The impact of using 2 beta-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 beta-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 beta-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 beta-agonists. The LM area was increased (P = 0.026) by ZH (75.2 cm(2)), but that of RH (72.2 cm(2)) was similar (P = 0.132) to the control steers (66.8 cm(2)). 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/cm(2)). Variables related to meat color indicated that both beta-agonists led to a similar redness of the LM area related to the control group. In general, feedlot performance was greatly enhanced by beta-adrenergic agonists, and meat tenderness from treated animals was classified as intermediate. Furthermore, meat color was not altered by beta-agonist supplementation.

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% 25% ) 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 ( < 0.025) G:F ratio, and their final BW ( 0.094) and ADG ( 0.084) tended to be enhanced. Feedlot performance of ZHG and ZHZ bulls was similar, although the DMI was ∼4% lower ( 0.05) in ZHG bulls vs. the ZHZ and Control groups. The HCW ( 0.001) and dressing percentage ( 0.015) were higher by 20 kg and 3%, respectively, in ZHG bulls vs. Control bulls. The KPH fat was lower ( 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 ( 0.007) water holding capacity and trended toward ( 0.06) increased chroma and reduced pH ( 0.09) compared to Control bulls. However, compared to ZHZ bulls, ZHG bulls had higher ( 0.02) chroma and a trend ( 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 aim was to evaluate effects of administration of two β-adrenergic agonists (β-AA) on growth performance, carcass characteristics and meat-quality traits of Zebu heifers finished in a feedlot. Fifty-four Zebu heifers weighing 397 ± 29.1 kg were used in a randomised complete block design with three treatments and six blocks (i.e. 18 pens with 3 heifers per pen). Treatments were as follows: (1) control (C; no supplement); (2) zilpaterol hydrochloride (ZH; 60 mg per heifer per day); and (3) ractopamine hydrochloride (RH; 300 mg per heifer per day). The β-AA were added to the diets during the final 33 days of the finishing period, after which the heifers were immediately slaughtered. Relative to C, average daily gain and gain : feed ratio were improved (P < 0.05) in heifers supplemented with ZH, but not in those supplemented with RH. Feed intake in C heifers was lower (P < 0.05) than in ZH heifers, but similar (P > 0.05) to RH heifers. The hot carcass weight showed a trend to be heavier (P = 0.096) in ZH than in C heifers. However, Longissimus dorsi (LM) area was increased (P < 0.05) by ZH (73.94 cm2), but RH (70.45 cm2) and C (66.3 cm2) groups had a similar (P > 0.05) LM area. The meat from the ZH- and RH-supplemented heifers had higher Warner–Bratzler shear-force values (P < 0.01) than that from C heifers (ZH = 5.11; RH = 5.50; C = 4.89 kg/cm2), and the meat from RH-supplemented heifers was classified as ‘tough’. Variables associated with meat colour indicated that ZH led to a lower b* average, which was related to a lighter LM area than in C. In general, feedlot performance was enhanced only by the β-AA ZH, with meat tenderness from RH heifers classified as ‘tough’. Meat colour was not altered by β-AA supplementation. These data suggested that while ZH supplementation to Bos indicus heifers offered advantages in feedlot performance and some carcass traits, RH supplementation did not positively affect these biological responses.