Effects of selected feed additives on rumen pH dynamics, physiological responses, and rumen and cecum morphometrics in feedlot beef cattle.

The search for alternatives to ionophore antibiotics such as monensin has increased due to regulatory restrictions and consumer demand for natural feed additives. The objective of this study was to evaluate the effects of feed additive combinations compared with sodium monensin on performance, carcass traits, ruminal and cecal health, and meat quality in feedlot-finished cattle. Twenty-four F1 Angus-Nellore crossbred bulls (BW = 456 ± 21.77 kg) were randomly assigned to one of three treatments: 1) sodium monensin (MON; 26 mg/kg DM); 2) a blend of essential oils plus α-amylase (BEO, 90 and 560 mg/kg of DM, respectively); and 3) BEO + 25-hydroxicholecalciferol (BEO + HyD; 1 mg/animal/day). Animals were considered the experimental units, and data were analyzed using a mixed model with treatment as fixed effect and animal as random effect. Feeding BEO and BEO + HyD increased DMI (P < 0.0001), HCW (P = 0.01), and dressing percentage (P = 0.03), and tended to increase ADG (P = 0.07) and final BW (P = 0.08) compared with MON, with no differences in feed efficiency (P > 0.50). Feed additives did not affect most meat quality traits (P > 0.05). During the adaptation period and overall, animals fed BEO and BEO + HyD maintained higher mean and minimum ruminal pH, spent less time below pH 6.2 and 6.0, and had a lower area under the curve below these thresholds, along with lower ruminal temperature compared with MON (P ≤ 0.05). Additionally, over the entire feeding period, animals fed BEO and BEO + HyD spent less time below pH 5.8 (P = 0.02). Animals fed MON and BEO exhibited greater ruminal absorptive surface area (P = 0.007) than those fed BEO + HyD. Cecal morphometric parameters were not affected by treatments (P > 0.05). In conclusion, the combination of a blend of essential oils and α-amylase improved ruminal health and enhanced performance and carcass production without affecting meat quality or cecal morphology compared with monensin. Supplementation with 1 mg of 25-(OH)D3 showed limited additional effects on carcass traits.