Effects of Butyric Acid Supplementation on the Gut Microbiome and Growth Performance of Weanling Pigs Fed a Low-Crude Protein, Propionic Acid-Preserved Grain Diet

Reducing crude protein (CP) in weaner pig diets lowers post-weaning diarrhoea risk but may impair growth performance. This study aimed to identify the beneficial effects of organic acid (OA)-preserved grain and butyric acid supplementation on gut health and growth in low-CP diets. At harvest, grain was divided into two batches: one dried at 65 °C, the other treated with a propionic acid. Ninety-six piglets (28 days old) were assigned to four treatments: (1) dried grain, (2) OA-preserved grain, (3) dried grain + 3% butyric acid, and (4) OA-preserved grain + 3% butyric acid. On day 8, microbial composition, inflammatory markers, volatile fatty acids, and intestinal morphology were assessed. The OA-preserved grain improved feed conversion ratio (<i>p</i> < 0.05) increased beneficial gut bacteria (<i>p</i> < 0.01), elevated caecal butyrate (<i>p</i> < 0.05), reduced jejunal <i>CXCL8</i> expression (<i>p</i> < 0.05), and enhanced nutrient digestibility (<i>p</i> < 0.01). Butyric acid reduced feed intake (<i>p</i> < 0.05), improved nutrient digestibility (<i>p</i> < 0.01), decreased colonic <i>Proteobacteria</i> (<i>p</i> < 0.05), and increased colonic propionate and butyrate (<i>p</i> < 0.01). Combining OA-preserved grain with butyric acid elevated ileal <i>Proteobacteria</i> and <i>Pasteurellaceae</i> (<i>p</i> < 0.05). In conclusion, while OA-preserved grain improves feed efficiency, nutrient digestibility, and gut microbiota, supplementing butyric acid enhances nutrient digestibility but reduces feed intake, and their combination may disrupt the microbial balance.