Species Choice Influences Weed Suppression, N Sharing and Crop Productivity in Oilseed Rape–Legume Intercrops

Increasing crop trait diversity in oilseed rape (OR, Brassica napus L.) cropping systems by introducing frost-sensitive legume species could improve weed suppression and crop productivity. Intercrops and sole crops were compared over two years in the field in Western France. Winter OR was intercropped simultaneously with either spring faba bean (Vicia faba L.) or common vetch (Vicia sativa L.) in a row replacement design without herbicides. Each species was sown at 50% of the recommended sole crop density in alternate rows. Due to the high values of faba bean aboveground traits (height, leaf area, and biomass) and the strong competitive ability for soil N of OR, both species appeared complementary in resource utilization, and thus less soil N and light were available for weeds. The OR–faba bean intercrop was able to reduce weed biomass by 41% compared to the OR–common vetch intercrop. Furthermore, growth and competitive ability of OR for soil N were increased when intercropped with faba bean. Both grain yield and number per plant were three times higher in OR–faba bean intercrops compared to OR sole crops. Under high weed infestation, the presence of faba bean with OR reduced weed aboveground biomass by 35% and weed N accumulation by 11% compared to the OR sole crop. No change was observed in the weed community composition. We observed that a level of aboveground biomass greater than 2 t ha−1 and a soil N uptake at approximately 80 kg ha−1 was needed to reduce biomass and N content of weeds.