Multi-year assessment of seed shedding for economically important grass weed species in Italy and the UK

Harvest Weed Seed Control (HWSC) tactics aim to reduce weed dissemination and are considered promising approaches for future Integrated Weed Management (IWM) strategies. To be effective however, HWSC requires that target species have high seed retention at crop harvest. Here, a multi-year assessment of seed shedding was conducted across large geographical areas in the UK and Italy, for pernicious grass weed species that infest winter wheat and soybean crops. In the UK, an eight year assessment of Alopecurus myosuroides seed shedding was carried out in winter wheat crops. In Italy, seed shedding studies were conducted for three years, assessing A. myosuroides, Avena spp. and Lolium perenne ssp. multiflorum in winter wheat, and Sorghum halepense and Echinochloa crus-galli in soybean crops. Our results demonstrate low levels of seed retention (approximately 20 %) for A. myosuroides and Avena spp. at harvest, while higher mean seed retention (49 %) was found for L. perenne ssp. multiflorum. As such, Avena spp. and A. myosuroides are not good targets for HWSC across the studied locations, while HWSC could significantly contribute to L. perenne ssp. multiflorum management if combined with further control tactics. Seed retention at soybean harvest was on average 50 % for E. crus-galli, but higher at approximately 75 % for S. halepense. HWSC could therefore have a considerable impact on S. halepense populations in Italian soybean fields, but only an intermediate-low impact on E. crus-galli populations. Importantly however, we also find evidence for significant spatial and temporal variability in the extent of seed retention for all species. This study demonstrates that the potential for HWSC varies considerably between target weed species and highlights the importance of inter-annual variation in determining its expected performance.