Evaluation of Bengal gram (Cicer arietinum) husk as a source of tannin and its interference in rumen and post-rumen nutrient digestion in sheep

Bengal gram (Cicer arietinum) husk (BGH) was evaluated for the content of tannin and its interference in digestion. The BGH contained (% DM) 76.0 NDF, 65.2 ADF, 6.1 ADL and 8.4 tannin. The gas production (ml/200 mg DM/24 h) from BGH incubated with rumen inoculum was 45.0, which increased to 61.3 when polyethylene glycol (PEG 6000) was added. The magnitude of the increase in gas production was not explained by the quantity of tannin-bound protein (acid detergent insoluble nitrogen×6.25) or the total protein. This suggested that carbohydrate could be the main substrate bound by tannin in BGH. The availability of tannin-bound substrate in the post-rumen digestive tract was assessed through a metabolic trial. Twelve male lambs aged between 6 and 8 months were divided into three groups of four animals in each group. BGH was incorporated in the diet at 0, 6.8 and 14% by replacing deoiled rice bran (DORB). Incorporation of BGH in the diet increased OM digestibility (P<0.002) from 63.3% (Group I) to 68.1% (Group II) and 72.4% (Group III). The observed differences among the diets was quantitatively explainable by the difference in ME content (8.5 MJ) of BGH and DORB after PEG addition. The faecal excretion (g/day) of organic neutral detergent solubles (ONDS) (54.5, 55.3 and 46.7), and neutral detergent soluble nitrogen (NDSN) (4.4, 4.4 and 3.9) for the three groups were similar. Therefore, it is speculated that the tannin-bound substrate in BGH is carbohydrate, protected from rumen fermentation but digested in the small intestine.