Assessing the combined effects of mechanical and chemical weeding on weed dynamics in winter wheat

AbstractMechanical weeding, such as harrowing, offers a promising approach for reaching the European Union's goal of a 50% reduction in pesticide use. To assess its potential, used alone or with foliar‐applied herbicides without residual activity, a 6‐year study was conducted on winter wheat cultivated in loamy soil under temperate conditions (Belgium). Weed density dynamics and percentage weed control (WC) were measured and compared between wheat tillering (BBCH 27‐29) and wheat canopy closure (BBCH 39‐75). Weeds were categorised as weeds maturing generally synchronously with wheat (OW), newly emergent (NE) weeds, or new weed species (NS) appearing in spring. The presence of OW was negatively correlated with yields (up to −0.44), while spring weeds had low or no impact on yield. Overall, one harrow pass reduced OW pressure significantly, with no statistical differences between one versus two passes (the second pass was performed several days after the first). The percentage of WC reached 92%–94% when herbicide was used in combination with harrowing (respectively for one and two harrow passes), and proved to be more effective than harrowing alone whose efficacy reached 64%–70% (respectively for two and one harrow passes). This study also found evidence effects of natural competition from wheat on weeds. Specifically, 54% WC was observed just after harrowing, and 70% was observed when the wheat canopy was closed. These findings suggest that an integrated weed management approach may involve an initial harrowing pass with subsequent evaluation of its effectiveness prior to commencing another intervention. In the event of poor efficacy, a full dose application of herbicide may be warranted.