Putatively resistant (PR) and putatively susceptible (PS) common waterhemp populations were grown in the greenhouse and sprayed at the three- to four-leaf stage with glyphosate (0.63 kg ae/ha). Surviving plants from PR populations and randomly selected plants from PS populations were clonally propagated and the clones were sprayed with 0.1 to 10.0 kg/ha glyphosate. The glyphosate rates required to reduce growth by 50% (GR50) among the clones were relatively similar, but the concentration required to reduce growth by 90% (GR90) ranged from 1.5 to 16.3 kg/ha. The concentration of glyphosate required to kill 90% of plants (LD90) ranged from 2.3 kg/ha to over 10.0 kg/ ha. This range of responses to glyphosate in common waterhemp clones from different parts of the Midwestern United States indicates a risk of evolution of resistance in common waterhemp populations that are repeatedly selected by applications of glyphosate in the field.
