Effects of monensin and tylosin in finishing diets containing corn wet distillers grains with solubles with differing corn processing methods.

A total of 3,632 crossbred steers were used in 3 separate randomized complete-block designed finishing experiments. Data from Exp. 1 were analyzed separately whereas data were combined for Exp. 2 and 3, based on corn processing method used [Exp.1 = equal combination of dry-rolled and high-moisture (DRC:HMC); Exp. 2 and 3 = steam-flaked (SFC)]. Steers were fed 1 of 5 treatments to evaluate the effects of monensin and tylosin in feedlot diets containing 25% corn wet distillers grains with solubles (WDGS; DM basis). Treatments included: 1) corn-based diet (no WDGS) with 360 mg/d monensin and 90 mg/d tylosin (CORN+MT), 2) 25% wet distillers grains with solubles (WDGS-CON), 3) 25% WDGS with 360 mg/d monensin (WDGS+M), 4) WDGS with monensin and tylosin at same levels as treatment 1, and 5) WDGS with 480 mg/d monensin and 90 mg/d tylosin (WDGS+HIMT). In Exp. 1, WDGS+MT increased (P < 0.01) ADG (6.5%), G:F (6.9%), and HCW (3.0%) compared with CORN+MT. Alternatively, in Exp. 2 and 3, WDGS+MT had no effect on ADG (P = 0.18), decreased (P < 0.01) G:F by 3.2%, and did not affect HCW (P = 0.57) compared with CORN+MT. In Exp. 1, addition of monensin to a WDGS diet increased G:F by 3.1% (P = 0.03) and tended to increase G:F in Exp. 2 and 3 (P = 0.09) compared with WDGS-CON. For all experiments, addition of monensin and tylosin in a diet containing WDGS increased G:F (P < 0.01) and reduced total and severe liver abscesses (P < 0.01) compared with WDGS-CON. Additionally, HCW was increased in Exp. 2 and 3 (P < 0.01) and tended to increase in Exp. 1 (P = 0.09) when monensin and tylosin were fed in a WDGS diet compared with WDGS-CON. Feeding WDGS+MT reduced total liver abscesses by 79.3% (Exp. 1) and 57.6% (Exp. 2 and 3) compared with WDGS-CON. Compared with WDGS+MT, minimal differences were observed when monensin was fed at 480 mg/steer daily with tylosin in diets containing WDGS. Inclusion of WDGS in finishing steer diets did not alter effectiveness of monensin with tylosin in improving G:F and reducing prevalence of liver abscesses, regardless of corn processing method. Corn processing method appears to be an important factor in performance response to WDGS with feeding value of WDGS, measured as an increase in feed efficiency, being greater in a DRC:HMC-based diet compared with a SFC-based diet.