Thermal manipulation and rosemary extract: Their combined effects on embryonic development, hatchability, and physiological responses of hatched chicks

This study explored the impact of rosemary extract and ascorbic acid sprays on eggs incubated at high temperatures, focusing on embryonic development, hatchability, and chick physiology. A total of 1,200 eggs were divided into four treatment groups: distilled water, rosemary extract, ascorbic acid, and no spray, under normal and heat stress conditions. Ascorbic acid under heat stress reduced embryo length by day 18. Using 30 g/L ascorbic acid increased egg weight, decreased water loss, reduced embryonic weight, and altered yolk sac to embryo weight ratio (<i>P</i> < 0.05). It also extended incubation time and increased embryonic mortality by day 21, negatively affecting hatchability (<i>P</i> < 0.05). Heat stress raised yolk sac weight and lowered cloacal temperature in chicks. Ascorbic acid increased yolk sac weight but resulted in shorter chicks at hatch. Rosemary extract and distilled water under heat stress increased red blood cell counts, while distilled water at normal temperatures had the highest hematocrit percentage (<i>P</i> < 0.05). Untreated eggs showed higher white blood cell counts and heterophil-to-lymphocyte ratios in chicks. Within 40 h post-hatch, ascorbic acid reduced chick weight but increased yolk sac metrics. Rosemary extract decreased cloacal and body surface temperatures (<i>P</i> < 0.05). By week's end, ascorbic acid reduced chick length, while heat stress improved chick weights and feed conversion ratios during broiler rearing (<i>P</i> < 0.05). In conclusion, rosemary extract and ascorbic acid significantly influence embryonic development and physiological responses. Ascorbic acid notably reduced hatchability and increased embryonic mortality under heat stress, highlighting its critical effects.