Use of in vitro dry matter digestibility and gas production to predict apparent total tract digestibility of total dietary fiber for growing pigs

In vitro DM disappearance (IVDMD) and gas production methods have been developed and used to measure in vivo nutrient digestibility of feed ingredients, but further validation is needed for ingredients containing high concentrations of insoluble fiber such as corn distiller’s dried grains with soluble (DDGS). A 3-step in vitro procedure and resulting gas production were used to predict in vivo apparent total tract digestibility (ATTD) of total dietary fiber (TDF) among 3 sources each of wheat straw (WS), soybean hulls (SBH), and DDGS. A total of 34 barrows and 2 gilts (84 ± 7 kg BW) were used in a changeover design to determine the ATTD of 9 dietary treatments. The WS, SBH, or DDGS sources were the only ingredients containing fiber in each diet, and all diets were formulated to contain the same TDF concentration (22.3%). The in vivo experiment was conducted in 2 consecutive 13-d periods, each including a 10-d adaptation and a 3-d collection period to provide 8 replications/dietary treatment, and 0.5% TiO₂ was added to each diet as an indigestible marker. Pigs had ad libitum access to water and were fed an amount of feed equivalent to 2.5% of initial BW in each period. The in vitro experiment was used to determine IVDMD and gas production of the 9 ingredients (5 to 8 replicates/ingredient) fed during the in vivo experiment. Gas production kinetics were fitted using a nonlinear model and analyzed using a mixed model, and predictions were evaluated using correlations and regression models. There were differences (P < 0.01) in ATTD of TDF among WS (26.7%), SBH (78.9%), and DDGS (43.0%) and among sources of DDGS (36.0 to 49.8%). Differences (P < 0.05) in IVDMD from simulated gastric and small intestinal hydrolysis were observed among WS (13.3%), SBH (18.9%), and DDGS (53.7%) and among sources of WS (12.8 to 13.8%), SBH (17.0 to 20.5%), and DDGS (52.0 to 56.9%). Differences (P < 0.05) in IVDMD from simulated large intestine fermentation (IVDMDf) were also observed among WS (23.3%), SBH (84.6%), and DDGS (69.6%) and among sources of WS (18.7 vs. 26.8%). In vitro DM disappearance from simulated total tract digestion of SBH (88.9%) and DDGS (86.1%) were greater (P < 0.01) than that of WS (33.5%). Differences (P < 0.01) in asymptotic gas production (A; mL/g DM substrate) were observed among WS (121), SBH (412), and DDGS (317), and ATTD of TDF was highly correlated with IVDMDf and A. In conclusion, low variability in ATTD of TDF and IVDMD among sources of WS and SBH evaluated in the current study may not justify the use of in vitro measurements, but in vitro fermentation accurately predicts ATTD of TDF among sources of corn DDGS.