Response of Giant Ragweed (Ambrosia trifida), Horseweed (Conyza canadensis), and Common Lambsquarters (Chenopodium album) Biotypes to Glyphosate in the Presence and Absence of Soil Microorganisms

Abstract In previous research conducted on nonweed species, the efficacy of glyphosate was shown to be greater in unsterile soils compared to sterile soils and soil microorganisms were found to play an important role in glyphosate efficacy. Conducting greenhouse studies in microbe-free soil may therefore produce unreliable data, leading to erroneous conclusions. The objective of this study was to determine the effect of soil microorganisms on the response of glyphosate-resistant and -susceptible biotypes of three problematic weeds of the midwestern United States: giant ragweed, horseweed, and common lambsquarters. A greenhouse dose–response study was conducted on each of the three weed species grown in sterile and unsterile field soil, and the dry weight response of roots and shoots was measured. The three weed species responded differently to glyphosate when grown in the sterile and unsterile soil; that is, in the presence and absence of soil microbes. Soil microbes influenced the response of the suscept...