Nutritive value assessment of the tropical shrub legume Acacia angustissima: Anti-nutritional compounds and in vitro digestibility

Abstract

The leguminous tropical multipurpose tree Acacia angustissima is a potential source of protein for ruminants fed roughage diets. Proximate analysis and in vitro techniques were used to evaluate the nutritive value of 15 accessions of A. angustissima in comparison with the tanniniferous legumes ( Calliandra calothyrsus and Leucaena leucocephala) and lucerne ( Medicago sativa). Freeze- and oven-dried (50 °C) leaf samples were used and polyethylene glycol (PEG) was added to some in vitro fermentations (5 mg PEG/50 mg plant substrate) to assess the effects of tannins on digestion of dry matter (DM) and nitrogen (N). The acacia accessions were also screened for secondary compounds that could effect the nutritive value of the plant. Average NDF and ADF content did not differ markedly between A. angustissima accessions (417 and 189 g/kg), L. leucocephala (447 and 178 g/kg), C. calothyrsus (416 and 205 g/kg) and lucerne (399 and 168 g/kg). Average N content of A. angustissima (38 g/kg) and C. calothyrsus (36 g/kg) were similar but tended to be lower than L. leucocephala (43 g/kg) and lucerne (50 g/kg). Oven drying at 50 °C compared with freeze drying did not significantly effect fermentation characteristics of NH 3, branched chain (BCVFA) and short chain volatile fatty acid (SCVFA) production in the plants. There was a two-fold difference in SCVFA production amongst acacia accessions without PEG while variation in NH 3 and BCVFA production was small due to negligible production or a net consumption of these metabolites during fermentation. Inclusion of PEG in fermentations of A. angustissima resulted in a marked and significant increase in DM digestibility and production of SCVFA (2–4.4-fold increase), NH 3 (>4-fold increase) and BCVFA (>10-fold increase). All acacia accessions had a high free condensed tannin content which ranged from 98 to 180 g/kg and only minor amounts of tannin were in a bound form (<2 g/kg). Dimethyltryptamine and N-methyltyramine alkaloids were present in the acacia accessions in very small quantities (1.1–10.2 ppm). Several non-protein amino acids were detected in all accessions of A. angustissima in the following order of concentration: γ-acetyldiaminobutyric acid > oxalylalbizziin > α-acetyldiaminobutyric acid > albizziin > oxalyldiaminobutyric acid and oxalyldiaminopropionic acid. The main non-protein amino acid was γ-acetyldiaminobutyric acid which ranged in concentration from 181 to 293 g/kg DM. In conclusion, in vitro digestibility was low for all acacia accession due to the presence of tannins. These accessions all contained significant amounts of non-protein amino acids that need to be evaluated further for potential toxicity.