Intensification of crop rotation affecting weed communities and the use of herbicides in the rolling Pampa.

Weeds are one of the main problems in the cropping systems of the Rolling Pampa (Argentina), where glyphosate resistant varieties of soybean sown with no-tillage system became the most important crop in the rotation. The challenge to solve this problem is to apply alternative approaches that both reduce weediness and the use of chemicals. Thus, the objectives of this work were i) to study the impact of crop rotation intensification on the species composition and richness of weed communities and to identify the relationship with some environmental (soil mineral organic matter) and agronomic variables (intensification, cereal crops in the rotation, biomass production and herbicide applications) and ii) to quantify the use and environmental risk of herbicides related to the intensification of crop rotations. From 2012 to 2019, four rotations were performed on three farms combining crops (soybean, maize, wheat and field pea), cover crops (oats and hairy vetch) and mixed pastures. During spring 2018 and autumn 2019 field and seedbank experiments were performed. PCA using presence-absence of species as response variable and intensification index of rotation (IIR), proportion of cereal crops in the rotation (C), biomass production (B), mineral organic matter (OM) and number of herbicide applications (HA) during the six years of the rotation as explanatory variables. The use and environmental risk of herbicides was also assessed. Surveys and seedbank analysis showed that intensification of crop rotations resulted in differences in the floristic composition of weed communities mainly related to IIR and C. Although the use of herbicides decreased as intensification grew, species richness and abundance did not change. Despite of all the variations considered in this study such as different approaches (emerged weeds and seedbank), locations, crops, pastures and sowing dates, intensification consistently filtered species conforming different weed assemblies and reducing the use of herbicides. Thus, promoting sustainable intensification by increasing cover crops, winter crops, cereal crops and pastures in the rotations would be a useful tool to manage weeds since the use of herbicides can be replaced by increasing the IIR without variations in weed abundance.