Effects of Dietary Crude Protein Level on Growth Performance, Immune Function, and Serum Metabolic Profile in Weaned Piglets

Dietary crude protein (CP) level is a critical nutritional factor influencing growth and intestinal health in weaned piglets. This study investigated the effects of graded dietary CP levels on growth performance, diarrhea incidence, serum immune indices, and metabolic profiles in weaned piglets. Five dietary CP levels (14%, 15%, 16%, 17%, and 18%) were tested, and growth performance and diarrhea incidence were recorded during a 14-day trial. Representative piglets from the 14%, 16%, and 18% CP groups were subsequently selected for serum biochemical and immunological analyses, as well as LC-MS-based untargeted metabolomics. The results showed that average daily gain (ADG) increased linearly (p = 0.004) and the feed-to-gain ratio (F/G) decreased linearly (p = 0.001) with increasing dietary CP level. However, diarrhea rate exhibited a quadratic trend in response to dietary CP level, with the lowest value observed for the 16% CP group, indicating a potential optimal intestinal-health response at this CP level. Serum total protein (TP) and immunoglobulins (IgA, IgG, and IgM) were significantly higher in the 18% CP group than in the 14% CP group (p < 0.05). Metabolomic analysis revealed clear separation among the 14% (n = 5), 16% (n = 4), and 18% (n = 4) CP groups in the PLS-DA model. Differential metabolites were mainly enriched in lipid and amino acid metabolic pathways, with amino acid metabolism being most strongly affected, involving several dipeptides and amino acid derivatives (e.g., Tyr-Pro, His-Leu, and Phe-Trp). Collectively, increasing dietary CP linearly improved growth performance and humoral immunity, whereas diarrhea incidence showed a quadratic numerical trend with the lowest value at 16% CP. These findings provide new metabolic insights for optimizing dietary CP formulation in weaned piglets.