Weeds are a major component of agricultural diversity affecting crop yield and ecosystem services. Compared to modern wheat (Triticum aestivum) varieties, ancient wheat cultivars (released before 1960) are taller, and this trait can be used to control weeds in organic farming systems. However, there is still a lack of quantitative assessments of the relative contribution of wheat breeding-history (ancient vs modern varieties) and synthetic inputs in explaining weed density and community structure. In this study, a field experiment was undertaken where five modern and five ancient wheat varieties were either treated as in a conventional system with synthetic inputs (nitrogen, herbicide and fungicide) or as in an organic system without the use of synthetic inputs. Crop light interception and weed density were recorded for 12 weeks until crop maturity. On average, ancient varieties reduced weed density by 17% compared with modern varieties, while the application of chemical inputs was responsible for an average reduction of 37%. The stronger competitive effect of ancient varieties was associated with increased sunlight interception. Species richness was higher in the absence of inputs for some weeks, but not by the end of the experiment. The field-based results illustrated that ancient varieties can be used to help control weed density in organic systems that do not rely on synthetic inputs for weed control Despite this effect of crop interference on weed density, a reduction in weed diversity was not observed. These findings could be of particular interest to promote agrobiodiversity in agricultural systems without synthetic inputs.