Effect of two beta adrenergic agonists and feeding duration on feedlot performance and carcass characteristics of finishing lambs

One hundred and twelve Dorper × Katahdin lambs (initial BW = 28.9 ± 0.9 kg) were used in a randomized complete block design, with the objective of evaluating the effects of feed supplementation with the β-adrenergic agonists (β-AA) ractopamine hydrochloride (RH) and zilpaterol hydrochloride (ZH) for the last 14, 28, or 42 d of feeding period, on growth performance and carcass characteristics of finishing lambs. Treatments consisted of the supplementation of 20.0 mg RH/kg of dietary DM or 6.0 mg ZH/kg of dietary DM for the last 14, 28, or 42 d of the feeding period, and a control group (without β-AA). Compared with controls, lambs fed β-AA, had greater (P < 0.001) final BW (9.6%), ADG (24.4%), and feed efficiency (28.2%). Growth performance was similar between RH and ZH. As ZH duration feeding increased, DMI increased (P = 0.04) linearly, but this tendency was vanishing (P = 0.30) when DMI was expressed on BW or metabolic weight basis. Lambs fed β-AA had greater (P ≤ 0.009) HCW, dressing percent, LM area, and reduced fat thickness when compared to lambs fed the control diet. Lambs fed ZH had greater HCW (3.9%, P = 0.001), dressing percentage (3.8%, P < 0.002) and lower fat thickness (20.6%, P = 0.006) than RH group. As the duration of ZH feeding increased, HCW and dressing percentage decreased (P = 0.001), and fat thickness increased linearly (P = 0.064); however, the lambs fed RH showed a linear increase (P < 0.001) in fat thickness as the duration of feeding increased, but did not show trends for other (P ≥ 0.142) carcass traits. The proportion of leaner carcasses were increased (P < 0.10) when lambs were fed ZH for 14 d; whereas, feeding of β-AA during 14 d, produced a higher proportion of carcasses classified as ≥ 16 cm2, but these proportions were reduced linearly (P ≤ 0.002) as the feeding duration became longer. β-AA supplementation increased growth performance in lambs as results of greater muscle accretion and body fat reduction. The β2-AA compound has a greater response in feedlot lambs than the β1-AA compound. Carcasses showed a more desirable combination of carcass characteristics if lambs were fed ZH during 42 d, and if lambs were fed RH between 28 and 42 d. Results of this study contribute to a better use of β-adrenergic agonists in feedlot lambs.