We conducted a greenhouse study to evaluate the differential response of Palmer amaranth to
glyphosate and mesotrione and to quantify the level of tolerance to mesotrione in recalcitrant
(difficult-to-control) accessions and their offspring. Seeds were collected from 174 crop fields (corn,
cotton, and soybean) across Arkansas between 2008 and 2016. Palmer amaranth seedlings (7 to
10cm tall) were treated with glyphosate at 840 g ae ha–1 or mesotrione at 105 g ha–1. Overall, 47%
of the accessions (172) were resistant to glyphosate with 68% survivors. Almost 35% of accessions
were highly resistant, with 90% survivors. The majority of survivors from glyphosate application
incurred between 31% and 60% injury. Mesotrione killed 66% of the accessions (174); the
remaining accessions had survivors with injury ranging from 61% to 90%. Accessions with the least
response to mesotrione were selected to determine tolerance level. Dose–response assays were
conducted with four recalcitrant populations and their F1 progeny. The average effective doses
(ED50) for the parent accessions and F1 progeny of survivors were 21.5 g ha–1 and 27.5 g ha–1,
respectively. The recalcitrant parent populations were three- to five-fold more tolerant to
mesotrione than the known susceptible population, as were the F1 progeny.

doi: 10.1017/wet.2018.34