Microbial strategies in the ruminal digestion of cereal grains

The rate and extent of ruminal digestion of cereal grains depends on the rate of passage and a number of complex interactions between cereal kernel tissues and rumen microorganisms. The pericarp of cereal grains is resistant to microbial attachment and penetration. This structure must be fractured by mechanical processing or mastication for digestion to proceed. The susceptibility of the exposed endosperm to digestion depends on the species of cereal grain. Corn and sorghum contain dense protein in the horny endosperm region which surrounds starch granules and resists microbial colonization and penetration. In contrast, the floury endosperm region in these two grains, and the endosperm of wheat and barley are rapidly colonized by a variety of ruminal bacteria. Primary colonizing bacteria produce end products which attract secondary colonizers to the site of digestion to form adherent multispecies microbial biofilms. With finely ground cereal grains microbial biofilms form at a rapid rate and the excessive production of acid end products can often lead to digestive disturbances. Substances that prevent microbial attachment or promote detachment can be used to regulate the rate of cereal grain digestion in the rumen. Future strategies aimed at regulating the rate of cereal grain digestion in the rumen must include an appreciation for the microbial processes involved in cereal grain digestion.