Retention, Uptake, and Translocation of14C-Glyphosate from Track-Spray Applications and Correlation to Rainfastness in Velvetleaf (Abutilon theophrasti)1

Three commercial formulations of glyphosate were spiked with 14C-glyphosate and applied via a spray nozzle to velvetleaf plants. The use of 14C-glyphosate as a marker caused minimal alteration to formulation properties, and the use of spray application simulated field practices. Formulation retention, calculated based on maximum plant-leaf area, showed that only 27 to 33% of the available area retained the spray. The small differences in retention among the formulations suggest that they contribute little to differences in efficacy. Following spray application, plants were harvested at various times to measure the levels of glyphosate uptake into the plant and translocation into roots. Significant differences were observed among the formulations in the rate of glyphosate uptake. The most efficient formulation absorbed about one third of the dose by 24 hr after treatment. Root translocation of glyphosate was approximately proportional to uptake and accounted for less than one third of the absorbed dose. The relationship between uptake and rainfastness was examined in greenhouse studies with simulated rainfall at various times after glyphosate application. A direct correlation was observed between rainfastness with the speed and quantity of glyphosate uptake.Nomenclature: Glyphosate, N-(phosphonomethyl)glycine. Velvetleaf, Abutilon theophrasti Medik., #3 ABUTH.Additional index words: Uptake, translocation, retention, track spray, glyphosate formulations.Abbreviations: DAT, days after treatment; GI, growth inhibition; HAT, hours after treatment.