Effects of Dietary Crude Protein Level on Growth Performance, Carcass Traits, Meat Quality, and Fatty Acid Composition of Ningxiang Finishing Pigs

This study investigated the effects of different crude protein (CP) levels on growth performance, carcass traits, meat quality, and fatty acid composition in Ningxiang finishing pigs. A total of 200 pigs (52.52 ± 0.41 kg) were assigned to five dietary treatments: high-protein (HP, 15.56%), moderate-high-protein (MHP, 13.99%), moderate-protein (MP, 12.94%), moderate-low-protein (MLP, 11.90%), and low-protein (LP, 10.31%). Feeding the MLP diet significantly improved average daily gain (ADG) compared to HP and LP diets (p < 0.05). Pigs fed the MP diet had higher lean meat percentage than those on the LP diet (p < 0.05), while both HP and MP diets reduced carcass fat percentage (p < 0.05). The LP diet significantly increased a*, pH45min, intramuscular fat (IMF), and C18:1n9c, while decreasing C23:0 and C20:3n6 in the longissimus dorsi muscle (p < 0.05). Liver metabolomics revealed that the LP diet inhibited nicotinate and nicotinamide metabolism (p < 0.05), accompanied by downregulation of genes related to the tryptophan–niacin metabolism and upregulation of genes involved with hepatic lipogenesis (p < 0.05). In muscle, the LP diet inhibited AMPK signaling via decreased p-AMPK expression, leading to increased IMF content (p < 0.05). In summary, the optimal growth performance of Ningxiang finishing pigs was achieved with a CP level of 11.90%. Additionally, the LP diet enhanced meat quality by modulating hepatic niacin metabolism and AMPK signaling.