Response to ractopamine-hydrogen chloride is similar in yearling steers across days on feed
2007
Winterholler, S.J. | Parsons, G.L. | Reinhardt, C.D. | Hutcheson, J.P. | Nichols, W.T. | Yates, D.A. | Swingle, R.S. | Johnson, B.J.
Yearling steers (n = 2,552; 314 kg of initial BW) were used to evaluate the effects of ractopamine-HCl (RAC) and days on feed on performance, carcass characteristics, and skeletal muscle gene expression in finishing steers. Treatment groups included serial slaughter dates of 150, 171, or 192 d on feed. Within each slaughter date, steers either received RAC (200 mg/steer) daily for the final 28 d or were not fed RAC. All steers were initially implanted with Revalor-IS and were reimplanted with Revalor-S after 75 d on feed. At slaughter, muscle samples from the semimembranosus were collected for mRNA analysis of the β-adrenergic receptors (β-AR). Ractopamine administration increased (P < 0.05) ADG, G:F, and HCW and increased (P = 0.08) LM area. Ractopamine did not affect the dressing percentage, USDA yield grade, or quality grade (P > 0.3). There was no change in overall feed intake across the entire feeding period; however, feed intake was increased during the 28-d period during which the steers were fed RAC (P <= 0.05). Greater days on feed decreased (P < 0.05) ADG, G:F, DMI, and the number of yield grade 1 and 2 carcasses. Also, greater days on feed increased (P < 0.05) HCW, dressing percentage, and the number of prime and choice carcasses, as well as the number of yield grade 4 and 5 carcasses. Increasing days on feed decreased (P < 0.05) the abundance of β₁-AR and β₃-AR mRNA and increased (P < 0.05) the abundance of β₂-AR mRNA in skeletal muscle samples obtained at slaughter. Ractopamine had no effect (P > 0.10) on the abundance of β₁-AR or β₃-AR mRNA, but tended (P = 0.09) to increase β₂-AR mRNA. Additional time-course studies with primary muscle cell cultures revealed that advancing time in culture increased (P < 0.001) β₂-AR mRNA but had no effect (P > 0.10) on β₁-AR or β₃-AR mRNA. We conclude that days on feed and RAC are affecting β-AR mRNA levels, which could, in turn, impact the biological response to RAC feeding in yearling steers.
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