Water Hyacinth Control by Glyphosate Herbicide and Its Impact on Water Quality

The aim of this study was to evaluate the herbicide glyphosate under aquatic environment conditions, in a controlled and closed experimental field, in the management of water hyacinth (Eichhornia crassipes) in reservoirs. Twenty reservoirs (polyethylene water tanks) with storage capacity of 1000 liters were used, without water flow and without evapotranspired water replacement (worst case), being 04 for each treatment. The adult plants were placed in the water tanks to provide 90% surface occupation of the reservoir. Five treatments with four repetitions were considered, being: 1) Reservoir colonized by water hyacinth without control;2) Reservoir colonized by water hyacinth, controlled by glyphosate;3) Reservoir colonized by water hyacinth, controlled by freezing;4) Reservoir without water hyacinth and glyphosate application and 5) Reservoir without water hyacinth and no glyphosate application. The glyphosate herbicide was used at the highest recommended dose, 7.0 L·ha-1 or 3360 g of acid equivalent per ha, applied using carbon dioxide precision equipment (backpack sprayer), providing a flow rate of 200 L·ha-1. The water samples were collected at the time of application, 6, 12, 18 and 24 hours after application and also at 2, 4, 8, 16, 32 and 64 days after application, in the morning, always at the same time, also between 8 and 9 h. The method used for determination of residues was by high performance liquid chromatography (HPLC) and mass spectrometry with a mass selective detector. Low concentrations of glyphosate and aminomethylphosphonic acid (AMPA) were found in both reservoirs that received application of the product. The half-life of glyphosate in water to the reservoirs with water hyacinth was 11 days and in the reservoirs without water hyacinth was 21 days. The results show a low potential of environmental impact of glyphosate use in the control of water hyacinth in reservoirs.

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.

Efficacy of Glyphosate in Water Management and the Effects on Water Quality

The present work aimed to evaluate the behavior of glyphosate herbicide in aquatic environment, considering some water quality parameters, in reservoir conditions without flow and without replacement of evapotranspired water. Twenty reservoirs (polyethylene water tanks) with a storage capacity of 1000 liters were used. The recommended herbicide dose according to the manufacturer was 7.0 L or 3360 grams of acid equivalent per hectare. For application in the reservoirs, a precision equipment was used to carbon dioxide (CO2), providing a flow of 200 L·ha-1. Freezing macrophyte death was achieved by storing the plants in a freezer at -18°C for a period of 48 hours. The analyses of control efficiency were performed at 7, 14, 21, 28, 35 and 42 days after application (DAA). Water samples for analysis of quality indicators, pH and dissolved oxygen were collected weekly for nine weeks. The obtained result allows concluding that the herbicide glyphosate presented excellent control efficiency of water hyacinth and did not significantly alter the water quality parameters pH and dissolved oxygen, indicating its use in the management of this macrophyte.