INFLUENCE OF FLAKE DENSITY ON THE FEEDING VALUE OF STEAM-PROCESSED CORN IN DIETS FOR LACTATING COWS

Four primiparous Holstein cows (112 d in lactation) with cannulas in t he rumen and proximal duodenum were used in a 4 x 4 Latin square desig n. Dry-rolled (DR) corn (density .52 kg/L) was compared with steam-fla ked (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 bicarbo nate, 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 < .0 1) 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 di gestibility was decreased by SF (P < .01); this effect was exacerbated by degree of processing (linear component, P < .05). Ruminal N effici ency (duodenal nonammonia N/N intake) was greater(P < .10) for SF than for DR corn because of an improved (P < .10) microbial efficiency. St eam flaking at all densities increased postruminal digestibility of OM (42.4%, P < .01), starch (84.9%, P < .01) N (13.6%, P < .01), and lip id (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) w ith degree of processing. However, this effect was offset by decreased (linear and quadratic components, P < .05) ADF digestibility. Steam f laking improved the feeding value of corn for lactating cows by improv ing diet acceptability and digestibility of lipid, starch, and N. Stea m flaking increased the estimated NE(I) of corn by 33%. Optimal flake density probably is between .32 and .39 kg/L.