Cover crops promote crop productivity but do not enhance weed management in tillage-based cropping systems

Cover crops (CC) have been proposed as a promising ecological tool to manage weeds and increase crop productivity. We hypothesized that the repeated use of CC could increase crop yield directly through nitrogen release and/or indirectly through a modification of weed communities. Data were collected on CC biomass, weed biomass, weed community composition, and crop yield during one complete rotation cycle (CC-sunflower-durum wheat-CC-maize-durum wheat) from 2011 to 2015, 18 years after the beginning of a long-term, single-site, split-split plot experiment focusing on tillage systems (conventional (CT) vs. reduced (RT)), nitrogen rates and CC species (Brassica juncea (L.) Czern. (Bj), Vicia villosa Roth (Vv), Trifolium squarrosum L. (Ts) and a winter baresoil control (C)). Univariate response variables were analyzed with generalized mixed effect models and community data were analyzed with multivariate linear models. During the fallow period, Bj suppressed weed biomass (with respect to C) by 79, 75, 34, and 28 % in CT:2012, RT:2012, CT:2014 and RT: 2014, respectively, whereas Vv only suppressed weed biomass by 69 and 37 % in CT and RT in 2012, respectively. Greater weed suppression for Bj than Vv or Ts at lower levels of CC productivity (200 g dry biomass m(-2)) was attributed to the importance of CC traits such as nitrophily, allelopathy and/or quick soil coverage. The weed suppressive effect of CC during the fallow period was greater in CT (beta(slope) = -0.28) than in RT (beta(slope) = -0.16), possibly due to contrasted weed flora and/or CC growth dynamics. Tillage and herbicides overrode the potential effect of CC on weed communities in the subsequent crops. The integration of a highly productive legume CC, such as Vv, allowed to increase maize productivity (with respect to C) by 65 % in absence of N fertilisation and by 23 % at the lowest N fertilisation level. CC effects on sunflower and durum wheat yield were limited due to dry weather conditions and quick nitrogen release in time, respectively. These results highlight the importance of legume CC for sustaining crop productivity while reducing nitrogen fertilisation. Further studies need to identify less intensive weed management practices that can complement potential CC effects rather than override them.