Abstract: Two trials were conducted to evaluate the effects of partial replacement of alfalfa hay with grass hay (sudangrass and elephant grass) on DMI, lactational performance, and digestive function. Cows were fed a steam-flaked corn-based diet containing (DMB): 1) 49% alfalfa hay; 2) 24% alfalfa and 16% sudangrass; 3) 24% alfalfa, 8% sudangrass, and 8% elephant grass; and 4) 24% alfalfa hay and 16% elephant grass. Diets were formulated to contain 30% NDF (DMB). In trial 1, four lactating Holstein cows (475 kg) with cannulas in the rumen and proximal duodenum were used in a 4 x 4 Latin square experiment to evaluate the treatments effects on digestive function. In trial 2, eight multiparous Holstein cows (567 kg) with 80 ? 8 DIM were utilized in a replicated 4 x 4 Latin square design to evaluate treatment effects of on DMI and lactational performance. There were no treatment effects (P > 0.10) on ruminal digestion of OM, and NDF, averaging 51 and 31%, respectively. Microbial efficiency (g microbial N/ kg OM fermented) was greater (20%, P = 0.07) for alfalfa (30) than for grass hay substituted diets (25). Nonammonia N flow to the small intestine, as a percentage of N intake (ruminal N efficiency) averaged 101%, and was lower (linear effect, P < 0.10) for elephant grass than for sudangrass hay. There were no treatment effects (P > 0.10) on total tract digestion of OM, and NDF, averaging 70 and 43%, respectively. Digestible energy content of the diet was lower (5%, P < 0.05) for alfalfa hay (64.5%) than for the grass hay substituted diets (67.5%). There were no treatment effects (P > 0.20) on DMI, milk yield, averaging 21.8 and 32.2 kg, respectively. Body condition scores were greater (linear, P > 0.05) for cows fed elephant grass than for cows fed sudangrass. Substituting grass hay for a portion of the alfalfa hay increased (1.2%, P < 0.10) milk fat percentage. We conclude that although substitution of a portion (40%) of alfalfa hay with grass hay in diets for lactating cows may slightly decrease ruminal microbial efficiency, the impact on ruminal and total tract digestion of OM and NDF are small. The feeding value of elephant grass is at least equivalent to that of sudangrass in diets for lactating dairy cows. Grass hay can replace up to 40% of the forage in lactation diets without detrimentally affecting fat corrected milk yield, and milk yield efficiency.