PS13-18. Effects of Monensin Alone or Combined with Ractopamine Hydrochloride, Fenbendazole, or Ivermectin on Growth Performance of Backgrounding and Finishing Beef Steers.

Abstract This study evaluated the effects of monensin (MON), MON and ractopamine hydrochloride (MON+RAC), MON and fenbendazole (MON+FBZ), and MON and ivermectin (MON+IVR) on growth performance of backgrounding and finishing beef steers and carcass merit. Two hundred weaned beef steers (303 ± 20.5 kg) were used in a completely randomized design with five treatments: control (CON), MON, MON+RAC, MON+FBZ, and MON+IVR (n = 4 pens/treatment; 10 steers/pen). Following a 14-d adaptation period, MON was included in all non-control dietary supplements to provide 33 mg rumensin/kg diet dry matter (DM) beginning on d 6. Ivermectin pour-on was administered at 500 µg/kg body weight (BW) on d 6. Fenbendazole oral drench was administered as a liquid suspension at 5 mg/kg BW on d 6. Ractopamine hydrochloride was included at 30 mg/kg diet DM beginning on d 265 for 42-d, with a 7-d withdrawal period prior to slaughter; therefore, MON+RAC steers received MON only during backgrounding. The study comprised an 84-d backgrounding phase, a 35-d transition, and a 191-d finishing phase. Steers were weighed on 2 consecutive days at the start and end of each phase, with interim weights collected approximately every 21 d during backgrounding and every 28 d during finishing. Feed was provided ad libitum and orts were measured weekly; feed intake was corrected for DM. Two animals were removed due to lameness (1 × MON; 1 × MON+RAC). Pen means were calculated for initial and final BW, average daily gain (ADG), dry matter intake (DMI), and gain to feed ratio (G:F) by phase and overall. Data were analyzed using the MIXED procedure of SAS with treatment as a fixed effect and pen as the experimental unit. During backgrounding, treatment did not affect initial or final BW, ADG, DMI, or G:F (P ≥ 0.12). During finishing, final BW was affected by treatment (P = 0.05), with MON+FBZ steers being heavier than MON steers, and ADG differed among treatments (P = 0.05), where MON and MON+RAC had lower ADG compared to all other treatment groups. Finishing DMI and G:F were not affected by treatment (P ≥ 0.24). Overall ADG, DMI, and G:F did not differ among treatments (P ≥ 0.13). Hot carcass weight (HCW) tended to be greater (P = 0.07) for MON+FBZ compared to MON steers. Quality grade, yield class, dressing percentage, retail cut yield, ribeye area, ribfat thickness, and marbling did not differ by treatment (P ≥ 0.15). In conclusion, supplementing feedlot steer diets with monensin combined with antiparasitic agents or ractopamine had limited effects on growth performance across backgrounding and finishing phases. Fenbendazole administered at feedlot entry was associated with greater finishing BW compared to monensin alone, suggesting a potential growth benefit of strategic parasitic control in incoming beef steers.