Effect of beta-agonist type and timing of Experior feeding on calculated cumulative ammonia gas emissions, live growth performance, and carcass outcomes, and objective tenderness outcomes of feedlot steers.

Abstract

Our objective was to compare beta-agonist feeding strategies and evaluate Experior (EXP) with added days on feed (DOF) for live growth performance and carcass outcomes. Steers (n = 2,517; initial BW = 336 ± 23 kg) were allocated to 36 pens in a randomized, complete-block design and assigned to 1 of 6 treatments. Treatments were negative control (no beta-ligand; CON), Optaflexx (ractopamine hydrochloride, 300 mg·head^-1·day^-1 for 35 d; OPT), and Experior (lubabegron fumarate, 36 mg·head^-1·day^-1 for 56 d + 4 d removal; 0EXP) with all three treatments fed for 198 d. Remaining treatments were fed EXP for same duration as 0EXP, but total DOF were adjusted by -14 d (-14EXP), +14 d (+14EXP) and +28 d (+28EXP). Statistical analyses were conducted using mixed models; treatment was the fixed effect, block was a random effect, means were separated with the Tukey-Kramer adjustment, and contrasts were calculated to test linear or quadratic effects of EXP across DOF. No treatment differences (P ≥ 0.72) were observed for daily dry matter intake. Calculated cumulative ammonia (NH₃) gas emissions (CCAGE) expressed either as total NH₃ emitted (g) or NH₃ emitted per kg of BW or HCW was reduced (P < 0.01) by 5.6-8.5%, 5.1-10.7%, and 6.3-13.6%, respectively for 0EXP, +14EXP and +28EXP compared to CON. Carcass ADG of 0EXP steers was 6.8% greater (P < 0.01) than CON, which had similar (P ≥ 0.10) ADG to OPT, +14EXP, and +28EXP steers. Carcass G:F of 0EXP steers was improved 5.0% compared to CON (P = 0.01) and was not different (P ≥ 0.10) from OPT, -14EXP or +14EXP steers. Hot carcass weight of 0EXP, +14EXP, or +28EXP steers was 14-37 kg heavier (P < 0.01) than CON, which did not differ (P = 0.81) from OPT. Steers fed EXP had 1.15-2.5% point increase (P < 0.01) in dressed yields. Longissimus muscle area was 4.03-6.23 cm² larger (P < 0.01) in steers fed EXP compared to CON, which tended to differ (P = 0.08) from OPT. Marbling score was 31-39 points lower (P ≤ 0.02) for -14EXP compared to CON and OPT, whereas 0EXP tended (P = 0.09) to differ from CON and +14EXP and +28EXP were similar (P ≥ 0.28) to CON and OPT. Slice shear force values were 20.1% greater (P < 0.02) for +14EXP compared to CON, whereas 0EXP tended (P = 0.09) to be greater and +28EXP, -14EXP and OPT were similar (P ≥ 0.28) to CON. This study illustrates EXP decreased CCAGE and increased HCW with minimal quality changes when fed the last 56 d on feed.