The non-NDF carbohydrates (NFC) are important sources of energy in the ration of high producing cows. The fibre must be proper quality and particle size to insure maximum DMI, optimal chewing activity, normal ruminal fermentation, and milk fat percentage. The NRC (1989) recommends 25 to 28% NDF in the rations of lactating cows; a minimum of 75% of the NDF should come from forages. These recommendations provide no adjustment for the physical effectiveness of the fibre, interactions among fibre sources and non fibre carbohydrates, or animal characteristics that may influence ration design. Few data are available to document the effect of the substitution of by-product NDF for forage NDF; our objective was to determine the effect of the substitution of alfalfa NDF from sugar beet on DMI, milk yield and composition, chewing activity, faecal and rumen pH, and apparent digestibility of DM.