Ionophore and Antimicrobial strategies alter live performance, carcass characteristics, and in vitro rumen fermentation parameters in beef cattle

One hundred ninety-two steers (BW = 354 ± 23.5 kg) were used in a randomized block design to evaluate the effects of ionophore and ractopamine hydrochloride (RH) supplementation strategies on performance and carcass characteristics. Twelve pens of 4 steers were assigned to each of the following treatments: unsupplemented control (CON), laidlomycin propionate (12.1 mg/kg DM) with or without RH (LPRH and LP, respectively), and monensin sodium (36.4 mg/kg DM) with RH (MSRH). Steers were fed for 151 d, of which, respective treatments received RH (Actogain; Zoetis, Florham Park, NJ) at a rate of 300 mg/(head • d) for the final 32 d. Laidlomycin was removed from the LPRH treatment during this period, as no combination feeding has been approved. Upon harvest, carcass data was collected by trained personnel, and subsequent analysis of the LM was conducted in order to estimate tenderness using Warner-Bratzler shear force (WBSF). Prior to RH supplementation, both LP and LPRH had greater ADG (P ≤ 0.02) and G:F (P < 0.01) than CON, while MSRH was intermediate. During the final 32 d, MSRH improved G:F (P ≤ 0.02) compared to all other treatments and tended to increase ADG over unsupplemented controls (P = 0.05). Cattle receiving LP without RH had significantly greater BW at d 151 than CON (P = 0.02), while both RH treatments tended to improve final BW (P ≤ 0.09). Ionophores improved ADG (P ≤ 0.03) and G:F (P < 0.01) for the entire feeding period, and although LP supplemented cattle had greater DMI for the final 32 d than both RH treatments (P ≤ 0.01), intakes for the 151 d trial were similar among treatments. Carcass weights were greater (P = 0.04) in cattle fed LP with no RH than CON, where cattle yielded an average of 12 kg more HCW. Ractopamine increased LM area in MSRH supplemented cattle (P = 0.03) and tended to increase LM area for steers receiving LPRH (P = 0.07). Longissimus steaks of MSRH supplemented cattle had greater WBSF values than CON (P = 0.04) after 7 d of postmortem aging, and greater WBSF values than ...