: Dietary inclusion of probiotics and synbiotics in broiler feed could improve the post-hatch growth of the spleen. However, the supplementation of both synbiotics, along with probiotics in the broiler diet had a profound influence on the post-hatch growth of the spleen which has been indicated by a significant increase in biometrical values. 12 15 2024 62 68 Ministry of Science and Technology, Government of the People’s Republic of Bangladeshhttp://dx.doi.org/10.13039/501100008804 Project ID: SRG-231351 https://creativecommons.org/licenses/by/4.0 10.58803/jwps.v3i4.42 https://jwps.rovedar.com/index.php/JWPS/article/view/42 https://jwps.rovedar.com/index.php/JWPS/article/download/42/71 https://jwps.rovedar.com/index.php/JWPS/article/download/42/71 10.1016/j.dci.2015.12.020 10.18632/oncotarget.14502 10.3389/fvets.2023.1153582 10.1016/j.tifs.2023.06.013 10.1016/j.psj.2020.11.073 10.14202/vetworld.2016.313-319 10.47278/journal.ijvs/2022.175 10.3382/ps/pey508 Raksasiri BV, Paengkoum P, Paengkoum S, and Poonsuk K. 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