EFFECTS OF FEEDING NONFORAGE FIBER SOURCES ON SITE OF FIBER DIGESTION

Although many nonforage fiber sources have high extents of neutral det ergent fiber (NDF) digestion, most have rates of digestion similar to or slower than the rates of forage NDF digestion, Rates of NDF digesti on vary considerably among and within sources of by-products. Digestio n kinetics also vary because of the technique used (in vitro versus in situ) and because of high amounts of dietary concentrate. Based on av ailable data for passage rate and specific gravity measurements, rates of passage of nonforage fiber sources from the rumen of high producin g cows appear to be faster than those of forages. Therefore, the poten tial to shift. NDF digestion to the hindgut has been discussed. To acc ount for variability in ruminal and total tract digestibility of NDF, multiple regression analysis was used to indicate that nonforage NDF p ercentage in the diet had about two-thirds the positive response on to tal tract NDF digestion that forage NDF percentage did. Although the l oss of potentially digestible NDF may occur, DMI does not appear to de crease much until forage NDF is below 14 to 16% of dietary DM. Convers ely, replacement of starch with nonforage NDF appears to increase dige stibility of fiber, mostly in diets with high concentrations of nonfib er carbohydrates, apparently because of reduced negative associative e ffects. Increasing the concentration of total NDF above 35% also can d ecrease DMI with little improvement in NDF digestibility. Increased kn owledge of the kinetics of digestion and the passage of various nonfor age fiber sources used to replace forage or concentrate should increas e the accuracy and precision of dynamic models, thereby increasing the flexibility and utility of nonforage fiber sources in dairy rations.