Effects of Glyphosate and Selective Herbicide Application Schemes on Weed Control and Species Diversity in Winter Wheat in the Zemgale Region of Latvia

This study aimed to compare different herbicide application schemes in winter wheat, including the application of glyphosate-containing herbicides in stubble and autumn or spring post-emergence application. Field experiments were conducted in 2022–2024, on two different farms, with two fields selected each year on each farm, where the pre-crop was either winter wheat (Triticum aestivum L.) or oilseed rape (Brassica napus L.). Glyphosate application had a significant effect on weed biomass reduction if the pre-crop was oilseed rape with herbicide application in one of the years, but not if it was winter wheat. The effect of glyphosate application also depended on the year and was most pronounced in the year when the sowing was performed later, reaching up to 57% weed biomass reduction without the application of post-emergence herbicides. A positive effect of glyphosate application on the grain yield was observed if the pre-crop was oilseed rape. The application of post-emergence herbicides resulted in a median weed biomass reduction of 95–96% of the untreated control. Additional herbicide application in spring did not improve weed control nor result in a yield increase. The likely causes of this are the competitive ability of winter wheat, as well as lower weed density and deeper stubble cultivation in the trials where the pre-crop was wheat. Further trials must be conducted in fields infested with grass-weed species to compare the effect of one versus two post-emergence applications. Post-emergence herbicide application, but not glyphosate application, had a negative effect on the alpha diversity of weed species, whereas higher weed diversity was associated with a higher yield increase when the pre-crop was winter oilseed rape, in the absence of highly competitive grass-weed species.