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Four primiparous Holstein cows (112 d in lactation) with cannulas in the rumen and proximal duodenum were used in a 4 x 4 Latin square design. Dry-rolled (DR) corn (density .52 kg/L) was compared with stream-flaked (SF) corn processed to three flake densities: .39, .32, and .26 kg/L. The basal diet contained (DM basis) 43.4% alfalfa hay, 39.5% corn (DR or SF), 2.2% yellow grease, 9.6% cane molasses, .7% sodium bicarbonate, and 4.6% protein-mineral supplement. Steam flaking increased DM intake (P < .01). There were no treatment effects on ruminal pH (P > .10). Ruminal molar ratios of acetate and propionate were lower (P < .01) for SF than for DR corn and declined (linear component, P < .01) as flake density decreased. Ruminal digestibility of OM was not affected (P > .10) by corn processing. Ruminal starch digestibility increased (linear component, P < .05) as flake density decreased. Ruminal ADF digestibility was decreased by SF (P < .01); this effect was exacerbated by degree of processing (linear component, P < .05). Ruminal N efficiency (duodenal nonammonia N/N intake) was greater (P < .10) for SF than for DR corn because of an improved (P < .10) microbial efficiency. Steam flaking at all densities increased postruminal digestibility of OM (42.4%, P < .01), starch (84.9%, P < .01) N (13.6%, P < .01), and lipid (11.8%, P < .05). Total-tract digestibilities of OM, N, starch, DE and ME were greater (15.2, 11.5, 25.2, 14.9, and 20.0%, respectively, P < .01) for SF (mean of the three densities) than for DR corn. Total-tract digestibility of starch increased (linear component, P < .10) with degree of processing. However, this effect was offset by decreased (linear and quadratic components, P < .05) ADF digestibility. Steam flaking improved the feeding value of corn for lactating cows by improving diet acceptability and digestibility of lipid, starch, and N. Steam flaking increased the estimated NEl of corn by 33%. Optimal flake density probably is between .32 and .39 kg/L.