Do we need specific breeding for legume-based mixtures?

Legume-based mixtures have considerable interest for modern agriculture, because they can increase or stabilize crop production and simultaneously provide important agro-ecosystem functions. Accordingly, there is increasing demand for legume cultivars adapted to this utilization. This paper reviews the available findings and discuss methodological, eco-physiological and diversity-related issues that can help define cost-efficient breeding strategies for cool- and warm-season annual legumes intercropped with cereals, and perennial legumes intercropped with forage grasses. On average, selection in pure stand (PS) exhibited about 40% lower predicted yield gains than selection in target mixed stand (MS) conditions, in a survey of case studies. Specific breeding for intercropping tends to be particularly important for species undergoing severe competition because of modest competitive ability or the targeted growing conditions, for which direct selection in MS or indirect selection for traits associated with greater competitive ability prove valuable. Breeding for compatibility with a wide range of plant companions is encouraged by the larger size of general-compatibility effects relative to specific-compatibility ones. There is fairly limited evidence for the advantage of greater intraspecific diversity in legume breeding for intercropping. On the whole, however, the available findings are insufficient for designing efficient breeding strategies for the majority of legume species. We present a range of informative and cost-efficient methodological approaches that could be exploited for future studies. There is an urgent need to assess the efficiency of novel and relatively low-cost breeding strategies, among which evolutionary breeding and genomic selection emerge as the most promising