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Six Acacia forage species–A. brevispica, A. elatior, A. mellifera, A. nilotica, A. senegal and A. seyal—were analysed for their chemical composition, including phenolics and rumen fermentation characteristics. In vitro gas production technique was used to study the rumen fermentation characteristics and the effect of tannins present in the browse forages on in vitro fermentation by including polyethylene glycol (PEG-6000) in the incubation. The forages had high crude protein content (145.7–270.1 g kg−1 DM) and low to moderate content of neutral detergent fibre (220.2–442.8 g kg−1 DM). The forages had variable content of total extractable phenolics ranging from 18.4 to 384.2 g kg−1 DM and total extractable tannins ranging from 7.1 to 364.8 g kg−1 DM (tannic acid equivalent). Fractionation of the condensed tannin flavonoids showed that the delphinidin/cyanidin ratio ranged from not detected:100 to 71:29. The gas production potential ranged from 28.4 to 40.8 mL gas 0.2 g−1 DM. The rate of gas production was highest in A. elatior and lowest in A. nilotica. Addition of PEG-6000 increased gas production significantly (P < 0.05) in all species except A. mellifera. PEG-6000 addition did not have any significant effect on in vitro true dry matter and organic matter degradability but decreased the partitioning factor in all the species. Gas production and degradability parameters correlated positively with the CP content and negatively with the fibre and phenolics contents. Based on chemical composition, gas production and degradability, the forages have high potential nutritive value, especially as supplements to low-quality feeds in the tropics and particularly during the dry season. However, the presence of tannins in terms of high levels and biological antinutritive activity may limit the nutritive potential of some of the forages, such as A. seyal and A. nilotica. Copyright © 2007 Society of Chemical Industry