Effect of feedlot management system on response to ractopamine-HCl in yearling steers

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 β-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 β₁-AR mRNA tended to increase (P = 0.09) with RAC, but RAC did not affect β₂-AR, β₃-AR, or IGF-I mRNA (P > 0.40). Management system did not affect β₁-AR, β₂-AR, β₃-AR, or IGF-I mRNA (P > 0.18), yet a trend (P = 0.06) for MS x RAC for β₂-AR mRNA was detected. These results indicate that response to RAC is affected by feedlot management pract