This study was conducted to investigate the effects of season, species and polyethylene glycol addition on gas production (GP) and GP kinetic parameters by in vitro incubation (72 h) of five plant species from the subhumid subtropical savannah, South Africa. Plant species used were Acacia natalitia, Acacia nilotica, Dichrostachys cinerea, Scutia myrtina and Chromolaena odorata, leaves of which were harvested during the dry (June/July), early wet (November/December) and late wet (February/March) seasons. An automated in vitro gas production technique was used in two experiments carried out with nine replicates. The first experiment was to test the effect of season and species, while the second experiment tested the effect of tannins using polyethylene glycol 4000 (PEG). The PEG treatment was applied to samples in the early wet and late wet seasons.
There were wide variations among seasons and species in crude protein (CP), neutral detergent fibre (NDF), acid detergent fibre (ADF) and condensed tannin (CT). Season and species affected the maximum GP and GP kinetic parameters. During the three seasons, C. odorata had the highest CP (186–226 g kg−1 dry matter (DM)) and GP (87–104 mL g−1 DM) and S. myrtina had the lowest CP (105–129 g kg−1 DM), while A. nilotica, A. natalitia, D. cinerea and S. myrtina had similar and low GP (23–50 mL g−1 DM). The maximum GP, its degradation rate and GP from the soluble fraction were positively correlated with CP both without and with PEG. With PEG, GP from the soluble fraction was negatively correlated with NDF, ADL and CT; without PEG, it was negatively correlated with CT.
Both season and species affected the GP parameters. The addition of PEG emphasises that the inhibitory effect of tannins on rumen microbes was greater for all but C. odorata, confirming that these browse species can be used as feed supplements.