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Response to ractopamine-HCl in heifers is altered by implant strategy across days on feed¹

E. K. Sissom^*, C. D. Reinhardt^*, J. P. Hutcheson, W. T. Nichols, D. A. Yates, R. S. Swingle and B. J. Johnson^*,2

^* Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506; and Intervet Inc., Millsboro, DE 19966; and and Cactus Research Ltd., Amarillo, TX 79116

² Corresponding author: bjohnson{at}ksu.edu

Two experiments were conducted to investigate the effects of ractopamine-HCl (RAC) and implant strategy or days on feed (DOF) on feedlot performance and expression of ß-adrenergic receptors (AR). In Exp. 1, 1,147 feedlot heifers weighing 282 ± 3 kg were used with implant treatments of Revalor-200 (R200) at arrival, or Revalor-IH at arrival with reimplantation with Finaplix-H on d 58 (RF). Ractopamine (0 vs. 200 mg/d) was fed the last 28 d in both experiments. Treatments were randomly assigned to 16 pens. At slaughter, semimembranosus muscle tissue was excised for RNA isolation. Ractopamine administration increased (P < 0.05) ADG, G:F, HCW, and LM; decreased (P < 0.05) 12th rib fat depth; and improved (P < 0.05) yield grade. There was no effect (P > 0.10) on the expression of ß₁-AR mRNA; however, there was a tendency (P = 0.10) for RAC feeding to increase ß₂-AR mRNA levels. For ß₃-AR mRNA, there was an implant by RAC interaction (P = 0.05), with RAC numerically increasing ß₃-AR mRNA in heifers implanted with RF, but a decrease (P < 0.05) in expression in heifers implanted with R200. Ractopamine also decreased (P < 0.05) IGF-I mRNA in heifers implanted with RF. In Exp. 2, 2,077 heifers were used to investigate the effects of RAC and DOF. Days on feed were 129, 150, and 170, and RAC was administered the last 28 d. Ractopamine improved (P < 0.05) G:F, but had no other effects (P > 0.05) on performance. Average daily gain decreased (P < 0.05) as DOF increased. Hot carcass weight, LM area, 12th rib fat, G:F, calculated yield grade, and marbling score increased (P < 0.05) and the percentage of KPH fat decreased (P < 0.05) as DOF increased. These data aid in our understanding of the effects of steroidal implants, DOF, and RAC administration in feedlot heifers.

Key Words: bovine • heifer • ractopamine • skeletal muscle • steroidal implant

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