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Beta-adrenergic agonists (β-AA) have been shown to positively impact finishing performance and some carcass traits of feedlot cattle. Our objective was to evaluate the efficacy of a β-AA on the basis of zilpaterol hydrochloride (Grofactor, Laboratorios Virbac México, Guadalajara, Mexico) on growth and DMI, carcass characteristics, and meat quality of finishing bulls. Forty-five bulls (75% Bos indicus,25% Bos taurus) initially weighing 448.7 ± 2.58 kg were blocked by BW and randomly assigned to 1 of 3 diets, using pens of 3 animals, in a randomized complete block design: 1) daily feeding without β-AA in the basal diet (Control), 2) daily feeding with 0.15 mg/kg BW of Grofactor added to the basal diet (ZHG), or 3) daily feeding with 0.15 mg/kg BW of Zilmax (MSD Salud Animal México, Mexico City, Mexico) added to the basal diet (ZHZ). The duration of the feeding period was 30 d with a subsequent 4-d withdrawal period. Compared with Control bulls, the group fed ZHG had a 12% better (P< 0.025) G:F ratio, and their final BW (P =0.094) and ADG (P =0.084) tended to be enhanced. Feedlot performance of ZHG and ZHZ bulls was similar, although the DMI was ∼4% lower (P <0.05) in ZHG bulls vs. the ZHZ and Control groups. The HCW (P <0.001) and dressing percentage (P =0.015) were higher by 20 kg and 3%, respectively, in ZHG bulls vs. Control bulls. The KPH fat was lower (P =0.007) in bulls fed ZHG than in nonsupplemented bulls, but other carcass characteristics were not different in the ZHG and ZHZ bulls, and noncarcass components were not affected by ZHG or ZHZ supplementation. At 48 h postmortem, ZHG bulls had lower (P =0.007) water holding capacity and trended toward (P =0.06) increased chroma and reduced pH (P =0.09) compared to Control bulls. However, compared to ZHZ bulls, ZHG bulls had higher (P =0.02) chroma and a trend (P =0.08) toward increased hue angle. At 14 d postmortem, meat quality variables did not differ between the 3 groups of bulls. Supplementation of ZH Grofactor improved feedlot performance and some carcass characteristics of finishing bulls without affecting meat quality. The effects of Grofactor on feedlot performance, carcass traits, and meat quality were similar to those of Zilmax.

The impact of using 2 β-adrenergic agonists in feedlot cattle fed finishing diets was evaluated using 54 steers (45 crossbred Charolais and 9 Brangus) initially weighing 424 ± 26.6 kg in a randomized complete block design with 3 treatments and 6 blocks (i.e., 18 pens with 3 steers per pen). Response variables were feedlot performance, carcass characteristics, and meat quality. Treatments were 1) control (no supplement added); 2) zilpaterol hydrochloride (ZH; 60 mg·steer−1·d−1); and 3) ractopamine hydrochloride (RH; 300 mg·steer−1·d−1). The β-agonists were added to the diets during the final 33 d of the experiment. The groups of steers fed ZH or RH improved (P< 0.01) ADG by 26 or 24%, respectively, compared with control steers. Steers supplemented with RH consumed less (P= 0.03) DM (8.37 kg) than control steers (8.51 kg), whereas intake was similar (P= 0.37) for ZH and control steers. Addition of either β-agonist to the diet considerably improved (P< 0.01) the G:F (ZH, 0.253 and RH, 0.248 vs. control, 0.185). Hot carcass weight and carcass yield were enhanced (P< 0.05) with both β-agonists. The LM area was increased (P= 0.026) by ZH (75.2 cm2), but that of RH (72.2 cm2) was similar (P= 0.132) to the control steers (66.8 cm2). Meat from the ZH- (P= 0.0007) and RH- (P= 0.0267) supplemented steers had greater shear force values than control steers (ZH = 5.11; RH = 4.83; control = 4.39 kg/cm2). Variables related to meat color indicated that both β-agonists led to a similar redness of the LM area related to the control group. In general, feedlot performance was greatly enhanced by β-adrenergic agonists, and meat tenderness from treated animals was classified as intermediate. Furthermore, meat color was not altered by β-agonist supplementation.