Abstract
Background:
Antibiotic growth promoters (AGPs) are being phased out in poultry production, leading to the search for functional non-nutritive additives. Synbiotic blends, such as the BBG complex, offer a strategy to enhance gut health and resilience against environmental stressors, particularly suboptimal water quality and heat stress.

Aim:
To evaluate the effect of a synbiotic blend (BBG) comprising Bacillus spp., β-glucans, and β-mannans as a non-nutritive immunostimulant on the productive performance and physiological parameters of broiler chickens.

Methods:
A total of 264 one-day-old Ross broiler chickens were distributed into two treatments (n=132 each) with three replicates of 44 birds each, covering three phases (Pre-starter, Starter, and Finisher). The Control treatment received a commercial diet, while the BBG treatment involved the dietary inclusion (5%) of a conglomerate comprising Bacillus subtilis (10-14%), Bacillus toyonensis (10-14%), Bacillus licheniformis (13-17%), Heyndrickxia coagulans (10-14%), Shouchella clausii (13-17%), β-glucans (1-5%), β-mannans (1-5%). Data were analyzed using a one-way Analysis of Variance. Physicochemical analyses were performed on the feed and water, while production parameters (initial and final body weight, daily feed intake, body weight gain, feed conversion ratio, and mortality) were evaluated. Hematological and biochemical analyses were performed weekly.

Results:
Physicochemical analysis confirmed that the drinking water deviated from regulatory standards, exhibiting a lack of residual chlorine, a slightly acidic pH, and elevated nitrate levels. BBG administration enhanced overall productive performance, yielding higher final body weight and weight gain, alongside a noteworthy reduction in mortality during the Pre-starter phase. Hematological analysis showed marked leukocytosis and elevated liver enzymes (ALT, AST, and bilirubin) in both treatments, confirming active environmental stress. While the BBG treatment did not completely normalize these stress indicators, it improved the birds´ resilience and enhanced feed efficiency during the initial stages.

Conclusion:
The BBG synbiotic blend is a viable alternative to conventional AGPs, improving productive performance and early viability under challenging field conditions characterized by high temperatures and suboptimal water quality.

Key words: Bacillus; Broiler chickens; Metabolic resilience; Non-nutritive additive; Productivity.