Characterisation and impacts of weed flora on maize crop in different organic cropping systems

Concerns about climate change and environmental degradation are raising and gaining concerns in Europe and the rest of the world. In this context, the European Union has introduced the “Green Deal’s Farm to Fork strategy”, with the aim to produce high quality food while reducing environmental impacts. One of its main objectives is to increase the proportion of farmlands managed under organic farming, by reaching 25% by 2030. However, management of weeds should be considered as one of the most challenging issues in organic agriculture. Indeed, the chemical control lever is prohibited and the control of weed flora must solved by using a multitude of action levers. Indeed, cropping system (CS) management plays an essential role in weed control, influencing the weed community through planted seed density, the use of cover crops or intercropping, the temporal succession of crops (crop rotation), reduced used of fertilier, mechanical weeding, etc (Colbach & Cordeau 2018; Adeux et al. 2019).
Since autumn 2018, a long-term organic farming experiment was launched where three different cropping systems are being tested within the experimental farm of the Walloon Agricultural Research Centre (CRA-W), Belgium. Within the first cropping system (CS1), soil is managed using conventional tillage (ploughing) and crop is fertilized using solely commercial exogenous organic fertilizer. The second cropping system (CS2) is managed with the use of conventional tillage but no external organic fertilizer is applied. Finally, the last cropping system (CS3) is managed without ploughing and without any fertilization involving external commercial organic fertilizer. For the management of their fertility, CS2 and CS3 rely on the introduction of more leguminous plants to increase atmospheric nitrogen fixation and on the establishment of associated crops in order to limit the development of weeds. In this project, the tested hypotheses are that (i) different CS would induce different weed communities and (ii) these communities would not have the same impacts on crops yields. To test these hypotheses different measurements have been performed. Weed community were collected in 2021 within maize crop, the crop cultivated commonly across the different CS. Weed density per species were determined at different moment of the cropping season. Furthermore, weeds and maize aboveground biomass were measured at the beginning of flowering stage of maize. Weeds diversity indices (Shannon diversity index, Species richness, Simpson’s Diversity Index…) were calculated. After only two years of management, a specific weed community seems to have developed within the CS3 (without tillage), as compared to the two other CS; it exhibited a greater Shannon diversity index. This weed community is characterized by more graminaceous species and a greater graminaceous biomass. At the same time, we observed the lower maize biomass within the CS3 compared to the other cropping systems. Maize yields were two times lower within CS3 compared to CS1. Although a negative correlation was observed between the biomass of grasses and maize biomass, further analyses should be conducted to try dissociating the effects of the cropping system from the negative effects of the greater weed biomass. These results represent only the first year of collected data within this long-term experiment. In the future, these findings must be confirmed, but they suggest a strong importance of cropping practices and rotations on weed flora with potentially different effect on crop yield.