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The aim of the study was to determine the effects of 3 feeding dose programs of the β-adrenergic agonists (β-AA) ractopamine hydrochloride (RH) or zilpaterol hydrochloride (ZH) for the final 30 d before slaughter on growth performance and carcass and meat characteristics of feedlot ram lambs. Eighty-four Dorper × Katahdin ram lambs (30.0 ± 1.6 kg) were blocked by BW and randomly assigned to pens (4 lambs per pen and 3 pens per treatment). Pens within a block were assigned randomly to 1 of 7 dietary treatments: 1) control (CTL) = diet without β-AA; 2) RH constant (RHC) = 20.0 mg/kg of RH, d 1 to 30; 3) RH increasing (RHI) = 10.0 mg/kg, d 1 to 10; 20.0 mg/kg, d 11 to 20; and 30.0 mg/kg, d 21 to 30; 4) RH decreasing (RHD) = 30.0 mg/kg, d 1 to 10; 20.0 mg/kg, d 11 to 20; and 10.0 mg/kg, d 21 to 30; 5) ZH constant (ZHC) = 6.0 mg/kg of ZH, d 1 to 30; 6) ZH increasing (ZHI) = 3.0 mg/kg, d 1 to 10; 6.0 mg/kg, d 11 to 20; and 9.0 mg/kg d 21 to 30; and 7) ZH decreasing (ZHD) = 9.0 mg/kg, d 1 to 10; 6.0 mg/kg, d 11 to 20; and 3.0 mg/kg, d 21 to 30. Overall, β-AA supplementation reduced DMI (P &lt; 0.001) compared with CTL lambs, but lambs fed RHI and ZHI programs had greater (P &lt; 0.05) total BW gain, ADG, and G:F. Carcass weight was improved (P &lt; 0.05) by RHI and ZHI programs, but dressing percentage was enhanced (P &lt; 0.05) by only ZHC or ZHI treatments. Fat thickness and yield grade were reduced (P &lt; 0.05) by ZH or RH regardless of feeding program. Most LM characteristics (pH, moisture loss, and chemical composition) were not different among treatments (P &gt; 0.05), with the exception of fat content that was reduced (P &lt; 0.001) in lambs fed β-AA, and diameter of muscle fibers that was increased (P &lt; 0.05) by ZHI treatment. Constant and increasing doses of ZH reduced (P &lt; 0.05) the a* value of LM and semitendinosus muscles, with no effects on L* or b* values. The mass of liver was reduced (P &lt; 0.05) in ZHI-treated lambs compared with CTL lambs, and plasma urea concentration was reduced (P &lt; 0.05) by RH or ZH administration regardless of feeding program, although there were no other differences in organ mass weight (P ≥ 0.35) or blood metabolites (P ≥ 0.16). Increasing doses of RH or ZH augmented the growth performance response without negative effects on organ mass weight or blood metabolites. Although a ZHI program improved carcass characteristics, the increased LM fiber diameter of lambs fed ZHI program could be unfavorable because of the potential negative effect on tenderness.

Effect of the administration program of 2 -adrenergic agonists on growth performance and carcass and meat characteristics of feedlot ram lambs