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.