Glyphosate-resistant giant ragweed has been confirmed in several Midwestern states. In some cases, weed resistance to glyphosate has been shown to carry a fitness penalty. Previous research has found that a glyphosate-resistant giant ragweed biotype from Indiana with a rapid necrosis response to glyphosate displayed early, rapid growth in the absence of glyphosate, flowered earlier, but produced 25% less seed than a sensitive biotype, suggesting that there may be a fitness penalty associated with the rapid necrosis resistance trait. In Wisconsin, we have recently identified a giant ragweed accession with a 6.5-fold level of resistance to glyphosate that does not demonstrate the rapid necrosis response. Our objective was to determine the noncompetitive growth and fecundity of the resistant accession in the absence of glyphosate, relative to a sensitive accession from a nearby field border population. In greenhouse experiments, plant height, leaf area, and dry shoot biomass were similar between the resistant and sensitive accessions during vegetative growth to the onset of flowering. The instantaneous relative growth rate, instantaneous net assimilation rate, and instantaneous leaf area ratio also did not differ between accessions. However, fecundity of resistant plants (812 seeds plant⁻¹) was greater (P  =  0.008) than sensitive plants (425 seeds plant⁻¹). The percentage of intact viable seeds, intact nonviable seeds, and empty involucres did not differ between resistant and sensitive accessions. These results indicate that resistance of this accession of giant ragweed to glyphosate has not affected its growth and development relative to a sensitive accession. The greater fecundity and similar viability of resistant plants relative to sensitive plants suggests that in the absence of selection by glyphosate, the frequency of the resistance trait for glyphosate may increase in the giant ragweed field population over time.

Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR.