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Cell-Wall Carbohydrates in Leaves, Stems, and Herbage of Alfalfa and Red Clover

Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
USDA-ARS, 1565 Agronomy Hall, Iowa State Univ., Ames, IA 50011

^* Corresponding author.

Digestion of herbage by ruminants is largely dependent on composition and organization of polysaccharides in plant cell walls (CW). This study was conducted to determine the concentration of CW neutral sugars from polysaccharides in maturing alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.) leaves, stems, and total herbage and to determine their relationship to digestibility. Field-grown plants were sampled during seven harvests over two growth periods. At each harvest, plants were divided into three-node (red clover) or six-node (alfalfa) stem segments, and leaves were removed. Total neutral-sugar (TNS) concentration increased with maturity in stem and total herbage samples of both species and was greater in alfalfa than in red clover. In both species, TNS concentration in stems was nearly twice the concentration found in leaves. Furthermore, rhamnose, arabinose, and galactose proportions in TNS generally decreased in stems and total herbage with increasing maturity, whereas xylose and glucose proportions increased. Leaves of both species had greater rhamnose, arabinose, mannose, and galactose proportions than stems. Cell-wall, CW lignin, and TNS concentrations were common variables in stepwise regression for both total herbage and stem models and accounted for nearly 95% of the variation of in vitro digestible dry matter (IVDDM). In leaves, TNS concentration explained more than half the variation in IVDDM. Compared with alfalfa leaves, red clover leaves had higher TNS concentrations and lower IVDDM. Relationships between individual neutral sugars and digestibility were inconsistent between leaves and stems. The high TNS concentration was more important in explaining the low digestibility of red clover leaves relative to alfalfa leaves than was lignin or CW concentration.

Received for publication July 18, 1988.