Response to ractopamine-hydrogen chloride is similar in yearling steers across days on feed

S. J. Winterholler, G. L. Parsons, C. D. Reinhardt, J. P. Hutcheson, W. T. Nichols, D. A. Yates, R. S. Swingle, B. J. Johnson

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54 Scopus citations


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 RAG (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.

Original languageEnglish
Pages (from-to)413-419
Number of pages7
JournalJournal of animal science
Issue number2
StatePublished - Feb 2007


  • Ractopamine-hydrogen chloride
  • Skeletal muscle tissue
  • Steer
  • β-adrenergic receptor


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