Feedlot performance, rumen and cecum morphometrics of Nellore cattle fed increasing levels of diet starch containing a blend of essential oils and amylase or monensin

**Background:** High-concentrate diets are used in feedlot cattle to maximize energy intake and performance, but they increase the risk of metabolic disorders such as acidosis. Monensin (MON) is widely used to improve feed efficiency and control rumen acidosis, but its classification as an antibiotic has prompted a search for natural alternatives. Blends of essential oils (BEO) have antimicrobial, anti-inflammatory, and antioxidant properties that may modulate rumen fermentation similarly to ionophores. Exogenous α-amylase may further improve starch utilization. However, no prior study had evaluated the combination of BEO plus α-amylase across increasing dietary starch levels in feedlot cattle.

**Methods:** The study used 210 Nellore bulls (initial BW 375 ± 13.25 kg) housed in 30 pens (7 animals/pen) at a feedlot in Brazil. The experiment used a completely randomized block design with a 3 × 2 factorial arrangement: three starch levels (25%, 35%, 45%) and two additives (BEO: 90 mg/kg DM essential oils + 560 mg/kg DM α-amylase; or MON: 26 mg/kg DM sodium monensin). Diets contained sugarcane bagasse, ground corn, soybean hulls, cottonseed, soybean meal, and a mineral-vitamin core. Animals underwent a 10-day pre-adaptation, a 14-day step-up adaptation (65% and 75% concentrate for 7 days each), and an 89-day finishing period (85% concentrate). Performance, carcass traits, feeding behavior, rumen and cecum morphometrics, and meat quality were evaluated. Data were analyzed using the MIXED procedure of SAS with Tukey test (P ≤ 0.05).

**Key Results:** A significant interaction between starch level and additive was observed for final BW (P = 0.04), ADG (P = 0.02), and DMI (P = 0.02). Cattle fed BEO35 had the highest final BW (516.72 kg), ADG (1.58 kg/d), and DMI (10.68 kg/d). MON improved feed efficiency (G:F: 0.150 vs. 0.146 for MON vs. BEO, P = 0.02). There was a significant linear decrease in G:F with increasing starch (0.151, 0.149, 0.145 for 25%, 35%, 45% starch, respectively; P = 0.05). BEO35 and BEO45 increased hot carcass weight (287.42 and 281.38 kg, respectively) compared to MON35 and MON45 (270.91 and 263.00 kg; P < 0.01). BEO resulted in greater dressing percentage (55.38% vs. 54.56%, P = 0.01), carcass gain (66.78% vs. 64.33%, P < 0.01), final rib-eye area (67.72 vs. 65.47 cm², P = 0.02), and rib-eye area gain (0.18 vs. 0.16 cm²/d, P = 0.01). MON25 improved carcass gain efficiency (0.102 kg/kg, P = 0.01), final marbling (2.72%, P = 0.04), and final subcutaneous fat thickness (4.33 mm, P < 0.01). MON reduced fecal starch (5.90% vs. 8.48%, P = 0.05). BEO increased rumen absorptive surface area (32.78 vs. 29.95 cm²/cm², P = 0.05), papillae area as % of ASA (97.06% vs. 96.74%, P < 0.01), and papillae height (4.20 vs. 3.97 mm, P = 0.03). MON reduced cecum lesions score (1.43 vs. 2.11, P = 0.02). No significant differences were found for meat color, shear force, or protein content.

**Clinical Implications:** The blend of essential oils plus α-amylase can serve as an effective alternative to monensin in high-starch finishing diets for Nellore cattle, particularly when aiming to increase carcass weight and dressing percentage. BEO at 35% starch optimized performance and carcass traits, while MON at 25% starch improved feed efficiency and fat deposition. However, BEO was associated with higher cecum lesion scores, suggesting that intestinal health should be monitored when using this additive in high-starch diets. These findings support the use of natural feed additives to replace ionophores in beef production systems, aligning with regulatory restrictions on antibiotic growth promoters.