Effect of maturity on rumen degradation of tropical and temperate forage cell wall polysaccharides from leaves and stems

Abstract

Two main experiments were conducted in this study with the aim of better understanding the factors that influence the degradability of forages by ruminants. In the first part of the study, the degradability of leaf and stem fractions of two mature tropical forages, bana grass (Pennisetum purpureuml and silverleaf desmodium (Desmodium uncinatum) were determined by means of the In situ nylon bag technique. The effective degradabilities of both bana grass stems and leaves DM (50h) were low (48.2% and 45.3% respectively) and did not differ (P > 0.05) for the two fractions. Desmodium stems were less degradable (P < 0.05) than the leaves (40.8% and 59.7% respectively). Neither grass leaves nor stems differed (P > 0.05) in effective degradability of NDF (49.0% vs 51.6% respectively) or its constituent polysaccharides. Bana leaves were higher (P < 0.05) in the potentially degradable DM, than the stems but did not differ (P > 0.05) in the rate at which this fraction was degraded. On the other hand, desmodium leaves were higher in the potentially degradable fraction and the rate at which it was degraded-than the respective stems. NDF, cellulose and hemicellulose showed a similar trend. The second part of this study was aimed at determining whether the degree of plant maturity has any effect on the distribution of cellulose and linear and branched fractions of hemicelluloses. Two temperate forages, orchard grass / i i (Dactvlis glomerata) and tall fescue (Festuca arundinacea) harvested at different stages of growth and separated into leaves and steins were used. The cell wall content (NDF) and hemicellulose A (linear xylan) showed an increasing trend with maturity in orchard grass but not in tall fescue. Hemicellulose B (branched xylan) in the sample used in this study did not seem to change with increasing maturity. Stems (DM) of medium and late cut orchard grass were less effectively degradable (P < 0.05) than leaves (58.1% vs 62.0% and 52.6% vs 59.2% respectively). The potentially degradable fraction did not differ (P > 0.05) with maturation for the two fractions. The rate of degradation of this fraction was higher for leaves (P < 0.1). Tall fescue fractions did not differ (P > 0.05) in effective degradability and rate of degradation of the potentially degradable fraction. Cellulose and hemicellulose A and B showed a declining trend in their effective degradabilities (50h). Among these polysaccharides hemicellulose B showed a relatively higher degradability than the other polymers. Hemicellulose A seemed the least degradable. It appeared that the polysaccharides of leaves were relatively more degradable than those of stems. Tall fescue polysaccharides seemed to differ in effective degradability in both leaves and stems at the two cutting dates other than for hemicellulose A. Hemicellulose B in orchard grass was associated with relatively high rates of degradation compared to the other polymers.