Three species of Desmanthus adapted to the heavy clay soils of northern Australia were studied to determine their nutritive value and effects on in vitro fermentation with rumen fluid, compared with Rhodes grass (Chloris gayana) hay. Leaves and stems of D. leptophyllus cv. JCU 1, D. virgatus cv. JCU 2 and D. bicornutus cv. JCU 4 were collected in summer, winter and spring of 2014 and analysed for chemical composition. Apparent digestibility as in vitro organic matter digestibility (IVD-OM) and fermentation parameters including methane (CH4) production were measured during 72-h fermentations using rumen fluid from steer donors grazing tropical grasses and legumes. Desmanthus bicornutus was on average more digestible than both D. leptophyllus and D. virgatus at 24, 48 and 72 h of incubation. This species also demonstrated an anti-methanogenic potential, in particular when harvested in summer with a reduction in CH4 production of 26% compared with Rhodes grass hay after 72 h of incubation. At this time point, D. leptophyllus produced higher volatile fatty acids (VFA per g of organic matter fermented) compared with the other forages. This legume also reduced the CH4 production up to 36% compared with the Rhodes grass hay reference. However, D. leptophyllus showed lower IVD-OM. Overall, Desmanthus species produced lower in vitro CH4 and lower volatile fatty acids concentration compared with the reference grass hay. These effects may be due to presence of secondary compounds such as hydrolysable tannins, condensed tannins and/or their combination in Desmanthus species. The IVD-OM was influenced by the season after 72 h of incubation; the digestibility was higher in plants collected in spring. This study suggests that contrasting fermentative profiles in Desmanthus cultivars may offer the opportunity to reduce the greenhouse gas contribution of the beef industry. The next step in demonstration of these promising in vitro results is demonstration of Desmanthus in vivo as proof of concept confirming the productivity and CH4 reduction ability of these legumes in the pastoral systems of northern Australia.