|Abstract:||Five experiments were conducted to investigate the influence of physicochemical characteristics of fiber and inclusion rate of high-fiber ingredients on the utilization of dietary fiber, energy, and nutrients in diets fed to pigs. Experiment 1 was conducted to quantify nutrient and fiber fractions of feed ingredients and to determine in vitro apparent ileal digestibility (IVAID) and in vitro apparent total tract digestibility (IVATTD) of DM and OM in each ingredient. Ten ingredients that vary in fiber concentration and composition were used: corn, wheat, soybean meal (SBM), canola meal, distillers dried grains with solubles (DDGS), corn germ meal, copra expellers, sugar beet pulp (SBP), synthetic cellulose, and pectin. Correlations between chemical and physical characteristics of ingredients and IVAID and IVATTD of DM and OM were determined. The physical characteristics measured included bulk density, water binding capacity (WBC), swelling, and viscosity. The analyzed GE was compared with values for GE calculated from all energy-contributing components. Results indicated that the analyzed chemical composition of most ingredients added to 100% or greater, and the difference between the sum of the calculated GE of the analyzed components and the analyzed GE of the ingredients ranged from -2.25 MJ/kg in DDGS to 1.74 MJ/kg in pectin. No correlation was observed between swelling, WBC, or viscosity and IVAID or IVATTD of DM or OM. The stronger correlations between insoluble dietary fiber (IDF), total dietary fiber (TDF), and insoluble non-starch polysaccharides and IVAID and IVATTD of DM and OM than between ADF and NDF and IVAID and IVATTD of DM and OM indicates that the concentration of TDF in feed ingredients is a better predictor of the digestibility of DM and OM than values for NDF and ADF. Experiment 2 evaluated effects of physicochemical characteristics of feed ingredients used in Exp. 1 on DE and ME and apparent total tract digestibility (ATTD) of GE, DM, and nutrients in growing pigs using ingredients with different IDF to soluble dietary fiber ratios. Results indicated that stronger correlations between TDF and DE and ME than between ADF or NDF and DE and ME were observed, indicating that TDF can be used to more accurately predict DE and ME than values for NDF or ADF. The DE, ME, and the ATTD of DM in ingredients were positively correlated (P < 0.05) with in vitro ATTD of DM generated in Exp. 1, indicating that the in vitro procedure may be used to estimate DE and ME in feed ingredients. Swelling and WBC were positively correlated (P < 0.05) with the ATTD of IDF, TDF, non-starch polysaccharides (NSP), and insoluble NSP, and viscosity was positively correlated (P < 0.05) with the ATTD of NDF, IDF, and insoluble NSP, indicating that some physical characteristics may influence digestibility of fiber but no correlations between physical characteristics and DE or ME were observed. Experiment 3 was conducted to determine the effects of inclusion rate on apparent ileal digestibility (AID), apparent hindgut disappearance (AHD), and ATTD of GE and on the concentration of DE and ME in fiber-rich ingredients fed to growing pigs. We hypothesized that increasing the inclusion rate of fiber decreases digestibility of GE and, thus, the contribution of DE and ME from hindgut fermentation because greater concentrations may reduce the ability of microbes to ferment fiber. A basal diet based on corn and SBM was formulated. A diet based on corn, SBM, and 30% corn starch was also formulated. Six diets were formulated by replacing 15 or 30% corn starch by 15 or 30% corn germ meal, SBP, or wheat middlings. Two additional diets were formulated by including 15 or 30% canola meal in a diet containing corn, SBM, and 30% corn starch at the expense of corn and SBM. Results indicated that inclusion rate did not affect the calculated DE and ME or AID, AHD, and ATTD of GE in canola meal, corn germ meal, SBP, or wheat middlings, indicating that concentration of DE and ME in ingredients were independent of inclusion rate and utilization of energy from test ingredients was equally efficient between diets with 15 and 30% inclusion. Increased inclusion of fiber in the diet did not influence transit time in the small intestine, but reduced the time of first appearance of digesta in the feces indicating that transit time was reduced in the hindgut of pigs fed high-fiber diets. In Exp 4 and 5, it was determined if values for AID, AHD, and ATTD of DM and nutrients in the high-fiber ingredients used in Exp. 3 measured at 15% inclusion are also accurate if 30% of that ingredient is used in diets fed to pigs. The hypothesis that much of the IDF is not fermented by the pig was also tested. Results indicated that AID, AHD, and ATTD of most nutrients measured at 15% inclusion were not different from values measured at 30% inclusion of the ingredients. The ATTD of IDF ranged from 52.9% in WM included at 15% to 86.2% in SBP included at 30% in the diet, which indicates that there was a relatively high digestibility of IDF under the conditions of this experiment. There was a reduction (P < 0.05) in AID of CP and all AA except Arg in canola meal and a reduction (P < 0.05) in AID of CP, Lys, Asp, Pro, and Ser in corn germ meal as inclusion rates of these ingredients increased in the diet. However, inclusion rate had no effect on the AID of CP and AA in sugar beet pulp or wheat middlings. In conclusion, DE and ME in feed ingredients may be predicted from some chemical constituents and from in vitro digestibility of DM, but not from physical characteristics. Inclusion rate of fiber-rich ingredients in diets did not affect calculated values for DE and ME in feed ingredients, but the AID, AHD, and ATTD of some nutrients and AID of AA measured at 30% inclusion is different from values obtained at 15% inclusion for some high fiber dietary ingredients in mixed diets.