Crop competition in winter wheat has a higher potential than farming practices to regulate weeds

Management of biotic interactions has been recognized as a potential substitute for costly agrochemical inputs. Competition is one of the most important biotic interactions known to regulate populations and govern species assemblages. However, although theoretical and empirical work has been produced on competition, in situ experimental evidence is much scantier, mainly because of the difficulty of manipulating competition in the field. Arable weeds offer an outstanding opportunity to meet this challenge, because of the relative ease of in situ experimental manipulation and because of the urgent need to find sustainable weed management strategies. Here, we assess the importance of crop competition and two main conventional farming practices (N fertilizer and weed control) on weed species richness, abundance, and biomass. We set up an experiment with a design with two factors, presence/absence of crops and presence/absence of N fertilizer and weed control, in working farm fields with winter cereals as the target crop. We found that the crop competition reduced weed biomass production by almost 65%, as a result of the crop's competitive advantage from its greater ability to take up N, while the effect on weed species richness was less important. Our results also show that the effect of crop competition on the weed assemblage was much stronger than the effect of N fertilizer and weed control. The decrease in weed abundance and biomass mainly resulted from a strong effect of the crop on the dominant species, while the abundance of intermediate species tended to be much less affected, a result consistent with studies in grasslands where the removal of the dominant species provides a competitive release for subordinate ones. Our results further give experimental support for crop competition as a way to reduce costly agricultural inputs for weed control. Conducting experiments with farmers in their field is a valuable approach to generate knowledge for the future delivery of sustainable management.