Weed Structure and Yield Response to Crop Sequence and Chemical Protection in Long-Term Experiment with Winter Wheat

Long-term simplification of cropping systems and crop protection practices promotes increased weed infestation and may lead to yield decline. The aim of this study was to assess the effect of crop sequence systems and levels of crop protection on weed infestation, weed community diversity, and grain yield of winter wheat under long-term field experiment conditions. The research was conducted in a static field experiment established in 1967 in Bałcyny, Poland. Two cropping systems were analyzed, monoculture and a six-course crop rotation, combined with three levels of protection: no protection, herbicide, and herbicide + fungicide. Weed density, biomass, and species composition were evaluated, as well as diversity indices (Shannon–Wiener and Simpson) and grain yield. Monoculture significantly increased weed density and biomass, promoting the dominance of competitive species such as Apera spica-venti and Centaurea cyanus. In the crop rotation system, lower weed infestation, higher species diversity, and a more even community structure were observed. The application of herbicide effectively reduced weed infestation; however, it led to a decline in species diversity and an increase in the dominance of tolerant species. Grain yield was strongly negatively correlated with the level of weed infestation. The highest yields were obtained in the crop rotation system with full chemical protection, whereas monoculture resulted in a significant yield reduction regardless of the level of protection. These results highlight the key role of crop rotation and integrated crop protection in reducing weed infestation and stabilizing winter wheat yields.