The ongoing simplification of agricultural production systems has resulted in several negative consequences, ranging from losses in soil organic carbon and biodiversity to a high dependency on external inputs to maintain high yields. We identify how grassland leys in crop rotations may help to mitigate these effects, by conserving soil organic carbon and enhancing nutrient efficiency. In particular, grasslands containing legumes enhance these benefits by providing nitrogen, and displacement of mineral N fertilizer. In crop rotations, these grasslands may transfer some of the acquired nitrogen to arable follow-on crops, thereby reducing the necessity for external inputs, while at the same time providing additional benefits, such as improvement of soil quality and reduction of weed pressure. However, there are still considerable knowledge gaps about how to optimize the community composition of grassland leys to best enhance the supply of these ecosystem services. Although the benefits of multi-species grasslands for the grassland crop have been shown repeatedly and across a large gradient of environments, further research is required to determine the benefits for follow-on crops, particularly across different environmental conditions. Here, we emphasize the importance of multi-site research, such as in the research network LegacyNet. Finally, we present management techniques that are optimized for both ecosystem services and agronomic performance in mechanically cut and grazed systems. For the latter, we consider how the inclusion of bioactive plant species can enhance animal health and lower methane emissions in grazing ruminants.