Effect of Glyphosate and Three Phenoxyacetic Acid Herbicides against Eichhornia crassipes (Mart) and Pistia stratiotes L.

Efficacy of four herbicides namely 2, 4-D Na salt, 2, 4-D ester, 2, 4-D amine salt and glyphosate against two floating aquatic weeds namely water hyacinth and Pistia is reported. Against tender and mature water hyacinth, the activity followed the order: 2, 4-D ester>2, 4-D Na salt = 2, 4-D amine salt = glyphosate. 2, 4-D ester @ 5 kg ha⁻¹ significantly controlled the water hyacinth in 3 days of contact. 2, 4-D Na and amine salts required 7 days, whereas glyphosate required 11 days of contact @ 10 kg ha⁻¹ to show significant effect on the mature weed. Glyphosate and 2, 4-D ester were equally and more effective in controlling Pistia than the other two herbicides at 5 and 10 kg ha⁻¹ for tender and mature weeds. It is judicious to apply the herbicides at the tender stage of the weeds to get maximum effect. 2, 4-D ester resulted in good control of water hyacinth at 2 to 5 kg ha⁻¹, but as it is toxic to fish, 2, 4-D Na and amine salts may be used at 5 to 10 kg ha⁻¹ in fisheries and aquaculture. In the field trial with mature water hyacinth and Pistia, no mortality of fish was observed in any treatment. 2, 4-D ester, and the sodium and amine salts @ 4, 10 and 10 kg ha⁻¹ respectively gave good control of water hyacinth (82.36 to 91.2%) within 15 days. Against Pistia, weed control efficiency of all the four herbicides was more or less similar (87.9 to 93.2%). In terms of effectiveness, environmental safety and cost of treatment, 2, 4-D Na salt was found to be best option for the control of both the weeds. However, to check herbicide resistance, glyphosate or 2, 4-D amine salt should also be used after few years of continuous application of 2, 4-D Na salt. Herbicide use to control aquatic weeds be avoided in areas where water is used for drinking purpose.