Technical Abstract: A 4-unit dual-flow continuous culture fermentor system was used to evaluate the effects of herbage, a total mixed ration (TMR) and flaxseed on nutrient digestibility and microbial N synthesis. Treatments were randomly assigned to fermentors in a 4 x 4 Latin square design. Each fermentor was fed a total of 70 g of dry matter (DM)/d of one of four diets: 1)100% freeze-dried orchardgrass herbage (Dactylis glomerata L.; HERB); 2) 100% freeze-driedTMR (100TMR); 3) 50% orchardgrass herbage supplemented with 50% TMR (50TMR); or 4) 90% orchardgrass herbage supplemented with 10% ground flaxseed (Linum usitatissimum L.; FLAX). Apparent DM and organic matter digestibilities were higher for the HERB diet while neutral detergent fiber digestibility was higher for the two herbage-based diets (HERB and FLAX). True DM and organic matter digestibilities were not affected by diet, averaging 78 and 81%, respectively. Mean and minimum pH was higher for the herbage-based diets (HERB and FLAX) while maximum pH was not affected by diet, averaging 6.66. Concentration of total volatile fatty acids, and molar proportions of acetate, propionate and isovalerate, as well as the acetate to propionate ratio, were higher, while molar propotions of isobutyrate were lower, for the two diets containing the TMR (100TMR and 50TMR). Ammonia-N concentrations tended to be lower for the 100TMR diet. Apparent crude protein digestibility and bacterial N synthesis did not differ across treatments. While TMR-based diets decreased nutrient digestibility slightly, TMR offered advantages in microbial fermentation in relation to volatile fatty acid production, which could potentially translate into better animal performance. Flaxseed shows promise as an energy alternative for herbage-based diets.