EVALUATION OF MULTIPURPOSE TREE GERMPLASM - THE USE OF GAS-PRODUCTIONAND RUMEN DEGRADATION CHARACTERISTICS

The rumen degradation and gas production characteristics (methane, car bon dioxide) of leaves of 20 accessions of multipurpose trees (MPTs) f rom six genera: Acacia, Cajanus, Chamaecytisus (L. fil.) Link, Erythri na, Leucaena and Sesbania and some poisonous plants were investigated in vitro and in sacco in rumen fistulated cows fed on a diet of grass hay ad libitum supplemented with cotton seed cake. The degradation con stants (i.e. the soluble fraction (a), the slowly degradable fraction (b) and the rate of degradation (c) and the potential degradability (a + b) (PD)) were calculated. The gas production constants were estimat ed following the equation: Volume = b(g)(1 - e(-c)g(t)) where b(g) is gas production and c(g) is the rate of gas production. Between and wit hin genus comparisons of degradability and gas production constants we re done. The soluble fraction (a) was significantly higher for Sesbani a (59) (P < 0.05) than the other genera, which had values between 45.1 8 and 40.38 units. There were no significant differences in the slowly degradable fraction (b) between genera. Sesbania was degraded signifi cantly faster (P < 0.05) than either Acacia, Leucaena or Cajanus. Simi larly, the potential degradability was significantly higher (P < 0.05) for Sesbania (92.7) than for the other genera. Acacia was the least d egradable genus. Degradation characteristics were similar between spec ies within the genera Acacia, Erythrina and Leucaena. However, within the Leucaena genus, L. revoluta had the highest soluble fraction (50.0 2) and the cross L. leucocephala x L. diversifolia had the lowest (29. 24). L. leucocephala had the highest slowly degradable fraction (57.32 ) and L. revoluta had the lowest (42.37). L. leucocephala x L. pallida had the highest rate of degradability (0.0626) and L. pallida had the lowest (0.0221). L. leucocephala had the highest potential degradabil ity (92.23) and the cross L. pallida x L. diversifolia the lowest (84. 81). Between the genera, more gas was produced from Sesbania than from any other genus. The effect of genus was significant (P < 0.003). On the other hand, the rate of gas production (c(g)) was higher with the genus Chamaecytisus than with any other genera (P < 0.0001). Although the two methods used agree in the position of a forage of a high degra dability like Sesbania, they fail to agree on the relative positions o f the forages of lower degradabilities. For some genera, drying of the tree foliage reduced the volume of gas produced, but increased the ra te of gas production.