Effects of Dietary Protein Level on Growth Performance, Serum Biochemistry, Intestinal Morphology, and Intestinal Microbiota in Ningxiang Finishing Pigs

ABSTRACT This study investigates the effects of different crude protein (CP) levels on growth performance, serum biochemistry, organ indices, intestinal morphology, colonic volatile fatty acids, and gut microbiota in Ningxiang finishing pigs. Ninety‐six pigs (53.20 ± 0.53 kg) were randomly assigned to three dietary treatments: high‐protein (HP, 15.56% CP), medium‐protein (MP, 12.94% CP), and low‐protein (LP, 10.31% CP), with four replicate pens per treatment and eight pigs per pen. Results showed that dietary CP levels had no significant effects on growth performance. However, the LP diet significantly reduced serum urea nitrogen, liver weight, and relative liver weight (p < 0.05). Additionally, jejunal crypt depth showed a linear decrease in response to graded reductions in dietary CP levels (Linear, p < 0.05). The LP diet significantly decreased the contents of isobutyric, isovaleric, and branched‐chain fatty acids in colonic fermentation products (p < 0.05). Furthermore, 16S rRNA sequencing revealed that the relative abundances of Terrisporobacter, Marvinbryantia, Turicibacter, Lachnospiraceae_AC2044_group, unclassified_f_Peptostreptococcaceae, norank_f_Eubacter_coprostanoligenes_group, Lachnospiraceae_UCG‐007, and UCG‐009 were significantly higher in the LP group (p < 0.05). Spearman correlation analysis indicated that isobutyric acid and isovaleric acid were negatively correlated with Lactobacillus and positively correlated with Streptococcus. In conclusion, the LP diet improved colonic microbiota composition while maintaining growth performance in Ningxiang finishing pigs. These results advance our understanding of protein nutrition in indigenous fat‐type pig breeds, providing a theoretical foundation for optimizing dietary formulations specifically in Ningxiang pigs.