Off-target: Herbicides in cereal fields favour competitive weeds over non-target species

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. However, less attention has been given to the specific effects of herbicides, insecticides, and fungicides on weed communities, including the species affected and the mechanisms driving these changes, based on farmer-recorded application data. 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 (QA) – on weed communities in 96 non-organic cereal fields over 4 years. We did not detect any effect of insecticide metrics on weed communities. Interestingly, our results indicate that TFI of fungicide decreased weed abundance, whereas QA did not. In contrast, for herbicides, QA had a stronger negative impact on weed communities than TFI. This suggests that TFI of fungicide, which may reflect low-dose but frequent applications, could exert indirect and long-term effects on weed suppression. In comparison, QA of herbicide more directly reflects the toxic load delivered to weeds and is therefore a better predictor of their suppression. 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 with competitive species. These findings point out the potential ineffectiveness of herbicide application on problematic weeds and emphasise the need to reconsider the use of herbicides to maintain weed diversity in agricultural landscapes. • We studied the impact of two indices of pesticides application on weed communities. • Herbicide quantity suppresses weeds more strongly than application intensity. • Fungicide intensity reduces weed abundance, but not fungicide quantity. • Herbicides decrease non-target weeds while replacing them by competitive weeds.