Supplementation of a Bacillus-based DFM mixture on growth performance, methane emissions, microbiome, and metabolome in backgrounding beef heifers.

Direct-fed microbials (DFM) have been used to improve livestock production efficiency and reduce environmental impact. Spore-forming bacteria such as Bacillus spp. present handling and processing advantages for use as a DFM; however, more information is needed about their effects in beef cattle. The objective of this study was to evaluate the effect of a DFM composed of a mixture of B. subtilis and B. licheniformis on growth performance, nutrient utilization, enteric methane (CH4) emissions, microbiome of ruminal fluid and feces, and metabolome of plasma and ruminal fluid. Angus crossbred heifers (n = 108; 318 ± 40 kg of body weight) were used in a generalized randomized block design. Heifers were fed a sorghum silage-based diet containing the following treatments: 1) control (CTL, no additive), or 2) Bacillus-based DFM [BOV, 310 mg/kg of diet dry matter (DM)], to provide an average of 3 g/animal/d. No effect of treatment (P > 0.05) was observed for DM intake, average daily gain, or gain-to-feed ratio. The DFM inclusion did not affect (P > 0.05) apparent total tract digestibility of nutrients or CH4 emissions. Alpha and beta diversity of fecal and ruminal microbiome did not differ with supplementation (P > 0.05). Multivariate analysis indicated minimal impact of the DFM on ruminal and plasma metabolomes. In ruminal fluid, supplementation with the DFM reduced the abundance of three metabolites after false discovery rate (FDR) correction (FDR < 0.05), and pathway analysis indicated that the ubiquinone biosynthesis pathway was enriched by DFM supplementation (P ≤ 0.05). In plasma, two metabolites were differentially abundant between treatments, with proline being more abundant and serine less abundant in heifers supplemented with the DFM than in control heifers (P ≤ 0.05). In summary, supplementation with a Bacillus-based DFM did not affect growth performance, efficiency of feed utilization, nutrient digestibility, or CH4 emissions in beef heifers under the experimental conditions of this trial. Minimal changes were observed in the ruminal and fecal microbiomes and the ruminal and plasma metabolomes, suggesting a limited microbial or systemic impact. Further research is needed to clarify the conditions and mechanisms by which Bacillus-based DFM may exert beneficial effects in beef cattle.