Physiological and Molecular Responses of Feeding Ractopamine toYearling Heifers Across Days on Feed
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Heifers comprise 30% of the annual beef harvest in the United States. Production pressures over the past several years have lead to a need to increase production efficiency. One mode currently available is the use of the β-agonist ractopamine. Ractopamine is designed to be fed 28 - 42 d prior to harvest and has been shown to increase REA and feed efficiency while decreasing yield grade. Seventy-one crossbred yearling heifers were allotted for days on feed (DOF) including 79 (n = 16), 100 (n = 16), 121 (n = 16), 142 (n = 16), and 163 (n = 7). Half of the heifers in each DOF were assigned to a ractopamine treatment of 300 mg/hd/d for 35 d prior to harvest (treatment phase) and half served as control. Leptin concentrations (LC) were determined for all animals at regular intervals. Real-time PCR was used to determine β2-receptor expression for comparison of treatment and control groups. Ractopamine had no significant effects on growth or carcass traits across DOF. During the treatment phase, animals fed ractopamine were found to have increased ADG (P < 0.05) (1.24 kg/d) compared to control animals (1.10 kg/d); (P < 0.02). Dry matter intake during the treatment phase across control and treatment groups were equal (5.53 kg/d). However, ractopamine significantly improved dry matter feed efficiency (P < 0.05) during the treatment phase when compared to control (7.57, 9.86 kg feed/kg gain, respectively). Ractopamine had no effect on leptin concentrations (P > 0.6). Mean LC were determined to be 24.9 ± 3.13 ng/mL 24 hrs prior to harvest. Correlations were found between leptin concentration and HCW (r = 0.27, P < 0.05), yield grade (r = 0.20 P < 0.05, and ADG (r = 0.28, P < 0.05). No differences were seen in expression of the β2-receptor or plasma IGF across treatments (P > 0.4).