Interactive effects of dietary crude protein reduction and resistant starch inclusion on growth performance and cecal microbial fermentation in broiler chickens

This study investigated the interaction between dietary crude protein (CP) reduction and supplemental raw potato starch (RPS) on growth performance, nutrient digestibility, cecal short-chain fatty acids (SCFA) profiles, and cecal microbiota in broiler chickens. On day 7, birds were allocated to a 2 × 3 factorial arrangement in a randomized complete block design with eight replicates (22 birds per pen). Six dietary treatments comprised three CP levels (standard diet or reductions of 15 or 30 g/kg CP) and two RPS inclusion levels (20 or 40 g/kg) during both grower (day 7-21) and finisher (day 21-35) stages. The main effects of RPS level, as well as the linear and quadratic effects of dietary CP and their interactions, were evaluated using orthogonal polynomial contrasts. An interaction (P < 0.05) between RPS level and the quadratic effect of dietary CP levels was observed for weight gain and feed intake during the experimental period. Birds fed 20 g/kg RPS showed linear reductions in weight gain (WG) and feed intake (FI) as dietary CP decreased, whereas those fed 40 g/kg RPS showed a quadratic response, with the highest performance at the intermediate CP level. Cecal concentrations of acetate, butyrate, straight-chain fatty acids, and total SCFA on day 35 showed interactions (P < 0.05) between RPS level and dietary CP, following trends similar to growth performance. Isobutyrate and branched-chain fatty acids exhibited an interaction (P < 0.05) between RPS level and linear CP reduction, with linear decreases observed in birds fed 40 g/kg RPS but not in those fed 20 g/kg RPS. Irrespective of RPS level, decreasing dietary CP reduced the abundance of protein-fermenting microbes and enriched fiber-fermenting microbes (P < 0.05). Correlation analysis showed that straight-chain fatty acids were positively correlated with WG and negatively correlated with feed conversion ratio, whereas branched-chain fatty acids were negatively correlated with WG and FI. In conclusion, growth performance responses to reduced-protein diets in broilers can be modulated by dietary RPS inclusion level through shifts in cecal microbiota and fermentation patterns.