Off target: herbicides applied in cereal fields exclude non-competitive species, while replacing them by competitive weeds.

Weed diversity plays an important role in maintaining resilient agroecosystems, yet agricultural practices, such as pesticide applications, significantly shape weed communities. Previous studies have primarily focused on comparing organic and conventional farming, with a particular emphasis on land-use intensification, with less attention on the impact of pesticides. Our study examined the impact of herbicide, fungicide, and insecticide use - both in terms of Treatment Frequency Index (TFI), i.e. intensity of application, and quantity - on weed communities in 96 non-organic cereal fields over 4 years. Interestingly, our results indicate that fungicide application intensity decreased weed abundance by 11.6% and species richness by 14.6% for each one-standard-deviation (1-SD) increase in TFI, whereas the total quantity applied (QA) did not. In contrast, for herbicides, QA had a stronger negative impact on weed communities than TFI, with a 14.3% decrease in weed abundance and a 12.4% decrease in species richness for each 1-SD increase in QA. This suggests that TFI of fungicide, which may reflect low-dose but frequent applications, could exert indirect and long-term effects on weed suppression, potentially by altering symbiotic arbuscular mycorrhizal fungi in the soil. In comparison, QA of herbicide more directly reflects the toxic load delivered to weeds and is therefore a better predictor of their suppression. Based on a combination of multivariate and univariate analyses and the study of underlying mechanisms in community changes, our results reveal that herbicides are the main factor shaping weed communities by decreasing the abundance of non-target/non-competitive species while replacing them by problematic, competitive to crops, weed species. These findings point out the potential ineffectiveness of herbicides application on problematic weeds and emphasize the need to reconsider the use of herbicides to maintain weed diversity in agricultural landscapes.