Alfalfa is generally considered to be among the highest quality forages for feeding ruminant livestock. Among the positive quality attributes of alfalfa are high protein and calcium concentrations, low cell wall concentration, and rapidly digested cell wall material. These cell wall traits contribute to the high intake potential of alfalfa by cattle. Alfalfa has an indeterminate growth habit and continues to accumulate leaf and stem material until the crop is harvested. The increase in stem mass exceeds leaf growth, which results in a shift of the leaf-to-stem ratio and a reduction in overall forage quality. This is because alfalfa leaves contains less cell wall material and more protein than stems. In addition, maturation does not alter alfalfa leaf composition appreciably whereas stems accumulate more cell wall material during maturation. Heavily lignified, thick-walled xylem fiber and vessel tissues are added to stems through cambial activity in stem internodes once internode elongation ceases. This deposition of additional xylem continues until the stem is harvested. Stem xylem cell wall material is rich in cellulose, xylans, and lignin. The cellulose and xylans are slowly degraded by ruminal microorganisms, and lignin interferes with the degradation of these polysaccharides, making the stem xylem tissue virtually non-degradable. In contrast, leaves contain little xylem tissue and most tissues are non-lignified and rich in pectin. These non-lignified cell walls are rapidly and virtually completely degradable. Stems contain a small amount of these non-lignified tissues that also remain degradable throughout development. Clearly the proportion of leaves in alfalfa when harvested is the dominant factor that determines the quality of this crop, both in terms of protein content and also the digestibility of it's cell wall material.