Leys of legume-grass mixtures, contribute significantly to sustainable plant production. The project EffMix aims at developing resource-efficient, robust seed mixtures of grasses, legumes and herbs adapted to future extreme weather conditions and at the application of plant species providing secondary metabolites that improve animal health and sustainability of animal production by reducing CH4 and nitrogen emissions. EffMix will further contribute to reducing feed-food competition by improving forage quality. Development will take place in two steps. Step 1 involves a detailed assessment of the potential of precisely defined seed mixtures in small-plot field trials, and Step 2 involves trials on farms with the best mixtures from Step 1, grown in large strips under practical conditions, using a co-creative approach. For maximum impact, the project is committed to making new seed mixtures available to agricultural practice as quickly and effectively as possible. The renowned ‘Swiss System of Standard Mixtures’ will be used to bring the mixtures into agricultural practice.