In vitro microbial fermentation of tropical grasses at an advanced maturity stage

Four tropical grasses (Cynodon dactylon, Cd, C. plectostachyus, Cp, Brachiaria humidicola, Bh and Pennisetum purpureum, Pp) harvested in Central Brazil during the dry season at 100-day age were compared in terms of chemical composition and in vitro gas production pattern in order to select the most promising species for feeding ruminants during the dry season. Because of its lower proportion of neutral detergent fibre (NDF, 709 g/kg dry matter, DM) and lignin (44 g/kg), and its higher nitrogen (N) content (15.3 g/kg), Pp appears to have the higher nutritive value in such conditions. In contrast, Cp showed a high NDF (843 g/kg) and lignin (115 g/kg) contents, and low N content (8.7 g/kg). Microbial fermentation, estimated by the gas production pattern agreed well with chemical composition of the studied species, and ranked them as follows: Pp > Cd ≥ Bh > Cp. The metabolic energy content of grasses was estimated from the volume of gas produced at 24 h and their crude protein content, and were (in MJ/kg DM): Cd, 5.76; Cp, 5.15; Bh, 5.91; Pp, 9.12. In another experiment, Cp, Bh and Pp were chosen to study the bacterial adhesion (estimated by 15N as microbial marker) and polysaccharidase and ß-glucosidase enzymatic activities after in vitro incubation of grasses for 4, 8, 12, 24 and 32 h. The pattern of microbial adhesion and total enzymatic activities agreed well with the chemical composition and gas production results. Although P. purpureum promoted a higher adhesion and total cellulolytic activity, bacterial enzymatic activity (per unit of adhered bacteria) showed that populations associated with the grasses do not differ qualitatively in their cellulolytic capability.