2.4. Fomesafen dose–response bioassay

Palmer amaranth seeds of the susceptible, original AR11‐LAW‐B, C1 and C2 populations were planted in 11 × 11 cm square pots filled with Sunshine Mix LC1 potting soil (Sun Gro Horticulture Canada Ltd, Vancouver, Canada). Seedlings were thinned to five per pot. Seedlings, 7.5–9 cm tall, were sprayed with eight doses of fomesafen from 0 to 2109 g ha⁻¹, which corresponds to 0–8 times the recommended field dose. The SS population was sprayed with seven doses from 4 to 264 g ha⁻¹, corresponding to 1/64 to 1× the recommended dose, with a non‐treated check. The herbicide was applied with a 0.5% NIS as described in Section 2.2. The experiment was conducted twice in a randomized complete block design with five replications. At 21 DAT, visible injury and the number of survivors were recorded. The aboveground plant tissue was harvested, placed in a brown paper bag and dried at 60 °C for 48 h, and dry weights were recorded. Data were expressed as percentage of biomass reduction relative to the non‐treated control. The biomass data generated from two runs were pooled, as the test for homogeneity of variance showed that the variance across runs was similar. Regression analysis was conducted using SigmaPlot v.13 (Systat Software, Inc., San Jose, CA, USA). The percentage biomass reduction and mortality were fitted to a non‐linear, sigmoid, three‐parameter Gompertz regression model defined by

where Y is the biomass reduction expressed as a percentage of the non‐treated control or mortality percentage, a is the asymptote, b is the growth rate, c is the inflection point and x is the fomesafen dose. The dose needed to kill 50% (LD₅₀) of the population, or cause 50% biomass reduction (GR₅₀), was calculated from the above equation.