Four experiments were conducted to determine the fermentation and energetic value of fiber in feed ingredients and diets fed to pigs. In Exp. 1, degradation of dietary fiber in the stomach, small intestine, and hindgut of pigs fed corn- or wheat-based diets without or with microbial xylanase was determined. Results indicated that the apparent ileal digestibility (AID) of GE in corn-soybean meal (SBM) or wheat-SBM diets was greater (P< 0.05) than in the corn-SBM-distillers dried grains with solubles (DDGS) and wheat-SBM-wheat middlings diets, but no difference was observed for the AID of dietary fiber between wheat-SBM and wheat-SBM-wheat middlings diets. The apparent total tract digestibility (ATTD) of dietary fiber was also greater (P < 0.05) in corn-SBM and wheat-SBM diets compared with corn-SBM-DDGS and wheat-SBM-wheat middlings diets, which indicates that the concentration of dietary fiber may influence the degree of fermentation of fiber. Inclusion of a microbial xylanase improved (P < 0.05) the apparent duodenal digestibility (ADD) and the ATTD of nutrients and dietary fiber in wheat-based diets, indicating activity of xylanase in the gastro-intestinal tract of pigs. The concentration of DE and ME in wheat-based diets was also improved (P < 0.05) by microbial xylanase, but that was not the case if microbial xylanase was added to the corn-based diets. In Exp. 2, the hypothesis that values for ileal and total tract digestibility of dietary fiber in mixed diets are more accurately predicted from values for standardized ileal digestibility (SID) and standardized total tract digestibility (STTD) of fiber in individual ingredients than from values for AID or ATTD of fiber. Results indicated that measured and predicted AID, SID, ATTD, and STTD of most dietary fractions in diets based on wheat middlings and soybean hulls were not different. Likewise, no differences were observed between the predicted and measured AID and SID of all dietary fiber components in diet based on DDGS, wheat middlings, and soybean hulls. However, the measured AID, SID, ATTD, and STTD of some dietary fiber components in diets based on DDGS and wheat middlings or DDGS and soybean hulls was different from the predicted values. The measured ATTD and STTD of some dietary fiber fractions in the diet based on DDGS, wheat middlings, and soybean hulls was also different from the predicted values. It was also concluded that values for SID or STTD did not improve additivity of digestibility values compared with values for AID or ATTD. Experiment 3 was conducted to test the hypothesis that pentoses that enter the small intestine may affect energy and CP utilization in pigs. Results indicated that AID of GE and OM in pigs fed a diet containing arabinose instead of glucose or xylose was reduced, but no difference was observed for the AID of CP and most AA among diets indicating that xylose and arabinose do not impact protein digestibility or utilization. No differences were observed for the ATTD of GE, DM, and OM, DE, N balance, and biological value of N among diets. However, more (P < 0.01) energy was excreted in the urine from pigs fed the xylose or arabinose diets compared with pigs fed the glucose diet, thereby reducing (P < 0.01) ME in these diets compared with the glucose diet. The AID of glucose, xylose, and arabinose was close to 100% and the ileal retention rates of xylose and arabinose were 82.57 and 82.41%, respectively, which may indicate that certain metabolites from xylose or arabinose were excreted in the urine and analyzed as energy. In Exp. 4, the hypothesis that pentoses cannot be absorbed from the hindgut, but instead are fermented in the large intestine, was tested. Arabinose or xylose was infused into the hindgut of pigs and the impact of xylose and arabinose on energy utilization, excretion of pentoses in urine and feces, and on concentration of fecal VFA were determined. No differences were observed for the ATTD of GE, DM, ash, and OM or in DE and ME of the diets. However, the pH in feces was reduced if arabinose was infused indicating increased fermentation in the hindgut. In conclusion, microbial xylanases used in this experiment improved the digestibility of dietary fiber in the stomach and hindgut of pigs and improved energy status of pigs fed wheat-based diets, but not of pigs fed corn-based diets. Bulk density and SDF of the diet are better predictors of DE and ME compared with other physical or chemical characteristics of the diet. Correcting for endogenous losses does not increase additivity of digestibility values for dietary fiber in mixed diets. Dietary pentoses are mostly absorbed prior to the end of the small intestine of pigs and may reduce energy utilization, but they do not affect CP utilization in pigs
