title: Response to ractopamine-hydrogen chloride is similar in yearling steers across days on feed.
abstract: 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 beta-adrenergic receptors (beta-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 < or = 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 beta(1)-AR and beta(3)-AR mRNA and increased (P < 0.05) the abundance of beta(2)-AR mRNA in skeletal muscle samples obtained at slaughter. Ractopamine had no effect (P > 0.10) on the abundance of beta(1)-AR or beta(3)-AR mRNA, but tended (P = 0.09) to increase beta(2)-AR mRNA. Additional time-course studies with primary muscle cell cultures revealed that advancing time in culture increased (P < 0.001) beta(2)-AR mRNA but had no effect (P > 0.10) on beta(1)-AR or beta(3)-AR mRNA. We conclude that days on feed and RAC are affecting beta-AR mRNA levels, which could, in turn, impact the biological response to RAC feeding in yearling steers.
authors.affiliation: Department of Animal Sciences and Industry, Kansas State University, Manhattan, Kansas 66506, USA.
authors.forename: S J
authors.initials: SJ
authors.lastname: Winterholler
authors.forename: G L
authors.initials: GL
authors.lastname: Parsons
authors.forename: C D
authors.initials: CD
authors.lastname: Reinhardt
authors.forename: J P
authors.initials: JP
authors.lastname: Hutcheson
authors.forename: W T
authors.initials: WT
authors.lastname: Nichols
authors.forename: D A
authors.initials: DA
authors.lastname: Yates
authors.forename: R S
authors.initials: RS
authors.lastname: Swingle
authors.forename: B J
authors.initials: BJ
authors.lastname: Johnson
mesh_terms: D000318:Adrenergic beta-Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D000821:Animal Feed / Q000032:analysis; D000818:Animals; D001823:Body Composition / Q000187:drug effects; D001835:Body Weight / Q000187:drug effects; D002417:Cattle / Q000378:metabolism*; D002478:Cells, Cultured; D017931:DNA Primers / Q000737:chemistry; D004435:Eating / Q000187:drug effects; D008297:Male; D008460:Meat / Q000592:standards; D018482:Muscle, Skeletal / Q000737:chemistry / Q000502:physiology*; D010627:Phenethylamines / Q000008:administration & dosage / Q000494:pharmacology*; D012333:RNA, Messenger / Q000032:analysis; D011943:Receptors, Adrenergic, beta / Q000096:biosynthesis / Q000187:drug effects* / Q000235:genetics; D013997:Time Factors
chemical_list: D000318:Adrenergic beta-Agonists; D017931:DNA Primers; D010627:Phenethylamines; D012333:RNA, Messenger; D011943:Receptors, Adrenergic, beta; C060897:ractopamine