176. Award Talk: Probiotics as Immunomodulator in Beef Cattle Exposed to Stress Challenges.

Abstract

A series of experiments evaluated the effects of Bacillus-based direct-fed microbials (DFM), alone or in combination with yeast-derived prebiotics, on rumen function, immune and metabolic responses, and productivity of beef cattle managed across forage-based, grazing, and feedlot systems. We hypothesized that Bacillus-based DFM enhances nutrient utilization and immunocompetence, thereby improving health and productivity, particularly under stress conditions. In Exp. 1, sixteen rumen-cannulated Angus-influenced heifers were used in a crossover design (35-d periods) and fed a forage-based diet with or without Bacillus-based DFM. Supplementation increased effective ruminal degradability and apparent total-tract digestibility of dry matter and neutral detergent fiber (P ≤ 0.04), shifted rumen fermentation toward greater propionate and butyrate proportions (P ≤ 0.05), and increased plasma glucose and insulin concentrations while reducing blood urea-N (P ≤ 0.04). Modifications to rumen microbiota were observed, whereas effects on reproductive tract microbiota were limited. In Exp. 2, yearling heifers receiving forage-based diets were subjected to vaccination, lipopolysaccharide (LPS) challenge, or transportation stress. Bacillus-based DFM reduced rectal temperature and plasma tumor necrosis factor-α following vaccination (P ≤ 0.05). Following LPS challenge, supplemented heifers had greater feed intake and reduced plasma cortisol, haptoglobin, and LPS- binding protein concentrations, as well as reduced expression of inflammatory and oxidative stress-related genes (P ≤ 0.05). Responses to transportation stress were largely unaffected. In Exp. 3, high-risk, auction-derived steers grazing native pasture for 90 d received dried distillers’ grains with or without Bacillus-based DFM. Although average daily gain did not differ (P ≥ 0.73), supplemented steers had reduced plasma haptoglobin during the initial grazing period (P ≤ 0.02), reduced mortality and removals (P = 0.01), and greater pasture-level total liveweight gain and productivity (P ≤ 0.04). In Exp. 4 and 5, feedlot steers were offered synbiotic supplements (yeast-derived prebiotic + Bacillus-based DFM) to replace or complement monensin and tylosin. Synbiotic supplementation increased feed intake during the receiving period (P ≤ 0.05), reduced the proportion of cattle requiring multiple antimicrobial treatments and removals due to severe morbidity (P ≤ 0.03), and maintained body weight gain and morbidity responses comparable to feed-grade antimicrobials. During the finishing period, synbiotic supplementation maintained growth performance, increased carcass marbling and proportion of carcasses grading Choice or better (P ≤ 0.05), and improved Longissimus muscle area, while effects on liver abscess incidence were dependent on inclusion of antimicrobials. In summary, Bacillus-based DFM, alone or combined with yeast-derived prebiotics, enhanced nutrient utilization, attenuated inflammatory responses to immune challenges, reduced health-related losses, and improved productivity across grazing and feedlot systems. These results support Bacillus-based DFM as effective nutritional strategies to promote cattle health and efficiency while reducing reliance on feed-grade antimicrobials.