Fiber Composition and Digestion Kinetics in Grass Stem Internodes as Influenced by Particle Size

Abstract

Our objective was to characterize discrete particle fractions in ground grass forage and determine the influence of differences in particle size among fractions on fiber composition and digestion kinetics. Three replicates of reed canarygrass and switchgrass were harvested and the fourth and fifth stem internodes below the panicle were removed, dried, and ground (3 mm). Samples were sieved to a constant weight and particles retained on .3, .425, .6, and .85-mm sieves were analyzed with half of each particle fraction reground to pass a 1-mm Udy mill screen. Particles were analyzed for length, width, and area using an image analyzer, and particle thickness was determined using a light microscope. Volume, surface area, and density measurements of defined particle fractions had an approximate geometric distribution for the four particle fractions, indicating that these measurements are related to particle size estimates based on logarithmic normal distributions. The four particle fractions were similar in composition based on density, crude protein, and fiber estimates. Digestion rate constants were not affected by particle size within the range of the four particle fractions. However, regrinding to pass a 1-mm Udy screen increased digestion rates, indicating that small particle size can affect digestion kinetics. Fiber values from sequential analysis of forage particles not ground to pass a 1-mm screen must be evaluated cautiously, because some potentially soluble components may remain in the residue after each extraction step and result in high analytical values. © 1988, American Dairy Science Association. All rights reserved.