Effect of Particle Size upon the in Vitro Cellulose Digestibility of Forages by Rumen Bacteria

Abstract

Rates of in vitro cellulose digestion were determined for mature timothy (seed ripe) which had been ball-milled 0, 6, 24, and 72 hr. The total amount of cellulose digested increased with the time of ball-milling up to 72 hr. Based on this observation four stages of timothy, three of alfalfa, and two each of brome and orchard grass, along with 72-hr. ball-milled samples of these same forages, were used as substrates for in vitro rumen fermentations. In all cases, ball-milling increased the amount of cellulose digested, and this increase became larger with advancing maturity and lignification of the forage. In further work, mature timothy was digested 48 hr. in vitro and the residue isolated. Less than 5% of the cellulose remaining in this residue was digested in a second 48-hr. fermentation; however, when a portion of this residue was ball-milled, 67% of the cellulose could be digested. Additional experiments which determined the amount of forage cellulose soluble in cupriethylene diamine, indicated that the amount of cellulose which can be dissolved decreases with forage maturity and increased deposition of lignin. The amount of cellulose dissolved was markedly increased after ball-milling, and again this increase became larger with advancing maturity of the forage. The experimental evidence presented in this paper substantiates the theory that the lignin in forages acts as a physical barrier between the cellulose and cellulolytic rumen bacteria.