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.