335. Effects of Supplemental Isoacids on Liver Health, Carcass Quality, and Performance in Finishing Beef Steers Fed High-Fiber Or High-Grain Diets.

Abstract Isoacids, branched-chain volatile fatty acids including isobutyric acid (IB) and 2-methylbutyric acid (2MB), support ruminal epithelial integrity and microbial protein synthesis in cattle, yet their effects on liver health and carcass characteristics under different dietary energy conditions remain unclear. The objective was to evaluate the effects of dietary fiber and starch levels, with or without supplemental isoacids, on performance, hematology, carcass traits, and liver and lung pathology. Twenty-four British crossbred steers (initial BW: 527 ± 7.8 kg) were allocated to individual pens in a 2 × 2 factorial: high-fiber [HF; 24.2% neutral detergent fiber (NDF), 37.4% starch, total digestible nutrients (TDN) 78.3%] or high-grain (HG; 15.4% NDF, 52.9% starch, TDN 82.8%) diets, with or without isoacids (7.5 g IB + 15 g 2MB/head/d, top-dressed). Following a 28-d adaptation, steers were fed for 75 d over three 25-d periods and harvested. Data were analyzed as a completely randomized block design using PROC MIXED of SAS; significance declared at P ≤ 0.05 and tendencies at 0.05 ≤ P ≤ 0.15. No effects of diet, isoacids, or their interaction were detected for overall ADG (1.99 ± 0.14 kg/d), final BW, or water intake (P > 0.05). DMI tended to be greater in HF than HG steers (15.05 vs. 14.00 kg/d; P = 0.08). In the final period (d 51–75), HF steers had greater ADG than HG steers (2.43 vs. 1.99 kg/d; P = 0.01). Hematological parameters were largely unaffected by fiber or isoacid supplementation; significant day effects were observed for multiple leukocyte and erythrocyte indices. Total white blood cell counts tended to be lower in isoacid-supplemented steers (P = 0.08). No treatment effects were detected for hot carcass weight, fat thickness, or yield grade. A diet × isoacid interaction was observed for marbling score (P = 0.044): isoacids increased marbling in HG steers (47.7 vs. 57.5) but not in HF steers (56.3 vs. 48.8). Ribeye area tended to be greater in isoacid-supplemented steers (93.9 vs. 90.4 cm²; P = 0.12). Pathology scoring at harvest revealed that non-edible liver incidence was lower in isoacid-supplemented steers (17% vs. 33%), no supplemented steer exhibited concurrent liver and lung pathology (0% vs. 17% in controls), and premium/choice quality grade rate was higher in supplemented animals (58% vs. 33%). These results suggest isoacid supplementation may support hepatic health and improve carcass quality in finishing cattle, with marbling response dependent on dietary energy density.