Specific Gravity of Various Feedstuffs as Affected by Particle Size and In Vitro Fermentation

Abstract

Experiments were designed to compare true and functional specific gravity of feed particles and to study the effects of particle size and in vitro fermentation on functional specific gravity. True specific gravity was estimated by centrifuging size fractions of various feedstuffs in ethanol-carbon tetrachloride density gradients. Functional specific gravity was determined in pycnometers. True specific gravity ranged from 1.3 to 1.5 but did not consistently differ with feedstuff or particle size. In contrast, functional specific gravity was affected by feedstuff, particle size (for forages), and in vitro fermentation. Functional specific gravity increased as forage particle size decreased and, for different forages, changed similarly over the in vitro fermentation period. A significant forage by particle size interaction suggested that differences in functional specific gravity between forages were not maintained as particle size changed. Part of the reduction in ruminal retention time achieved by decreasing the particle size of a feedstuff may result from its effects on functional specific gravity. Manipulating the diet to alter the functional specific gravity of particles in the rumen may help to obtain the desired effects on digestion in and passage from the rumen. © 1991, American Dairy Science Association. All rights reserved.