| dc.description.abstract | Giant ragweed (Ambrosia trifida L.) is a competitive annual plant found in disturbed landscapes and is the most troublesome weed in Indiana and the US Corn Belt. It is one of the most common and problematic weeds in corn and soybean production. The introduction of herbicide glyphosate, N-(phosphonomethyl) glycine in early 1970’s provided farmers with a better and low-cost tool to control weeds. The use of glyphosate drastically increased after the development of glyphosate resistant agronomic crops in 1996 and was use as a post-emergence selective herbicide. This led to overreliance and repeated use of glyphosate for weed control especially in roundup ready corn and soybean cropping systems and resulted in tremendous selection pressure for evolution of glyphosate resistant weeds and specifically giant ragweed in Indiana. Our research investigated the response of glyphosate-resistant (GR) and glyphosate-susceptible (GS) giant ragweed to glyphosate treatment under varying environmental conditions and the fitness and competitiveness of glyphosate resistant giant ragweed. Greenhouse studies investigated response of GS and GR giant ragweed biotypes to various doses of glyphosate and to light and temperature. GR plants had a unique response when treated with glyphosate, exhibiting initial rapid necrosis of mature leaves within 12 hours of treatment. GR plants do not die from a glyphosate treatment but resumed normal-growth from axillary meristems and reproduce. The progression of the response and symptoms resemble a typical hypersensitive response similar to that observed on some plants after pathogen attack. GS plants do not exhibit rapid leaf necrosis but their leaves become xiii chlorotic, then necrotic and plants die over a 2-3 week period. The GR50 for GS and GR biotypes were 426.49 g ae ha-1 and 860.87 g ae ha-1 and the estimated GR90 for GS and GR biotypes were 515.28 g ae ha-1 and 3338. 39 g ae ha-1 respectively indicating that the dose required to achieve 90% of GS biotype was lower than the recommended field rate of 700 g ae ha-1 and amount to achieve 90% control of GR biotypes was almost 5 times greater than the recommended glyphosate rate. The GR50 was two times greater for GR than GS and GR90 was 6.5 times greater for GR than GS. The results also show that short period dark treatment and different temperatures did not improve glyphosate activity on giant ragweed but delayed the injury caused by the herbicide. Field study results showed that in absence of glyphosate, when GR and GS plants were grown independently, the GS plants grow taller than GR plants and GR plants were wider than GS plants. However, no differences occur in leaf area, seed production and total plant biomass. Competition studies showed that when grown in mixtures, the two biotypes were similar in total biomass and seed production. The results show that resistance trait in GR plants is not associated with growth penalty and do not incur ecological cost in field. Results of this research confirm the presence of glyphosate resistant giant ragweed in Indiana and show that giant ragweed can survive high doses of glyphosate. Results also show that manipulating environmental conditions specifically during the short-term, post spraying period have no major influence in glyphosate performance on giant ragweed. | en_US |
