The inheritance, fitness, and control of glyphosate-resistant giant ragweed

Giant ragweed is one the most competitive and problematic weeds in corn and soybean fields in the Midwest. Giant ragweed is difficult to control because of an early and extended germination period, rapid growth rate, inherent tolerance to herbicides, and has now evolved resistance to glyphosate. The main goal in this study was to determine the fate of glyphosate resistance in a giant ragweed population. Our objectives were to determine the inheritance of glyphosate resistance, fitness, and control of a glyphosate-resistant (GR) giant ragweed biotype from Indiana. In a glyphosate dose response experiment, the LD 50 values of GR, glyphosate-susceptible (GS), R[female]S[male] F 1, and S[female]R[male] F1 plants at 21 DAT were 3,142 g, 73 g, 2,181 g, and 774 g ae ha-1, respectively. At 400 g ae ha-1 of glyphosate, the backcross progeny of GS x S[female]R[male] F1 segregated according to our null hypothesis of 1 R: 1 S for mortality at 21 DAT, but not for necrosis at 3 DAT. The results indicated glyphosate resistance is manifested by a single major, semi-dominant to dominant gene, depending on the maternal parent, and is transferable through pollen and/or seed. Also, our results demonstrated that rapid necrosis did associated with plant survival at 21 DAT, but the continuous phenotypic variation observed suggested resistance is influenced by environmental conditions, epistatic interactions, or by additional minor genes. In the absence of glyphosate, GR and GS plants had a similar growth pattern, but during reproduction, GR plants flowered earlier and produced 25% less seed than GS plants. In the presence of glyphosate at 840 g followed by 2,520 g ae ha-1, plants with the GR trait were the superior biotype and required additional methods of control, rather than glyphosate alone. Tank mixtures of glyphosate plus fomesafen controlled 15 cm tall GR plants ≤65%; however, reduced rates of fomesafen tank mixed with glyphosate or glufosinate at the label field use rate did provide ≥90% control of 20 to 30 cm tall GS giant ragweed. Our results demonstrate control of GR giant ragweed will not be easy and the fate of glyphosate resistance in giant ragweed will continue to persist, and could potentially spread rapidly via pollen and/or seed mediated gene flow in our current glyphosate-dominated agricultural ecosystem