title: Effect of feedlot management system on response to ractopamine-HCl in yearling steers.
abstract: Two experiments evaluated the effects of conventional and natural feedlot management systems (MS) on ractopamine-HCl (RAC) response in yearling steers. Feedlot performance, carcass characteristics, skeletal muscle gene expression, and circulating IGF-I concentrations were measured. The conventional system included a combined trenbolone acetate and estradiol implant, Revalor-S (IMP), as well as monensin-tylosin feed additives (IA). Treatments were arranged in a 2 x 2 factorial and included: 1) natural (NAT): no IMP-no IA, no RAC; 2) natural plus (NAT+): no IMP-no IA, RAC; 3) conventional (CON): IMP-IA, no RAC; and 4) conventional plus (CON+): IMP-IA, RAC. In Exp. 1, one hundred twenty crossbred steers (initial BW = 400 +/- 26 kg) were allotted randomly to treatment in a randomized complete block design (BW was blocking criteria); pen was the experimental unit. In Exp. 2, twenty-four individually fed crossbred steers (initial BW = 452 +/- 25 kg) were used in a randomized complete block design (BW was blocking criteria) and assigned to the same treatments as Exp. 1, with 6 steers/treatment. In Exp. 2, serum was harvested on d 0 and 31 and within the 28-d RAC feeding period, at d 0, 14, and 28. Longissimus biopsy samples were taken on d 0, 14, and 28 of the RAC feeding period for mRNA analysis of beta-adrenergic receptors and steady-state IGF-I mRNA. In Exp. 1, ADG, G:F, final BW, and HCW were greatest for CON+ (P < 0.01). During the final 37 d, RAC increased ADG (P = 0.05) and increased overall G:F (P = 0.02). Marbling score was reduced (P = 0.02), and yield grade was improved with RAC (P = 0.02), but RAC did not affect dressing percentage (P = 0.96) or HCW (P = 0.31). In Exp. 2, MS x RAC interactions were detected in ADG and G:F the last 28 d, overall ADG and overall G:F, final BW, and HCW (P < 0.01). Dressing percentage, yield grade, and marbling score were not altered by MS or RAC (P > 0.10). Circulating IGF-I concentration was increased on d 31 by the conventional MS, and concentration was greater throughout the study than NAT steers (P < 0.01). Circulating IGF-I concentrations were not changed by RAC (P = 0.49). Abundance of beta(1)-AR mRNA tended to increase (P = 0.09) with RAC, but RAC did not affect beta(2)-AR, beta(3)-AR, or IGF-I mRNA (P > 0.40). Management system did not affect beta(1)-AR, beta(2)-AR, beta(3)-AR, or IGF-I mRNA (P > 0.18), yet a trend (P = 0.06) for MS x RAC for beta(2)-AR mRNA was detected. These results indicate that response to RAC is affected by feedlot management practices.
authors.affiliation: Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506, USA.
authors.forename: S J
authors.initials: SJ
authors.lastname: Winterholler
authors.forename: G L
authors.initials: GL
authors.lastname: Parsons
authors.forename: D K
authors.initials: DK
authors.lastname: Walker
authors.forename: M J
authors.initials: MJ
authors.lastname: Quinn
authors.forename: J S
authors.initials: JS
authors.lastname: Drouillard
authors.forename: B J
authors.initials: BJ
authors.lastname: Johnson
mesh_terms: D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000822:Animal Husbandry / Q000379:methods*; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D001706:Biopsy / Q000662:veterinary; D001835:Body Weight; D002417:Cattle / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis / Q000235:genetics; D008297:Male; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism; D010627:Phenethylamines / Q000494:pharmacology*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D011897:Random Allocation; D011943:Receptors, Adrenergic, beta / Q000096:biosynthesis / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000662:veterinary
chemical_list: D000318:Adrenergic beta-Agonists; D010627:Phenethylamines; D012333:RNA, Messenger; D011943:Receptors, Adrenergic, beta; C060897:ractopamine; D007334:Insulin-Like Growth Factor I