Galactooligosaccharides (GOS) that are delivered in ovo improve intestinal microbiota composition and mitigate the negative effects of heat stress in broiler chickens. Hubbard hybrids are slow-growing chickens with a high resistance to heat. In this paper, we determined the impact of GOS delivered in ovo on slow-growing chickens that are challenged with heat. The experiment was a 2 × 2 × 2 factorial design. On day 12 of incubation, GOS (3.5 mg/egg) was delivered into the egg (n = 300). Controls (C) were mock-injected with physiological saline (n = 300). After hatching, the GOS and C groups were split into thermal groups: thermoneutral (TN) and heat stress (HS). HS (30 °C) lasted for 14 days (days 36−50 post-hatching). The spleen (n = 8) was sampled after acute (8.5 h) and chronic (14 days) HS. The gene expression of immune-related (IL-2, IL-4, IL-6, IL-10, IL-12p40, and IL-17) and stress-related genes (HSP25, HSP90AA1, BAG3, CAT, and SOD) was detected with RT-qPCR. Chronic HS up-regulated the expression of the genes: IL-10, IL-12p40, SOD (p < 0.05), and CAT (p < 0.01). GOS delivered in ovo down-regulated IL-4 (acute p < 0.001; chronic p < 0.01), IL-12p40, CAT and SOD (chronic p < 0.05). The obtained results suggest that slow-growing hybrids are resistant to acute heat and tolerant to chronic heat, which can be supported with in ovo GOS administration.
