Forages provide physically effective fiber, which is required for proper rumen function. However, forage neutral detergent fiber (NDF) is filling and can limit dry matter intake (DMI) by distention from undigested feed residues within the gastrointestinal tract. The rumen is generally regarded as the site within the gastrointestinal tract at which distention limits DMI. A signal from distention is integrated in brain feeding centers and increasingly limits DMI as milk yield increases. Therefore, less filling rations should be fed to cows with high milk yield. Forage NDF has a longer ruminal retention time than NDF from other sources because of longer initial particle size. While grinding and pelleting forages generally increases DMI, decreasing particle size of chopped forages generally has less effect unless chopped very finely. Digestibility of NDF varies among forage type (e.g., grasses vs. legumes) and generally decreases as forages mature and become more lignified. Greater in vitro or in situ NDF digestibility of forages has been related positively to DMI and milk yield of lactating cows. Within a forage type, NDF that is more fermentable clears from the rumen faster and is less filling, allowing greater DMI when limited by distention. However, this applies only within forage type; NDF from perennial grasses is generally much more digestible than NDF from legumes but is also more filling and more likely to limit DMI. This is because grasses are more resistant to particle size reduction by chewing during eating and ruminating. Faster particle size reduction will increase the mass of particles below the threshold size to pass from the rumen as well as decrease the ability of the rumen to selectively retain those particles by decreasing the size of the rumen mat. Therefore, in vitro NDF digestibility should be used to compared within forage type only. The overall filling effect of diets is determined primarily by forage NDF content, forage particle size, fragility of forage NDF, and NDF digestibility within a forage family.