Effects of feeding a Bacillus-based probiotic to grazing Nellore bulls finished on tropical pasture

Abstract Interest in probiotics as an alternative to ionophores has increased, particularly in pasture-based systems using energy supplementation. While these additives aim to improve feed efficiency and mitigate acidosis risk, probiotics may also modulate rumen function by enhancing fiber digestion. This study evaluated the effects of including a Bacillus -based probiotic in an energy supplement on intake, performance, total tract digestibility, and rumen and cecum morphometrics of finishing Nellore cattle. Twenty-eight bulls (502.6 ± 7.0 kg) were blocked by initial body weight and assigned to four paddocks of Urochloa brizantha cv. Marandu. A ground corn-based supplement was offered daily at 0.3% of body weight for 112 d with either no probiotic (CON; n = 14) or 2 g/animal of Bacillus licheniformis and Bacillus subtilis (6.4 × 10⁹ CFU/g; n = 14) using a Calan gate system. Forage intake and fecal output were estimated using indigestible neutral detergent fiber and titanium dioxide. Probiotic supplementation reduced pasture intake (6.32 vs. 7.18 kg/d; P = 0.04), total dry matter intake (8.23 vs. 9.25 kg/d; P = 0.02), and total crude protein intake (0.83 vs. 0.92 kg/d; P = 0.02), as well as NDF intake (4.40 vs. 5.04 kg/d; P = 0.02), without affecting nutrient digestibility ( P > 0.10). Average daily gain was reduced (0.75 vs. 0.91 kg/d; P = 0.02), as was final P8 fat thickness (6.47 vs. 7.49 mm; P = 0.05), whereas feed efficiency (0.098 vs. 0.101; P = 0.76), hot carcass weight (318.61 vs. 320.32 kg; P = 0.70), and dressing percentage (53.88 vs. 52.65%; P = 0.30) were unchanged. Rumen and cecum morphometrics were not affected. Supplementation with this Bacillus-based probiotic did not improve performance of pasture-finished Nellore bulls, indicating limited benefits under these conditions and highlighting the need to clarify interactions among probiotic strains, supplementation level, and rumen kinetics.