Modelling the long-term impact of harvest weed seed control for species like Bromus diandrus and Hordeum spp. that shed a portion of seed prior to harvest

Harvest weed seed control (HWSC) is an increasingly popular integrated weed management tactic to catch, destroy or remove weed seeds at crop harvest. However, most weed species shed some seed prior to crop harvest, and we have little data on the value of HWSC for those species with high or variable rates of seed shedding. In the current study, Bromus diandrus and Hordeum spp. were identified as problematic weeds with variable rates of seed shedding. Field data on B. diandrus and Hordeum spp. density, seed production and resulting crop yield were used to validate the output of a decision support tool: the Weed Seed Wizard. The tool was then used to generate a six-year crop rotation scenario for each weed species. The level of annual HWSC in the scenario was set at 1%, 20%, 40%, 60%, 80% and 100% seed removal. The results indicated that B. diandrus in particular was difficult to control with herbicide alone and a conventional harvest (1% weed seed removed as a contaminant of the grain). By comparison, the total weed seed bank in the soil at the end of six years could be reduced by 46% using HWSC to annually destroy 20% of seed at crop harvest. Hordeum spp. were easier to control with herbicide, but HWSC still changed final seed density in the soil seed bank. Therefore, HWSC may be an effective management tactic for those weed species with early seed shedding, but this needs to be confirmed with field based research