PS8-26. Residual Feed Intake Phenotype Modulates Growth Performance and Ruminal Microbiome Responses to a Bacillus-based Direct-fed Microbial in Beef Steers.

Abstract Dietary supplementation with direct-fed microbials may improve cattle performance through rumen microbiome modulation; however, responses are inconsistent and may depend on host feed efficiency phenotype. This study evaluated the effects of a Bacillus-based direct-fed microbial (DFM) on growth performance and the ruminal microbiome of beef steers with divergent residual feed intake (RFI) phenotypes. Following a 49-d RFI test for 108 Angus × Hereford beef steers (initial BW = 242 ± 14.4 kg), 40 steers (BW = 332 ± 18 kg) representing negative (n = 20; RFI = -1.66, more efficient) or positive (n = 20; RFI = 1.62, less efficient) RFI were used in a 70-d feeding trial arranged as a 2 × 2 factorial with RFI phenotype (negative vs. positive) and diet (control vs. Bacillus DFM; 14 g/steer/d; 1.5 × 10⁸ CFU/g, Papillon, Easton, MD). Treatments were replicated across 12 pens (3 pens/treatment; 3–4 steers/pen). Rumen fluid collected on d 70 was used for 16S rRNA gene (V3–V4) sequencing; sequences were processed in QIIME 2 with DADA2, and MicrobiomeAnalyst (microbiomeanalyst.ca). A diet × RFI interaction was detected for final BW and ADG (P = 0.02), with Bacillus supplementation increasing final BW (489 vs. 467 kg) and ADG (2.19 vs. 2.05 kg/d) in negative-RFI steers but not in positive-RFI steers. Dry matter intake differed by RFI (P = 0.03) but was not affected by diet. Chao1 richness and beta diversity index exhibited diet × RFI interactions (P = 0.05 and P = 0.01, respectively), with corresponding interactions for Bacteroidota, Firmicutes, Prevotella, Ruminococcus, and Lachnospiraceae_NK3A20_group. These results indicate that Bacillus-based DFM responses are phenotype dependent as more efficient beef cattle benefited more from the supplementation, supporting precision supplementation strategies based on feed efficiency.