Effects of wet distillers grains with solubles in diets based on steam-flaked corn containing moderate or high roughage levels on nutrient digestibility in feedlot cattle and in vitro fermentation

Previous research with roughage and wet distillers grains with solubles (WDGS) concentrations in feedlot diets based on steam-flaked corn has not extensively evaluated measurements of digestibility. Two experiments were conducted to evaluate the effects of concentrations of alfalfa hay (AH) and WDGS on apparent total-tract nutrient digestibility by finishing cattle and in vitro fermentation characteristics. In both studies, treatments were arranged in a 2 × 3 + 1 factorial and consisted of diets based on steam-flaked corn including AH (6 or 12% of DM) and WDGS (15, 30, or 45% of DM), and a control diet without WDGS that contained 9% AH. In Exp. 1, 56 beef steers (initial BW = 395 ± 7 kg) were used in a randomized complete block design and fed for 28 d. Intakes of DM, OM, NDF, ADF, and CP were greater (P ≤ 0.048) by steers fed 12 versus 6% AH. Increasing the dietary concentration of WDGS increased NDF, ADF, and CP intakes (P ≤ 0.046), and an AH × WDGS interaction (P ≤ 0.02) was detected for starch intake. Digestibility of DM and OM was greater for control steers than for the average of all treatments containing WDGS (P < 0.01). In Exp. 2, ruminal fluid was collected from 2 ruminally cannulated steers adapted to a 60% concentrate diet to evaluate in vitro total gas and methane production, nutrient disappearance, and VFA concentrations after 24 h of incubation. Total gas production decreased linearly as WDGS increased (P = 0.04). The control substrate had decreased NDF disappearance versus the average of other treatments (P < 0.01), and consistent with in vivo total-tract digestibility, in vitro NDF disappearance increased linearly as WDGS increased (P < 0.01) regardless of AH concentration. The concentration of AH and WDGS in feedlot diets had interactive effects on total-tract digestibility of DM, OM, and starch, but not NDF, in feedlot steers.