Tall weeds escaping early weed management, such as weeds resistant to one or multiple herbicides, are an increasing concern. When weeds reach a certain size, few options other than hand weeding will limit the production and dispersal of seeds. The objective of this project was to evaluate the efficacy of the Bourquin Organic Weed Puller® (a rotating series of wheels that grab and pull) at removing tall weeds before they shed seeds in soybean and adzuki bean. Trials were set up in Canada at the Agriculture and Agri-Food Canada research farms at Saint-Jean-sur-Richelieu (SJR), QC (2 yr), and Harrow (HAR), ON (1 yr), on a loamy and a sandy soil, respectively. The experimental design included crops of different potential heights (different soybean cultivars and adzuki bean), two weed species per location (common lambsquarters [both locations] and common ragweed or redroot pigweed at HAR), and two pulling dates. The set-up also included weedy and herbicide-treated control plots. Weeds overtopping the crop canopy by at least 10 cm were tagged and characterized. Damage from the weed puller was rated as 1, pulled (desired effect); 2, cut; 3, folded; 4, stripped; and 5, intact. The seed production of damaged and intact weeds was also recorded. Less than one-third of common ragweed or redroot pigweed plants were pulled during any treatment combination. The highest pulling rates were observed for common lambsquarters at SJR (43%), but very few were pulled at HAR (3.1% max). Pulling rates were not high enough to potentially control seed inputs from herbicide-resistant populations, and successfully pulled common lambsquarters left on the ground produced thousands of viable seeds.

Small weed patches may be noticed in fields after herbicide application, but they typically do not have a significant impact on the season’s crop yield. As a result, they are usually not treated as a threat to future yields. However, if these patches harbour weed biotypes resistant to one or multiple herbicides, resistance alleles can spread both spatially (via various dispersal pathways, including seed transport by machinery and commodity contamination) and temporally (through seed persistence). This poses a significant threat to herbicide-based weed management. Once these populations spread and cover a large enough area, eradication becomes improbable despite all the resistance management efforts. Therefore, a proactive and collaborative endeavour is needed to detect and manage small and patchy resistant weed populations. In this paper, we review the current potential of weed resistance detection using imagery and molecular markers as well as possible weed management approaches. Finally, we advocate for the use of a combination of these techniques to manage herbicide-resistant weeds when populations are small. This multifaceted approach is presently not applicable to all resistance mechanisms, and all weed species located in any crop, but could initially focus on biotypes and species that are easy to detect and represent the greatest threat.

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 artemisiifolia L.) 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 aestivum L.), soybean [Glycine max (L.) Merr.], or corn (Zea mays L.). 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 artemisiifolia seed 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% of A. artemisiifolia seeds were dispersed before harvest in corn and soybean.