Forage Digestibility: The Intersection of Cell Wall Lignification and Plant Tissue Anatomy

Ruminants evolved their multi-compartment digestive tract to effectively process plant material containing large amounts of cell wall material. We rely on forages as the sole or major feed resource for domestic ruminant animal production, even in the grain intensive North American beef and dairy systems. Unfortunately, digestibility of forages is extremely variable and almost always less than grains. Because cell wall material is the single largest component of forages, the digestibility of forage cell walls is a primary determinant of animal productivity and efficiency. The plant cell wall is a complex matrix of polymers that surrounds every plant cell. Walls provide the physical support required for plants to grow upright in terrestrial habits and serve many other important functions, such as being a barrier to attack by pathogens and insects. While all cell walls share some basic chemical characteristics, striking differences exist among plant tissues in cell wall concentration, composition, and structural organization. Cell walls of many tissues undergo dramatic shifts in chemical characteristics during maturation. Leaves and stems differ in the types and proportions of tissues they contain, and grasses and legumes differ in cell wall composition and tissue profiles. Growth environment also modulates plant and cell wall development. These sources of variation must be understood if performance of ruminants fed forages is to be accurately predicted. Cellulose and the other cell wall polysaccharides can be completely digested by the rumen microflora, but only when isolated from the wall. Understanding how cell wall component interactions limit microbial digestion is critical. My objective is to integrate knowledge of cell wall composition and structure, plant and tissue anatomy, and plant maturation as factors influencing the digestion of forage cell wall material in the rumen.