PS13-23. Effect of Monensin and Narasin on Performance of Finishing Beef Cattle.

Abstract This study aimed to evaluate the effects of monensin (Rumensin; Elanco Animal Health, São Paulo, Brazil) and narasin (Zimprova; Elanco Animal Health) on performance of feedlot-finished Nellore cattle. A total of 156 animals were ranked by initial body weight (BW = 336 ± 8 kg) and randomly assigned to 52 pens (3 animals/pen), which served as the experimental units and were allocated to 1 of 4 dietary treatments: 1) a total mixed ration with no feed additives administered throughout the 100-d experimental period (CON; n = 13), 2) CON + monensin at 28 mg/kg dry matter (DM) from d 1 to 100 (MON; n = 13), 3) CON + narasin at 13 mg/kg DM from d 1 to 97 d (NAR; n = 13), or 4) CON with sequential monensin (28 mg/kg DM; d 1–49) followed by narasin (13 mg/kg DM; d 50–97) supplementation (MNS; n = 13). During the first phase (d 0–49), DMI was greater for NAR (9.9 kg/d) vs. MON (9.2 kg/d; P = 0.01), whereas CON (9.5 kg/d) and MNS (9.2 kg/d) did not differ from either treatment (P > 0.05). During phase 2 (d 50–100), DMI differed among treatments (P = 0.03), being greater for NAR (11.4 kg/d) than MON (10.5 kg/d), while CON (10.9 kg/d) and MNS (11.2 kg/d) were intermediate (P > 0.05). Across the entire period (d 0–100), DMI was greater for NAR (10.7 kg/d) than MON (9.8 kg/d; P = 0.01), with CON (10.2 kg/d) and MNS (10.2 kg/d) remaining intermediate (P > 0.05).During phase 1 (d 0–49), ADG differed among treatments (P < 0.01), with greater gain for NAR (2.070 kg/d) vs. CON (1.85 kg/d), MON (1.91 kg/d), and MNS (1.91 kg/d), which did not differ (P > 0.05). No treatment effects were detected for ADG during the second phase (d 50–100; P = 0.13) or across the overall experimental period (d 0–100; P = 0.11). An effect of treatment was observed for intermediate BW on d 50 (P < 0.01), with greater BW for NAR (439.0 kg) vs. CON (428.4 kg), MON (431.1 kg), and MNS (431.2 kg), whereas CON, MON, and MNS did not differ from one another (P > 0.05). No treatment effects (P = 0.11) were detected for final BW among treatments (514.1, 516.9, 524.3, and 522.3 kg for CON, MON, NAR, and MNS, respectively). In conclusion, narasin supplementation increased DMI and early growth performance, resulting in greater BW at mid-trial; however, monensin supplementation, alone or followed by narasin, improved feed efficiency without compromising overall ADG. These results indicate that ionophore source and supplementation strategy differentially influence intake, growth, and efficiency responses, highlighting opportunities to optimize feedlot performance through targeted additive use.