Supplementation with Bacillus-based direct-fed microbials for developing beef heifers: impacts on fecal and vaginal microbiome, growth, and reproductive performance.

This 2-year study evaluated the effects of Bacillus-based direct-fed microbial (DFM) supplementation on fecal and vaginal microbiomes, growth, and reproduction of beef heifers. On day 0 (30 days after weaning), Brangus crossbred beef heifers (n = 64 per year) were stratified by body weight (BW; 261 ± 29 kg) and age (286 ± 11 days) and randomly allocated into 1 of 16 bahiagrass pastures (1 ha and 4 heifers per pasture). Treatments (8 pastures per treatment) consisted of daily concentrate supplementation at 1.50% of BW (dry matter basis), with (BAC) or without (CON) a DFM supplement containing Bacillus subtilis 810 and B. licheniformis 809 (3 g per heifer; 6.6 × 109 CFU per day) from day 0 to 244. Heifers were estrus-synchronized from day 98 to 112, artificially inseminated (AI) on days 109 to 112, and exposed to Angus bulls from day 120 to 210 (1 bull per pasture). Fecal and vaginal swab samples were collected on days 0, 112 (timed-AI), and 210 (end of the breeding season). Effects of treatment × year were not detected (P ≥ 0.18) for any variable. Treatments did not affect herbage utilization, heifer BW, or plasma concentrations of glucose and insulin-like growth factor 1 (IGF-1). Percentage of pubertal heifers and reproductive tract scores were greater on day 60 (P = 0.05), but not on day 90 (P ≥ 0.22), for BAC vs. CON heifers. Pregnancy per AI tended (P = 0.10) to be greater for BAC vs. CON heifers. Final pregnancy percentage did not differ (P = 0.65) between treatments, whereas a greater (P ≤ 0.05) proportion of BAC heifers calved within the first 28 days of the calving season. Effects of treatment × day were detected for fecal Firmicutes, Bacteroidetes, and Clostridium, Butyrivibrio, and Blautia, which were greater in BAC vs. CON heifers on day 112 (P ≤ 0.02). Fecal Shannon and Simpson diversity indexes on day 112 were lower (P < 0.01) for BAC vs. CON heifers. Effects of treatment were detected (P ≤ 0.04) for vaginal Methanobrevibacter and Euryarchaeota, both of which were lower overall in BAC vs. CON heifers. These findings indicate that Bacillus-based DFM supplementation modulated fecal and vaginal microbial communities and improved reproductive outcomes in developing beef heifers, suggesting that reproductive benefits may be partly mediated through microbiome-host interactions.