Twelve lactating, multiparous Holstein cows were used in two experiments, designed as 6 x 6 Latin squares with 28-d periods, to evaluate the effects of increasing the NDF concentration of diets consisting of barley-based concentrates and various forages. Diets were designed in a 3 x 2 factorial set with three concentrations of total dietary NDF (32, 36, and 40%) combined with two sources of forage. In Experiment 1, cows received alfalfa hay (45.1% NDF) or orchardgrass hay (54.0% NDF). In Experiment 2, cows received alfalfa hay (42.5% NDF) or corn silage (50.5% NDF). Cows ate more DM when fed alfalfa hay than when fed orchardgrass hay or corn silage. Intake of DM decreased as NDF concentration increased for alfalfa hay and corn silage diets, but a positive quadratic relationship between DMI and NDF occurred for orchardgrass hay. As dietary NDF increased, milk yield decreased, but fat percentage increased. Time spent ruminating was associated positively with concentration of NDF and was similar for diets formulated with various forages at the same dietary NDF concentration. Regardless of forage source, increasing dietary NDF concentration did not affect FCM yields because, as NDF intake increased, energy intake decreased. With consumer demand for low fat milk products, high yield of low fat milk is advantageous, and the minimal amount of fiber necessary in dairy cattle diets may be less than currently recommended. Further research is required to determine the long-term effects on health and longevity of low fiber diets fed to cows.