Assessment of the feeding value of five shrub legumes of local interest

Abstract

The intake and digestibility o f tree and shrub foliage by ruminants are limited due to the presence o f undesirable secondary plant metabolites. In particular, tannins have been implicated in the failure o f ruminants to consume some forage legumes. Therefore, the aim o f the present experiment was to determine the extractable condensed tannin (CT) contents o f five leguminous shrubs and to examine the relationship o f CT with in vivo dry matter digestibility (IVDMD). The presence o f CT, which ranged from 6.92 (A. auriculiformis) to 0.78% (G. sepium), affected the nutritive value o f these plants to different degrees. The shrubs used in this study can be classified according to their CT contents (determined by butanol-HCl method) as follows. CT free, traces (6%) CT levels found in Erythrina indica, Gliricidia sepium, Leucaena leucocephala, Calliandra calothyrsus and Acacia auriculiformis respectively In contrast, Erythrina indica was free o f condensed tannins. In vivo DMD rangedfrom 49% in C. calothyrsus to 59.6% in E. indica. Inverse relationships between concentrations o f condensed tannins and IVDMD (r=- 0.925) and crude protein (r=0.7359) indicated negative effects o f CT. However, CT correlated positively with NDF (r=0.49) andAD F (r=0.75) fractions. Therefore, it could be assumed that CT bound with fibre fractions reduces digestibility and CP. CT contents o f tender leaves and stems (0- 30 cm from tip) and mature leaves and stems (30-60 cm from tip) in A. auriculiformis and C. calothyrsus were 7.39%, 6.05% respectively a n d 5.46%, 4.38% respectively, which may be due to the chemical defense mechanism fo u n d in the younger plant parts o f browse leguminous plants. Comparedwith other species, E. indica with a low ADF, lowest ADL and C T and having the highest IVDMD with a high CP content was quantitatively the best. However, feeding strategies should be developed to improve the synergistic effects offeeding C. calothyrsus and Acacia auriculiformis. It is advisable to mix tender and mature leaves as much as possible when used under cut and fe d systems fo r ruminants.