Common ragweed (Ambrosia artemisiifolia) seed shattering in wheat, corn, and soybean

Before any late-season weed control operations are planned to manage herbicide-resistant weeds, it is essential to evaluate the plants' maturity and shattering potential. Our goal was to assess the seed-shattering phenology of common ragweed (Ambrosia artemisiifoliaL.) using pollination bags as seed traps. A secondary goal was to evaluate the efficiency of these traps. Trials were conducted from 2014 to 2017 at two locations in eastern Canada (Saint-Jean-sur-Richelieu, QC, and Harrow, ON). At each location, three adjacent fields were seeded with spring wheat (Triticum aestivumL.), soybean [Glycine max(L.) Merr.], or corn (Zea maysL.). Each field was divided into four replicate blocks that included two treatment plots with 5 weeds m(-2)planted on the same date as the crop or when crop plants had two leaves (early or late emergence). To evaluate shattering in time, the experiment included up to 12 weekly collection dates (subplots). In each subplot, weeds were individually bagged at flowering (using mesh bags) until collection, when the number and viability of shattered and retained seeds per plant was recorded. Weather data as well as crop and weed stages were recorded. The effect of the pollen bags on seed retrieval and viability was evaluated by installing open and closed bags in corn and uncropped (bare) plots at a single location.Ambrosia artemisiifoliaseed biomass was equivalent or higher in closed bags, and seed viability was equivalent or slightly reduced. No seeds were produced before harvest in spring wheat, as dispersal started in September. The percentage of seeds retained on the plant decreased linearly (1 site-year) or followed a logistic equation (4 site-years) with day of year or growing degree days. Dispersal in time was similar between early- and late-emerging weeds and similar in both corn and soybean. On average, more than 50% ofA. artemisiifoliaseeds were dispersed before harvest in corn and soybean.