DEGRADATION CHARACTERISTICS OF ISOLATED AND IN-SITU CELL-WALL LUCERNEPECTIC POLYSACCHARIDES BY MIXED RUMINAL MICROBES

Two pectic polysaccharide fractions were isolated from lucerne (Medica go sativa L) leaves and used in fermentation experiments with mixed ru minal microbes. Both fractions were similar in chemical composition, c ontaining galacturonic acid (52-58 mol%) and the neutral sugars arabin ose (14-18 mol%), galactose (6-8 mol%) and rhamnose (8-12 mol%). Ferme ntation of both fractions was rapid and complete with a half-life of a pproximately 4 h. Production of total volatile fatty acids matched the degradation profile reaching a maximum level shortly after the rate o f degradation began to decrease. The fermentation characteristics of c itrus pectin and polygalacturonic acid were similar to those of the lu cerne pectic fractions but galacturonic acid was much slower in its ra te of degradation while soluble arabinogalactan from larchwood was vir tually undegraded. Leaves of early bud stage lucerne and lower nodes a nd internodes of stems from full bloom lucerne were also fermented by mixed ruminal microbes. Pectic polysaccharides were rapidly and extens ively degraded from both tissues. Initial rates were faster for leaves than for stems and the extent of pectic degradation was greater in le aves (8% residual) than in stems (17% residual). Selection of forage l ines with increased pectic polysaccharides would provide greater amoun ts of rapidly available energy that could result in more efficient uti lisation of the rapidly degraded protein in lucerne.