Abstract

The advent of crops resistant to glyphosate allows for a potential three to four crop rotation while using glyphosate as the sole weed management tactic. Increased selection pressure from continuous use of glyphosate in glyphosate resistant crops will likely cause a shift in the weed spectrum towards more naturally tolerant species. Common lambsquarters (Chenopodium album ) has been noted recently as having the ability to escape glyphosate treatment in resistant crops to produce seed either by avoidance mechanisms or a natural low-level tolerance to the herbicide. This research has elucidated several factors that contribute to reduced efficacy of glyphosate on common lambsquarters. It has been shown: (a) that biotypes exist within irrigated sites that exhibit differential tolerance to glyphosate, (b) that herbicide use history influences glyphosate tolerance, (c) that ammonium sulfate has a variable effect on glyphosate efficacy on common lambsquarters, and (d) environmental conditions strongly influence the ability to differentiate between biotypes. Numerous factors influence efficacy of glyphosate on this weed, including environmental conditions, genetic makeup, growth stage, glyphosate rate, commercial formulation, and addition of ammonium sulfate. Interactions between these factors further confound our ability to easily explain poor control or make uniform recommendations.