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Effects of two ß-adrenergic agonists on finishing performance, carcass characteristics, and meat quality of feedlot steers^1,2

L. Avendaño-Reyes^*,3, V. Torres-Rodríguez, F. J. Meraz-Murillo, C. Pérez-Linares, F. Figueroa-Saavedra and P. H. Robinson

^* Instituto de Ciencias Agrícolas, Universidad Autónoma de Baja California, Ejido Nuevo León, Baja California, México; and Asociación Ganadera Local Especializada en Bovinos para Engorda de Baja California, Mexicali, Baja California, México; and Instituto de Investigaciones en Ciencias Veterinarias, Universidad Autónoma de Baja California, Mexicali, Baja California, México; and and Department of Animal Science, University of California, Davis 95616

³ Corresponding author: lar62{at}uabc.mx

The impact of using 2 ß-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 ß-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 ß-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 ß-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 ß-agonists led to a similar redness of the LM area related to the control group. In general, feedlot performance was greatly enhanced by ß-adrenergic agonists, and meat tenderness from treated animals was classified as intermediate. Furthermore, meat color was not altered by ß-agonist supplementation.

Key Words: ß-adrenergic receptor agonist • carcass characteristic • feedlot cattle • meat quality

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