In the last decade, intercropping of frost-sensitive service plants with winter oilseed rape has become common in Western Switzerland and France. Currently, many farmers grow increasingly complex service plant mixtures. However, the composition of these mixtures is often based on empirical experiments, and little is known about the role of each species in these complex intercropping systems. In this study, we addressed this question by performing two experiments in field conditions. The first consisted of removing one service plant species from a four-legume mixture intercropped with oilseed rape. This four-species mixture consisted of berseem clover, grass pea, lentil, and common vetch. In the second experiment, a fifth species was added to the mixture. The added species was either a legume such as faba bean or a non-legume such as niger, buckwheat, or mustard. Our results show that the specific composition, especially the addition of a non-legume, could strongly affect the fall biomass production of oilseed rape and service plants and potential ecosystem service provision. The addition of buckwheat or mustard to a legume service plant mixture increased early ground cover and service plant dry weight which are known to contribute to weed control. However, it also decreased oilseed rape dry weight in late fall by 29 to 66% and did not favour total nitrogen accumulation by the plant cover. In contrast, the addition or removal of a legume service plant had a limited impact on plant growth and ground cover. Finally, our results highlight that some dominant species could have a big impact on the intercropping growth even when included in small proportion in a complex mixture. The impact of specific composition on cover growth also varied depending on the growing conditions. It remained a key factor in explaining the fall biomass acquisition of service plants and oilseed rape.