Digestive Fermentation in Herbivores: Effect of Food Particle Size

We examined the effect of reduction in food particle size on the rate and extent of fermentation of dry matter and cell walls in ruminants using in vitro and in sacco techniques. Large particles were digested at significantly lower rates, and time to initiation of cell wall degradation by microbes was signifcantly longer for large particles. Even after 144 h, large particles were digested to a lesser extent than were small particles. These results have important implications for digestive processing in herbivores, such as reptiles, that only minimally reduce food particle size. In these herbivores, digesta must be retained for much longer periods of time to achieve digestive efficiencies similar to those of herbivores that reduce food particle size to a greater extent. Length of retention time is a function of the amount of surface area exposed to microbial attack, not total surface area. Microbial activity concentrates in areas not protected by the cuticle. Because of the complexity of plant structure, relative rates of fermentation of diferent particle sizes cannot be predicted from relative particle size alone.