The nutritive value of the leguminous browse Calliandra calothyrsus and the role of condensed tannins in ruminant feeds
Experiments were conducted to assess the potential of Calliandra calothyrsus as a feed for ruminants. The role of the condensed tannins in the nutritional quality of Calliandra and the ways of reducing the adverse effects were also investigated. Separate experiments were also conducted to evaluate the effects of condensed tannins in vivo in sheep and in vitro on rumen microbial activities. Lastly, the possibility of using tannins as silage additives was investigated. Calliandra samples were harvested from Zimbabwe and Kenya. The chemical composition of the plant showed that the leaves had a high crude protein (CP) content and had a balanced amino acid profile. These qualities appear to make Calliandra especially suited for the dry season supplement or feed for ruminants. However, In sacco dry matter (DM) and nitrogen degradability of the leaves and stems of samples harvested from Zimbabwe were very low. In contrast, leaves harvested from Kenya had a high rumen degradability. Differences were also observed between old and young plants from Kenya. The Calliandra samples also have a high flavanol as well as proanthocyanidin (PA) content, and the concentration of PAs in Calliandra was found to be inversely related with DM, nitrogen and amino acids digestibility in the gastro-intestinal tract. Extraction of the tannins in Calliandra and treatment with the binding agent polyethylene glycol (PEG) improved DM losses and gas production in vitro in a CBC system. Aqueous methanol extracts of Calliandra leaves were demonstrated to have differential effects on the growth of selected anaerobic gut bacteria. The cellulase and xylanase activities of the rumen fungus Neocallimastix frontalis strain RE1 was also reduced by aqueous extracts of Calliandra. Less than 50% of the bindings between Calliandra tannins and BSA was found to be pH dependent. However treatment of precipitates formed between tannins in Calliandra and BSA with PEG released over 70% of the bound protein into solution.