In vitro quality assessment of tannin-containing tropical shrub legumes: protein and fibre digestion

In vitro techniques were evaluated to determine the nutritive value of a selection of tanniniferous tree and shrub legumes (Calliandra calothyrsus, Leucaena leucocephala L. diversifolia and L. pallida) compared with lucerne (Medicago sativa). Polyethylene glycol (PEG) was also added to some in vitro fermentations (10 mg PEG/50 mg plant substrate) to assess the effects of tannins on digestion of dry matter (DM), neutral detergent fibre (NDF) and nitrogen (N). Total tannin content was poorly correlated with digestibility of dry matter and nitrogen. Apparent digestibility of dry matter and N were significantly different between plants and ranked in the following order; lucerne > L. leucocephala > L. diversifolia > L. pallida > C. calothyrsus. Ammonia was not produced (net accumulation) during 72 h fermentation of C. calothyrsus, L. diversifolia and L. pallida although apparent nitrogen digestion in these plants ranged from 36.9 to 44.3%. Acid-pepsin digestion resulted in a further 17±22% of dry matter digestion in the shrub legumes compared with 8% in lucerne following 72 h fermentation. The amount of acid-pepsin digestible N available was lowest for lucerne (8.4%) and highest for L. pallida (38.9%) with the other legumes ranging from 26.5 to 36.8%. The PEG addition caused a significant increase in rate and extent of DM and NDF digestibility and ammonia production for all the tannin containing shrub legumes but not for lucerne. However, DM loss and fermentability of these plants appeared to be poorly correlated because PEG addition resulted in an increase in volatile fatty acid production ranging from 3.7 to 202% compared with an increase in apparent DM digestibility of 9.1±30%. It is concluded that in vitro evaluation of apparent DM and N digestibility of tannin containing plants provides a poor indication of true digestion (fermentability) and thus measurements of Animal Feed Science and Technology