In vitro assessment of the suitability of replacing the low-tannin legume Vigna unguiculata with the tanniniferous legumes Leucaena leucocephala, Flemingia macrophylla or Calliandra calothyrsus in a tropical grass diet

An in vitro experiment was carried out to evaluate the effects on ruminal fermentation and crude protein (CP) degradation of a partial replacement of Vigna unguiculata (CIAT 391), a high quality, herbaceous legume, by tanniniferous shrub legumes of contrasting quality in combination with the tropical grass Brachiaria humidicola. All mixtures consisted of B. humidicola and legume foliage [1:1, on dry matter (DM) basis]. The legume part of the mixtures consisted either of V. unguiculata alone or of mixtures (3:2) of this legume with Leucaena leucocephala, Flemingia macrophylla or Calliandra calothyrsus. Contents of total condensed tannins (g/kg DM) of L. leucocephala, F. macrophylla and C. calothyrsus were 51, 124 and 311, respectively. The experiment was conducted using a rumen simulation technique (RUSITEC) (n = 8). The partial replacement of V. unguiculata by any tanniniferous shrub decreased (P<0.05) ammonia concentration in the fermenter fluid and apparent fibre degradation. However, the decrease was more pronounced with C. calothyrsus and F. macrophylla than with L. leucocephala. Apparent degradation of organic matter and CP decreased (P<0.05) when V. unguiculata was partially replaced by C. calothyrsus or F. macrophylla, but not (P>0.05) when L. leucocephala was included. These results indicate that V. unguiculata may be partially replaced by L. leucocephala without seriously affecting the ruminal fermentation characteristics of the complete diet. In contrast, the same level of inclusion of F. macrophylla or C. calothyrsus, suppressed in vitro fermentation and nutrient degradation.