ORIGINAL ABSTRACT

Glyphosate is a widely used herbicide in commercial crop production for both conventional and herbicide-resistant crops. Herbicide-resistant crops, like glyphosate-resistant sugar beet, are often exposed to multiple applications of glyphosate during the growing season. The fate of this herbicide in resistant crops has not been publicly documented. We investigated the fate of glyphosate and main metabolite aminomethylphosphonic acid in glyphosate-resistant sugar beet grown in northern Colorado. Glyphosate residues were measured via directed ultra-high-performance liquid chromatography tandem mass spectrometry analysis of sugar beet shoots and roots throughout the growing season, from samples collected at various steps during sugar beet processing, and from flow-through samples of greenhouse-grown beets. Sugar beet rapidly absorbed glyphosate after foliar application, and subsequently translocated the herbicide to its roots, with between 2 and 3 μg/g fresh weight measured in both tissue types within 1 week of application. However, only trace amounts of glyphosate remained in either the shoots or the roots 2 weeks after application. Analysis of irrigation flow-through in pot assays confirmed that the herbicide readily exuded out of the roots. Processing of the beets removed glyphosate and herbicide levels were below the limit of detection in the crystalline sugar final product.