The concentration and molecular weights (MW) of condensed tannins (CT) of three locally available tropical plant species leaves which have potential to be used as ruminant feed, and their effects on in vitro gas, including methane, production were investigated. Leaves of three plant species, namely, leucaena (Leucaena leucocephala), cassava (Manihot esculenta, Cranzt), and Siamese neem (Azadirachta indica A.Juss. var. Siamensis Valeton) were used in the present study. CT contents ranged from 1.2% in the leucaena to 5.0% in Siamese neem. The weight-average molecular weights (Mw) of the purified CTs, determined using gel-permeation chromatography, were 3222, 3409 and 3612 Da for leucaena, cassava and Siamese neem respectively. The above values were within the range reported for CTs of various tropical plant species. We know of no published data on MW of CTs from leaves of cassava and Siamese neem and, thus, this research, for the first time, reported the MWs of the above two plant materials. Subsequently, two CT extracts with differing MWs, such as CTs extracted from leaves of Siamese neem and leaves of leucaena, were selected as the representative of high and low MWs of CT respectively, and used for investigation of the effect of the MW of CT on in vitro gas production and fermentation parameters. Supplementation of CTs of a higher MW extracted from leaves of Siamese neem (at 2–6 mg/100 mg DM) significantly inhibited in vitro total gas and methane production, while supplementation of CTs with a lower MW extracted from leaves of leucaena had no effect, except for total gas production at the highest level (6 mg/100 mg DM) of supplementation. Similarly, CT from Siamese neem leaves had a stronger inhibitory effect (P < 0.001) on in vitro volatile fatty acid, acetic acid and butyric acid production. The above results indicated that concentrations and the MW of CT varied among the plant species; in addition, the efficacy of CTs to inhibit ruminal CH4 emission are influenced by their MW.
