THE INFLUENCE OF INTAKE LEVEL AND CORN PROCESSING ON DIGESTIBILITY AND RUMINAL METABOLISM IN STEERS FED ALL-CONCENTRATE DIETS

The effects of intake (ad libitum and 70% of ad libitum) and corn proc essing (whole or rolled) on digestion, ruminal metabolism, and blood m etabolites were determined in a 2 x 2 factorial experiment using eight ruminally fistulated steers. All-concentrate diets were fed with adju stments to dietary concentrations of N, vitamins, and minerals to ensu re equal daily intake of these nutrients. Digestion of DM, OM, and sta rch were affected by an intake x processing interaction (P < .03). Sta rch digestion for the low-intake, whole-corn diet was much lower than that for the other three diets. This decrease in starch digestion was the major reason for the interaction. Nitrogen digestion was improved (P < .03) with limited intake but was not affected by processing. Rumi nal volume and ruminal turnover (percentage/ hour) were both reduced ( P < .03) on the low-intake diet. Molar proportion of acetate was reduc ed and propionate was increased (P < .01) for steers receiving high in take of rolled corn compared with the other three diets. Butyrate conc entrations were increased (P < .01) when corn was rolled before feedin g. Ruminal pH was higher immediately before feeding for steers receivi ng whole corn than for those receiving rolled corn. Decreases in pH we re observed for rolled corn 2 to 9 h after feeding, suggesting a faste r rate of fermentation of rolled corn than of whole corn. Concentratio ns of blood glucose and insulin were not affected by intake or process ing. It was concluded that processing corn can improve DM and OM diges tibility if intake is restricted and that N digestion is improved with restricted feeding. Reductions in intake of up to 30% do not have an effect on ruminal pH, nor do they reduce blood glucose or insulin conc entrations.