38. Effects of Feeding a Bacillus-based Probiotic on Nutrient Utilization and Microbiota Profile in the Rumen and Reproductive Tract of Beef Heifers Receiving a Forage-based Diet

Abstract 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/head daily of Bacillus-based probiotic (BAC; n = 16). Blood samples were collected on d 0, 7, 14, 21, 28, and 35. Rumen fluid samples were collected on d 0, 14, and 28. Vaginal and uterine swabs were collected on d 28. On d 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 d 32 to 35. Data were analyzed using heifer as the experimental unit, and results from d 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 d 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.