Development of lucerne root morphology traits in lucerne-grass mixture in relation to forage yield and root disease score

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It is a common practice to grow lucerne (Medicago sativa L.) in mixture with grass. The development of lucerne root morphology along with concomitant forage mixture growth, however, has been rarely studied in multi-year field experiments. Our objectives were: (i) to assess the effect of a grass component on lucerne root trait development, (ii) to evaluate changes in forage yield and aboveground traits in the lucerne monocultures and lucerne-grass mixtures and (iii) to investigate the relationships between root traits and lucerne yield, total forage yield, as well as forage aboveground traits, using multivariate methods. Treatments were a factorial combination of four lucerne varieties and three lucerne sowing ratios [100% (L100), 90% (L90) and 75% (L75)] with grass hybrid Festuca pratensis × Lolium multiflorum under a three-cut harvesting regime in a 4-year experiment. Lucerne yield, total forage yield and forage aboveground traits were evaluated at each cut. Each autumn, lucerne plants were evaluated for root morphology traits and scored for root disease. The negative impact of the presence of a grass component on lucerne root morphology development was observed only in combination with the higher lucerne sowing ratio (L90), with reduced intensity of root branching and increased depth of the first lateral root. In two post-seeding years out of three, total forage yield was 4–8 % higher for lucerne-grass mixture in comparison with monoculture. From all lucerne yield components in the mixture, stem density was the best predictor for lucerne yield. Lucerne root morphology explains about 10% of forage yield and aboveground traits variability where calculated root potential index, lucerne plant density and root dry matter mostly correlated with lucerne yield in the mixture. Root potential index was an effective indicator for monitoring root development and was even effective for comparing lucerne varieties in mixture. Understanding the relationships between root morphology development and lucerne stand performance in a mixture can help in optimization of sowing ratio and stand evaluation.