Cell‐Wall Composition and Digestibility of Alfalfa Stems and Leaves1

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Abstract
The cell walls of alfalfa (Medicago sativa L.) are a major source of energy for both dairy and beef cattle. Despite this, little is known about composition of the cell walls. Our objective was to characterize changes in the proportion of, and cell‐wall carbohydrate and lignin composition in, stems and leaves of maturing alfalfa. Field‐grown ‘Tempo’ and ‘Spredor 2’ alfalfa (upright and semiprostrate cultivars, respectively) were sampled at weekly intervals seven times in 1981 and eight times in 1982. Sampling both years commenced approximately 1 week before early‐bud stage and was terminated when seed pods formed. Plants were divided into stem and leaf fractions before cell‐wall and in vitro digestible dry matter (IVDDM) analyses were conducted. Concentrations of IVDDM in leaves were approximately 800 g kg‐1 and changed little with maturity. Stem IVDDM declined from approximately 750 g kg‐1 before bud development to 480 g kg‐1 in 1981 and 520 g kg‐1 in 1982. The concentration of cell‐wall material (CWM) increased by nearly 60% in stems in both seasons, whereas leaf CWM increased by only 10%. The proportion of lignin in the stem CWM increased by 30% with a corresponding decrease in the neutral‐sugar fraction as the plants matured. Xylose and glucose (low digestibility) increased with maturity as a proportion of total cell wall neutral sugars while arabinose and galactose (high digestibility) decreased. Soil‐moisture deficits in 1981 seemed to increase the amount of stem cell‐wall arabinose and xylose and decrease glucose concentrations compared with 1982. This work confirms that digestibility of alfalfa decreases with maturity as a result of increased concentration of CWM in stems, decreased stem digestibility, and increased proportions of stems. Significant compositional changes in CWM in stems and little change in leaves occurred as plants matured.