Background and aimsRoot phenotyping and breeding for desirable root traits have been limited in small seed forage species. The objectives of this study were to analyse the degree of within-species variation regarding initial root allocation in two forage legumes and to assess the covariation between root allocation patterns and root and shoot morphogenetic traits involved in carbon source-sink relationships.MethodsExperiments were carried out at a phenotyping platform to grow alfalfa and red clover plants of five contrasting cultivars under nutrient conditions non-limiting for growth. An automatic image analysis pipeline was used to characterise dynamic root-shoot allocation over 50 days during plant establishment. Measurements were also performed to characterise plant traits related to potential shoot and root development.ResultsIn both species, genetic variability was found regarding the allocation of biomass to roots and root morphological traits controlling root elongation and branching. The degree of variability was high for most root traits, within a range similar to that found for the shoot traits usually targeted by breeders. A clear dependency of initial biomass allocation to roots on the shoot traits controlling plant leaf area, and favouring carbon acquisition, was identified. Analysis of covariation between root traits revealed they were largely independent, suggesting considerable potential for their recombination to achieve improved root phenotypes capturing more soil resources at a given biomass.ConclusionThis study demonstrates the feasibility of direct breeding for root traits in legumes and could help to identify trait combinations that promote their rapid establishment and competitive ability.