Effects of embryonic thermal manipulation and post-hatch synbiotic supplementation on growth performance, gut morphology, skeletal traits and caecal microbiota of broiler chickens under tropical conditions

1. This study investigated the effects of embryonic thermal manipulation and post-hatch synbiotic supplementation on growth performance, intestinal morphology, foot health, leg strength and tibial characteristics of broiler chickens reared in tropical conditions.2. A total of 840 fertile Ross 308 eggs were incubated at either the optimal temperature (37.5°C) or subjected to embryonic thermal manipulation (38.5°C from embryonic days (ED) 8 to 14). After hatch, chicks received either a basal diet or a basal diet supplemented with a synbiotic (1 g/kg) until 42 d of age.3. From d 21–28 all birds experienced an outbreak of leucocytozoonosis and received identical antiparasitic treatment, enabling comparisons of treatments under disease challenge.4. Embryonic thermal manipulation reduced hatchability (87.7 vs. 95.7%), starter feed intake and early body weight, but did not affect final body weight. Synbiotic supplementation improved overall feed conversion ratio. Under optimal incubation conditions, synbiotic supplementation increased body weight gain and duodenal villus height, whereas, following embryonic thermal manipulation, it increased tibial phosphorus content but reduced villus height.5. Neither embryonic thermal manipulation nor synbiotic supplementation affected gait score, latency-to-lie or caecal bacteria. Footpad dermatitis and hock burn scores increased only in synbiotic-supplemented broilers incubated at optimal temperature.6. Embryonic thermal manipulation altered the response to post-hatch synbiotic supplementation, shifting the benefits from enhanced intestinal morphology and growth performance to improved tibial mineralisation. These findings underscored the importance of incubation conditions when devising nutritional strategies for broiler production.