Effect of dose and duration of ractopamine hydrochloride supplementation on growth performance and carcass characteristics of feedlot heifers1

Our objectives were to determine the effects of dose and duration of ractopamine hydrochloride (RAC) supplementation on growth performance and carcass characteristics of feedlot heifers. Heifers (n = 128) were allotted to 20 pens. Pens were randomly assigned 1 of 4 treatments: (1) 0 mg of RAC/heifer per d for 28 d, (2) 300 mg of RAC/heifer per d (Optaflexx 45; Elanco Animal Health, Greenfield, IN) for 28 d, (3) 0 mg of RAC/heifer per d for 41 d, or (4) 300 mg of RAC/heifer per d for 41 d. Heifers were fed a basal diet of 50% corn, 20% dried distillers grains with solubles, 20% corn silage, and 10% supplement (DM basis). Dry rolled corn, 0.454 kg/heifer, was removed from the diet, and 300 mg of RAC/0.454 kg of ground corn carrier was top-dressed to heifers fed RAC, daily. There were no day × RAC interactions (P ≥ 0.31) on growth performance or carcass characteristics, nor main effects (P ≥ 0.11) of duration of RAC feeding on carcass characteristics. There were no effects (P ≥ 0.22) of RAC on DMI, ADG, or G:F; however, heifers fed 300 mg of RAC had 8.4 kg heavier (P = 0.03) final BW than those not supplemented with RAC, corresponding to 9 kg greater (P < 0.01) HCW. Carcasses from heifers fed RAC had increased (P ≤ 0.03) LM area and DP compared with those from heifers not fed RAC. Feeding RAC did not affect (P ≥ 0.14) marbling, back fat, KPH, or YG. There were no interactions of RAC dose and duration; however, supplementing 300 mg of RAC/heifer per d increased final BW, HCW, LM area, and DP.