Various supplements and unconventional feeds (supplement-type: Acacia tannins, Yucca saponins, extruded linseed, lauric and myristic acid; medicinal-type plant parts: garlic bulbs, hop cones, grape seeds, maca hypocotyls; legumes: lupine seeds and lucerne meal) were screened with the in vitro Hohenheim gas test for their methane mitigating potential in an incubation period of 48 h. The control diet consisted of maize silage and concentrate (0.85:0.15). Replacing 150 g/kg dry matter of the control diet with garlic bulbs (Allium sativum) decreased methane by 50% and the methane-to-total gas ratio by 15 to 17% at 75 and 150 g garlic/kg DM. This was accompanied by a decline in acetate proportion of total short-chain fatty acids. Methane formation tended to be reduced by 15% with a high-tannin extract from Acacia mearnsii (50 g/kg diet). The methane-to-total gas ratio was not significantly affected by the other treatments. Accordingly, garlic turned out most promising for the mitigation of methane without negatively affecting rumen microbial fermentation.