Effects of dietary protein level on growth performance, intestinal morphology, and vaccine-induced antibody responses in Sakini chickens during the starter phase.

Indigenous chickens are widely presumed to perform adequately on low-protein diets; however, their protein requirements under semi-intensive production conditions remain poorly defined. The present study evaluated the effects of graded dietary crude protein (CP) levels on growth performance, intestinal morphology, physiological stress, and Newcastle disease (ND) vaccine-induced antibody responses in Sakini chickens. A total of 300 one-day-old chicks were fed a commercial pre-starter diet for 7 days and then allotted to isocaloric diets (2800 kcal ME/kg) containing 18%, 19%, 20%, 21%, or 22% crude protein (CP) from days 8-63 in a completely randomized design with three replicates per treatment. Feed intake was recorded daily, body weight was measured weekly, and ND antibody titers were determined on days 7, 14, and 21 post-vaccination. At the end of the experiment, intestinal histomorphometry and the heterophil-to-lymphocyte (H/L) ratio were analyzed. Birds fed ≥20% CP showed significantly greater body weight and improved FCR compared with those fed 18-19% CP (p < 0.05). Duodenal villus height and the villus height-to-crypt depth ratio were increased in the higher CP groups, whereas the jejunal morphology remained similar. The lowest H/L ratio was observed in birds fed 22% CP (p < 0.05), indicating reduced physiological stress. Protective ND hemagglutination-inhibition titers (≥ log₂4) were observed only in birds fed 21-22% CP, whereas birds fed ≤20% CP did not consistently reach this level. In contrast, carcass characteristics and muscle pH remained unaffected by dietary CP. These findings indicate that dietary protein level influences growth performance, selected intestinal traits, physiological stress, and vaccine-induced antibody responses in Sakini chickens during the starter phase. Diets containing ≥20% CP support improved growth performance, while higher levels (21-22% CP) are associated with more favorable physiological status and vaccine-induced antibody responses under the conditions of this study.