Legume breeding for intercropping is indispensable for species expected to face high competition

Legume-based intercrops exploit plant functional diversity to raise and/or stabilize crop yields and quality while enhancing ecosystem services. The need for specific legume breeding for intercropping depends crucially on the genetic correlation (r_G ) for genotype yields across pure stand (PS) and target mixed stand (MS) conditions. This review aimed to assess the size of r_G as a function of competitive stress exerted on the legume and discuss its implications along with complementary information supporting cost-efficient breeding strategies. Following a literature review, we retained 13 data sets of perennial legumes, warm-season or cool-season grain legumes including at least 14 genotypes (lines, cultivars, etc.) evaluated in MS and PS. Their pooled information revealed a significant (P < 0.001) linear decrease of r_G with the increase of competitive stress that explained 67% of r_G variation. The predicted efficiency of indirect selection in PS relative to direct selection in MS averaged 0.61, with wide variation (0.42-1.07) mainly determined by r_G variation (0.40-1.00). These results and other indications suggested that specific breeding for intercropping is indispensable for legume species expectedly facing high competition (as determined by associated species/cultivars and crop management), which seemingly are the majority. In this situation, large-scale legume selection could rely on genotype yield in MS with a few (possibly mixed) highly productive non-legume testers, since greater legume competitive ability could also improve the total yield of intercrops with likely modest negative impact on indirect genetic effects leading to higher yield of non-legume companions, whereas specific mixing ability effects are expectedly small.