Intake limitations, feeding behavior, and rumen function of cows challenged with rumen fill from dietary fiber or inert bulk.

Twelve multiparous, rumen-cannulated cows (17 DIM) were fed 25 or 35% NDF diets without or with added rumen-inert bulk as water-filled plastic containers (500 ml each) within three 4 x 4 Latin squares (21-d periods). Added bulk equaled 25% of pretrial rumen volume for each cow. Objectives were to challenge cows with rumen fill in the form of dietary NDF and inert bulk to determine whether bulk affects intake similarly for diets of different NDF content or whether changes in behavior or rumen function allow cows to adapt to higher fill. Inert bulk had no effect on DMI for cows fed 25% NDF but decreased DMI for cows fed 35% NDF (18.7 vs. 16.6 kg/d). Volume of rumen digesta plus inert bulk, 102 L, was similar for 35% NDF treatments regardless of presence or absence inert bulk. Additional NDF or inert bulk increased the number of ruminating bouts per day, chewing time per unit of DMI or NDF intake, frequency of reticular contractions during rumination, and fractional passage rate of NDF from the rumen. Although these changes may help to alleviate fill, they were insufficient to maintain intake under conditions of high fill. Intake by cows receiving high fiber diets can be limited by reticulorumen capacity. Future study is required to determine the effect of rumen fiber digestion, fiber particle size, and animal characteristics on fill-limited intake.