Growing mixtures of species instead of sole crops is expected to increase the ecosystem services provided
by cover crops. This study aimed at understanding the interactions between species and investigating
how they affect the performance of the mixture. Four species were combined in six bispecific mixtures in
a field experiment. The performance of each species when grown in a mixture was compared to its
performance as a sole crop at different sowing densities, to characterise the influence of intra- and
interspecific competition for each species. Intra- and interspecific competition coefficients were
quantified using a response surface design and the hyperbolic yield-density equation. Interactions
between the four species ranged from facilitation to competition. Most of the mixtures exhibited
transgressive overyielding. Without nitrogen (N) fertilisation, high complementarity between species
allowed to achieve the highest biomass. With N fertilisation, high dominance of one mixture component
should be avoided to achieve good performance. A revised approach in the use of the land equivalent ratio
for the evaluation of cover crop mixtures is also proposed in this study. It allows to better identify
transgressive overyielding in mixtures and to better characterise the effect of one species on the other
within the mixture.