134 Effect of Rumen Degradable Protein Intake on Performance, Carcass Quality, and Methane Emissions of Yearling Finishing Steers

Abstract

Abstract The performance, economic, and environmental consequences of feeding rumen degradable protein (RDP) levels above and below calculated maximum energy-dependent microbial crude protein (MCP) synthesis potential are poorly understood. Yearling steers (n = 54; initial BW = 484.1 ± 26.03 kg) were assigned to one of two pens that each contained an automated head chamber system (GreenFeed; C-Lock Inc., Rapid City, SD) for individual measurement of enteric methane (CH4) emissions. Each steer was assigned to an individual feed bunk (Calan Gate; American Calan, Northwood, NH) and randomly assigned to one of three dietary treatments: low RDP intake (LRDPI; n = 8), neutral RDP intake (NRDPI; n = 8), or high RDP intake (HRDPI; n = 8) in a completely randomized design. Steers were transitioned to treatment diets utilizing a two-ration blend system for 20-day followed by an 80-day finishing period. Diets were formulated to provide cattle with similar metabolizable protein (MP) and net energy for gain (NEg) but different RDP supply. Diets were formulated to provide approximately 87% (LRDPI), 100% (NRDPI), or 113% (HRDPI) of calculated RDP requirements to maximize energy-allowable MCP-yield. Paired-day, unshrunk BW was collected on d 0, 50, and 80, and cattle were transported to a commercial abattoir for carcass data collection. Statistical analyses were conducted using Analysis of Variance in JMP Pro6.0 using the Fit Model procedure. Least-squares means differences were considered statistically significant if P < 0.05 and considered a tendency toward significant if 0.05 ≤ P < 0.15. Treatment rations provided approximately 94% (LRDPI), 106% (NRDPI), and 118% (HRDPI) of the RDP required to maximize energy-allowable MCP yield, respectively. Daily DMI was similar (P > 0.15) for HRDPI and NRDPI cattle, but LRDPI cattle tended (P = 0.06) to have less DMI than NRDPI cattle. Dietary treatment did not affect (P ≥ 0.32) ADG, feed conversion efficiency, ribeye area, marbling score or carcass yield grade. Yet, LRDPI cattle had lighter hot carcass weight (P < 0.05) and less subcutaneous fat thickness (P < 0.05) than NRDPI cattle, whereas HRDPI cattle were not different (P ≥ 0.15) from the other treatments. On average, LRDPI cattle tended to emit 7.0% more daily CH4 (g×d-1; P = 0.05) than both NRDPI and HRDPI. Moreover, the LRDPI treatment elicited a 14.7% and 18.1% increase in CH4-yield (g CH4×kg DMI-1; P < 0.01) and emission intensity (g CH4×kg ADG-1; P < 0.05) compared with the other treatments, respectively. These data indicate that limiting MCP-yield by restricting RDP intake may reduce subcutaneous fat thickness and carcass weight while increasing enteric CH4 emissions. These results reiterate the importance of meeting RDP requirements and suggest no performance or methane emission benefits of overfeeding RDP.