Effect of feeding ractopamine hydrochloride (Optaflexx) with or without supplemental zinc and chromium propionate on growth performance, carcass characteristics, and LM quality of finishing steers

Abstract Text: Objectives of this study were to determine the effects of feeding ractopamine hydrochloride (Optaflexx; Elanco Animal Health) with or without supplemental Zn-propionate and Cr-propionate (Kemin Industries Inc.) to finishing steers on growth performance, carcass characteristics, and LM quality. Angus-cross steers (n = 142; BW = 527 ± 6 kg) were blocked by BW into 4 blocks (6 pens/block, 5 to 6 head/pen). Steers were randomly assigned to 1 of 3 dietary supplements for the final 35 d before slaughter, targeted at an average of 0.9 cm 12^th rib backfat, by block, via ultrasound: (1) no supplementation (CONT), (2) 300 mg ractopamine hydrochloride steer^-1·d^-1 (RAC), or (3) 300 mg ractopamine hydrochloride, 1.0 g Zn-propionate, and 3 mg Cr-propionate steer^-1·d^-1 (RAC+TM). All steers were fed a similar basal diet of 20% corn silage, 20% dried distillers grains, 50% corn, and 10% mineral/vitamin supplement (DM basis). Data were analyzed as a randomized complete block design with a fixed effect of treatment and a random effect of block. There were no differences (P ≥ 0.09) between steers fed RAC when compared to steers fed RAC+TM; therefore, data are presented as the contrast of steers fed the average of these 2 treatments versus CONT. Steers fed RAC and RAC+TM had 15% greater (P < 0.01) ADG and G:F compared to steers fed CONT. Dry matter intake did not differ (P = 0.89) between steers fed RAC and RAC+TM compared to steers fed CONT. Carcasses from steers fed RAC and RAC+TM were 6.23 kg heavier (P = 0.02) than carcasses from steers fed CONT. Furthermore, 12^th rib fat thickness was 0.13 cm greater (P = 0.03) and calculated yield grade was 0.18 units greater (P = 0.04) for steers fed RAC and RAC+TM than for steers fed CONT. Warner-Bratzler shear force was 0.39 kg greater (tougher; P = 0.01) for steers fed RAC and RAC+TM when compared with steers fed CONT. However, loin quality, as measured by pH, color, and moisture composition, did not differ (P ≥ 0.19) when steers fed RAC and RAC+TM were compared to steers fed CONT. While steers fed RAC had improved feed efficiency, ADG, and HCW in this study, feedlot growth performance and carcass characteristics were not further improved by additional supplemental dietary zinc and chromium. 

Keywords: ractopamine, chromium, zinc