Shahid Bahonar University of Kerman and Iranian Society of Animal Science Journal of Livestock Science and Technologies 2322-3553 15 1 2027 07 01 Differential responses of Ross and Arian broilers to embryonic thermal manipulation and post-hatch heat stress 1 8 5233 10.22103/jlst.2025.26221.1691 EN Zarbakht Ansari Department of Animal Science, Sari Agricultural Sciences and Natural Resources University (SANRU), Sari, Iran Hamid Deldar Department of Animal Science, Faculty of Agriculture, University of Guilan, Rasht, Iran Essa Dirandeh Department of Animal Science, Sari Agricultural Sciences and Natural Resources University (SANRU), Sari, Iran Mojtaba Momennejad Department of Animal Science, Sari Agricultural Sciences and Natural Resources University (SANRU), Sari, Iran Journal Article 2025 11 02 This study investigated the effects of heat stress and embryonic thermal manipulation (TM) on physiological, biochemical, and heat shock protein 70 (HSP70) gene expression in two broiler strains. A total of 400 fertilized eggs were incubated under either normal (37.8 °C) or thermally manipulated (39.5 °C; embryonic day 7 to 16) conditions. Post-hatch chicks were reared under either normal (25 °C) or heat stress (32 °C; from day 21 onward) conditions. The four experimental groups were defined as follows: Arian strain subjected to embryonic thermal manipulation but not to post-hatch heat stress (ATC); Arian strain without embryonic manipulation but subjected to post-hatch heat stress (ACH); Ross 308 strain subjected to embryonic thermal manipulation but not to post-hatch heat stress (RTC); and Ross 308 strain without embryonic manipulation but subjected to post-hatch heat stress (RCH). Blood parameters, lipid profiles, thyroid hormone (T3) levels, and HSP70 gene expression were analyzed at 42 days of age. Significant differences were observed in red and white blood cell counts, hematocrit, and T3 levels among treatments. RCH exhibited higher cholesterol (157.75±2.21) and LDL levels (59.67±2.88), whereas ATC showed higher T3 levels (8.85±0.32) and spleen weight (0.17±0.02). HSP70 gene expression was highest in RCH. The feed conversion ratio (FCR) was significantly lower in RCH (1.93±0.01). In conclusion, RCH demonstrated superior FCR under heat stress, while ATC modulate the thyroidal activity and immune responses following TM. Thermal manipulation during incubation reduced cholesterol, LDL, and HSP70 levels, enhancing thermotolerance. These findings highlighted the potential of TM to mitigate the heat stress effects, emphasizing the importance of strain-specific strategies for optimizing the broiler performance in heat-stressed environments. https://lst.uk.ac.ir/article_5233_248f63cb3f58a7dd17fd7eddf35af168.pdf