When a highly digestible food is progressively diluted with one of greater ‘bulk’ the prevailing view is that the rate of food intake will initially increase at such a rate such that DE intake remains constant and, that beyond a critical point, intake of food and DE will fall as the dilution proceeds further. The critical point has been assumed to reflect the capacity of the pig for ‘bulk’. Current models which attempt to predict the voluntary food intake (VFI) of pigs use either food dry matter (DM) as a measure of bulk (eg Whittemore, 1983) or the undigested organic matter (eg Roan, 1991), or ignore the problem (ARC, 1981). It has long been known however, that such views are likely to be inadequate across the complete range of foods (eg the significant reduction in VFI of pigs on foods based on sugar beet pulp, compared to that on other more undigestible materials).

The experiment described here was designed (i) to try to identify the property of ‘bulky’ foods which might be responsible for limiting their VFI and (ii) to see the extent to which the capacity for a ‘bulky’ food might be modified by prior experience.