Effects of Dietary Bacillus spp. and Bacillus spp.-Fermented Soybean Hull on Growth Performance and Apparent Nutrient Digestibility in Growing–Finishing Pigs

This study investigated the fermentation improvement of soybean hull by Bacillus subtilis and Bacillus licheniformis from Ningxiang pigs and evaluated different feeding methods in growing-finishing pigs. The cellulase-producing capacity of the two strains and their fermentation effects on soybean hull were determined. Compound Bacillus preparation (200 and 400 g/t) and fermented soybean hull (5% and 10%) were supplemented into finishing pig diets, and their effects on growth performance; apparent nutrient digestibility; and serum biochemical, immune, and antioxidant indices were evaluated. In vitro results showed that combined fermentation significantly decreased crude fiber content and increased crude protein and ether extract concentrations. Animal trial results showed that supplementation with 400 g/t compound Bacillus preparation or 5% fermented soybean hull significantly increased final body weight but did not significantly affect average daily gain (ADG), average daily feed intake (ADFI), or feed-to-gain ratio (F/G). In the compound Bacillus preparation trial, supplementation increased the apparent digestibility of crude protein and gross energy and was associated with changes in several serum antioxidant and inflammatory indicators. In the fermented soybean hull trial, supplementation improved crude protein utilization efficiency, with no significant effects on the measured serum immune parameters. Supplementation with 200 g/t compound Bacillus preparation or 10% fermented soybean hull showed no growth-promoting effects. Neither method adversely affected the measured growth performance, nutrient digestibility, or serum parameters. The present study demonstrates that appropriate doses of Ningxiang pig-derived compound Bacillus preparation or its fermented soybean hull can increase final body weight and nutrient digestibility in growing–finishing pigs. These findings provide experimental support for the application of Bacillus spp. in feed fermentation and finishing pig production.