WS Effects of protein concentration and degradability on performance and carcass characteristics of finishing heifers receiving 0 or 400 mg ractopamine hydrochloride

Abstract Text: This study evaluated if excess protein decreases performance and carcass quality of finishing cattle fed diets with or without ractopamine hydrochloride (RH). Heifers were assigned to 48 pens in a randomized complete block design and pens of cattle were randomly assigned to 3 protein and 2 RH (0 vs 400 mg/day) treatments. Protein treatments were steam-flaked corn-based diets containing 13.9% CP, 8.8% RDP, and 5.0% RUP (CON), 20.9% CP, 13.4% RDP, 6.1% RUP (High RDP), or 20.9% CP, 9.1% RDP, 10.4% RUP (High RUP). Cattle were weighed at initiation of RH and at shipping. No RH × CP interactions (P ≥ 0.11) occurred for performance or carcass traits. Excess CP did not affect (P ≥ 0.12) final BW or ADG. Carcass-adjusted final BW and ADG tended to be greater (P = 0.06) for cattle receiving High RDP than High RUP and CON. Water intake, DMI, G:F, and carcass-adjusted G:F were not different (P ≥ 0.12) among CP treatments. Hot carcass weight tended to be greater (P = 0.06) for cattle receiving High RDP than High RUP and CON. Dressing percentage was lower (P ˂ 0.01) for cattle fed High RUP than High RDP and CON. Marbling score, 12^th rib fat depth, LM area, and yield grade were not different (P ≥ 0.16) among CP treatments. Heifers receiving High RUP tended to have lower (P = 0.10) KPH than CON. Percentage choice tended to be greater (P = 0.09) for heifers receiving High RDP vs. High RUP. Water and DMI were not different (P ≥ 0.36) for RH vs. no RH. Cattle receiving RH had greater (P ˂ 0.01) final BW, ADG, carcass- adjusted final BW, and carcass-adjusted ADG, and lower (P ˂ 0.01) G:F and carcass-adjusted G:F compared with no RH. Hot carcass weights were greater (P ˂ 0.01) and dressing percentage tended to be greater (P = 0.09) for cattle receiving RH, while marbling score was not affected (P = 0.11) by RH. Twelfth-rib fat depth tended to be lower (P = 0.08), and KPH was lower (P = 0.02) for RH vs. no RH. The LM area was greater (P = 0.03) for cattle receiving RH vs. no RH. Excess CP does not negatively impact performance or carcass traits of finishing cattle, and no interactions between CP and RH suggest that CP requirements are not affected by RH.

Keywords: cattle, protein, ractopamine hydrochloride