Glyphosate resistance screening
A population ID was assigned to each locality (Table 1). Each population ID consists of the abbreviated state/province of origin, a unique identifying number, and its designation as GR or GS (e.g., CO1R = Colorado resistant population 1). To determine glyphosate susceptibility and resistance, a screening was performed in the greenhouse. Seeds from each population were planted in germination flats. After emergence, seedlings were transplanted into 18-insert (7×7 cm pots) flats containing custom mix potting soil (Fafard, Sun Gro Horticulture, Agawam, MA, USA), and grown at 23°C under a 14 h light/10 h dark cycle with supplemental light from sodium halide lamps. Plants were watered daily and fertilized once (Miracle-Gro, Scotts Miracle-Gro Company, Marysville, OH). When plants were 3 weeks old, nine to 18 plants per population (Table 1) were sprayed with commercially formulated glyphosate (Roundup WeatherMax) in distilled water at 0.84 kg ae ha-1. Glyphosate applications were performed using a moving flat-fan nozzle (8002EVS) in a laboratory spray chamber at 156 L ha-1 spray volume. Three weeks after herbicide treatment, individual survival for each population was assessed. Populations for which at least one individual survived were classified as GR.