In vitro microbial fermentation and protein utilisation of tropical forage legumes grown during the dry season

Abstract

The nutritive value of four tropical forage legumes grown during the dry season was assessed by gas production, microbial adhesion and in vitro N degradation and intestinal digestion. Species tested were: Leucaena leucocephala ( Ll), Cajanus cajan ( Cc), Neonotonia wightii ( Nw) and Stylosanthes macrocephala ( Sm). Chemical analysis showed higher lignification of Sm (187 g acid detergent lignin, ADL/kg dry matter, DM) compared with Nw (92 g ADL/kg DM), and a higher concentration of extractable and fibre-bound condensed tannins (CT) in browse ( Ll and Cc) than in herbaceous ( Nw and Sm) legumes (82, 110, 6 and 6 g total CT/kg DM, respectively). A higher total gas volume occurred for Nw (188, 163, 146 and 130 ml gas/g DM for Nw, Ll, Cc and Sm, respectively; P<0.001). Rates of fermentation at 24 h were 0.069, 0.027, 0.046 and 0.051 h −1 for Nw, Ll, Cc and Sm ( P<0.001). The DM degradation of the legume species ranked them similar to gas production. There were no differences among legumes in microbial adhesion patterns, nor major differences in enzymatic activity against structural polysaccharides until 32 h, but then Nw showed higher total cellulase ( P<0.001), xylanase ( P<0.001), β-glucosidase ( P<0.01) and β–xylosidase ( P<0.05) activities than Ll and Cc. In vitro nitrogen disappearance after 32 h incubation was higher for Nw, intermediate for Sm and lowest for Ll and Cc ( P<0.001), probably because of the high CT concentration in the latter. Intestinal digestion of Ll and Cc was also low, suggesting that CT may still be impeding digestion to a certain extent at this site.