Electronic Thesis and Dissertation Repository


Master of Science




Dr. Robert Nurse


Glyphosate-resistant giant ragweed can be found in agricultural fields throughout the mid-western United States and southwestern Ontario. Environmental factors can influence growth and herbicide efficacy in C3 plant species. I measured the aboveground injury to resistant and susceptible seedlings for 28 d following glyphosate treatment to test the hypothesis that young leaf stages would be more susceptible to glyphosate under warm, dry, low-CO2 conditions. Glyphosate-resistance was not affected by environmental factors, leaf stage, or glyphosate dose, but plants grown at the highest temperature after spray had the least glyphosate injury. Resistant alleles may be associated with fitness penalties because they are rare in populations without herbicide selection pressures; however, in a greenhouse trial, resistant biotypes recovered from glyphosate injury and produced seeds. My results suggest that potentially stressful growth conditions and treatment at young growth stages will not improve the control of resistant giant ragweed biotypes.