Mtui et al., 2009. Livest. Res. Rural Dev., 21 (10): 158

The nutritive values of three grasses (Cynodon plectostachyus, Panicum maximum, Pennisetum purpureum), three creeping legumes (CL) (Calopogonium mucunoides, Macroptilium atropurpureum, Neonotonia wightii) and three multipurpose trees (MPTs) (Leucaena leucocephala, Gliricidia sepium, Morus alba) native to Eastern Tanzania, collected during two seasons (i.e., wet and dry) were evaluated. Forages were analysed for chemical composition including polyphenolics and in vitro gas production characteristics. Tannin bioassay studies were done on CL and MPTs after in vitro incubation with or without addition of polyethylene glycol (PEG) as a tannin binding agent. The forages had variable crude protein contents, which were influenced by seasonal variations. The CP content ranged from 120 (Cynodon plectostachyus, wet season) to 69 g/kg DM (Pennisetum purpureum, dry season) in grasses; from 217 (Macroptilium atropurpureum, dry season) to 141 g/kg DM (Neonotonia wightii, wet season) in CL and from 317 (Leucaena leucocephala, dry season) to 212 g/kg DM (Morus alba, wet season) in MPTs. Forage type and seasonal interaction affected neutral detergent fibre (NDF) content but not acid detergent (ADF) and acid detergent lignin (ADL) levels. Grasses had high levels of NDF and ADF while MPTs had the lowest levels in each season. The ADL contents were highest in CL but lowest in grasses in both seasons. The contents of total extractable tannins (TET) in CL and MPTs were influenced by the type of season and species interaction and ranged from 67.9 (Neonotonia wightii, dry season) to 9.3 mg/g DM (Morus alba, wet season). The season variation had no effect (P>0.05) on the in vitro gas production volumes and fermentation characteristics. At 12 h of incubation, grasses produced least gas compared to CL and MPTs. From 24 h to 72 h of incubation, Calopogonium mucunoides produced highest volume of gas. At 96 h of incubation, Cynodon plectostachyus produced the highest gas volume whilst Neonotonia wightii produced least gas volume throughout incubation period. The potential gas production was highest (P<0.05) in grasses compared to other forages and the high rate of gas production was observed in MPTs. Addition of PEG improved (P<0.05) 24 h gas production, organic matter digestibility (OMD) and metabolizable energy (ME) in CL and MPTs and both in wet and dry seasons. In both seasons, high levels of 24 h gas production, OMD and ME were observed in Calopogonium mucunoides. The lowest responses were in Gliricidia sepium. Results indicate that seasonal variation affected protein content but not in vitro fermentation characteristics of the studies species. Creeping legumes (CL) and MPTs were observed to have high CP content (>100 g/kg DM) and moderate NDF levels (<550 g/kg DM) suggesting their potential to be used as protein supplements. Polyethylene glycol (PEG) treatment improved the digestibility of Neonotonia wightii and Leucaena leucocephala (dry season) previously depressed by tannin's anti-nutritive activity, thus, improves their nutritive potential.