Feeding zilpaterol hydrochloride (ZH) with ruminally protected AA was evaluated in a small-pen feeding trial. Crossbred steers ( = 180; initial BW = 366 kg) were blocked by weight and then randomly assigned to treatments (45 pens; 9 pens/treatment). Treatment groups consisted of no ZH and no AA (Cont-), ZH and no AA (Cont+), ZH and a ruminally protected lysine supplement (Lys), ZH and a ruminally protected methionine supplement (Met), and ZH and ruminally protected lysine and methionine (Lys+Met). Zilpaterol hydrochloride (8.3 mg/kg DM) was fed for the last 20 d of the finishing period with a 3-d withdrawal period. Lysine and Met were top dressed daily for the 134-d feeding trial to provide 12 or 4 g·hd·d, respectively, to the small intestine. Carcass characteristics, striploins, and prerigor muscle samples were collected following harvest at a commercial facility. Steaks from each steer were aged for 7, 14, 21, and 28 d, and Warner-Bratzler shear force (WBSF) was determined as an indicator of tenderness. Prerigor muscle samples were used for immunohistological analysis. Cattle treated with Met and Lys+Met had increased final BW ( < 0.3) and ADG ( < 0.05) compared to Cont- and Cont+. Supplementation of Lys, Met, and Lys+Met improved G:F ( < 0.05) compared to Cont- during the ZH feeding period (d 111 to 134) as well as the entire feeding period ( < 0.05). Zilpaterol hydrochloride increased carcass ADG ( < 0.05) when compared to non-ZH-fed steers. Methionine and Lys+Met treatments had heavier HCW ( < 0.02) than that of Cont-. Yield grade was decreased ( < 0.04) for Cont+ steers compared to steers treated with Lys, Lys+Met, and Cont-. Tenderness was reduced ( < 0.05) with ZH regardless of AA supplementation. Lysine, Met, Lys+Met, and Cont+ had less tender steaks ( < 0.05) throughout all aging groups compared to Cont-. Steaks from Lys-treated steers were less tender ( < 0.05) than those of Cont+ during the 7- and 14-d aging periods. Nuclei density was the greatest with Cont- cattle compared to all other treatments suggesting a dilution effect of the nuclei in the larger muscle fibers with ZH feeding. Supplementation of Met in conjunction with ZH feeding increased ADG and HCW although this may lead to decreased tenderness even after aging for 28 d. These findings indicated that steers fed ZH may require additional AA absorbed from the small intestine to maximize performance.

Objectives were to evaluate benefits of yeast cell wall (YCW) supplementation on performance, carcass traits and tenderness of steers finished with zilpaterol hydrochloride (ZH). A randomized complete block design was used. British X Continental steers (n = 72; initial BW = 305±13 kg) were blocked by BW and allotted randomly to 24 pens (8 pens/treatment; 3 pens/block; 3 steers/pen). Treatments were: 1) control (CON); 2) YCW containing 100,000 IU vitamin D 2 /g (5.0 g (Y-C). Steers were supplemented with respective treatments for 55 d, of which ZH was fed d 30-49. Cattle were weighed at d 0, 21, and 55. Carcass data was collected at the plant, and strip loins were obtained. Strips were cut into steaks and assigned to one of four aging periods (7, 14, 21 or 28 d). Tenderness was examined using Warner-Bratzler shear force (WBSF). Shrunk performance showed no differences.Carcass adjusted average daily gain (ADG) from d 21-55 was 0.29 kg greater for Y-D and 0.35 kg greater for Y-C when compared to CON (P = 0.04 and 0.01, respectively).Additionally, YCW increased G:F from d 21-55 with a 20.77% improvement for Y-D and 28.46% for Y-C over CON (P = 0.06 and 0.01, respectively). Carcass data revealed no differences, yet there was a trend for a 6 kg increase in HCW by both Y-D and Y-C compared to CON (P = 0.16 and 0.12). The treatment × aging interaction with the WBSF data was not significant (P = 0.20) and no differences were found in cooking loss (P = 0.88). Treatment Y-C displayed WBSF values 0.30 kg higher than CON and 0.29 kg greater than Y-D (P = 0.0062 and 0.0075). Within the 7 d aging period, Y-C steaks were 0.62 kg (P = 0.005) and 0.54 kg (P = 0.014) less tender than CON or Y-D, respectively. For 14 d steaks, Y-C WBSF values were 0.58 kg greater than CON (P = 0.008). No CHAPTER I LITERATURE REVIEW

The objective of this study was to determine if the addition of zinc (Zn) in combination with zilpaterol HCL (ZH) affected the interaction of ZH with the beta 2 -adrenergic receptor (β-AR) by altering cAMP production, gene expression, and protein abundance in cultured skeletal muscle cells. Cultures of muscle bovine satellite cells were established and treated at 120 h with: 1) 0 µM Zn/zilpaterol hydrochloride (ZH; CON); 2) 0 µM Zn/10 µM ZH (ZH); 3) 1 µM Zn from Zn chloride/0 µM ZH (Zn); 4) 1 µM Zn from Zn chloride/10 µM ZH (ZN/ZH) in differentiation media for an additional 0, 6, 24, 48 and 96 h. Protein and mRNA were isolated and quantified at 24 and 96 h, and cAMP was measured at 0, 6, 24, 48 and 96 h. At 0, 24, 48 and 96 h, no differences (P > 0.05) were detected in cAMP production. At 6 h, Zn cells had the greatest concentration of cAMP (P < 0.05) compared to ZH treatments. No differences (P > 0.05) were detected in mRNA abundance at 24 h. At 96 h, 0 µM Zn/10 µM ZH cells had an increased abundance of myosin heavy chain (MHC)-I mRNA (P < 0.05) compared to CON. Furthermore, ZH had a greater abundance of MHC-IIX mRNA (P < 0.05) and a tendency for a greater abundance of IGF-1 mRNA (P < 0.15) compared to CON and ZN/ZH. No differences (P > 0.05) were detected in the protein abundance of β1AR and the β2AR. These results indicated Zn and ZH in combination did not have an additive effect on β 2 -AR function as indicated by cAMP concentrations.

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