Broiler performance, intestinal adaptation, and environmental responses to graded dietary inclusion of bacterial single-cell protein.

We evaluated the effects of replacing soybean meal (SBM) with Methylococcus capsulatus bacterial single-cell protein (BSCP) at varying inclusion levels and early dietary exposure on broiler performance, gut physiology, nutrient utilization, and environmental emissions. A total of 480-day-old Ross 308 broilers were allocated to six dietary treatments across starter (d0-10), grower (d11-21), and finisher (d22-35) phases. During the starter phase, birds received either an SBM control diet (N) or a BSCP diet containing 3.125% BSCP (E). In the grower and finisher phases, a 2 × 2 factorial arrangements evaluated low (6.25%; L) or high (12.5%; H) BSCP inclusion within each starter treatment, alongside an SBM control (C). An adaptation treatment (E-l-H) received incremental BSCP inclusion (3.125→6.25→12.5%) across phases. Starter ADG and ADFI were unaffected by diet, although FCR tended to be better in N birds (1.19 vs. 1.23; P = 0.07). Overall growth performance was maintained across treatments (P > 0.05). Orthogonal contrasts showed higher ADG (67.5 vs. 64.1 g/bird/d; P = 0.002) and a tendency for improved FCR (1.31 vs. 1.37; P = 0.08) in C birds during the grower phase, whereas BSCP-fed birds tended to consume more feed during the finisher phase (160.4 vs. 152.3 g/bird/d; P = 0.08). Apparent ileal dry matter digestibility increased with high BSCP inclusion during the grower and finisher phases (P < 0.01). Lean tissue accretion was maintained, although fat mass increased during the finisher phase (P = 0.03) with BSCP inclusion. Early BSCP exposure interacted with dietary inclusion level to increase intestinal aminopeptidase-N gene expression during the grower phase (P = 0.03), suggesting enhanced digestive adaptation. Expression of ileal tight junction genes claudin-1, claudin-5, and zonula occludens-1 reduced in BSCP-fed birds, while claudin-2 expression increased in grower phase (P < 0.05). Starter excreta uric acid concentration was elevated in BSCP-fed birds (29.8 vs. 21.9 mg/g; P = 0.03), whereas litter gas emissions were unaffected (P > 0.05). Collectively, these findings demonstrate that early BSCP exposure facilitates adaptation to dietary inclusion levels up to 12.5% without compromising growth performance or environmental emissions, although a transient metabolic response during the grower phase supports a phased inclusion strategy.