Novel strategies to improve nutrient-poor tropical diets for ruminants should aim to increase feeding value and, simultaneously, reduce emissions of the greenhouse gas methane. Both aims were addressed in the present in vitro experiment when supplementing a low quality, tropical grass (Brachiaria humidicola; Centro Internacional de Agricultura Tropical accession number 6133) with foliage from various leguminous multi-purpose shrubs, all of them containing plant secondary metabolites in different concentrations. In detail, foliage of Acacia angustissima from the International Livestock Research Institute ( ILRI; accessions no. 459 and 15132), Sesbania sesban (ILRI 10865 and 15019), Samanea saman (ILRI 14884), and leafy crop residues of the grain legume Cajanus cajan (ILRI 16555) were supplemented at 200 g/kg dry matter. Additionally, a combination of C. cajan and S. sesban 10865 was tested. Effects on methanogenesis, ruminal nitrogen turnover and other fermentation traits were determined with the rumen simulation technique Rusitec. All supplements enhanced the fermentable nutrient supply, especially ruminally degradable crude protein, and improved the calculated microbial efficiency in nitrogen utilisation. Methanogenesis was limited by one accession of S. sesban (10865) and, less clearly, by one A. angustissima accession (459), while the other supplements remained ineffective. The first mentioned accessions proved to be far richer in several plant secondary metabolites, especially saponins and tannins. Provided in combination, C. cajan and S. sesban 10865 supported each other in their effects on nitrogen usage and total methane release. Accordingly, a combination strategy might provide, after being verified in vivo, a particularly promising option to improve low quality, tropical diets at limited environmental impact thus facilitating its adoption by stakeholders.