Bacillus velezensis CML532 supplementation improved growth performance, protein digestion, and gut health in broilers fed with corn-miscellaneous meal-based low-protein diets

The poultry industry faces challenges due to its high reliance on imported soybean meal (SBM) and the rising costs of conventional protein ingredients, prompting interest in using miscellaneous meals as alternative protein sources. However, low-protein diets formulated with these meals often impair broiler growth performance and protein digestion. In the present study, the effects of <i>Bacillus velezensis</i> CML532 supplementation on growth performance, protein digestion, and gut health were investigated in broilers fed corn-miscellaneous meal-based diets without SBM during the grower and finisher phases. A total of 252 one-d-old healthy male Shengze 901 plus broiler chicks (initial body weight 43.76 ± 0.61 g) were randomly assigned to 3 dietary treatments, each comprising 6 replicates of 14 birds: control group, corn-miscellaneous meal-based normal-protein diet (CM); corn-miscellaneous meal-based low-protein diet (CML) group; and CML diet with supplemental 5 × 10⁹ colony-forming unit (CFU)/kg <i>B. velezensis</i> CML532 (CMLB) group. The experiment lasted 42 d. The results showed that, compared with the CM group, the lower crude protein level (-2%) in the CML group significantly increased feed conversion ratio (FCR), and decreased half-eviscerated rate, eviscerated rate, and breast muscle rate (<i>P</i> < 0.05). Meanwhile, the mRNA expression of calcium sensing receptor (<i>CaSR</i>) and cationic amino acid transporter 1 (<i>CAT1</i>), as well as the concentration of anti-inflammatory factor interleukin-10 (<i>IL-10</i>), were significantly downregulated (<i>P</i> < 0.05). Compared with the CML group, <i>B. velezensis</i> supplementation significantly decreased FCR, improved slaughter traits, increased apparent protein and amino acid digestibility (<i>P</i> < 0.05), and reduced serum uric acid levels (<i>P</i> < 0.001). Notably, growth performance in the CMLB group in terms of final body weight and FCR of the whole growth period was comparable to that in the CM group (<i>P</i> > 0.05). In the jejunum, the CMLB group significantly increased the expression of intestinal epithelial barrier proteins, and elevated the activities of α-amylase and chymotrypsin in the digesta compared with both the CM and CML groups (<i>P</i> < 0.05). Furthermore, the mRNA expression levels of amino acid sensing receptors <i>CaSR</i>, taste receptor type 1 member 1 (<i>T1R1</i>), G protein-coupled receptor class C group 6 member A (<i>GPRC6A</i>), and transporter <i>CAT1</i> were markedly upregulated in the CMLB group (<i>P</i> < 0.05). Ileal microbial analysis revealed a decreased relative abundance of <i>uncultured_bacterium_g_Lactobacillus</i> in the CML group, whereas <i>B. velezensis</i> supplementation increased the enrichment of <i>Coriobacteriaceae</i> <i>_bacterium_CHKCI002</i> and <i>B. velezensis</i>, accompanied by enhanced His metabolism and biosynthesis pathways. These results indicated that <i>B. velezensis</i> CML532 supplementation improved protein digestion and utilization by enhancing digestive enzyme activity and modulating gut microbial composition, thereby promoting growth performance and intestinal health in broilers fed with low-protein corn-miscellaneous meal-based diets to levels comparable with those fed normal protein diets.