Response to ractopamine-hydrogen chloride is similar in yearling steers across days on feed1

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.