Effect of exogenous glucoamylase inclusion on in vitro fermentation and growth performance of feedlot steers fed a dry-rolled corn-based diet

The objective was to evaluate the effect of adding an exogenous glucoamylase (GA) enzyme from the fungus Trichoderma reesei on in vitro fermentation, growth performance, and carcass characteristics of feedlot steers fed a dry-rolled corn (DRC)-based diet. Experiment 1 evaluated three levels of added enzyme (0, 0.24, and 0.72 GA enzyme units) and two corn particle sizes (CPS; 2 and 4 mm) in a factorial arrangement using a 7 h in vitro batch culture fermentation. Addition of GA increased (P < 0.01) in vitro dry matter disappearance by 13% and decreased final pH (P < 0.01). Molar proportion of propionate increased with GA inclusion (P < 0.01). A smaller CPS increased (P < 0.01) in vitro dry matter disappearance and total volatile fatty acid and decreased final pH (P < 0.01). A smaller CPS also decreased (P < 0.01) the molar proportion of acetate and increased (P < 0.01) the molar proportion of butyrate. In experiment 2, Angus × Simmental steers (n = 105; initial body weight [BW] = 329 ± 38 kg) were used to evaluate the inclusion of an exogenous GA on growth performance and carcass characteristics. Steers were fed a basal diet consisting of 60% DRC, 17.5% modified distillers grains with solubles, 12.5% corn silage, and 10% dry supplement on a dry matter basis for 136 d. Steers were blocked by weight and allotted to pens. Pens were randomly assigned to one of three treatments (5 pens/treatment): diet with no GA (CON), low inclusion of GA (122 enzyme units/kg dry matter [DM]; LGA), or high inclusion of GA (183 enzyme units/kg DM; HGA). Inclusion of GA did not affect (P ≥ 0.23) final BW, dry matter intake (DMI), or average daily gain (ADG) for the 136-d feeding period. Feed conversion was affected (P = 0.02) by treatment with steers fed HGA having ~8% greater G:F compared with LGA and CON. Treatment did not affect (P = 0.32) fecal starch. Inclusion of GA did not affect (P ≥ 0.19) carcass traits including hot carcass weight, 12th rib fat thickness, yield grade, longissimus muscle area, or marbling score. Overall, results suggest inclusion of exogenous GA enzyme increased in vitro dry matter disappearance in batch culture and improved feed conversion in steers fed 183 enzyme units/kg DM during the finishing phase.