This study was conducted to evaluate the effects of different levels of the probiotic Ecobacto‑P on the performance, carcass characteristics, blood parameters, intestinal microbial composition, and histological features of Ross 308 broiler chickens exposed to heat stress. The experiment was arranged in a completely randomized design with four dietary treatments and five replicates per treatment (12 birds per replicate; a total of 240 chicks) in two separate environmentally controlled houses: one maintained under standard rearing temperature and the other under a controlled heat‑stress regimen. The experimental treatments included: (1) positive control (standard temperature conditions), (2) negative control (heat stress without additive), (3) negative control + 200 g/t Ecobacto‑P, and (4) negative control + 300 g/t Ecobacto‑P.
The results indicated that during the starter and grower phases, supplementation with Ecobacto‑P significantly increased body weight and improved feed conversion ratio under heat‑stress conditions. During the finisher phase and the entire rearing period, the Ecobacto‑P‑1 group (200 g/t of feed) exhibited the best feed conversion ratio and the highest European Production Efficiency Factor (EPEF). Carcass trait evaluation showed that the probiotic had no significant effect on breast and spleen weights; however, differences were observed in carcass yield and certain internal organs. Analysis of ileal microflora revealed that a higher level of probiotic increased the population of Gram‑negative bacteria (Escherichia coli and Salmonella), whereas the Lactobacillus count remained unchanged.
Regarding blood metabolites, the lower supplementation level of Ecobacto‑P significantly enhanced total protein and reduced alkaline phosphatase activity, while the higher level increased uric acid and creatinine concentrations. Hematological analysis demonstrated that probiotic supplementation increased lymphocyte count and decreased the heterophil‑to‑lymphocyte