Effects of continuous and step-up ractopamine hydrochloride supplementation protocols on feeding performance and carcass characteristics of finishing steers1

The objectives of this experiment were to evaluate the effects of continuous (CNT) and step-up (STP) ractopamine hydrochloride (RH) supplementation protocols during the last 42 d of the finishing period on feeding performance and carcass characteristics of market steers. Thirty-six Angus-Simmental cross steers (510 ± 4.99 kg initial BW) were blocked by BW and randomly assigned to 1 of 3 isocaloric, isonitrogenous treatments containing 0 (CON) or 200 (CNT) mg of RH from d 0 to 42 or daily supplementation of 100 mg of RH from d 0 to 21, no RH from d 21 to 28, and daily supplementation of 300 mg of RH from d 28 to 42 (STP). Steers were fed for ad libitum intake, BW were measured at 14-d intervals, and final BW were measured on d 41 and 42 before slaughter to monitor feedlot performance. Carcass characteristics were collected following a 24-h chill. There were no differences across treatments for initial BW or DMI throughout the feeding period (P ≥ 0.26). Likewise, ADG did not differ (P ≥ 0.20), and there were no changes in total BW gain (P = 0.52) or G:F (P = 0.36) due to dietary treatment. The lack of performance differences was reflected in several carcass measurements often affected by BW gain with no differences across treatments for HCW (P = 0.31), DP (P = 0.80), 12th-rib fat thickness (P = 0.35), LM area (P = 0.19), percentage of KPH (P = 0.97), and YG (P = 0.38). However, CON had greater marbling scores (P = 0.04) than did CNT, with STP being intermediate to both treatments. This last effect likely contributed to the trend in QG differences (P = 0.08) between CON and CNT, with STP again being intermediate. Feeding performance was not altered because of RH supplementation at either a constant level or a step-up protocol over a period of 42 d. Carcass quality was not improved and continuous RH administration had a negative effect on marbling and potentially final QG of beef steers.