Rethinking variety testing to recognise mixing ability: bridging breeding, policy, and practice in diversified agriculture.

Agricultural diversification through species mixtures offers proven ecological and agronomic benefits, from improved nutrient cycling and yield stability to enhanced resilience against pests, diseases, and climatic stresses. Yet, the widespread uptake of these systems remains constrained by a fundamental gap: the absence of information on variety suitability for intercropping. Under current legislation, variety testing frameworks are designed for pure stand cultivation, thereby discouraging breeders from selecting within and for species mixtures, let alone registering mixture-adapted varieties. This Perspective article examines how current principles, policies, and procedures underpinning variety testing constrain the development and uptake of crop varieties suited for species mixtures. We synthesise evidence suggesting that integrating mixing ability into variety testing and registration could better align breeding incentives with farmer needs. Building on insights from a EU CAP Network Focus Group and long-standing forage and cereal-legume research, we outline conceptual and operational pathways to enable adaptation. A framework for action identifies key leverage points across five levels: breeding, regulation, farm networks, data infrastructures, and policy. Breeding must shift from isolated to interaction-based variety evaluation and selection, supported by traits and genomic predictors relevant for intercropping. When appropriate, official testing should add mixture sub-trials and recognise mixing ability as a sustainability trait, while linking practitioner networks and open data systems. Policy incentives and eco-labelling can help make diversity-oriented breeding economically viable. Rethinking breeding and variety testing in this way is pivotal for aligning breeding innovation, agricultural policy, and on-farm practice with the goals of resilient, diversified agroecosystems.