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 se lection of tanniniferous tree and shrub legumes (Calliandra calothyrsus, Le ucaena 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 t annins on digestion of dry matter (DM), neutral detergent fibre (NDF) and n itrogen (N), Total tannin content was poorly correlated with digestibility of dry matter and nitrogen. Apparent digestibility of dry matter and N were significantl y different between plants and ranked in the following ol der: 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 the se plants ranged from 36.9 to 34.3%. 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 fermentatio n. 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 fr om 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 shr ub legumes but not for lucerne. However, DM loss and fermentability of thes e plants appeared to be poorly correlated because PEG addition resulted in an increase in volatile fatty acid production ranging from 3.7 to 202% comp ared 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 offermentation end-products (ammonia and short and branched chain fatty acids) should also be undertaken to eval uate nutritive value. Also, addition of PEG to in vitro fermentations can b e used to determine the effect of tannins on N digestibility. Based on the in vitro methods of rumen fermentation used in this study, nutritive value of the tanniniferous forages is ranked as follows: L. leucocephala > L. div ersifolia and L. pallida > C. calothyrsus. (C) 1999 Elsevier Science B.V. A ll rights reserved.