Abstract
Background:
Climate change and rising temperatures present significant challenges for the poultry industry, particularly in broiler production. The high metabolic rates of broilers elevate their internal body temperatures, a situation exacerbated by increasingly hot weather conditions.

Aim:
This study investigates the effects of incorporating powdered rosemary leaves into broiler chicken feed at various levels on mortality and biochemical indicators under heat stress conditions.

Methods:
The experiment utilized 180 one-hundred and eighteen one-old unsexed Ross 308 chicks. The birds were raised under controlled environmental conditions until 25 days of age and then divided into four treatment groups. Four experimental groups were subjected to heat stress with varying levels of rosemary powder (0.0%, 0.5%, 1.0%, and 1.5%). Each treatment comprised three replicates, each containing 15 birds. Heat stress exposure for 6 hours with temperature up to 34 C° at 34 and 35 days of age. Key parameters investigated were mortality during the experiment,
enzymatic activity Creatine kinase, Lactate dehydrogenase, and Gamma-glutamyl transferase.

Results:
Results indicated that adding 0.5% and 1.0% rosemary powder significantly reduced mortality rates during the heat stress period and decreased Creatine kinase and lactate dehydrogenase levels in groups receiving rosemary leaves for a period of 10 days before heat stress. Furthermore, the addition of powdered rosemary leaves did not demonstrate significant changes in Gamma Glutamyl Transferase levels.

Conclusion:
These findings suggest that rosemary powder may contribute beneficially to the health of broiler chickens under heat-stress conditions.

Key words: Broiler; Creatine kinase; Gamma-glutamyl transferase; Heat stress; Lactate dehydrogenase; Rosemary leaves.