Effects of feeding a <i>Bacillus</i> -based probiotic on nutrient utilization and microbiota profile in the rumen and reproductive tract of beef heifers receiving a forage-based diet

This experiment evaluated the effects of feeding a Bacillus-based probiotic on rumen fermentation traits, apparent nutrient digestibility, microbiota profile in the rumen and reproductive tract of beef heifers receiving a forage-based diet. Sixteen rumen-cannulated, pubertal Angus-influenced heifers were used in a crossover design (35-d periods). Treatments included hay and concentrate (CON; n = 16) or CON with the addition of 3 g/heifer daily of Bacillus-based probiotic (BAC; n = 16). Blood samples were collected on days 0, 7, 14, 21, 28, and 35. Rumen fluid samples were collected on days 0, 14, and 28. Vaginal and uterine swabs were collected on day 28. On day 32 of each period, ruminal liquid fill, liquid dilution rate, forage dry matter (DM) and neutral detergent fiber (NDF) disappearance were assessed for 96 h. Apparent total-tract digestibility (ATTD) of nutrients was calculated using fecal samples collected from days 32 to 35. Data were analyzed using heifer as the experimental unit, and results from day 0 as covariate when appropriate. Ruminal fluid pH and ammonia concentrations did not differ between treatments (P ≥ 0.28). Acetate proportion in the ruminal fluid was reduced (P < 0.01), whereas rumen propionate, butyrate, and iso-valerate proportions were increased (P ≤ 0.05) in BAC heifers. No treatment differences were observed (P ≥ 0.14) for rumen fluid phyla. Relative abundances of the genera Bacteroides, Pedobacter, and Clostridium were less (P ≤ 0.04) in the rumen fluid of BAC heifers. Rumen liquid volume and dilution rate were not affected (P ≥ 0.63) by treatments. Heifers assigned to BAC had greater (P ≤ 0.04) effective ruminal degradability of DM and NDF, as well as ATTD of DM, CP, and NDF. Mean plasma glucose and insulin concentrations were greater (P ≤ 0.04) while mean blood urea-N concentration was less (P < 0.01) in BAC heifers. Plasma IGF-I concentration was greater (P = 0.04) in BAC heifers on day 21 (treatment × day; P = 0.04). Heifers receiving BAC had reduced (P = 0.05) abundance of the phylum Bacteroidetes, and reduced (P ≤ 0.05) abundance of the genera Pedobacter and Bacteroides in vaginal swabs. No treatment effects were observed (P ≥ 0.29) for phyla or genera relative abundances in uterine swabs. Supplementing a Bacillus-based probiotic improved nutrient utilization, rumen fermentation traits, and physiological responses of beef heifers consuming a forage-based diet, with effects on the reproductive tract limited to the vaginal microbiota.