346. Evaluation of Partial Replacement of Distillers’ Grains with Heat-treated Full-fat Soybeans on Growth and Carcass Characteristics in Finishing Beef Steers.

Abstract Distillers’ grains (DGS) are widely used as a protein and energy source in feedlot diets. Complete replacement of DGS with heat-treated soybeans has resulted in enhanced growth performance. Soybeans are rich in essential amino acids, particularly lysine. Heat treating full-fat soybeans increases ruminal undegradable protein (RUP) concentration and replacing a portion of DGS with heat-treated soybeans may be a cost-effective approach to tailor amino acid supply to animal requirements. The objective of this study was to determine the effects of partially replacing modified distillers' grains (MDGS) with full-fat extruded (heat-treated) soybeans (ESB) on growth performance and carcass characteristics of finishing beef steers. Simmental x Angus steers (n = 128; 411 ± 24.5 kg) were allotted by initial BW to one of 16 drylot pens (n = 8 pens/treatment; 8 steers/pen) in a randomized complete block design. Basal diets (DM basis) were based on dry-rolled corn (DRC) and differed by protein supplement: 17.1% MDGS (ESB-0) or 10.4% MDGS and 5.8% ESB (ESB-6). Diets were isonitrogenous and ESB-6 contained 1% greater dietary fat. Steers were implanted with 200 mg TBA and 20 mg E2-17B (Revalor-200) on d 28. Lubabegron fumarate (Experior) was fed for 54 days at 36 mg/hd daily followed by a 4-d voluntary withdrawal. Growth performance and carcass traits were analyzed using the MIXED procedure of SAS 9.4; distributions of QG, YG, and liver abscess incidence were analyzed as multinomial distributions using the GLIMMIX procedure. During the first 28-d, ESB-6 did not affect DMI or ADG (P ≥ 0.22) but tended to improve G:F by 4.5% (P = 0.08). For the 28-d period post-implanting (d 29 to d 56), feeding ESB-6 increased ADG by 10.4% (P = 0.01) and increased G:F by 8.5% (P = 0.03) with no difference in DMI (P = 0.53). Treatment did not affect DMI, ADG, or G:F (P ≥ 0.52) from d 57 to d 84. Feeding ESB-6 with lubabegron (d 85 to d 143) increased ADG by 11.5% (P = 0.05) and tended to improve G:F by 9.5% (P = 0.10). Cumulatively, feeding ESB-6 increased ADG by 7.3% (P = 0.01) and tended to increase G:F by 5.6% (P = 0.08) but did not affect DMI (P = 0.58). Feeding ESB-6 increased final body weight and HCW by 19.5 and 12.2 kg, respectively (P = 0.01) and tended to increase rib fat thickness (P = 0.06) and calculated USDA YG (P = 0.09), with a greater proportion of cattle in YG 4 and 5 (P = 0.02). Treatment did not affect ribeye area, marbling scores, QG distribution, or liver abscess incidence (P ≥ 0.22). Replacing one-third of MDGS on a DM basis with ESB increased ADG and carcass weight and tended to improve feed efficiency.