Half-Life of Glyphosate on the Control of Water Hyacinths in Water Tanks

The present study had the purpose to assess the behavior of the herbicide glyphosate in the control of water hyacinths, in a water environment through half-life, in a controlled and closed experimental field, in order to contribute to the analysis of the risk of the environmental impact of the use of this product. Eight fiber-cement tanks with the storage capacity of 250 liters were used under the conditions without water flow and without replacement of water. The experiment was designed in two treatments with four repetitions, the first being colonized by water hyacinths and the second without the presence of the macrophyte. The recommended dose, according to the manufacturer, was 7.0 L·ha-1 or 3402 g·ha-1 of equivalent acid. For the application in the tanks, we have used carbon dioxide (CO2) precision equipment, providing a flow rate of 200 L·ha-1. The samples were collected on the day of the application (day zero) and 2, 4, 8, 16, 32 and 64 days thereafter (DAP). The method used for determination of residues was by high performance liquid chromatography (HPLC) and mass spectrometry with a mass selective detector. From the mathematical model of charge decay, the half-life of glyphosate in water was estimated to six days for the tanks without water hyacinths and sixteen days for the tanks colonized by macrophytes. The result obtained permits to conclude that the glyphosate in water is quickly degraded in closed water environments and does not leave residues that would prevent its use.